Narrow QRS Tachycardia with ST Depression

ECG Quiz 21

A narrow QRS tachycardia is seen at a rate of around 150/min. ST segment depression is seen in inferior and lateral leads, with T wave inversion in leads I, aVl and V6. These ST – T changes during tachycardia need not indicate associated coronary artery disease. It is quite common in supraventricular tachycardia even in young individuals. It could be a manifestation of a prominent Ta wave which is the atrial repolarization wave. Another situation in which Ta wave becomes prominent and produces ST segment change is during exercise test. The upsloping ST segment depression seen during exercise test has low specificity for coronary artery disease due to this. In this case there is evidence of left ventricular hypertrophy which can also contribute to the ST – T changes.

The rate of 150/min should make us suspect atrial flutter at a rate of 300/min with 2:1 conduction as the underlying cause. Carotid sinus massage can reduce the conduction rate and bring out flutter waves in a doubtful case.  In this case no features of flutter waves are evident. Instead there is a suggestion of a retrograde P wave at the end of the QRS in inferior leads. This will qualify for the description of a short RP tachycardia. Two important differential diagnoses of short RP tachycardia are AVNRT and AVRT.