Ventricular hypertrophy may be defined as an increase in ventricular mass as determined at post-mortem exinations or as estimated pre-mortem by one of several cardiac imaging techniques including echocardiography, and magnetic resonance. Included in this definition is an increase in ventricular wall thickness, which reflects an increase in the size of the individual myocytes, and an increase in ventricular volume which occurs in association with ventricular dilatation.
An increase in the thickness of the ventricular wall is a common response to a pressure or systolic overload of the ventricles. This occurs in the setting aortic or pulmonic valvular or sub-valvular stenosis and with systemic or pulmonary hypertension. Ventricular dilatation is a common response to a volume or diastolic overload of the ventricles such as that which accompanies valvular insufficiency or intracardiac shunts. Not infrequently, hypertrophy and dilatation occur together.
The anatomic and physiologic changes are associated with electrophysiologic changes that together produce the electrocardiographic changes interpreted as ventricular hypertrophy. The electrocardiographic changes involve the shape and duration of the P wave, the amplitude, duration and axis of the QRS complex, and abnormalities in the ST segment and the T wave.
Studies correlating ECG patterns to increases in ventricular mass have spawned multiple sets of ECG criteria for the diagnosis of left, right and combined ventricular hypertrophy Most rely on changes in QRS amplitude as the primary criterion but some include multiple factors including frontal plane axis, QRS duration, P wave abnormalities and abnormalities in the ST segment and T wave. As was pointed out in chapter 2, QRS amplitude in healthy individuals is influenced by multiple factors including age, gender, race and body habitus and the range of normal values is quite large. Moreover, changes in left ventricular surface area, an increase in intracavitary blood volume and the closer proximity of the ventricle to the chest wall, as well as the increase in ventricular mass, may contribute to the changes in QRS amplitude associated with chamber enlargement. The various ECG criteria for hypertrophy are a trade-off of between specificity and sensitivity and the search for the most accurate and clinically relevant criteria is ongoing.
In this chapter, the ECG changes associated with ventricular hypertrophy and dilatation will be discussed and examples provided to illustrate the various changes induced by diseases that alter ventricular mass and volume.