Biomedical Instrumentation- II

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This section covers the need for cardiac pacing, external and implantable pacemakers, the need for defibrillation, the DC defibrillator and its waveforms, and the implantable pacer-cardioverter-defibrillator.

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Need for a Cardiac Pacemaker • The heart's own conduction system (1.4) fires from the SA node at 60-100/min, the AV node and Purkinje system acting as slower escape pacemakers.

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When that system fails, the ventricles beat too slowly or not at all, producing fatigue, dizziness, syncope (Stokes-Adams attacks), heart failure or death. • Indications: symptomatic sinus bradycardia and sick sinus syndrome, complete (third-degree) and symptomatic second-degree atrioventricular block, bifascicular and trifascicular block, drug- or infarction-related bradycardia, carotid sinus hypersensitivity, and — for the specialised devices — cardiac resynchronisation in heart failure and overdrive pacing for certain tachyarrhythmias. • Principle: a pulse generator delivers a brief electrical stimulus that depolarises the myocardium (capture).

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The threshold is the minimum energy that reliably captures; typical outputs are 2-5 V for 0.4-0.5 ms, programmed with a safety margin of about twice the threshold voltage.

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Type Description and use External (temporary) transcutaneous Large adhesive pads on the chest wall delivering 40-80 mA over 20-40 ms; immediate, non-invasive and available on any modern defibrillator, but painful and requires sedation, and capture must be confirmed mechanically (a pulse), not merely by the ECG spikes.

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For emergency use only External temporary transvenous A pacing wire passed through a central vein to the right ventricle, connected to an external pulse generator; reliable and better tolerated, used for days while awaiting recovery or a permanent system Epicardial temporary Wires sutured to the heart at cardiac surgery and brought out through the chest wall Implantable (permanent) Titanium can with a lithium-iodine battery (7-12 years) and one or more transvenous leads; single chamber (VVI or AAI), dual chamber (DDD) or biventricular (CRT).

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See the NBG code in 3.1 Leadless A self-contained capsule implanted directly in the right ventricle, avoiding a pocket and leads • Modes of operation: asynchronous (fixed rate, VOO/AOO) — paces regardless of the intrinsic rhythm, with the danger of competition and a stimulus landing on the T wave (R-on-T), which can provoke ventricular fibrillation; and demand (synchronous, inhibited — VVI/DDD) — the generator senses the intrinsic beat and withholds its own stimulus, which is the normal mode.

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Rate-responsive (R) devices raise the rate with activity, using an accelerometer or minute-ventilation sensor. • Problems: failure to capture (output too low, lead displacement, fibrosis, electrolyte disturbance), failure to sense (undersensing, giving competition) or oversensing (of myopotentials or interference, causing inappropriate inhibition), lead fracture and insulation failure, pocket infection, pacemaker syndrome (loss of AV synchrony in VVI pacing) and electromagnetic interference, including from surgical diathermy (6.2) and MRI unless the system is MR-conditional.

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Need for a Defibrillator • Ventricular fibrillation is chaotic, disorganised electrical activity in which the myocardium quivers without coordinated contraction, so cardiac output ceases immediately; it is the commonest initial rhythm in sudden cardiac arrest and is invariably fatal within minutes unless corrected.

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Ventricular tachycardia without a pulse has the same consequence. • Principle of defibrillation: a brief high-energy current passed through the heart depolarises a critical mass of myocardium simultaneously, placing it all in refractoriness so that the fibrillation wavefronts are extinguished and the SA node can resume control.

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Defibrillation does not 'restart' a stopped heart — it stops a chaotically active one.

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Consequently asystole and pulseless electrical activity are not shockable rhythms, a point examiners like. • Survival falls by roughly 7-10 % for every minute of delay, which is why early defibrillation and the automated external defibrillator matter so much.

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The DC Defibrillator • Why DC and not AC: early AC defibrillators were dangerous and ineffective; the DC defibrillator, which discharges a capacitor through the chest, is more effective, needs less energy, causes less myocardial damage and can be synchronised. • Circuit: a step-up transformer and rectifier charge a high-voltage capacitor (about 16-32 μF) to several thousand volts; a relay or solid-state switch then discharges it through an inductor (which shapes the pulse and prevents an excessively sharp spike) and the paddles or pads into the patient.

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The delivered energy in joules is ½CV², and only part of it reaches the heart, the rest being dissipated in the chest wall. • Waveforms: the monophasic damped sinusoidal (Lown) waveform delivers current in one direction, typically requiring 200 → 300 → 360 J.

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The biphasic waveform reverses polarity partway through the discharge and is now standard: it is more effective at lower energy (120-200 J), causes less myocardial injury and automatically compensates for transthoracic impedance. • Transthoracic impedance (typically 70-80 Ω) determines how much of the delivered energy reaches the heart, and is reduced by conductive gel or gel pads, firm paddle pressure (about 8 kg), correct pad size and position, and shaving excessive hair; shocks are delivered in expiration. • Electrode position: anterolateral (right parasternal second-third intercostal space and left mid-axillary line at the apex) or anteroposterior.

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Internal paddles for use directly on the heart require only 10-50 J. • Defibrillation versus cardioversion — the key distinction: defibrillation is asynchronous, given for VF and pulseless VT, because there is no organised R wave to synchronise with; synchronised cardioversion is used for organised rhythms with a pulse (atrial fibrillation and flutter, SVT, VT with a pulse), the shock being timed to the R wave to avoid the vulnerable period of the T wave, where a shock could provoke ventricular fibrillation.

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Cardioversion usually requires lower energy and sedation. • The automated external defibrillator (AED) analyses the rhythm itself through the pads, decides whether a shock is indicated and guides the operator by voice prompts — designed so that a layperson can use it safely. • Safety: warn and ensure that nobody is touching the patient or the bed before discharge, keep away from oxygen and flammable agents, avoid placing pads over a pacemaker (at least 8 cm away), dry the chest, and test and maintain the device regularly (capacitors and batteries degrade).

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Defibrillator-protected patient connections are required on all monitoring equipment (5.1).

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Pacer-Cardioverter-Defibrillator (ICD) • The implantable cardioverter-defibrillator combines the functions of a pacemaker, a cardioverter and a defibrillator in one implanted device.

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It is indicated for secondary prevention (survivors of cardiac arrest or sustained VT) and, more commonly now, primary prevention in patients at high risk — typically those with a left ventricular ejection fraction below about 35 %, and in inherited arrhythmia syndromes. • Construction: a titanium can (which itself acts as one electrode), a high-voltage capacitor and transformer, a battery, sensing and pacing circuitry, and a lead carrying both pace/sense electrodes and one or two high-voltage shocking coils. • Operation — tiered therapy: the device continuously monitors the rate and morphology of ventricular events and classifies them into programmed zones.

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For slower VT it first attempts anti-tachycardia pacing (ATP) — a rapid burst of pacing that can terminate re-entry painlessly; if that fails, or for faster VT, it delivers a synchronised cardioversion shock; and for VF it delivers an unsynchronised defibrillation shock, typically up to 30-40 J (far less than external shocks because the current is delivered directly).

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It also provides bradycardia backup pacing after the shock and stores electrograms for later review. • Problems: inappropriate shocks — the major clinical problem, usually caused by atrial fibrillation with rapid ventricular response, supraventricular tachycardia, T-wave oversensing or lead fracture — which are painful and psychologically distressing; lead failure; infection; and battery depletion.

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Subcutaneous ICDs avoid intravascular leads at the cost of being unable to pace. • Related devices:

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CRT-P and CRT-D (biventricular pacing with or without defibrillation) for heart failure with dyssynchrony, and the wearable cardioverter-defibrillator vest for temporary risk.

1

This section covers the principle of surgical diathermy, monopolar and bipolar electrosurgical machines, safety aspects of electrosurgical units, the principle of lasers, the main medical laser types and laser safety.

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Principle of Surgical Diathermy • Surgical diathermy (electrosurgery) uses a high-frequency alternating current passed through tissue to generate heat by the tissue's own resistance, the power dissipated being P = I²R per unit volume.

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Because the current density is enormously concentrated at the small active electrode tip and very low at the large return plate, the heating — and therefore the surgical effect — occurs only at the tip. • Why high frequency is the central examinable point: nerve and muscle can only be stimulated by currents below about 100 kHz; above that the depolarisation of excitable membranes cannot follow the alternations, so the current produces heat without stimulating nerve or muscle and without causing electrocution or ventricular fibrillation.

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Diathermy therefore operates at 300 kHz to about 3 MHz (commonly 400-500 kHz).

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It must not be confused with medical diathermy for heating (6.3) or with mains-frequency current, which at these levels would be lethal. • Tissue effects and waveforms: cutting uses a continuous, low-voltage, high-current sinusoidal waveform that heats the cells so rapidly that their water flashes to steam and they explode (vaporisation), parting the tissue with little haemostasis; coagulation uses an interrupted (damped, modulated) high-voltage, low-duty-cycle waveform that heats more slowly, denaturing protein and sealing vessels; blend mixes the two; fulguration sprays sparks across an air gap to char a bleeding surface; and desiccation dries the tissue by direct contact.

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Typical outputs are 50-400 W at several hundred to several thousand volts.

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Monopolar diathermy Bipolar diathermy Current path From the active electrode through the whole patient to a large dispersive return (patient) plate and back to the generator Between the two tips of the forceps only — the current path is a few millimetres of tissue Return plate Essential — large area, good contact, well vascularised muscle, away from bony prominences, scars, metal implants and ECG electrodes Not required Power Higher (up to several hundred watts) Much lower (typically under 50 W) Effects available Cutting, coagulation, blend, fulguration, desiccation Mainly coagulation; some cutting on modern units Precision Less precise; deeper and wider thermal spread Very precise, minimal lateral spread Safety Risk of burns at the plate or at alternative earth points, capacitive coupling, and interference with pacemakers Much safer — the preferred choice in neurosurgery, ophthalmic and microsurgery, on appendages (digits, penis), and in patients with pacemakers Safety Aspects in Electrosurgical Units • Burns are the commonest injury.

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At the return plate they arise from partial detachment, poor or dried contact, a plate that is too small, hair, scar or bone beneath it, or fluid soaking the adhesive; modern generators therefore incorporate a contact quality monitor (return electrode monitoring, REM) using a split plate whose impedance is measured continuously, cutting power if contact deteriorates.

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Burns may also occur at alternative earth paths — ECG electrodes, temperature probes, metal table parts — which is why isolated (floating) generator outputs replaced earth-referenced designs. • Other hazards: direct coupling (the active tip touching another instrument); capacitive coupling in laparoscopy, where the active electrode's insulated shaft capacitively transfers energy to a metal cannula or to bowel — especially with hybrid metal-plastic cannulas and high-voltage coagulation current; insulation failure of long laparoscopic instruments, causing unseen burns outside the field of view; fire and explosion, since the tip is an ignition source in an oxygen-enriched atmosphere or near alcohol-based skin preparation and bowel gas; surgical smoke (plume), which contains viable particles, viral DNA and carcinogens and must be evacuated; interference with pacemakers, ICDs and monitors; and stimulation of muscle and nerve if low-frequency components leak. • Practical rules: place the plate correctly and as close to the operative site as practical; use the lowest effective power and report to the surgeon if repeated increases are requested (a sign of a plate problem); keep the active electrode in an insulated holster when not in use; inspect insulation; activate only under direct vision; use bipolar wherever possible; avoid pooling of flammable preparations; and test and maintain the generator, including its alarms.

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Principle of Lasers • LASER — Light Amplification by Stimulated Emission of Radiation.

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The process rests on Einstein's stimulated emission: an atom already in an excited state, struck by a photon of exactly the transition energy, emits a second photon identical in wavelength, phase, polarisation and direction.

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To amplify rather than absorb, a population inversion is required — more atoms in the excited than in the ground state — which is produced by pumping (optical flash lamp or diode, electrical discharge, chemical or injection current).

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Amplification takes place in an optical resonant cavity between two mirrors, one fully and one partially reflecting, the partially reflecting mirror allowing the beam to emerge. • The four properties that make laser light surgically useful: monochromaticity (a single wavelength, so absorption by a chosen chromophore can be selected), coherence (all waves in phase), collimation (a parallel beam with minimal divergence, so it can be delivered over distance and through fibres) and high intensity/brightness (enormous power concentrated into a small spot). • Tissue interaction depends on wavelength (which determines the absorbing chromophore — water, haemoglobin or melanin), power density, spot size and exposure time.

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Effects in ascending order of energy are photochemical (photodynamic therapy), photothermal (coagulation 60-100 °C, vaporisation above 100 °C, carbonisation above 200 °C), photoablation, and photodisruption by plasma formation (as in Nd-YAG capsulotomy).

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Operation may be continuous wave or pulsed (including Q-switched and mode-locked), pulsing allowing very high peak power with limited thermal spread — the basis of selective photothermolysis.

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Laser Wavelength / medium Absorption and clinical use Pulsed ruby 694.3 nm, chromium-doped aluminium oxide crystal, flash-lamp pumped, pulsed; the first laser ever built (Maiman, 1960) Deep red light strongly absorbed by melanin; used for tattoo and pigmented lesion removal and hair reduction; historically for retinal photocoagulation Nd-YAG 1,064 nm, neodymium in yttrium aluminium garnet; continuous or Q-switched Poorly absorbed by water, so it penetrates 3-5 mm and scatters widely — the deepest-acting surgical laser, giving excellent coagulation of large vessels and tumour debulking through an endoscope; the Q-switched form performs posterior capsulotomy after cataract surgery; frequency-doubled to 532 nm (KTP) it is absorbed by haemoglobin Helium-neon (He-Ne) 632.8 nm, gas, continuous, low power (mW) Visible red; used as an aiming beam for invisible infrared lasers, in alignment, in laboratory instruments (flow cytometers, bar-code readers) and in low-level laser (biostimulation) therapy Argon ion 488 nm (blue) and 514.5 nm (green), gas, continuous Strongly absorbed by haemoglobin and melanin and transmitted by clear ocular media — the classic laser for retinal photocoagulation in diabetic retinopathy, trabeculoplasty, and vascular skin lesions such as port-wine stains Laser Wavelength / medium Absorption and clinical use CO₂ 10,600 nm (10.6 μm) far infrared, gas, continuous or pulsed Very strongly absorbed by water, so almost all energy is deposited within 0.1 mm — the precise cutting/vaporising laser with minimal thermal damage.

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Used as a light scalpel in ENT, gynaecology and neurosurgery, and for skin resurfacing and wart/lesion vaporisation.

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Cannot be transmitted by ordinary glass fibre, requiring an articulated mirror arm or special waveguide, and needs a He-Ne aiming beam Semiconduct or (diode) 800-980 nm and others, p-n junction pumped by injection current Compact, efficient, cheap, robust and fibre-deliverable; used in photocoagulation, endovenous vein ablation, dentistry, hair removal, pulse oximeters, and as the pump for solid-state lasers Excimer 193 nm (ArF) ultraviolet Photoablation without heat — LASIK and PRK corneal refractive surgery, and coronary atherectomy Er-YAG / dye / holmium 2,940 nm / tunable / 2,100 nm Water absorption even higher than CO₂ (skin and dental ablation); tunable dye lasers for vascular lesions and photodynamic therapy; holmium for lithotripsy of urinary stones and prostate enucleation Laser Safety • Classification (IEC 60825):

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Class 1 safe under all conditions;

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Class 1M/2/2M visible, safe because of the blink reflex;

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Class 3R/3B hazardous to the eye on direct viewing;

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Class 4 — all surgical lasers — hazardous to eye and skin from direct, reflected and even diffusely scattered radiation, and a fire risk. • The eye is the critical organ, and the injury depends on wavelength: visible and near infrared (400-1,400 nm — the 'retinal hazard region', which includes ruby, argon, He-Ne, diode and Nd-YAG) are transmitted by the ocular media and focused by the lens onto the retina, amplifying the irradiance about 100,000 times and causing a permanent retinal burn or blind spot; whereas far infrared (CO₂) and ultraviolet (excimer) are absorbed by the cornea, causing corneal burns and keratitis. • Controls: a designated controlled area with warning signs and interlocked doors, a laser safety officer, restricted access, wavelength-specific protective eyewear of the correct optical density for everyone present, including the patient, matt/anodised non-reflective instruments, removal of flammable and reflective materials, wet drapes and swabs, a fire extinguisher and water immediately available, a key switch and standby mode with foot-switch control, the lowest effective power, and smoke evacuation of the laser plume, which carries the same hazards as diathermy smoke. • Airway fire during laser surgery of the larynx is the classic catastrophe: it requires laser-resistant endotracheal tubes, cuffs filled with saline (often dyed), the lowest tolerable inspired oxygen concentration, avoidance of nitrous oxide, and a rehearsed drill for extinguishing the fire.

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This section covers high-frequency heat therapy, short-wave and microwave diathermy, therapeutic ultrasound, electrodiagnostic and therapeutic stimulators, and pain relief by electrical stimulation.

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High-Frequency Heat Therapy • Therapeutic heating (medical diathermy) raises tissue temperature to about 40-45 °C in order to produce vasodilatation and increased blood flow, increased metabolic rate, relief of muscle spasm, increased extensibility of collagen and connective tissue, reduction of pain and acceleration of tissue repair.

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This is quite distinct from surgical diathermy (6.2), which destroys tissue. • Superficial heating (hot packs, paraffin wax, infrared) reaches only a few millimetres; deep heating requires short-wave, microwave or ultrasound diathermy. • General contraindications to deep heating, which are heavily examined: metal implants and metallic foreign bodies (which concentrate current and burn), cardiac pacemakers and ICDs, pregnancy (the gravid uterus), malignancy, acute inflammation, infection, haemorrhage or thrombophlebitis, impaired sensation or impaired circulation (the patient cannot warn of overheating), over the eyes and testes, over epiphyses in children, and in the confused or uncooperative patient.

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Short-Wave Diathermy (SWD) • Frequency 27.12 MHz (wavelength 11 m), internationally allocated; older machines also used 13.56 and 40.68 MHz.

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It is the deepest-penetrating of the electrical modalities, capable of heating tissue several centimetres down. • Two methods of application: the capacitive (condenser) field method, in which the part is placed between two capacitor plates and becomes the dielectric — the oscillating field causes rotation of dipoles and vibration of ions, generating heat, and because fat has high impedance and is heated preferentially, this method heats superficial fat more than muscle; and the inductive (cable or drum/monode) method, in which a coil carrying the current induces eddy currents in the tissue, heating tissues of low impedance — muscle and other fluid-rich tissue — preferentially, and so giving deeper, more useful heating. • Machine: a high-frequency oscillator (push-pull or Colpitts) and power amplifier, an output tuning circuit that must be resonated with the patient circuit, electrodes or a drum applicator, a timer and output meter, all within a screened cabinet.

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The patient circuit is tuned to resonance for maximum energy transfer. • Pulsed short-wave diathermy (PSWD) delivers the same energy in short bursts with long intervals, so the mean temperature rise is negligible; it is claimed to have non-thermal effects on healing and oedema and can be used where heat is contraindicated (for example over recent injury and, in some protocols, over metal). • Safety: a safe distance of 1-2 m from the machine and cables for the operator (and further for pregnant staff), no metal (including jewellery, watches, underwired garments, metal chairs or beds), dry towelling to absorb perspiration, no concentration of field on moist skin folds, and careful questioning about sensation and implants.

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Microwave Diathermy (MWD) • Frequency 2,450 MHz (wavelength 12.25 cm), sometimes 915 MHz.

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Energy is generated by a magnetron and radiated from a directional antenna in an applicator (circular, rectangular or contact) at a set distance from the skin — so, unlike short-wave, it is a radiated (antenna) rather than a conducted field, needs no tuning to the patient, and is simpler to apply. • Absorption is greatest in tissues of high water content — muscle, skin and blood — while fat is relatively transparent, which is an advantage; but its penetration is limited to about 3 cm, less than short-wave. • Particular hazards: the eye is highly susceptible because the lens is avascular and cannot dissipate heat — microwave exposure can cause cataract, so the eyes must be shielded or the head avoided; the testes are similarly vulnerable; overheating of subcutaneous fat-muscle interfaces and of oedematous or moist tissue; and the same list of general contraindications as short-wave, with special emphasis on metal implants, pacemakers and pregnancy.

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Ultrasonic Therapy Unit • Therapeutic ultrasound uses a piezoelectric (PZT) transducer driven by a high-frequency oscillator to produce a mechanical wave transmitted into the tissue through a coupling gel, which is essential because air reflects virtually all the energy (4.4). • Parameters: frequency 1 MHz for deep structures (penetration about 3-5 cm) and 3 MHz for superficial ones (about 1-2 cm) — the higher frequency being absorbed more rapidly; intensity 0.5-3 W/cm² (spatial average); continuous mode for thermal effects and pulsed mode (for example 1:4 duty cycle) for predominantly non-thermal effects; and a treatment time of 5-10 minutes with the applicator kept constantly moving to avoid hot spots and standing waves. • Effects: thermal — deep heating, greatest at interfaces of differing impedance and especially at bone-soft tissue interfaces (causing periosteal pain if the intensity is too high), with collagen-rich tissue (tendon, ligament, capsule, scar) absorbing strongly; and non-thermal — stable cavitation, acoustic streaming and micromassage, which are held to stimulate membrane transport, fibroblast activity and repair. • Quality measures: the beam non-uniformity ratio (BNR) — the ratio of peak to average intensity, which should be low (ideally under 6) because a high BNR means damaging hot spots — and the effective radiating area (ERA). • Uses: soft-tissue injury, tendinopathy, adhesions and scar tissue, joint contracture, and phonophoresis (driving topical drugs through the skin).

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Contraindications: over the eye, gravid uterus, heart, cancer, active epiphyses, testes, spinal cord after laminectomy, ischaemic or anaesthetic areas, thrombophlebitis and over a pacemaker; also never hold the transducer stationary at high intensity.

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Electrodiagnostic and Therapeutic Apparatus • Electrodiagnosis in physiotherapy assesses the integrity of nerve and muscle by their response to stimulation.

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The classical tests are the strength-duration curve — a plot of the stimulus intensity needed to produce a minimal contraction against the pulse duration, from which are read the rheobase (the minimum intensity that will excite with a very long pulse) and the chronaxie (the duration required at twice the rheobase) — a denervated muscle shows a curve shifted up and to the right with a greatly increased chronaxie; faradic and galvanic stimulation and the reaction of degeneration, in which denervated muscle fails to respond to short faradic pulses but still responds sluggishly to long galvanic ones; and, in modern practice, nerve conduction studies and EMG (5.1). • Therapeutic stimulation: neuromuscular electrical stimulation (NMES/FES) for muscle re-education, prevention of disuse atrophy, improvement of circulation and functional electrical stimulation of paralysed muscle; interferential therapy, in which two medium-frequency currents (for example 4,000 and 4,100 Hz) are crossed within the tissue so that they beat to produce a low-frequency (0-250 Hz) therapeutic current deep inside, while the medium frequency crosses the skin with low impedance and little discomfort;

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Russian and diadynamic currents; iontophoresis (driving ionised drugs through the skin with direct current); and biofeedback. • Stimulator parameters: waveform (monophasic or, preferably, biphasic and charge-balanced, to avoid electrochemical skin damage), pulse amplitude, pulse duration (width) and frequency, on-off ratio, ramp, and electrode size and placement.

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Pain Relief Through Electrical Stimulation • TENS (transcutaneous electrical nerve stimulation) delivers pulsed current through surface electrodes to relieve pain, and is one of the standard examination topics because its two modes rest on two different mechanisms. • Conventional (high-frequency, low-intensity) TENS:

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50-100 Hz at short pulse width (50-100 μs), producing a comfortable tingling without muscle contraction.

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It works by the gate control theory of Melzack and Wall — stimulation of large-diameter A-beta afferents closes the 'gate' in the substantia gelatinosa of the dorsal horn and inhibits transmission of nociceptive impulses in the small A-delta and C fibres.

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Its onset is rapid but its effect lasts only while the stimulation continues. • Acupuncture-like (low-frequency, high-intensity) TENS:

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1-5 Hz at long pulse width (150-250 μs), strong enough to produce visible muscle twitching.

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It acts by stimulating the release of endogenous opioids (endorphins and enkephalins) and descending inhibition, with a slower onset but longer-lasting relief, and it can be reversed by naloxone — the classic proof of the mechanism. • Burst and modulated modes combine the two and reduce accommodation. • Uses: chronic musculoskeletal pain, low back pain, neuropathic pain, labour pain, postoperative pain and dysmenorrhoea.

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Contraindications and cautions: demand pacemakers and ICDs, over the anterior neck (carotid sinus — risk of hypotension and laryngospasm), over the eyes, transcerebrally, over the gravid uterus in early pregnancy, on broken or anaesthetic skin, and in patients unable to report discomfort.

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Skin irritation under the electrodes is the usual minor complication. • Other electrical pain therapies: spinal cord stimulation (an implanted epidural electrode for refractory neuropathic pain and failed back surgery syndrome), peripheral nerve and deep brain stimulation, and percutaneous electrical nerve stimulation.

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This section covers the artificial kidney, dialysers and membranes, the haemodialysis machine and its monitoring systems, conventional and modern lithotripters, and extracorporeal shock wave therapy.

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The Artificial Kidney:

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Principles • Haemodialysis substitutes for the excretory function of the failed kidney (1.5) by passing blood on one side of a semipermeable membrane and dialysate on the other.

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Three transport processes are involved, and their distinction is examinable: • Diffusion — movement of solute down its concentration gradient across the membrane; this removes urea, creatinine, potassium and phosphate and supplies bicarbonate and calcium.

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It is most effective for small molecules, and is maximised by counter-current flow of blood and dialysate, which maintains the concentration gradient along the whole length of the dialyser. • Ultrafiltration — movement of water across the membrane under a transmembrane pressure (TMP), which removes the fluid accumulated between treatments.

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The ultrafiltration coefficient KUF (mL/h/mmHg) characterises the membrane; high-flux membranes have a high KUF and require volumetric ultrafiltration control. • Convection (solvent drag) — solute carried along with the ultrafiltered water, which is the main route for middle molecules such as β₂-microglobulin and is exploited in haemofiltration and haemodiafiltration. • Indications for dialysis are remembered as AEIOU — Acidosis, Electrolyte disturbance (hyperkalaemia), Intoxication, Overload (fluid), Uraemia (pericarditis, encephalopathy), plus end-stage renal disease.

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Adequacy is measured by Kt/V and the urea reduction ratio, a typical schedule being 4 hours three times a week with blood flow 300-400 mL/min and dialysate flow 500 mL/min.

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Vascular access is by arteriovenous fistula (the best), graft or central venous catheter.

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Dialysers and Membranes Dialyser type Description Coil The original Kolff design — membrane tubing wound in a coil; obsolete, with high blood volume and poor ultrafiltration control Parallel plate Alternating layers of membrane and support; superseded Hollow fibre (capillary) The universal modern design:

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8,000-15,000 hollow fibres of about 200 μm internal diameter in a cylindrical housing, blood inside the fibres and dialysate counter-current outside, giving a large surface area (1.0-2.2 m²) in a small priming volume with low resistance and efficient exchange • Membrane materials: cellulosic (regenerated cellulose, cuprophane) — cheap and effective for small solutes, but low flux and strongly complement-activating because of their surface hydroxyl groups, causing the first-use reaction described in 2.6; modified cellulose (cellulose acetate, Hemophan) — less activating; and synthetic (polysulphone, polyethersulphone, polyacrylonitrile, PMMA, polyamide) — now standard, being far more biocompatible, available as high-flux membranes with larger pores that clear middle molecules, and mechanically stronger. • Membrane characteristics: clearance (KoA) for a given solute, ultrafiltration coefficient, surface area, priming volume, sieving coefficient (the fraction of a solute passing with the filtrate), biocompatibility and sterilisation method (steam, gamma or ethylene oxide — the last carrying a risk of hypersensitivity if residues remain).

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Membranes must be permeable to uraemic toxins but impermeable to albumin and cells. • Dialysate is prepared on line by proportioning purified water with acid and bicarbonate concentrates, and contains sodium, potassium, calcium, magnesium, chloride, bicarbonate and glucose at physiological concentrations, with potassium and calcium adjusted to the patient.

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Water quality is critical, since a patient is exposed to some 300-400 litres a week across a membrane: the treatment train is filtration, softening, carbon adsorption (removing chlorine and chloramine), reverse osmosis, deionisation and ultraviolet/ultrafilter, with regular chemical and microbiological testing.

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Historic disasters — aluminium encephalopathy and dialysis dementia, chloramine-induced haemolysis, and endotoxin-related pyrogenic reactions — all arose from inadequate water treatment.

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The Haemodialysis Machine • The machine comprises an extracorporeal blood circuit and a dialysate delivery circuit, with an extensive monitoring and alarm system. • Blood circuit: arterial line from the access → blood pump (a peristaltic roller pump, chosen because it does not contact the blood) → heparin infusion pump → dialyser → venous line with air/bubble trap and venous pressure monitor → air detector (ultrasonic) and venous line clamp → back to the patient.

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Arterial and venous pressure monitors detect access problems, disconnection and clotting. • Dialysate circuit: treated water → heater (to 37 °C) → deaerator → proportioning pumps mixing acid and bicarbonate concentrate → conductivity meter and pH monitor → flow past the dialyser counter-current to the blood → blood leak detector (photo-optical) → ultrafiltration control → drain. • The essential monitors and what each protects against: conductivity — incorrect dialysate composition, which can cause fatal haemolysis or osmotic disturbance (with a bypass valve that diverts dialysate away from the dialyser when out of range); temperature — haemolysis from overheating and hypothermia from cold dialysate; air detector with venous clamp — fatal air embolism; blood leak detector — membrane rupture; arterial and venous pressure — disconnection, infiltration and clotting; transmembrane pressure and volumetric ultrafiltration control — over- or under-removal of fluid. • Anticoagulation is required because the whole circuit activates the intrinsic pathway (2.6): usually unfractionated or low-molecular-weight heparin, with heparin-free saline-flush or citrate regional anticoagulation in patients at risk of bleeding. • Complications of dialysis: hypotension (the commonest), cramps, nausea, headache, disequilibrium syndrome (cerebral oedema from too-rapid urea removal in the first sessions), arrhythmia, haemolysis, air embolism, reactions to the membrane or sterilant, infection of the access, and long-term β₂-microglobulin amyloidosis. • Related therapies: haemofiltration (pure convection with replacement fluid), haemodiafiltration (both), continuous renal replacement therapy (CRRT) for the haemodynamically unstable patient in intensive care, and peritoneal dialysis, which uses the patient's own peritoneum as the membrane and needs no machine — an important option in countries with limited dialysis infrastructure.

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Lithotripters • Extracorporeal shock wave lithotripsy (ESWL) fragments urinary stones with focused shock waves generated outside the body, so that the fragments can be passed in the urine — introduced in the early 1980s, it transformed stone surgery. • Principle: a shock wave is a very short, high-amplitude pressure pulse (a steep positive spike of tens of megapascals lasting about a microsecond, followed by a negative tail).

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Because it is a mechanical wave it passes through water and soft tissue of similar acoustic impedance with little effect, but at the interface with the much harder stone the large impedance mismatch produces reflection, compressive and tensile stresses, spallation and cavitation — and it is cavitation on the stone surface that does much of the damage.

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Typically 2,000-4,000 shocks are delivered, at a rate of 60-120 per minute. • Three methods of generating the wave: electrohydraulic (spark gap) — a spark discharged under water creates a plasma bubble whose expansion generates the shock, focused by an ellipsoidal reflector, the spark at the first focus (F1) and the stone at the second focus (F2) — the original Dornier principle, powerful but with a short-lived electrode and variable output; electromagnetic — a coil repels a metal membrane, creating a plane wave focused by an acoustic lens, giving reproducible output and long life; and piezoelectric — thousands of small crystals on a spherical dish converge their output at the centre of curvature, giving a very precise focus and low entry pain but limited power. • The conventional (first-generation) lithotripter — the original Dornier HM3 — required the patient to be immersed in a water bath for coupling, used X-ray fluoroscopic localisation in two planes, generally needed general or regional anaesthesia because of the large entry aperture and pain, and was a large dedicated installation. • Modern lithotripters: a water cushion with coupling gel instead of a bath, a smaller focal spot with a larger aperture, so entry pressure and pain are much reduced and treatment is possible under sedation alone, dual localisation by fluoroscopy and ultrasound, ECG gating (firing on the R wave in the refractory period to avoid inducing arrhythmia), respiratory tracking, multifunctional tables allowing endourological procedures, and compact mobile units that can be moved between hospitals. • Indications and limits: stones up to about 2 cm in the renal pelvis or upper ureter.

19

Contraindications: pregnancy, uncorrected coagulopathy or anticoagulation, untreated urinary infection or obstruction distal to the stone, severe obesity or skeletal deformity preventing focusing, and an abdominal aortic aneurysm near the field; caution with pacemakers.

20

Complications: haematuria (almost universal), perirenal haematoma, steinstrasse (a column of fragments obstructing the ureter), residual fragments, infection and sepsis, and hypertension after repeated treatment.

21

Alternatives are ureteroscopy with a holmium laser and percutaneous nephrolithotomy. • Extracorporeal shock wave therapy (ESWT) applies the same technology at much lower energy to musculoskeletal and other soft-tissue conditions, where the aim is not fragmentation but biological stimulation — neovascularisation, growth factor release, altered pain signalling and breakdown of calcific deposits.

22

It is delivered as focused or radial (a ballistic pressure wave, which is not strictly a shock wave) therapy, in sessions of 1,500-2,500 pulses.

23

Uses include plantar fasciitis, calcific tendinitis of the shoulder, lateral epicondylitis, greater trochanteric pain, non-union of fractures, chronic wounds and erectile dysfunction.

24

It is avoided over lung tissue, large vessels and nerves, the gravid uterus, growth plates in children, and in coagulopathy or local infection and malignancy.

1

This section covers the anaesthesia machine and its electronic components, artificial ventilators with their types, terms and modes, and drug infusion systems including syringe and volumetric pumps.

2

The Anaesthesia Machine • The anaesthesia machine (anaesthesia workstation) delivers a precisely controlled mixture of medical gases and volatile anaesthetic agent at a safe pressure and flow, provides ventilation, scavenges waste gas and monitors the patient. • Gas supply enters from the pipeline (about 4 bar / 400 kPa) or from cylinders (oxygen at about 137 bar) with a pressure regulator reducing it to working pressure.

3

Safety features here include the pin index system for cylinders and the non-interchangeable screw thread (NIST/DISS) for pipelines, colour coding, non-return valves, and a pressure gauge for each gas. • Flow control: flowmeters (rotameters), tapered glass tubes with a bobbin, individually calibrated for each gas, with oxygen conventionally last (downstream) in the manifold so that a leak in another tube cannot dilute it.

4

Modern machines use electronic flow sensors and gas mixers. • Vaporiser: converts liquid volatile agent into vapour at an accurate concentration.

5

The standard design is a temperature-compensated, flow-compensated, agent-specific plenum vaporiser with a variable bypass and a splitting ratio, and keyed filling systems to prevent the wrong agent being put in.

6

Desflurane requires an electrically heated and pressurised vaporiser because of its low boiling point. • Breathing system: the common gas outlet feeds a circuit — the circle system with a soda-lime absorber being standard, allowing low-flow anaesthesia — with unidirectional valves, a reservoir bag, an APL valve and the ventilator. • Essential safety systems, which are directly examinable: the oxygen failure alarm and cut-off device (which alarms and shuts off nitrous oxide if oxygen pressure falls); the hypoxic guard / oxygen-nitrous oxide ratio interlock, which makes it mechanically or electronically impossible to deliver less than about 25 % oxygen; the oxygen flush (35-75 L/min); non-return and pressure-relief valves; and scavenging of waste anaesthetic gases. • Electronic parts: the modern workstation is a computer-controlled system containing electronic gas mixers and flow sensors, an electronically controlled ventilator with pressure and flow transducers, agent and gas analysers (infrared absorption for CO₂ and volatile agents, paramagnetic or fuel-cell/galvanic oxygen analysers, and side-stream or main-stream sampling), integrated patient monitoring (ECG, NIBP, SpO₂, capnography, temperature, neuromuscular and depth-of-anaesthesia monitoring), alarms and displays, an automatic self-test and leak check at start-up, an uninterruptible power supply and battery backup, and data logging with electronic record keeping. • Capnography deserves emphasis: it measures end-tidal CO₂ by infrared absorption, and is the definitive confirmation of tracheal intubation and of continuing ventilation, as well as reflecting cardiac output and detecting circuit disconnection, rebreathing, bronchospasm and air embolism from the waveform shape.

7

Artificial Ventilators • An artificial ventilator moves gas in and out of the lungs when the patient cannot ventilate adequately.

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Indications: apnoea, respiratory failure (hypoxaemic or hypercapnic), airway protection, general anaesthesia with muscle relaxation, raised intracranial pressure and exhaustion. • Negative versus positive pressure — the fundamental distinction: negative pressure ventilators (the 'iron lung', cuirass and jacket types) apply subatmospheric pressure to the outside of the chest, expanding it and drawing air in through the natural airway — physiologically identical to normal breathing, needing no tracheal tube; but they are bulky, restrict nursing access, do not protect the airway and are now used only in a few neuromuscular patients.

9

Positive pressure ventilators instead push gas into the lungs through a tracheal tube, tracheostomy or a tight-fitting mask, reversing the normal pressure relationship.

10

This is universal in modern practice but accounts for the characteristic complications: reduced venous return and cardiac output, barotrauma and volutrauma, ventilator-associated pneumonia, and fluid retention. • Classification of positive-pressure ventilators by the cycling variable — what terminates inspiration: volume cycled (stops when a set volume has been delivered — guarantees tidal volume but pressure varies with compliance); pressure cycled (stops at a set pressure — tidal volume varies, so a fall in compliance reduces ventilation); time cycled (stops after a set time — used in neonatal and pressure-controlled ventilation); and flow cycled (stops when inspiratory flow falls to a set fraction of its peak — the basis of pressure support).

11

Ventilators are also classified by the control variable (pressure, volume or flow) and by the trigger (time for a mandatory breath; pressure or flow triggering for a patient-initiated breath).

12

Term Meaning Tidal volume (VT) Volume delivered per breath — now set at 6-8 mL/kg predicted body weight, and 6 mL/kg or less in ARDS (lung-protective ventilation) Respiratory rate and minute volume Breaths per minute; minute volume = VT × rate FiO₂ Fraction of inspired oxygen, 0.21 to 1.0 — kept as low as possible to avoid oxygen toxicity PEEP Positive end-expiratory pressure — pressure maintained at the end of expiration to prevent alveolar collapse, recruit lung and improve oxygenation; excessive PEEP reduces venous return and risks barotrauma CPAP Continuous positive airway pressure in a spontaneously breathing patient — the equivalent of PEEP without mandatory breaths Peak and plateau pressure Maximum airway pressure and the pressure during an inspiratory pause; plateau pressure reflects alveolar distension and should be kept below about 30 cmH₂O I:E ratio Inspiratory to expiratory time, normally 1:2, lengthened in obstruction and sometimes inverted in severe ARDS Compliance and resistance ΔV/ΔP of the respiratory system, and the resistance of the airways — derived from the pressure and flow measurements Sensitivity / trigger The effort required for the patient to initiate a breath Sigh, dead space, auto-PEEP A periodic large breath; wasted ventilation; gas trapped by incomplete expiration • Basic modes:

13

CMV / controlled mandatory ventilation — the ventilator does all the work at a set rate and volume or pressure, used in the paralysed or apnoeic patient; assist-control (A/C) — every breath is fully supported, whether triggered by the patient or by time;

14

SIMV (synchronised intermittent mandatory ventilation) — a set number of mandatory breaths synchronised with the patient's effort, between which the patient may breathe spontaneously, usually with pressure support;

15

PSV (pressure support ventilation) — every breath is patient-triggered and supported to a set pressure and flow-cycled, the patient controlling rate, timing and to a large extent volume, which makes it the standard weaning mode;

16

PCV and VCV (pressure- and volume-controlled);

17

CPAP/BiPAP for non-invasive ventilation; and the advanced modes PRVC, APRV and high-frequency oscillatory ventilation. • Components of a ventilator: gas supply and blender, a drive mechanism (piston, bellows, or, in modern machines, a high-performance proportional solenoid valve or turbine), inspiratory and expiratory valves, a humidifier (heated or HME), filters, a patient circuit with water traps, and the sensors — pressure, flow (pneumotachograph, hot wire or ultrasonic) and oxygen — with a microprocessor controller, display of waveforms and loops, and a comprehensive alarm set (high and low pressure, disconnection, apnoea, high and low minute volume, FiO₂, and power or gas failure, with battery backup). • Biomedical applications and hazards: intensive care, anaesthesia, neonatal care, transport and home ventilation.

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Hazards include disconnection (the commonest and most lethal), barotrauma, oxygen toxicity, ventilator-associated pneumonia, tube blockage or displacement, humidification failure and alarm mismanagement — so regular checking, circuit management and staff training are inseparable from the machine.

19

Drug Infusion Systems • Why infusion pumps: many drugs must be given continuously, accurately and at low rates — inotropes, sedatives, analgesics, insulin, chemotherapy, anticoagulants, parenteral nutrition — where gravity drip sets are far too imprecise, especially in neonates and in critically ill patients. • Components of an infusion system: the drug reservoir (syringe, bag or cassette), the pumping mechanism, a drive motor with a controller, the administration set and patient access, the user interface for programming rate, volume and dose, sensors and alarms (occlusion pressure, air-in-line, end of infusion, door open, low battery), a battery, and, increasingly, a drug library with dose error reduction software ('smart pump'), hard and soft limits, and network connectivity for infusion data. • Syringe pumps: a lead screw driven by a stepper motor advances the plunger of a standard syringe.

20

They give very accurate, smooth, low-rate delivery (as little as 0.1 mL/h) with small volumes, making them the choice for potent drugs, neonates and target-controlled anaesthetic infusions; their limits are the small reservoir volume and the mechanical slack and syringe compliance that cause a start-up delay and a bolus on release of an occlusion — which is clinically important with vasoactive drugs. • Volumetric (peristaltic) pumps: usually linear peristaltic fingers acting on the giving set, delivering larger volumes (up to 1,000 mL/h or more) for fluids, nutrition and antibiotics, with accuracy typically ±5 %.

21

Other types are rotary peristaltic, cassette, piston and elastomeric (balloon) pumps, the last being mechanical, disposable and needing no power — useful for ambulatory chemotherapy and postoperative analgesia.

22

Implantable pumps deliver intrathecal baclofen or morphine and insulin. • Patient-controlled analgesia (PCA) lets the patient self-administer a preset bolus with a lockout interval and optional background infusion and maximum dose limits, exploiting the fact that the patient titrates to their own requirement — a straightforward example of feedback by the patient. • Closed-loop control in infusion systems: the pump adjusts its own rate from a measured physiological variable, rather than from a rate set by the clinician — the control engineering topic of the section.

23

Examples: insulin delivery driven by a continuous glucose monitor (the artificial pancreas / closed-loop insulin system); vasopressor infusion driven by arterial blood pressure; anaesthetic infusion driven by a depth-of-anaesthesia index such as the BIS; target-controlled infusion (TCI), which is in fact open-loop but model-based, computing the rate from a pharmacokinetic model to achieve a target plasma or effect-site concentration; and closed-loop oxygen therapy driven by SpO₂.

24

The essential elements are a reliable sensor, a control algorithm (PID, model-predictive or fuzzy) and safety limits, and the critical engineering problems are sensor failure and artefact, time delays and lag between infusion and effect, patient variability, and the need for fail-safe behaviour and clinician override. • Hazards: free flow when the set is removed from the pump without an anti-free-flow clamp, programming and dose errors (the commonest, hence smart pumps), occlusion with a delayed bolus, air embolism, extravasation, siphoning if the syringe is placed above the patient, and drug incompatibility in a shared line.

1

This section covers electric shock hazards including macroshock and microshock, leakage currents and their limits, methods of protection, and the electrical safety analyser and routine testing.

2

Electric Shock Hazards • It is current, not voltage, that injures.

3

The effect depends on the magnitude of the current, its path through the body, the duration of contact, the frequency, and the area and condition of contact — and the current that flows is determined by Ohm's law with the body's impedance, which is dominated by the skin: dry intact skin may be 15,000-100,000 Ω, but wet or broken skin falls to about 1,000 Ω, and internal body resistance is only about 300-500 Ω.

4

This is why moisture, gel, sweat and broken skin are so dangerous in a clinical setting. • Frequency matters: the body is most sensitive around 50-60 Hz, which is precisely the mains frequency; sensitivity falls sharply above a few kilohertz, which is why surgical diathermy at 300 kHz-3 MHz can pass large currents safely (6.2).

5

Direct current requires about five times the alternating current to produce the same effect.

6

Current (50/60 Hz, hand to hand) Effect ~1 mA Threshold of perception — a faint tingle ~5 mA Maximum harmless current; accepted as the safe let-go limit for the general public 10-20 mA 'Let-go' current — sustained involuntary muscle contraction; the victim cannot release the conductor ~50 mA Pain, possible fainting, respiratory muscle paralysis and exhaustion 100 mA - 3 A Ventricular fibrillation — the usual cause of death in electrocution > 6 A Sustained myocardial contraction, then normal rhythm may return; burns and respiratory paralysis dominate ~10 μA delivered directly to the myocardium Microshock — ventricular fibrillation • Macroshock is current applied to the intact skin surface, spreading through the body so that only a small fraction passes through the heart; the danger threshold is therefore in the milliampere range. • Microshock is current delivered directly to the myocardium through a low-resistance intracardiac pathway — a pacing wire, an intracardiac catheter, a fluid-filled pressure line or a central venous catheter — so that the entire current is concentrated in a small volume of heart muscle.

7

As little as 10 μA can cause fibrillation — a current far too small to be felt, which is what makes it so insidious.

8

Any patient with such a connection is electrically susceptible, and every device connected to them must be of the CF (cardiac floating) class. • Where the hazard comes from: faulty equipment (a live conductor touching the case), broken or missing protective earth (the classic cause of a case becoming live), damaged flexible cords and plugs, wet environments and spills, multiple devices connected to one patient creating potential differences between their earths, unauthorised repairs, and the use of domestic equipment in the clinical area. • Other electrically related hazards: burns from diathermy and from high-density current, fire and explosion in oxygen-enriched atmospheres, electromagnetic interference with monitors and implanted devices, and failure of life-support equipment on power loss — hence emergency supplies and battery backup.

9

Leakage Currents • Leakage current is the small current that flows from the mains parts of equipment to earth, or to accessible parts, by paths other than the intended circuit — even when the equipment is working perfectly.

10

It arises unavoidably from the capacitance and imperfect resistance of insulation, the capacitance between the mains transformer windings, and stray capacitance of the wiring and chassis, and it increases with the size and age of the equipment. • The four types defined by the standards, which are commonly asked: earth leakage current — from the mains parts through the insulation into the protective earth conductor; enclosure (chassis/touch) leakage current — from the enclosure through a person touching it to earth; patient leakage current — flowing from or to the patient through the applied part to earth; and patient auxiliary current — flowing between parts of the applied part through the patient (for example between two ECG electrodes) in normal use. • Conditions of measurement: values are measured both in normal condition (NC) and in single fault condition (SFC) — such as an open protective earth, an open supply conductor or mains voltage appearing on an applied part — and the limits are relaxed in SFC, because the philosophy of the standard is that no single failure should create a hazard. • Typical order of the limits under IEC 60601-1, which should be quoted as approximate and verified against the current edition: earth leakage 500 μA NC and 1,000 μA SFC; enclosure leakage 100 μA NC and 500 μA SFC; patient leakage 100 μA NC and 500 μA SFC for type B and BF applied parts, but only 10 μA NC and 50 μA SFC for type CF applied parts — the CF figures being set directly by the microshock threshold. • Classification of the applied part:

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Type B — earthed applied part, suitable for external use only;

12

Type BF — floating (isolated) applied part, for contact with the body including invasive contact but not the heart;

13

Type CF — floating with the lowest leakage limits, suitable for direct cardiac application.

14

The symbols (a human figure, a figure in a box, and a heart in a box) appear on the equipment label.

15

Equipment is separately classified by protection class — Class I (protective earth), Class II (double insulation), and internally powered.

16

Protection Against Electrical Hazards • Protective earthing (Class I): all exposed conductive parts are bonded to earth by a low-resistance conductor, so a fault current flows to earth and operates the fuse or circuit breaker instead of passing through a person.

17

Its integrity is the single most important routine test — earth continuity resistance should be below about 0.1-0.2 Ω. • Double insulation (Class II): two independent layers of insulation, so that no single failure can make an accessible part live; no protective earth is needed. • Isolated (floating) patient circuits: the applied part is separated from earth and from the mains by transformer or optical isolation, so that current cannot flow from the patient to earth through the equipment.

18

This is what makes BF and CF classification possible and is fundamental to every modern patient monitor. • Equipotential bonding: all metal surfaces and equipment earths in the patient's vicinity are connected to a common bus, so that no potential difference can exist between two items the patient or staff might touch simultaneously — the key protection against microshock. • Isolated power supply with a line isolation monitor, used in operating theatres and other wet locations: the supply is floated with respect to earth, so a first fault does not cause current to flow, and the monitor alarms so the fault can be corrected before a second one occurs. • Residual current device (RCD/GFCI): trips when the current in live and neutral differs by more than about 30 mA (or 6-10 mA for personal protection) — effective against macroshock but not against microshock, since 10 μA is far below its threshold. • Administrative and practical measures: purchase to standards, commissioning tests, a planned preventive maintenance and electrical safety testing programme with records, staff training, prompt removal of faulty equipment from service, prohibition of unauthorised repairs and of domestic multiway adaptors, cable and plug inspection, keeping fluids away from equipment, and never using two-pin plugs or defeating the earth pin.

19

The Electrical Safety Analyser • An electrical safety analyser is a dedicated test instrument that performs, in a defined sequence, the electrical safety measurements required by IEC 62353 (in-service testing) and IEC 60601-1 (design and type testing).

20

It replaces a collection of meters with one instrument that applies the correct measuring circuit and records the results. • The standard tests it performs:

21

(1) visual inspection — the most valuable single check, covering the plug, cord, strain relief, case, controls and labels;

22

(2) protective earth (ground) continuity resistance, measured with a high test current (typically 10-25 A or 200 mA depending on the standard) between the earth pin and exposed metal, with a limit of about 0.1-0.2 Ω (0.3 Ω including a detachable cord);

23

(3) insulation resistance, measured at 500 V DC between mains parts and earth or applied parts, expected in the megohm range;

24

(4) earth leakage current;

25

(5) enclosure (touch) leakage current;

26

(6) patient leakage current, including with mains voltage applied to the applied part for BF and CF parts;

27

(7) patient auxiliary current; and (8) functional/performance checks of the device itself. • Key features of the instrument: a measuring device (MD) network that mimics the frequency-dependent response of the human body (so that the reading represents the physiologically relevant current rather than the raw value); the ability to reverse the mains polarity and to open the earth and neutral conductors in order to create the single fault conditions; selectable applied part type (B, BF, CF) and class; automatic test sequences; and storage, printing and transfer of results to the equipment management database. • When testing is done: on acceptance/commissioning of new equipment, after any repair or modification, at defined intervals determined by risk and by the manufacturer's recommendation (commonly 6-12 months, more often for high-risk or mobile devices), and whenever a fault is suspected.

28

Records must be kept, and equipment failing a test must be labelled and withdrawn from service immediately. • Safety of the tester: the analyser itself applies mains voltage under fault conditions, so it must be used by trained staff on a properly earthed supply, never with a patient connected, and with the device under test isolated from the patient at all times.