Electrocardiography (ECG) is the acquisition of electrical activity of theheart captured over time by an external electrode attached to the skin. Eachof the cell membranes that form the outer covering of the heart cell has anassociated charge which is depolarized during every heartbeat. These appearas tiny electrical signals on the skin which can be detected and amplifiedby an electrical circuit.

History of ECG

Alexander Muirhead is reported to have recorded a patient’s heartbeatinitially in 1872. In the early twentieth century, Willem Einthoven used astring galvanometer to measure the electrical activity of the heart. Inthose days, patients had to immerse their limbs into salt solutions fromwhich the ECG was recorded. Nowadays, the ECG machine can be carried aroundin a patient’s pocket and has become truly mobile. Einthoven assignedthe letters P, Q, R, S, T to various deflections in the wave and identifiedfeatures of a number of disorders based on the deflections. He was awardedthe Nobel Prize in medicine for his discovery. Even though many advanceshave been made in the field of ECG, the basic principles are still valideven today.

Sample waveforms of ECG

The ECG signal is characterized by peaks and troughs as shown in Figure 28.1.For each beat of the heart, a similar sequence is generated owing tocontraction and relaxation of various muscles of the heart. The ECG signalis plotted on graph paper as shown in Figure 28.2.