The ECG enables physicians to infer the course of cardiac electrical impulses by recording the variations in electrical potential at various loci on the surface of the body. By analyzing the details of these fluctuations in electrical potential, the physician gains valuable insight into (1) the anatomical orientation of the heart; (2) the relative sizes of its chambers; (3) various disturbances in rhythm and conduction; (4) the extent, location, and progress of ischemic damage to the myocardium; (5) the effects of •Fig. 16.25 Transmembrane Action Potentials Recorded From Purkinje Fibers. Acetylstrophanthidin,adigitalisglycoside,wasaddedtothebath,andsequencesofsixdrivenbeats(denotedbythedots)wereeachproducedatabasiccyclelength(BCL)of800(A), 700(B), 600(C), and500(D) msec.NotethatdelayedafterpotentialsoccurredafterthedrivenbeatsandthattheseafterpotentialsreachedthresholdafterthelastdrivenbeatinB toD. (FromFerrierGR,etal.Circ Res. 1973;32:600.) altered electrolyte concentrations; and (6) the influence of certain drugs (notably digitalis, antiarrhythmic agents, and calcium channel antagonists). Because electrocardiography is an extensive and complex discipline, only the elementary principles are considered in this section.
P-R QRS Q P R S T QT ST 0.1 sec •Fig. 16.26 Importantdeflectionsandintervalsofatypicalscalarelectrocardiogram.
In electrocardiography, a lead is the electrical connection from the patient’s skin to a recording device (electrocardiograph) that measures the electrical activity of the heart. The system of leads used to record routine ECGs is oriented in certain planes of the body. The diverse electrical events that exist in the heart at any moment can be represented by a three-dimensional vector (a quantity with magnitude and direction). A system of recording leads oriented in a given plane detects only the projection of the three-dimensional vector on that plane. The potential difference between two recording electrodes represents the projection of the vector on the line between the two leads. Components of vectors projected on such lines are not vectors but scalar quantities (having magnitude but not direction). Hence, a recording of changes in the difference in potential between two points on the skin surface over time is called a scalar ECG.
A scalar ECG detects temporal changes in the electrical potential between some point on the surface of the skin and an indifferent electrode or between pairs of points on the skin surface. The cardiac impulse progresses through the heart in a complex three-dimensional pattern. Hence, the precise configuration of the ECG varies from individual to individual, and in any given individual, the pattern varies with the anatomical location of the leads. The graphic display of the electrical impulse recorded in an ECG is called a tracing.
In general, a tracing consists of P, QRS, and T waves (
Fig. 16.26). The P wave reflects the spread of depolarization through the atria, the QRS wave (or complex) reflects depolarization of the ventricles, and the T wave represents repolarization of the ventricles (repolarization of the atria occurs, and is therefore masked, during ventricular depolarization). The PR interval (or more precisely, the PQ interval) is a measure of the time from the onset of atrial activation to the onset of ventricular activation; it normally ranges from 0.12 to 0.20 second. A large fraction of this time involves passage of the impulse through the AV conduction system. Pathological prolongations of the PR interval are associated with disturbances in AV conduction. Such disturbances may be produced by inflammatory, circulatory, pharmacological, or neuronal mechanisms.