The electrocardiogram

It is possible to measure the electrical activity of the human heart simply by attaching leads to the wrists and ankles of the subject and connecting them to

1 mV

Fig. 11.6. A human electrocardiogram, recorded between electrodes applied to the right wrist and left ankle.

a suitable recording device. The resulting record is known as an electrocardiogram, or ECG for short. Conventional ECGs were first obtained by Einthoven, using the string galvanometer which he invented for the purpose. Their measurement is now usually done by means of a hot wire pen recorder, and has long been a standard procedure in medical practice. Fig. 11.6 shows a typical ECG, recorded between the right arm and the left leg. The different peaks in the electrical cycle of events were labelled the P, Q, R, S and Twaves by Einthoven. The events in the heart cycle to which these electrical waves correspond can be worked out by recording with surface electrodes from exposed hearts in experimental animals.

The P wave is produced by currents associated with the spread of excitation over the atria. During the plateau of the atrial action potentials there is little current flow and so the level of the ECG approximates to zero. The net currents involved in the excitation of the atrioventricular node and the specialized conducting tissue in the ventricle are small, because the number of cells involved is small, and so are not evident in the ECG. Then the depolarization of the large mass of ventricular cells is accompanied by large net currents which are seen as the QRScomplex in the ECG. After this the whole of the ventricle is depolarized and there is very little electrical current flow. The ventricular muscle is contracting at this time to pump blood out along the aorta and pulmonary artery. Then repolarization of the ventricular fibres occurs, at slightly different times in different places, and the current flow associated with this is seen as the Twave. After this the heart is electrically at rest except in the pacemaker regions, its muscle cells are relaxing and it is refilling with blood ready for the next cycle. The pacemaker potentials preceding the next P wave are not visible in the ECG.

We can relate the form of the ECG to the time course of the action potentials in the cardiac muscle cells. If the action potentials were synchronous in all the muscle cells, there would be no current flow from one cell to another (because they would all be at the same potential at any one instant) and so there would be no external current flow and therefore no ECG. But since the excitation and repolarization move progressively across the heart there is an external current flow which is detected as the ECG. In other words the voltage recorded in the ECG at any moment is related to the differences in membrane potential of muscle cells in different regions of the heart at that moment. Hence we find that the rapidly changing currents constituting the QRS complex are related to the rapid depolarizations at the beginning of the ventricular muscle cell action potentials, and the slower and smaller changes constituting the T wave correspond to their repolarization.

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