The initial impression of tachycardia was largely misled by the unusually large amplitude P waves accompanied by atrial T waves with an opposite polarity to that of the P waves because atrial repolarization does not normally register a visible wave on an ECG. However, the quick bedside findings, including markedly slow heart sound by auscultation and pulse wave on the pulse oximeter, were additional clues for a diagnosis of complete AV block. Additionally, the heart sound and pulse wave that occurred simultaneously with those slow and wider QRS complexes on the ECG (Figure) further supported the diagnosis.
The P waves with amplitude up to 8 mm plus prominent atrial T waves indicated either an isolated markedly enlarged right atrium or possible combined right and left atrial dilatation. The bedside 2-dimensional echocardiography revealed that the right and left atriums were measured at 42 mm and 38 mm, respectively, which are considerably larger than the top normal limits at the patient’s age in the setting of left ventricular hypertrophy. These echocardiographic findings were consistent with hypertrophic restrictive cardiomyopathy. The lesson learned in this case is that a P wave resulting from marked biatrial enlargement can be large enough that it could be mistakenly viewed as a QRS complex, particularly when prominent atrial T waves are also present. Failure to recognize it would have led to wrong treatments for pseudo–ventricular tachycardia in the presence of complete AV block, for example, that could have potentially put the patient at life-threatening risk if an intravenous antiarrhythmic drug such as amiodarone or direct current cardioversion was started.