The devices: defibrillators, AEDs and ECG machines from the inside

Name the types of defibrillator and ECG equipment, explain energy, waveform and impedance in practical terms, and trace the main assemblies of any defibrillator from battery to pads.

Why this family of devices is different

Most medical equipment works every day, and when it breaks somebody notices within hours. A defibrillator is the opposite. It can sit on a crash trolley or a wall bracket for months, switched off or in standby, and then it must work at full output within seconds, in the hands of a stressed nurse or a passer-by. Nobody finds out that it does not work until the moment it is needed. That changes how you service it: your job is less about repairing what users report and more about proving, regularly and with records, that the device is ready.

ECG machines share the front end of the defibrillator - the electrodes, cables and amplifier that pick up the heart's electrical signal - and many of their faults are the same. That is why this course treats them together. Patient monitors also measure ECG; the course mon-l1 covers them, and we refer to it rather than repeat it.

The types you will meet

A manual defibrillator is operated by a trained clinician who reads the ECG, chooses the energy, charges and delivers the shock. Most current manual units are monitor-defibrillators: the same box contains an ECG monitor, often with SpO2, non-invasive blood pressure, CO2 and a printer, and frequently a transcutaneous pacer. Examples in this course include the Philips Efficia DFM100, the Nihon Kohden TEC-5600 series and the ZOLL R Series and X Series.

An automated external defibrillator (AED) analyses the rhythm itself through the pads and decides whether a shock is advised. A semi-automatic AED tells the user to press the shock button; a fully automatic one delivers the shock after a warning. Many hospital monitor-defibrillators also have an AED mode. Public-access AEDs such as the Nihon Kohden AED-3100 or the Physio-Control LIFEPAK 1000 are built to be used by untrained people, so voice prompts, pad readiness and self-testing are central to their design.

An electrocardiograph records a diagnostic ECG, usually 12 leads, and prints or stores it. A Holter recorder is a small battery-powered ECG recorder worn by the patient for one or more days; the recording is downloaded to a PC and analysed by software. Holters bring their own faults: memory cards, small batteries, worn connectors and software versions.

Energy, waveform and impedance in plain terms

The defibrillator stores energy in a high-voltage capacitor and then releases it through the patient's chest for a few milliseconds. The energy is given in joules. What actually stops fibrillation is the current that flows through the heart muscle, and that current depends on the voltage and on the resistance between the pads, which we call transthoracic impedance. A patient with large body mass, dry skin or badly applied pads has higher impedance, so the same stored energy pushes less current through the heart.

Older devices used a monophasic waveform: the current flows in one direction only. Monophasic units typically needed higher energies, up to 360 J. Modern devices use biphasic waveforms: the current flows one way and then reverses. Biphasic shocks achieve the same effect with lower energy, and the device usually measures the patient's impedance just before or during the shock and adjusts the voltage or the duration of the phases to compensate. Each manufacturer has its own biphasic shape and its own energy range; for example, the ZOLL R Series service manual gives a maximum of 200 J into 50 ohms, while the LIFEPAK 1000 can be configured up to 360 J. Never assume one brand's energy steps or limits apply to another.

Impedance is measured continuously through the pads by passing a tiny, harmless high-frequency signal, and many devices measure it again during the shock itself. That measurement serves three purposes: it tells the device whether the pads are attached ('connect pads' or 'check pads' messages), it lets the device compensate the waveform, and in an AED it helps to detect movement or CPR artefact. When a defibrillator says 'no contact' on a patient and also on a test load, the fault is in the device, not in the patient. Remember that sentence; we will use it in lesson 6.

Synchronised cardioversion and pacing

Some rhythms are treated with a shock that must be timed to the patient's R-wave, so it does not fall in the vulnerable part of the cardiac cycle. This is synchronised cardioversion. The device detects the R-wave on its ECG input, places a marker on it, and when the operator holds the shock button it waits for the next R-wave and fires within a very short delay. If the device cannot see the R-waves clearly, it will not fire. A device left in sync mode by mistake looks exactly like a device that 'will not shock'. Many devices drop sync mode after each shock or after power-off by default, but this is configurable, so you must know how each unit on your site is set up.

Transcutaneous pacing delivers repeated, smaller current pulses through pads to make the heart beat at a set rate. The operator sets the rate and the current in milliamps. Pacing uses the same pads and cable as defibrillation and needs a working ECG to run in demand mode. You test it with the pacer function of the defibrillator analyser, as described in lesson 3.

The chain inside a defibrillator

Every defibrillator, from a small AED to a full monitor-defibrillator, follows the same chain. Energy comes from a battery, from mains through an internal power supply, or both. A high-voltage charger converts the low battery voltage into several thousand volts and charges the capacitor. While charging, the control board measures the capacitor voltage and stops at the level that matches the selected energy. When the operator presses shock, a switching stage - relays, or semiconductor switches such as IGBTs in a bridge for biphasic waveforms - connects the capacitor to the patient output and shapes the waveform. If the shock is not delivered, an internal dump circuit discharges the capacitor safely into a resistor inside the device after a timeout or when the operator disarms.

Around that chain sit the ECG front end, which must survive a shock delivered through the same patient and recover quickly; the pads or paddles and their cable; the user interface and the voice prompts; the event memory; and the printer. The battery is part of the energy path, not just a power source for the electronics. Charging the capacitor draws a large current from the battery for several seconds. A tired battery may run the screen perfectly well and still collapse during charging. That is the most common reason a defibrillator 'works on the bench' but switches off when the user charges it.

An ECG machine has a simpler chain: patient cable and leads, a protected input stage, amplifiers and filters, an analogue-to-digital converter, a processor that builds the leads and measurements, a thermal printer, storage and export, and a battery and charger. Lesson 5 covers it in detail.

• Energy path: battery or mains - charger - HV capacitor - switching - pads.
• Safety path: dump circuit and timeout that empty the capacitor if no shock is given.
• Sensing path: pads or ECG leads - protected front end - rhythm detection and impedance.
• Records path: event memory, self-test log, printer and export.

What Level 1 covers, and where it stops

At Level 1 you are expected to inspect, clean, test, replace user-replaceable parts such as batteries, pads, cables, paddles, paper and fuses where the manufacturer allows it, read error and self-test logs, run the manufacturer's performance verification with an analyser, and find faults down to the assembly. You record what you found and what you did.

You do not open the high-voltage section, repair or replace the HV capacitor, charger or switching boards, or change firmware or calibration constants unless you have been trained and authorised for that model and you follow the manufacturer's procedure. High-voltage repair is beyond Level 1: diagnose, record, escalate. Lesson 2 explains why this line exists.

On-site checklist

• Identify each unit on site: type (manual, AED, monitor-defibrillator, ECG, Holter), model, serial, waveform and maximum energy.
• Note how each unit is configured: AED or manual mode at power-on, sync behaviour after a shock, energy steps.
• Find the manufacturer's service manual and the performance verification procedure for each model.
• List user-replaceable parts and their correct part numbers: batteries, pads, cables, paddles, paper.
• Agree with your manager which work is Level 1 and which goes to authorised service.

Check yourself