The Electrocardiogram

The electrocardiogram (ECG) records electrical fields created during depolarization and repolarization of the atrial and ventricular myocardium that are referred to as vectors. The multiple leads define vector magnitude and direction in space in relation with the anatomy and the electrophysiology. The ECG does not detect activation of the specialized conduction tissues but indirect data such as changes in activation vectors and activation times may point to specific abnormalities. Atrial activation is a ‘minor’ portion of the ECG but the P wave can still help identify chamber enlargement and, very significantly, the mechanisms of bradycardias and tachycardias. The QRS complex, generated by ventricular activation is divided in time windows for easier analysis. Initial vectors show the direction of septal activation and the function of the left bundle branch. Abnormal negative waves pinpoint the site of myocardial infarction scars. The middle QRS vectors express the dominance of either ventricle and reflect left or right ventricular enlargement. Block of a bundle branch widens the QRS and the direction of the delayed vectors will point at the blocked branch. The ST segment and the T wave have a unique ability to show acute metabolic or inflammatory changes, and the evolution of these changes help to follow the clinical course of the disease. In its ‘old age’ the ECG is still an essential tool for the cardiologist, the internist, and the general practitioner, and its evaluation in the clinical context can offer information essential to make diagnostic and therapeutic decisions both cheaply and quickly.