9.13: Electrocardiogram (ECG)

Electrical signals produced by the heart can be captured through the strategic positioning of electrodes on specific areas of the body. This process of tracking the heart's electrical activity is referred to as an electrocardiogram (ECG), also sometimes abbreviated as EKG (with "K" representing "kardiology," the German term for cardiology). Unlike recording signals from individual muscle fibers, a standard ECG showcases the overall fluctuations in electrical current flow, offering a comprehensive view of heart function which makes it a crucial tool in clinical diagnosis.

The term "lead" describes the voltage difference between two electrodes. An electrocardiograph can vary with 3, 5, or 12 leads. The level of detail an ECG provides increases with the number of leads used. A 12-lead electrocardiograph employs ten electrodes placed strategically on the patient's body, as illustrated in Figure 16.

Figure 16 | Standard Placement of ECG leads in a 12-lead ECG| ²⁹ six electrodes are placed on the chest, and four electrodes are placed on the limbs 

In an ECG, the key features are the P wave, the QRS complex, and the T wave ( Figure 17). The small P wave signifies atrial depolarization, while the larger QRS complex represents ventricular depolarization. The QRS complex has a more robust electrical signal due to the larger size of the ventricular cardiac muscle. The T wave corresponds to ventricular repolarization. Although atrial repolarization occurs during the QRS complex, it is concealed by the larger QRS complex on the ECG, which masks the atrial repolarization wave. The segments refer to the regions between two waves, intervals include a segment with one or more waves in an electrograph. For instance, the PR segment starts at the end of the P wave and extends until the start of the QRS complex. Meanwhile, the PR interval comprises the P wave and the PR segment. The P-R interval, lasting 0.16 seconds, thus measures the duration from the onset of the P wave to the onset of the QRS wave. It signifies the time elapsed between the initiation of atrial contraction and the commencement of ventricular contraction.

Q-T interval, spanning 0.35 seconds, indicates the timeframe from the onset of the Q wave to the conclusion of the T wave. It provides an approximation of the duration of ventricular contraction.

Atrial contractions commence approximately 25 ms after the onset of the P wave. Similarly, the ventricles begin their contraction around the peak of the R wave within the QRS complex. Clinically, the PR interval holds more significance as it measures the duration from the start of atrial depolarization (represented by the P wave) to the onset of the QRS complex.³⁰

Figure 17 | Electrocardiogram | ³¹ A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments.

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