The atrial to His (A-H) interval, which represents atrioventricular (AV) nodal conduction time, is plotted against the pacing cycle length or A1-A2 interval. The normal response is a progressive lengthening of the AH interval with a shorter pacing cycle length (ie, faster heart rate), a phenomenon known as decremental conduction. In patients with enhanced AV nodal conduction (EAVNC), four patterns of response are seen. Type I represents reduced AH interval prolongation in response to atrial pacing; type II shows an initial increase in the AH interval followed by a plateau and then a further increase in the AH interval with faster pacing rates; type III reveals no change in AH interval in response to increasing atrial pacing rates; type IV represents no change in AH interval until the pacing cycle lengths are very short, thereby resulting in marked AH prolongation.
