Figure 11-29: QRS changes (location of the electrical axis and polarity of lead Vi) produced by pacing from right ventricular apex (RVA), right ventricular outflow tract (RVOT), great cardiac vein (GCV), and middle cardiac vein (MCV).

Left Atrial Hypertrophy

Munuswamy et al.,109 using M-mode echocardiography as the "gold standard," evaluated the specificity and sensitivity of the most important clues for determining left atrial hypertrophy. These included (1) P wave duration greater than 0.11 s and notched P wave with an interpeak interval in excess of 0.04 s and (2) negative phase of P in V1 longer than 0.04 s and greater than 1 mm in lead V1. There are, however, problems when applying these criteria in a given ECG. For example, according to Josephson,110 prolonged duration of the P wave and of (posteriorly directed) terminal forces reflected delayed left atrial activation, not left atrial enlargement. In fact, most criteria mentioned above also apply for intraatrial block. Moreover, a negative P wave in lead V1110 may reflect improper (high) placement of this lead, a common error made by ECG technicians. Generally, if the previously mentioned findings are found in patients with LV enlargement or mitral stenosis, left atrial hypertrophy is most likely present, but in their absence, such findings usually indicate an intraatrial conduction defect. In any case, the ECG pattern of left atrial hypertrophy results from a hypertrophy-induced (stretching) intraatrial conduction delay.


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Essentials of Human Physiology

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