Complete heart block – narrow QRS

Complete heart block – narrow QRS

Complete heart block - narrow QRS
Complete heart block – narrow QRS

ECG tracing showing severe bradycardia due to complete heart block. RR interval is 1.8 s, corresponding to a ventricular rate of 33 per minute. PP interval is 720 ms, indicating an atrial rate of 83 per minute.

The four criteria for diagnosis of complete heart block in an ECG are:

  1. Regular PP interval
  2. Regular RR interval
  3. Varying PR interval
  4. PP interval less than the RR interval.

The T waves are very broad and the QT interval is prolonged, suggesting the potential for torsades de pointes and Stokes Adam attacks. QT interval is 760 ms, when corrected by Bazett’s formula gives a QTc of 564 ms. Torsades de pointes is a polymorphic ventricular tachycardia occurring in the setting of bradycardia with QT prolongation. Here it is acquired QT prolongation secondary to bradycardia. If there had been a Stokes Adam attack, the anoxia would contribute to worsening of QT prolongation. In such cases, post anoxic giant T wave inversions are noted, which is not seen here. 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 ventricular systole. Ventriculophasic sinus arrhythmia is a form of non-respiratory sinus arrhythmia. This is different from the respiratory sinus arrhythmia which is quite common in children.