What does a R to S duration greater than 100 ms in precordial leads indicate?

A duration of R to S greater than 100 ms in precordial leads is indicative of ventricular tachycardia (VT). This finding suggests a single ectopic focus in the ventricles that is firing at a rapid rate, leading to a wide QRS complex. The prolonged duration reflects a disruption in the normal conduction through the ventricles, which is characteristic of VT.

In the context of cardiac rhythm, a wide QRS complex typically arises from abnormal conduction, and a duration exceeding 100 ms signals a significant alteration, making it less likely to be associated with normal sinus rhythms or atrial fibrillation. Atrial fibrillation, for instance, would typically show irregularly timed R-R intervals without a wide QRS complex.

It is essential in clinical settings to recognize such findings and take appropriate actions, as prolonged R to S durations can correlate with more severe arrhythmias that may require intervention, but specifically, this measurement strongly supports the diagnosis of ventricular tachycardia.