3.10.3.1 - Simple ECG Machines and the Normal ECG Waveform

Electrical activity of the heart

The heart has four chambers: the left and right atria at the top and the left and right ventricles below. Blood enters the atria, passes through one-way valves into the ventricles, and is then pumped away from the heart when the ventricles contract.

The sequence of contraction is coordinated by electrical impulses rather than by instructions from the brain. The impulse originates at the sino-atrial (SA) node, a small region of specialised tissue in the wall of the right atrium. This node acts as the heart's natural pacemaker.

From the SA node the electrical signal spreads across both atria, causing depolarisation that triggers their contraction, pushing blood into the ventricles. The signal then reaches the atrioventricular (AV) node, located between the atria and ventricles. The AV node introduces a short delay of order 0.1 s, ensuring that the atria finish contracting before the ventricles are stimulated.

When cardiac muscle is at rest it is polarised, meaning there is a stable potential difference across the cell membranes due to ion concentration gradients. When an electrical impulse arrives, ion channels open, ions move across the membrane, and the cell depolarises, triggering contraction. Repolarisation restores the membrane potential toward its resting value as the cells recover for another cycle. Electrical excitation triggers mechanical contraction, but the ECG does not measure force, blood flow or contraction directly. The impulse passes from the AV node down conducting tissue in the septum toward the apex and then through the ventricular walls. The atria are activated before the ventricles. Detailed conduction anatomy is supporting context; the assessed link is from coordinated electrical activity to a time-varying surface potential difference.

Obtaining the ECG waveform — electrodes and noise reduction

An electrocardiograph works by measuring the very small potential differences, typically of the order of 1 mV, that appear at the skin surface as the heart's electrical impulses spread through the body. Electrodes are attached in standard positions on the chest and limbs so that the heart's activity can be detected from different directions.

An ECG lead measures the potential difference between a chosen pair or arrangement of electrode positions on the body. The variation of this potential difference with time is the ECG trace.

Because the signals are so small, excellent electrical contact and rigorous noise reduction are essential. The following steps are taken to optimise electrode contact:

• Skin preparation: suitable cleaning and preparation of the electrode site so the contact resistance between the electrode and the skin is reduced.
• Conducting gel: a layer of conductive gel is applied between the electrode and the skin to remove air gaps and improve electrical coupling.
• Firm attachment: electrodes are secured firmly to the skin so movement of the electrodes does not introduce unwanted signals.

Even with good contact, various sources of electrical noise can corrupt the trace. Noise is minimised by:

• Keeping the patient relaxed and still, because electrical signals from skeletal muscle contractions can also be detected.
• Shielding the electrocardiograph and its connecting leads from external alternating-current sources such as mains electricity.
• Using stable electrode–gel interfaces (commonly silver/silver-chloride) to limit varying electrode potentials and polarisation artefacts.

The detected signal must be amplified before it can be displayed or recorded. Because amplification boosts noise as well as signal, the amplifier must satisfy three key requirements:

In exam answers, focus on the stable physics points: standard electrode positions, good electrical contact, reduced noise, and careful amplification of a very small signal.

A differential amplifier amplifies the potential difference between its two inputs while rejecting a signal common to both, such as much mains interference. High input resistance reduces loading: most of the small source potential difference remains across the amplifier input. Gain means $V_\text{out}/V_\text{in}$; for example a gain of 500 turns $0.80\,\mathrm{mV}$ into $0.40\,\mathrm{V}$. High gain by itself cannot distinguish cardiac signal from noise.