Twelve-Lead Electrocardiogram in Dogs: When and How to Use Precordial Leads

摘要

Precordial lead system has been firstly standardised in human beings by a joint committee of the AHA and the Cardiac Society of Great Britain and Ireland in 1938.1 The system was then reviewed by Wilson et ah, system actually in use in human beings3 that consists in 6 leads placed on different positions of the thoracic wall as follows: VI, fourth intercostal space at the right sternal border; V2, fourth intercostal space at the left sternal border; V3, midway between V2 and V4; V4, fifth intercostalspace in the midclavicular line; V5, in the horizontal plane of V4 at the anterior axillary line, or if the anterior axillary line is ambiguous, midway between V4 and V6; and V6, in the horizontal plane of V4 at the midaxillary line. Chest leads enhancethe pattern recognition comparing the projections of the resultant cardiac vectors in two orthogonal planes at different angles. Since over 90% of the electrical activity of the heart can be explained as a dipole source model, to evaluate it can be sufficient to measure at least two planes. Lead I, II and III reflect the frontal plane components, while precordial leads the anteroposterior component with lead V2 directed closest to the X axis and roughly orthogonal to the standard limb leads.4 Precordialleads register the potential at each of the six torso sites placing the positive exploring electrodes on the designed area above reported. The negative or reference input is composed by the so-called Wilson central terminal, which combines the output ofthe right arm, left arm, and left leg electrodes. The potential recorded by the Wilson central terminal is relatively constant during the cardiac cycle, and therefore the output of the precordial lead is determined by the changes overtime at the precordial site and reflects the changes of the potentials in the cardiac regions close to the torso sites. For this reason, even small variation of the position induces major alteration of the waves, with considerable amount of variability of amplitude measurements as found in serial electrocardiographic tracings.5 In humans, some studies suggested to correlate precordial leads with the region of the heart that is analysed: lead VI records the right ventricular potentials, V2 through V4 the lateral or antero-basal part of the left ventricle while V5 and V6 the apical part.