Basic Principles

ECG traces are commonly used to examine the function of the heart. Simple electrodes are stuck to the skin to measure the electrical signals generated during the cardiac cycle.

It is a simple non-invasive test that is quick and simple to perform. In order to understand and interpret the trace, you need to understand the concepts behind what an ECG is showing.

This session will explain what an ECG trace is and how it is obtained.

The Electrocardiogram is a way of measuring electrical activity in the heart.

The principle of the ECG tracing is a visual display of the path of electricity during the cardiac cycle.

Normal conduction in the heart originates with an electrical signal at the Sinoatrial (SA) node. This propagates through the walls of the atria causing atrial depolarisation and contraction. This is represented on the ECG as the P wave. The signal reaches the Atrioventricular (AV) node where there is a delay of approximately 0.1s.

The signal then travels down the bundle of His to the bundle branches, travelling towards the apex. This is represented on the ECG as the QRS complex. It then spreads up through the ventricular walls causing ventricular depolarisation and contraction.

The ventricles then begin to repolarise at the apex, which is represented on the ECG as the T wave.

The ECG trace on the paper represents the movement of electrical charge.

Think of the ECG machine is a voltmeter with 2 electrodes. The electrical signal in the heart is a wave of electrons that move in a direction (vector) between those 2 electrodes.

If the electrical charge moves directly across the electrodes this will be recorded on the tracing. Electrons near the positive node show a ‘positive deflection’ in the trace and those near the negative node show a ‘negative deflection’ (Fig 1-4).

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If the electrical charge moves in a different orientation it will be detected differently by the electrodes. For example if it moves perpendicularly to the electrodes it will barely be detected as a deflection in the ECG trace (Fig 5-8).

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