Variations in Cardiac Excitability: Evidence from P Wave Synchrony Studies

¹ Section of Cardiology, Department of Medicine, Mount Sinai Hospital, and the University of Miami School of Medicine (Coral Gables)

The technique of P-wave synchrony utilizing bipolar atrial and ventricular catheters has been employed to determine the variations in cardiac excitability in different phases of the cardiac cycle. In this study,

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a bipolar intraatrial catheter is used to detect a P-wave which is then transmitted via a Medtronic paired pulse pacemaker to a bipolar ventricular catheter. The stimuli can therefore be applied to the ventricles at various intervals after the P-wave and at various intervals within the cardiac cycle. When the stimulus is applied to the ventricle, shortly after the P-wave, a Wolff- Parkinson-White syndrome results. When the stimulus is applied to the ventricle, within the absolute refractory period, the stimulus is not effective. When the stimulus is applied to the ventricle, during the latter portions of the T-wave, a ventricular rhythm develops with retrograde P-wave conduction. Variations in the strength of the stimulus, as well as in its timing, were utilized in this study to demonstrate that there are considerable variations in cardiac excitability in different portions of the cardiac cycle. Thus, there is an increase in cardiac excitability in the latter portion of diastole, as opposed to early diastole, since the minimum stimulus strength that results in the onset of ventricular pacing is smaller later in diastole and conversely is larger earlier in diastole.