When recording ECG, the interbeat interval is measured between successive what?

The correct answer is that the interbeat interval is measured between successive R peaks. The interbeat interval specifically refers to the time between consecutive heartbeats, and in an electrocardiogram (ECG), the R peak corresponds to the highest point of the R wave in the QRS complex, which reflects the depolarization of the ventricles and is considered a reliable point for measuring heartbeats. By measuring the intervals between these R peaks, one can better assess heart rate variability and overall cardiac function, as this timing is critical in understanding the rhythm of the heart.

The other options, such as P waves and T waves, represent different electrical activities within the cardiac cycle but do not provide the same clarity in defining the intervals between heartbeats. P waves represent atrial depolarization, and T waves indicate ventricular repolarization, neither of which serves as a standard measure for heart rate intervals.