An ECG, also sometimes referred to as an EKG from the original German word ‘electrokardiogram’, measures the electrical activity of the heart . This electrical activity produces the contractions and relaxations of the cardiac muscles required to pump blood around the body. An ECG is recorded over a series of cardiac cycles (heartbeats) and shows the different phases of the cardiac cycle. The ECG indirectly measures transmembrane voltages in myocardial cells that depolarize and repolarize within each cardiac cycle. These depolarization and repolarization events produce ionic currents within the body, and these are transduced into voltages by electrodes (described in Chapter 2) placed on the surface of the chest and thorax, as shown in Figures 5.1(a) and (b). Up to twelve different lead voltages are recorded, with the magnitude of the voltages being in the low mV range, Figure 5.1(c), and a frequency spectrum between 0 and 30 Hz, as shown in Figure 5.1(d). The ECG signal has many distinct features, such as the P-wave, QRS-complex and T-wave, illustrated in Figure 5.1(c). The amplitude, shape and relative timing of these features can be used to diagnose different clinical conditions.
An ECG is an essential part of diagnosing and treating patients with acute coronary syndromes and is the most accurate method of diagnosing ventricular conduction disturbances and cardiac arrhythmias. It is also used to diagnose heart conditions such as myocardial infarcts, atrial enlargements, ventricular hypertrophies and blocks of the various bundle branches. An ECG is universally used to monitor a patient's cardiac activity during surgery.
Most ECG machines are now digital and automated, meaning that the data is analyzed automatically. Software algorithms measure different aspects (such as delays, durations and slopes) of the ECG waveform and provide a set of keyword interpretations of the scan such as ‘abnormal ECG’ or more specific suggested diagnoses such as ‘possible sinoatrial malfunction’. The most common software package is the University of Glasgow algorithm, which is outlined in detail in section 5.4.1. The major interference signals are caused by electromagnetic coupling from external sources such as power lines, which must be removed by appropriate filtering.