The electrocardiogram (ECG) provides vital clues to the size of the cardiac chambers, which may enlarge in response to a variety of cardiac disorders. Electrocardiographic evidence of chamber enlargement represents an abnormal increase in mass, which may be due to hypertrophy of the muscular walls, dilation of the internal cavity, or a combination of both processes (Figure 7-1). Concentric hypertrophy is a circumferential increase in the wall thickness relative to the internal dimension of the myocardial chamber. This is the characteristic response to pressure overload, where the heart is forced to pump against increased resistance. Cardiac enlargement may also result from dilation, which is an increase in the internal chamber dimension. Eccentric hypertrophy is the adaptive response of the heart to volume overload, where the heart needs to pump an added quantity of blood with each contraction. In this situation, wall thickness increases proportional to the degree of dilation. Cardiac disorders frequently involve both pressure and volume overload; therefore, enlargement of either the atria or ventricles may involve both hypertrophy and dilation.
Forms of cardiac chamber enlargement. (a) Normal. (b) Concentric hypertrophy with increased wall thickness. (c) Dilation with increased internal dimension. (d) Eccentric hypertrophy with increased wall thickness proportional to chamber dilation.
Aortic stenosis and hypertension are examples of pressure overload. Left ventricular hypertrophy (LVH) is the physiologic adaptation to the added workload of systolic contraction against the restricted aortic valve or elevated blood pressure. Examples of volume overload include aortic and mitral insufficiency. In this situation, the left ventricle dilates to accommodate the added quantity of blood flowing into the chamber during diastole.
ATRIAL ENLARGEMENT: GENERAL CONCEPTS
The normal sinus P wave has two components representing depolarization of the right atrium followed by the left atrium (Figure 7-2). Remember that the right atrium is really an anterior structure and the left atrium is actually posterior. The initial right atrial vector is directed leftward, inferiorly, and anteriorly. The terminal left atrial vector follows, directed leftward, inferiorly, and posteriorly. In the frontal plane, the contour of the normal P wave is smooth, with an amplitude ≤2.5 mm, a duration ≤0.11 seconds, and an axis between 0 and +75 degrees. In the horizontal plane, the normal P wave may be upright, inverted, or biphasic (upright followed by inverted) in lead V1. The inverted terminal portion of a normal biphasic P wave is <0.1 second in duration and <1 mm in depth. Analysis of the P wave is best performed by first inspecting lead II in the frontal plane and lead V1 in the horizontal plane. The thin-walled atria dilate in response to both pressure and volume overload, so avoid using the term hypertrophy when referring to atrial enlargement (which will immediately identify you as an ECG rookie). Left ...