A Technical Look at Rhythm Detection, Artefact and Clinical Context

There is a commonly repeated statement that “an AED will not shock a conscious person.”

It sounds reassuring. It supports public confidence. But like many simplified messages in resuscitation, the full picture is more nuanced.

This article is not about discouraging AED use. Early defibrillation saves lives. Public access defibrillation works.

It is about understanding how these devices function, where their limits sit, and why context matters in professional discussions.

How AED Rhythm Analysis Actually Works

Automated External Defibrillators analyse surface ECG signals through adhesive pads placed on the chest. The internal algorithm evaluates multiple parameters, including:

  • Rate

  • Amplitude

  • Frequency content

  • Waveform morphology

  • Regularity

  • Signal stability

Based on this analysis, the device determines whether the rhythm falls within programmed shockable parameters.

For public access AEDs, shockable rhythms are typically:

  • Ventricular Fibrillation (VF)

  • Pulseless Ventricular Tachycardia (VT)

Importantly, AEDs do not detect pulses. They detect electrical patterns.

The device assumes that if the electrical signature meets shock criteria, the patient is in cardiac arrest.

That assumption is usually correct.

It is not infallible.

VF vs VT: Not All Tachycardia Is the Same

VF is chaotic electrical activity with no organised cardiac output. The patient is unconscious and pulseless.

VT is more complex.

VT may be:

  • Pulseless and immediately life threatening

  • Perfusing, where cardiac output is maintained

  • Associated with reduced output but preserved consciousness

Modern AEDs use rate thresholds and morphology analysis to identify VT. Some devices allow programmable rate cut-offs, for example 150 bpm, 170 bpm or 180 bpm depending on manufacturer and configuration.

Here lies the clinical nuance.

A patient in perfusing VT at 180 bpm may be conscious. In a monitored clinical environment, this is often treated with synchronised cardioversion, antiarrhythmics, or rate control strategies depending on stability.

That is not the same as an unresponsive patient in cardiac arrest.

Public AED algorithms are designed for unresponsive, non-breathing patients. They are not designed to assess haemodynamic stability.

Motion Artefact and Environmental Factors

AED algorithms incorporate filtering systems to reject noise and artefact. However, real-world environments are rarely ideal.

Motion artefact can be introduced by:

  • Chest compressions

  • Patient movement

  • Vehicle transport

  • Poor pad adhesion

  • External electrical interference

There are documented cases in pre-hospital settings where artefact has mimicked VF, leading to inappropriate shock recommendations.

These events are rare, but they are not theoretical.

Anyone who has worked with monitor defibrillators in moving ambulances knows how dynamic ECG interpretation can become.

Algorithms are sophisticated. They are not omniscient.

Fully Automatic vs Semi Automatic AEDs

Some manufacturers avoid fully automatic shock delivery for a reason.

Semi automatic devices require a rescuer to press a shock button after the device advises treatment. This introduces a final human confirmation step.

Fully automatic devices remove that step.

The safety of either approach depends on correct patient selection. The device is intended for someone who is unresponsive and not breathing normally.

When used according to BLS guidelines, the risk of shocking a conscious patient is extremely low.

When used outside that context, risk increases.

The Critical Distinction: BLS vs Clinical Defibrillation

For lay rescuers, the message must remain simple:

If the person is unresponsive and not breathing normally, attach the AED and follow the prompts.

Do not overthink rhythm discrimination.

Do not delay defibrillation.

However, in professional environments, we must acknowledge:

  • AEDs analyse electrical activity, not perfusion

  • VT can exist with preserved consciousness

  • Shock thresholds are programmable

  • Artefact can occur

  • Inappropriate shocks, while rare, are possible

A shock delivered to a perfusing rhythm is not benign. It can destabilise the myocardium and potentially precipitate arrest.

That is why clinical defibrillation outside of cardiac arrest is deliberate, synchronised, and carefully assessed.

Why This Discussion Matters

This is not about undermining AED use.

It is about intellectual honesty.

Oversimplified statements may reassure the public, but professionals must understand both the strengths and the limitations of the tools we teach others to use.

Early defibrillation saves lives.
Equitable access saves lives.
Clear training saves lives.

But clarity should not require ignoring nuance.

When we teach, we must do two things at once:

Keep the public message simple.
Keep the professional discussion accurate.

Both can coexist.