heart: the electrocardiogram
Electrical activity is a basic characteristic of the heart and is the stimulus for cardiac contraction. Disturbances of electrical function are common in heart disease. Their registration as an electrocardiogram (ECG) plays an essential role in the diagnosis and management of heart disorders. THE GENESIS OF THE ELECTROCARDIOGRAM Pathways of conduction and the electrocardiogram
The sinus node is situated in the right atrium close to the entrance of the superior vena cava. The atrioventricular node lies in the right atrial wall immediately above the tricuspid valve. The fibres of the AV bundle (of His) arise from the atrioventricular node and run along the posterior border of the septum between the ventricles (Fig. 1.1). On reaching the muscular part of the septum, they split into right and left bundle branches and then spread out in the subendocardium of the ventricles as the Purkinje system. The right bundle is a slender, compact structure. The left bundle soon splits into two or more divisions or fascicles, one of which proceeds anteriorly, sharing the same blood supply as the right bundle, and another is directed posteriorly. In the usual sequence of events, the electrical impulse arises in the sinus node and spreads across the atria to reach the atrioventricular node. It can then only reach the ventricles by passing into the rapidly conducting atrioventricular bundle and its branches. The first part of the ventricles to be activated is the septum, followed by the endocardium. Finally, the impulse spreads outwards to the epicardium. The spread of the cardiac impulse gives rise to the main deflections of the electrocardiogram: P, QRS and T waves (Fig. 1.2):
The P wave represents atrial depolarization.
The PR interval represents the time taken for the cardiac impulse to spread over the atrium and through the AV node and HisPurkinje system.
The QRS complex represents ventricular depolarization.
The T wave represents ventricular repolarization.
Electrodes and leads
A conventional ECG consists of tracings from 12 or more leads. The term lead' refers to the ECG obtained as a result of recording the difference in potential between a pair of electrodes.
The bipolar (standard) leads
In these leads, the electrodes are attached to the limbs. In lead I the positive electrode is attached to the left arm and the negative to the right arm. In lead II the positive electrode is attached to the left leg and the negative to the right arm. In lead III the positive is attached to the left leg and the negative to the left arm. They may thus be depicted as:
lead I = left arm minus right arm (LARA)
lead II = left leg minus right arm (LLRA)
lead III = left leg minus left arm (LLLA).
It can be deduced from these equations that lead II should be equal to the sum of leads I and III. The position from which the heart is viewed by each of these leads is shown in Figure 1.3.
These have an exploring electrode placed on a chosen site linked with an indifferent electrode with a very small potential. In an attempt to obtain a
Fig. 1.3 Diagram of the effective position of the bipolar (standard) leads. In lead I, the positive electrode is attached to the left arm and the negative to the right arm. In effect, lead I is the sum of the potentials from the left arm with those that would be obtained from an electrode diametrically opposite the right arm. The resultant force is directed midway between these two points. Similar principles can be applied to derive the effective direction of the leads II and III. central terminal with zero potential', Wilson connected all three limb electrodes through 5000 Ω resistances to form the indifferent electrode. Unipolar chest leads
When unipolar leads are recorded from the chest wall, the exploring electrode...