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In nine patients without clinical or laboratory evidence of heart disease, premature atrial depolarization (PAB) induced a complete block of conduction in the intraventricular conducting system. In these patients the functional refractory period AV (FRPav)) gave short and very similar results to the effective refractory period of the His-Purkinje system (ERPhp), and the effective refractory period AV (ERPAV) was found to be shorter than the ERPhp in all cases. A linear correlation between the ERPhp and the basic cycle length (BCL) was also observed. These special functional properties of the AV node justify the occurrence of intraventricular block after PAB. In fact, the stimulus, rapidly conducted through the AV node, finds a complete or incomplete refractoriness in the ventricular conducting system, and therefore bundle branch or complete intraventricular block occurs. The linear correlation between the ERPHP and the BCL explains why the atrial pacing is not always useful for pointing out intraventricular conducting defects.

Sinoatrial conduction was investigated in 18 normal subjects, using premature atrial depolarizations. The results obtained in this investigation were evaluated plotting the test cycle (expressed as difference between the basic sinus cycle and the test cycle as a percentage of the basic sinus cycle) as a function of the return cycle (expressed as difference between the return cycle and the basic cycle as a percentage of the basic sinus cycle). In normal subjects, premature atrial depolarizations elicited in the last 10-20% of the spontaneous sinus cycle, produced a progressive prolongation of the return cycle and the points correlating the return cycle index to the test cycle index fell above the diagonal of the plotting system. After earlier premature atrial stimulations, the return cycle stayed the same length, and the points correlating the return cycle index to the test cycle index fell along a line parallel to "y" axis (plateau). The mean value of the returning cycle (expressed as above) corresponding to the test cycles (evaluated as above) included in the first 5% of the "plateau" can be defined as the "sinoatrial conduction index". This index, the sum of conduction into and out of the sinus node, ranged from 79 msec to 187 msec. By assuming similar anterograde and retrograde conduction, the sinoatrial conduction time ranged from 39.5 msec to 97.5 msec (mean value 70 msec).