ECG Quiz Answer – First degree AV block, sinus pause, QT prolongation

ECG Quiz Answer – First degree AV block, sinus pause, QT prolongation

PR interval is prolonged and measures 240 ms. A sinus pause of 1880 ms is seen at the end of the tracing. Together these indicate dual nodal dysfunction. An interesting finding is the recovery of AV conduction after the long pause so that PR interval in the beat following the pause is only 120 ms.

QT interval is 440 ms with a cycle length of 640 ms, indicating a QTc of around 550 ms. It may be noted that the QT prolongation is predominantly due to prolongation of ST segment. ST prolongation contributing to QT prolongation is characteristic of hypocalcemia. This is because calcium is the ion predominantly involved in the phase 2 (plateau phase) of the myocardial action potential.

A case of hypocalcemia induced ST prolongation causing QT prolongation has been reported by Gardner JD and colleagues [1]. In that case, serum calcium was 5.0 mg/dL and QT interval 440 ms with correct QT interval (QTc) 533 ms. In another case reported in the same article, with serum calcium of 12 mg/dL, there was no discernible ST segment. QT interval was 320 ms and QTc was 341 ms. The patient had primary hyperparathyroidism due to parathyroid adenoma and hypercalcemia reverted after surgical removal of the adenoma.

1. Gardner JD, Calkins JB Jr, Garrison GE. ECG diagnosis: The effect of ionized serum calcium levels on electrocardiogram. Perm J. 2014 Winter;18(1):e119-20.