Complete heart block – narrow QRS
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ECG tracing showing severe bradycardia due to complete heart block. RR interval is 1.8 sec, corresponding to a ventricular rate of 33 per minute. PP interval is 720 msec, indicating an atrial rate of 83 per minute. The four criteria for diagnosis of complete heart block in an ECG are: regular PP interval, regular RR interval, varying PR interval and PP interval less than the RR interval.
The T waves are very broad and the QT interval is prolonged (760 msec), suggesting the potential for torsades de pointes and Stokes Adam attacks. Torsades de pointes is a polymorphic ventricular tachycardia occurring in the setting of bradycardia with QT prolongation.
The odd feature about this ECG is the very slow ventricular rhythm in spite of being a narrow QRS escape rhythm. Sometimes a PP interval enclosing a QRS complex is shorter than that which does not enclose a QRS complex in complete heart block. Such an instance is called ventriculophasic sinus arrhythmia and is supposed to be due to shortening of the sinus cycle length by the mechanical stimulus of the ventricular systole.