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....