Where do you measure the His?

• A. From the earliest His deflection and earliest ventricular activation
• B. From the latest His deflection and latest atrial activation
• C. From the earliest atrial activation
• D. From the distal coronary sinus potentials

Answer: (A)

Measuring the His means recording from the His bundle region and identifying the His potential—the small deflection that sits between the atrial signal and the ventricular activation on the intracardiac electrogram. You time from the onset of that His deflection to the onset of ventricular activation, which defines the HV interval and reflects conduction through the His-Purkinje system. This is best demonstrated by the earliest His deflection that accompanies the earliest ventricular activation.

Choosing the latest His deflection would reflect delayed or non-His conduction, not the true His bundle signal. The earliest atrial activation does not represent the His bundle itself, and distal coronary sinus potentials come from the coronary sinus, not the His bundle.