Principles of Defibrillators, Pacemakers, Ventilators and Pulse Oximetry

The four devices at a glance

All four devices either measure a body signal or support a failing organ: the defibrillator and pacemaker act on the heart's electrical system, the ventilator moves air for the lungs, and the pulse oximeter watches how well the blood is oxygenated.

Device	Organ system	What it does	Core physical principle
Defibrillator	Heart (electrical)	Delivers one large shock to stop a chaotic rhythm	Capacitor stores energy, E = ½CV², discharged through the chest
Pacemaker	Heart (electrical)	Delivers small, repeated pulses to keep a minimum heart rate	Sense the heart's own beat; stimulate only when it is missing
Ventilator	Lungs	Pushes gas into the lungs and lets it out	Positive pressure overcomes airway resistance and lung compliance
Pulse oximeter	Blood	Measures oxygen saturation (SpO₂) and pulse rate without a needle	Beer–Lambert law: oxy- and deoxyhaemoglobin absorb red and infrared light differently

Each section below follows the same order: principle → main parts (with a diagram) → how it works step by step → key numbers → safety points. A revision table and practice questions close the notes.

1. Defibrillator

A defibrillator stops a chaotic heart rhythm by sending one brief, high-energy current through the chest so that most of the heart muscle depolarises at the same instant; the heart's own pacemaker (the SA node) can then restart a normal beat.

Principle

•         In ventricular fibrillation (VF) different patches of the ventricle contract at random, so the heart quivers instead of pumping. Blood pressure drops to zero within seconds.

•         A shock depolarises a critical mass of the myocardium at once. All those cells enter their refractory period together, the chaos stops, and the heart is briefly electrically silent.

•         During that silence the SA node, which fires on its own, takes over again and a normal rhythm can return.

•         A shock does not "jump-start" a stopped heart. It only resets disorganised activity, so asystole and PEA are not shockable.

Where the energy comes from

The energy is built up slowly in a capacitor and released in milliseconds. The stored energy is:

E = \frac{1}{2} C V^{2}

E is energy in joules, C is capacitance in farads and V is the capacitor voltage in volts.

Worked example: to store 200 J in a 100 µF capacitor, V = √(2E / C) = √(400 / 0.0001) = 2000 V.

The current that reaches the heart depends on the transthoracic impedance, typically about 50 to 150 Ω in adults. Firm pad contact, gel and a dry chest keep it low.

Main parts

Defibrillator block diagram · charge path above, discharge path below

The top row charges the capacitor over a few seconds; the bottom row delivers the stored energy to the patient in a few milliseconds when the relay closes.

In real use

Figure 1b · Defibrillator in real use: pad placement, AED and the monitor trace before and after a shock

On a real monitor, VF looks like an irregular wavy line with no clear QRS complexes. The shock appears as one tall spike, and a successful shock is followed by organised beats with P, QRS and T waves.

How a shock is delivered, step by step

1.       Attach pads or paddles to the bare chest: one below the right clavicle, one at the cardiac apex (left mid-axillary line).

2.       Analyse the ECG. An AED does this automatically; with a manual device the operator reads the monitor.

3.       Select energy and press CHARGE. The charging circuit raises the capacitor to kilovolts in a few seconds.

4.       Stand clear: nobody touches the patient or the bed; oxygen is moved away from the chest.

5.       Press SHOCK. The relay closes and the capacitor discharges through the wave-shaping circuit and pads, typically over about 5 to 20 ms.

6.       Resume CPR at once and reassess the rhythm after the next cycle.

Monophasic vs biphasic waveforms

Waveform	Current direction	Typical adult energy	Notes
Monophasic damped sine	One direction only	360 J	Older design: an RLC circuit (capacitor + inductor) shapes the pulse
Biphasic truncated exponential / rectilinear	Reverses halfway through	120 to 200 J	Modern standard: works at lower energy, causes less heart-muscle injury, adjusts for patient impedance

Types of defibrillator

Type	Where the electrodes are	Who uses it
Manual external	Paddles or pads on the chest	Doctors and trained staff, in hospital and ambulances
AED (automated external)	Adhesive pads on the chest	Anyone: it analyses the rhythm and gives voice prompts
ICD (implantable)	Leads inside the heart	Implanted under the skin in high-risk patients; shocks automatically
Internal paddles	Directly on the heart	Open-chest cardiac surgery; very low energy needed
Wearable (vest)	Electrodes in a garment	Patients waiting for an ICD

Synchronised cardioversion

For rhythms that still have organised QRS complexes (atrial fibrillation, atrial flutter, VT with a pulse), the shock is synchronised to the R wave. A shock that lands on the T wave, the heart's vulnerable period, can itself trigger VF. In sync mode the controller waits for the next R wave before firing.

Safety points

•         Shock only VF and pulseless VT; never asystole or PEA.

•         Shout "stand clear" and check before every shock.

•         Wipe the chest dry; remove medication patches; keep pads at least 8 cm from an implanted pacemaker.

•         Children: use paediatric pads and about 4 J/kg.

Clinical example: cardiac arrest at a railway station

Illustrative teaching case. A 58-year-old man collapses on a platform. He is unresponsive and not breathing normally. A bystander starts chest compressions and station staff bring the AED.

Time	What happens	What the device shows or does
0 min	Collapse; bystander starts CPR	—
3 min	AED switched on, pads placed (right upper chest + left side)	Voice prompt: "Analysing heart rhythm, do not touch the patient"
3.5 min	Rhythm analysed	VF detected: "Shock advised. Charging." Capacitor charges in a few seconds
4 min	Everyone stands clear; SHOCK pressed	First biphasic shock, about 150 J (device default)
4–6 min	CPR resumed immediately for 2 minutes	AED counts the cycle, then re-analyses
6 min	Still VF	Second shock delivered; CPR continues
8 min	Rhythm check	"No shock advised": organised rhythm on the trace
9 min	Pulse returns (ROSC); ambulance takes over	Pads left on; monitor shows sinus rhythm

Why it worked: VF is a shockable rhythm, and every minute without a shock lowers survival sharply. Early CPR kept blood flowing to the brain; the shock reset the chaotic ventricles so the SA node could take over.

Second example: synchronised cardioversion. A 64-year-old woman in the emergency department has atrial fibrillation at 160 beats/min with low blood pressure. She is sedated, the defibrillator is switched to SYNC mode (a marker appears on each R wave), and a synchronised biphasic shock of about 120–150 J is delivered. The monitor then shows sinus rhythm at 82 beats/min. Without sync, a shock landing on a T wave could have triggered VF.

2. Pacemaker

A pacemaker keeps the heart from beating too slowly by delivering small electrical pulses (a few volts, under a millisecond) through a lead; a modern "demand" pacemaker watches every beat and fires only when the heart's own beat does not arrive in time.

Principle

•         The heart's natural pacemaker is the SA node. If it fails, or the signal is blocked between atria and ventricles (heart block), the heart rate falls and the patient feels dizzy, faints or becomes breathless.

•         A pacing pulse at the lead tip depolarises a small area of muscle. That depolarisation then spreads cell to cell through the whole chamber, producing a full contraction. This is called capture.

•         The pacemaker does two jobs: pacing (sending pulses) and sensing (listening to the heart's own activity so it does not compete with it).

Main uses: symptomatic bradycardia, sick sinus syndrome, complete (third-degree) heart block, and cardiac resynchronisation in heart failure.

Main parts

Pacemaker block diagram · pacing path and sensing feedback loop

If a natural beat is sensed before the timer runs out, the pacemaker stays quiet and restarts its timer; if not, the output circuit fires a pulse down the lead.

In real use

Figure 2b · Pacemaker in real use: implant position, lead route and a paced ECG

On a chest X-ray the generator sits below the left collarbone with the lead curving into the heart. On the ECG, a ventricular-paced beat shows a sharp vertical spike followed by a broad QRS, because the impulse spreads cell to cell instead of through the normal conduction system.

•         Pulse generator: battery, timing circuit, output circuit and sense amplifier sealed in a titanium can.

•         Lead(s): insulated wires from the generator through a vein into the right atrium and/or right ventricle. The tip electrode touches the heart wall.

•         Programmer: an external unit that talks to the generator wirelessly to read data and change settings.

Key electrical ideas

•         Stimulation threshold: the smallest pulse that reliably captures the heart, often under 1 V at 0.4 ms. The output is set at about 2 to 3 times the threshold as a safety margin.

•         Strength–duration curve: a shorter pulse needs a higher voltage. Rheobase is the lowest voltage that captures with a very long pulse; chronaxie is the pulse width at twice the rheobase, where energy use is close to its minimum.

•         Sensing: the ventricular R wave inside the heart is roughly 5 to 20 mV and the atrial P wave about 1.5 to 5 mV. The sensitivity setting (in mV) is the smallest signal the device will count as a beat.

•         Rate and intervals: a lower rate of 60 beats/min means a pacing interval of 60 000 / 60 = 1000 ms.

\text{Pacing interval (ms)} = \frac{60\,000}{\text{rate (beats/min)}}

The pacing code (NBG code)

Pacing modes are written with up to five letters. The first three are the ones students must know.

Position	Meaning	Letters used
I	Chamber paced	O = none, A = atrium, V = ventricle, D = dual (A + V)
II	Chamber sensed	O, A, V, D
III	Response to a sensed beat	O = none, I = inhibit, T = trigger, D = dual (I + T)
IV	Rate modulation	O = none, R = rate-responsive (speeds up with activity)
V	Multisite pacing	O, A, V, D

Examples:

•         VVI: paces the ventricle, senses the ventricle, inhibits when a beat is sensed. The commonest single-chamber mode.

•         AAI: the same in the atrium; used when AV conduction is normal.

•         DDD: paces and senses both chambers, keeps the atria and ventricles in step. Common for heart block.

•         VOO / DOO: fixed-rate (asynchronous) pacing with no sensing. A magnet placed over many devices switches them into this mode.

•         DDDR: DDD plus a sensor that raises the rate during exercise.

Types of pacemaker

Type	How it is applied	When it is used
Transcutaneous (external)	Large pads on the chest	Emergency, short term; painful at high outputs
Temporary transvenous	Lead through a vein to the RV, external box	Hours to days, until a permanent device is fitted
Permanent single / dual chamber	Generator under the skin, 1 or 2 leads	Long-term bradycardia and heart block
Biventricular (CRT)	Third lead to the left ventricle	Heart failure with poorly coordinated ventricles
Leadless	Capsule implanted inside the RV	Single-chamber pacing without a lead or pocket

Safety points

•         Check for capture on the ECG: every pacing spike should be followed by a wide QRS complex.

•         Failure to sense (pacing over the patient's own beats) and failure to capture (spike with no QRS) are the two classic faults.

•         Electrocautery, MRI and strong magnets can interfere; only MRI-conditional systems may be scanned, under protocol.

•         The battery's elective replacement indicator warns months before the battery runs out.

Clinical example: complete heart block in an elderly patient

Illustrative teaching case. A 76-year-old woman comes to the emergency department after fainting twice at home. She is pale and drowsy. Pulse 34 beats/min, blood pressure 80/50 mmHg.

Stage	Finding or action	Pacing detail
ECG	P waves at 80/min march through with no link to QRS complexes; ventricular rate 34/min	Diagnosis: complete (third-degree) heart block
Emergency	Atropine has little effect	Transcutaneous pacing: pads on chest and back, rate set to 70/min, current raised step by step until every spike is followed by a wide QRS (capture) and a pulse is felt
Same day	Transcutaneous pacing is painful and not reliable long term	Temporary transvenous lead passed through a vein into the right ventricle; external box set to VVI 70/min
Next 1–2 days	Heart block persists	Permanent dual-chamber pacemaker implanted under the left collarbone, programmed DDD at a lower rate of 60/min
Follow-up clinic	Feels well, no more faints	Programmer checks threshold (e.g. 0.75 V at 0.4 ms), output set to 2.5 V for a safety margin, battery status good

Why DDD? The atria are still beating normally. DDD senses each P wave and paces the ventricle after a set delay, so the atria and ventricles stay in step and the heart rate rises naturally with exercise.

Troubleshooting example. Two weeks later the same patient feels dizzy again. Her ECG shows pacing spikes with no QRS after some of them. This is failure to capture. A chest X-ray shows the ventricular lead has moved (lead displacement). The lead is repositioned and capture returns.

3. Mechanical ventilator

A mechanical ventilator breathes for a patient by raising the pressure at the airway above the pressure in the alveoli, so gas flows into the lungs; when the pressure is released, the elastic lungs and chest wall recoil and push the gas back out.

Principle

•         Normal breathing uses negative pressure: the diaphragm pulls down, pressure in the chest falls and air is sucked in.

•         A modern ventilator uses positive pressure: it pushes gas in. Old "iron lung" machines used negative pressure around the body.

•         Expiration is passive. The ventilator simply opens the expiratory valve and the lungs empty by elastic recoil.

•         How much pressure is needed depends on two properties of the respiratory system:

o        Compliance (C): how easily the lungs stretch. C = change in volume / change in pressure. Normal is roughly 50 to 100 mL/cmH₂O; stiff lungs (ARDS, fibrosis) have low compliance.

o        Resistance (R): how hard it is for gas to flow through the tubes and airways. It rises with a narrow ET tube, secretions or bronchospasm.

These are combined in the equation of motion:

P_{aw} = \frac{V_T}{C} + R \times \dot{V} + PEEP

P_aw is airway pressure, V_T is tidal volume and V̇ is gas flow.

Main parts

Mechanical ventilator circuit · inspiratory and expiratory limbs

Gas travels out along the top row and comes back along the bottom row; the controller opens one valve at a time so the patient breathes in and then out.

In real use

Figure 3b · Ventilator in real use: patient circuit and a typical volume-control screen

A real ventilator screen shows pressure and flow against time. In volume control the inspiratory flow is a flat-topped square block; the expiratory flow is negative and decays back to zero as the lungs empty. The numbers on the side are the measured values the nurse records each hour.

The four phases of every breath

1.       Trigger: what starts the breath. Time (the machine's set rate) or the patient (a small drop in pressure or flow when the patient tries to breathe).

2.       Target / limit: what is controlled during inspiration. Either a set volume or a set pressure.

3.       Cycle: what ends inspiration. Set time, delivered volume, or flow falling to a set fraction.

4.       Expiration and baseline: the expiratory valve opens and pressure falls to PEEP, which keeps the alveoli open at the end of each breath.

Reading the pressure waveform

Airway pressure during a volume-controlled breath (illustrative shape)

The gap between peak and plateau reflects airway resistance; the gap between plateau and PEEP (the driving pressure) reflects lung stiffness. A rising peak with a steady plateau points to a blocked tube or bronchospasm; a rising plateau points to stiffer lungs or a pneumothorax.

Common modes

Mode	What is set	How it behaves	Typical use
Volume control (AC-VC)	Tidal volume, rate	Fixed volume every breath; pressure varies with the lungs	Sedated, paralysed patients
Pressure control (AC-PC)	Inspiratory pressure, time, rate	Fixed pressure; volume varies with the lungs	Stiff lungs, limiting pressure
SIMV	Volume or pressure, rate	Set mandatory breaths, patient can breathe between them	Weaning (older approach)
Pressure support (PSV)	Support pressure, PEEP	Every breath started by the patient, machine assists	Weaning, awake patients
CPAP	PEEP only	Constant pressure, patient does all the breathing	Spontaneous breathing trial, sleep apnoea
BiPAP / NIV	Two pressures (IPAP, EPAP)	Delivered through a tight face mask, no tube	COPD flare-ups, avoiding intubation

Key settings and starting values (adult)

Setting	Meaning	Typical starting value
Tidal volume (V_T)	Volume of each breath	6 to 8 mL/kg of predicted body weight
Respiratory rate	Breaths per minute	12 to 20 /min
FiO₂	Fraction of oxygen in the gas	0.21 to 1.0, lowered as SpO₂ allows
PEEP	Pressure left in the lungs at end of expiration	5 cmH₂O, higher in ARDS
I:E ratio	Inspiration time : expiration time	1:2
Plateau pressure	Pressure in alveoli during a pause	Keep below 30 cmH₂O
Trigger sensitivity	Effort needed to start a breath	about −1 to −2 cmH₂O or 1 to 3 L/min

Minute ventilation = tidal volume × rate. Example: 500 mL × 14 /min = 7 L/min.

Alarms and safety

•         High pressure: blocked or kinked tube, secretions, patient biting or coughing, bronchospasm, pneumothorax.

•         Low pressure / low volume: disconnection or leak, the most dangerous alarm to ignore.

•         Apnoea, low minute volume, FiO₂ and gas-supply / power failure alarms are also standard.

•         If the patient deteriorates suddenly, think DOPE: Displaced tube, Obstructed tube, Pneumothorax, Equipment failure. Disconnect and hand-ventilate with a bag if in doubt.

•         Complications to know: barotrauma and volutrauma (lung injury from pressure or over-stretch), low blood pressure (less venous return), ventilator-associated pneumonia, and oxygen toxicity.

Clinical example: severe pneumonia leading to ARDS

Illustrative teaching case. A 45-year-old man with severe pneumonia is breathing 38 times a minute. His SpO₂ is 82% even on a high-flow oxygen mask, and he is exhausted. He is intubated and put on a ventilator in the ICU. His predicted body weight is 60 kg.

Setting	Value chosen	Reason
Mode	Volume control (AC-VC)	Sedated patient; guaranteed volume each breath
Tidal volume	360 mL (6 mL/kg × 60 kg)	Low volume protects stiff, injured lungs
Rate	20 /min	Keeps minute ventilation about 7.2 L/min to clear CO₂
FiO₂	0.8 at first, lowered later	Correct severe hypoxia quickly
PEEP	10 cmH₂O	Keeps collapsed alveoli open, improves oxygenation

Checking the screen: plateau pressure 28 cmH₂O (below the 30 limit), so the settings are lung-protective. SpO₂ rises to 94% and FiO₂ is reduced to 0.5 over the next hours.

Alarm on day 2 — reading the waveform:

What the screen shows	Likely cause	Action
Peak pressure jumps from 32 to 45 cmH₂O, plateau unchanged at 28	Airway resistance has risen: secretions blocking the tube	Suction the tube; peak falls back to 32
Peak and plateau both rise suddenly, SpO₂ falls, breath sounds absent on one side	Lung compliance has dropped: tension pneumothorax	Emergency chest decompression, then chest drain
Low-pressure and low-volume alarms, no chest movement	Disconnection of the circuit	Reconnect at once; hand-ventilate with a bag if needed

Weaning: as the pneumonia improves he is switched to pressure support, then passes a spontaneous breathing trial on CPAP with low support, and is extubated on day 6.

4. Pulse oximetry

A pulse oximeter measures the percentage of haemoglobin carrying oxygen (SpO₂) by shining red and infrared light through a fingertip and comparing how much of each is absorbed by the pulsing arterial blood; normal SpO₂ is about 95 to 100%.

Principle

Pulse oximetry combines two ideas.

1. Spectrophotometry (the Beer–Lambert law). The amount of light a substance absorbs depends on how much of it is in the path and how far the light travels:

A = \varepsilon \, c \, l \qquad\qquad I = I_0 \, e^{-\varepsilon c l}

A is absorbance, ε is the absorption coefficient of the substance, c its concentration, l the path length, I₀ the light going in and I the light coming out.

•         Deoxyhaemoglobin (Hb) absorbs much more red light (660 nm). Deoxygenated blood looks dark.

•         Oxyhaemoglobin (HbO₂) absorbs more infrared light (940 nm). Oxygenated blood looks bright red because it lets red light through.

•         At about 805 nm (the isosbestic point) both absorb equally.

2. Plethysmography. With each heartbeat a small extra volume of arterial blood enters the finger, so the absorbed light rises and falls. The oximeter uses only this pulsing (AC) part, which belongs to arterial blood. The steady (DC) part, from skin, bone, venous blood and non-pulsing arterial blood, is cancelled out.

A: absorption of light by Hb and HbO2 (schematic) · B: AC and DC parts of the absorbed light

Panel A is why two wavelengths are used; panel B is why the device needs a pulse. Only the shaded bumps in B change with each beat, and they come from arterial blood alone.

Ratio of ratios

The processor divides the pulsing part by the steady part at each wavelength, then compares the two:

R = \frac{AC_{660} / DC_{660}}{AC_{940} / DC_{940}}

•         Low R (around 0.5) means well-oxygenated blood: about 100% SpO₂.

•         R = 1 means about 85% SpO₂.

•         A common classroom approximation is SpO₂ ≈ 110 − 25 × R.

The real conversion uses a calibration curve stored in the device, made by testing healthy volunteers breathing low-oxygen mixtures. Below about 70 to 80% the curve is less reliable, because volunteers cannot safely be desaturated further.

Main parts

↗ Pulse oximeter block diagram · light path and signal processing

Light travels across the top row through the finger; the electronics in the bottom row turn the received light into the number on the screen.

In real use

↗ Figure 4b · Pulse oximeter in real use: finger clip probe and the monitor display

The red glow you see through a fingertip under the clip is the red LED; the infrared LED is invisible to the eye. Trust the number only when the pleth wave is regular and the displayed pulse rate matches the patient's heart rate.

•         Probe types: transmission (LEDs and photodiode on opposite sides of a finger, toe or ear lobe) and reflectance (both on the same side, e.g. forehead).

•         Display: SpO₂ (%), pulse rate, and the plethysmograph (pleth) waveform. A clean, regular pleth wave is the best sign that the reading can be trusted.

SpO₂, SaO₂ and PaO₂

Term	Meaning	Measured by
SpO₂	Oxygen saturation estimated from pulse oximetry	Pulse oximeter (non-invasive)
SaO₂	True arterial oxygen saturation	Arterial blood gas with co-oximetry
PaO₂	Partial pressure of oxygen dissolved in arterial blood (mmHg or kPa)	Arterial blood gas

Because of the S-shaped oxygen–haemoglobin dissociation curve, an SpO₂ of about 90% corresponds to a PaO₂ of about 60 mmHg (8 kPa). Below this point saturation falls steeply.

Sources of error

Problem	Effect on reading	Why
Poor perfusion (cold hands, shock, vasoconstriction)	Unreliable or no reading	The AC signal is too small
Movement, shivering	False readings	Motion creates fake pulses
Carbon monoxide poisoning	Falsely high	Carboxyhaemoglobin absorbs red light like HbO₂
Methaemoglobinaemia	Reads close to 85%	Absorbs red and IR about equally, so R ≈ 1
Dark nail polish, dyes (e.g. methylene blue)	Falsely low	Extra absorption of light
Darker skin pigmentation	May overestimate at low saturations	Calibration largely done on lighter skin
Bright ambient light, loose probe	Erratic readings	Stray light reaches the photodiode
Severe anaemia	Less reliable	Very little haemoglobin to measure
Venous pulsation (e.g. tricuspid regurgitation)	Falsely low	Venous blood is counted as arterial

Safety points

•         SpO₂ shows oxygenation, not ventilation. A patient on oxygen can have a normal SpO₂ while CO₂ rises dangerously.

•         A finger probe reacts later than an ear probe because the blood takes longer to reach it.

•         Move the probe every few hours on long-term monitoring to avoid pressure injury or burns.

•         Always look at the patient and the pleth wave, not just the number.

Clinical examples: when the number is right, and when it lies

Illustrative teaching cases.

Case	Oximeter reading	What was really happening	Lesson
Pneumonia in the ward. A 70-year-old with fever and cough; breathing 28 /min	SpO₂ 88%, clean pleth wave	Genuine low oxygen. Oxygen by nasal cannula raised SpO₂ to 94%	A good pleth wave means the reading can be trusted; act on it
Charcoal stove in a closed room. Family found drowsy with headache in the morning	SpO₂ 99%	Carbon monoxide poisoning. Blood-gas co-oximetry showed carboxyhaemoglobin 25%; true oxygen content low	CO fools the oximeter (falsely high). Give 100% oxygen; never be reassured by SpO₂ alone
After surgery, cold hands. Patient shivering in recovery	SpO₂ 84%, flat noisy pleth wave	Poor finger perfusion, not true hypoxia. Probe moved to the ear lobe and hands warmed: SpO₂ 97%	Check the pleth wave before acting on a low number
After a methylene blue injection during surgery	SpO₂ drops suddenly to 85% for a few minutes	Blue dye absorbs red light; the patient's real oxygen was normal	Dyes give falsely low readings; check the patient, not just the monitor
Opioid painkiller on the ward, patient on 4 L/min oxygen	SpO₂ steady at 97%	Breathing slow and shallow; blood CO₂ rising dangerously	SpO₂ does not show ventilation. Count the breathing rate; capnography detects rising CO₂
Home monitoring of a viral lung infection	SpO₂ falls from 96% to 91% over two days, patient feels fine	Early silent hypoxia	A steady downward trend means "seek medical review", even without symptoms

Quick revision and practice questions

	Defibrillator	Pacemaker	Ventilator	Pulse oximeter
Key principle	Simultaneous depolarisation resets chaotic rhythm	Small pulse captures muscle; sense before pacing	Positive pressure vs compliance and resistance	Beer–Lambert law + pulsatile (AC) signal
Key formula	E = ½CV²	Interval (ms) = 60 000 / rate	P = V/C + R × flow + PEEP	R = (AC/DC)red / (AC/DC)IR
Key numbers	Biphasic 120–200 J; monophasic 360 J; child 4 J/kg	Output 2.5–5 V, 0.4 ms; lower rate ~60 /min	V_T 6–8 mL/kg; PEEP 5 cmH₂O; Pplat < 30 cmH₂O	660 nm and 940 nm; normal 95–100%
Main hazard	Shock to bystanders; burns	Failure to sense or capture	Barotrauma, disconnection	False reading (CO, motion, low perfusion)

Practice questions

1.       Calculate the voltage needed to store 360 J in a 200 µF capacitor. (Answer: V = √(2 × 360 / 0.0002) = 1897 V, about 1.9 kV.)

2.       Why is a shock synchronised to the R wave during cardioversion, but not during defibrillation of VF?

3.       Explain why asystole is not treated with a shock.

4.       Decode the pacing mode DDD and describe what the pacemaker does when it senses an atrial beat.

5.       A VVI pacemaker is set at 70 beats/min. What is its pacing interval? (Answer: 60 000 / 70 ≈ 857 ms.)

6.       On a volume-controlled ventilator the peak pressure rises sharply but the plateau pressure is unchanged. What is the likely cause?

7.       A patient has V_T 450 mL and rate 16 /min. Find the minute ventilation. (Answer: 7.2 L/min.)

8.       Why does a pulse oximeter need two wavelengths of light, and why does it use only the pulsing part of the signal?

9.       A fire victim has SpO₂ 99% but is confused and breathless. Explain why the reading may be wrong.

10.   Distinguish SpO₂, SaO₂ and PaO₂, and state the PaO₂ that matches an SpO₂ of 90%.