Dynamic counterbalance between direct and indirect vagal controls of atrioventricular conduction in cats

Atrioventricular conduction variability in coronary patients

Variability in atrioventricular (AV) conduction has been described in healthy subjects, whereas data for coronary patients are lacking. This study was designed to evaluate AV conduction variability in patients with stable coronary artery disease. Beat-to-beat PR interval variability with 5-minute resting high-resolution electrocardiogram recordings was measured in 30 men (mean age 50.9 years) with effort angina pectoris and 30 age-matched men without clinically evident coronary artery disease (controls). To evaluate the degree of coronary artery disease in coronary patients, coronary angiography was performed. Coronary patients displayed significantly lower values of PR interval variability compared with control subjects (P <.001). Relative PR variability (PRPP) index, calculated as the ratio of normalized PR variability to cycle length variability, was significantly higher in coronary patients (P =.001) and did depend on the degree of coronary artery disease. We concluded that coronary artery disease is associated with decreased beat-to-beat fluctuations in AV conduction. Increased relative PR variability index suggests inhomogeneous AV conduction in stable coronary atherosclerosis.

Dynamic sympathetic control of atrioventricular conduction time and heart period

Although power spectra of R-R and P-R intervals in response to random respiration show similar frequency distributions, the way in which dynamic sympathetic regulation contributes to such similarity remains unknown. We estimated the transfer function from sympathetic stimulation to the atrioventricular interval (AV conduction time; T(AV)) with and without constant atrial pacing in seven anesthetized cats. The transfer function from sympathetic stimulation to T(AV), except for absolute gain values, approximated a low-pass filter similar to that from sympathetic stimulation to the A-A interval (heart period; T(AA)). The 90%-rise times did not differ between the T(AA) and T(AV) step responses (32.3 +/- 1.8 vs. 29.6 +/- 3.2 s). Constant pacing augmented the T(AV) step response (-0.58 +/- 0.10 vs. -0.86 +/- 0.12 ms/Hz, P < 0.05) without affecting the 90%-rise time. These findings suggest that the dynamic characteristics of sympathetic control are similar between T(AA) and T(AV) despite the different electrophysiological mechanisms determining T(AA) and T(AV). A numerical simulation indicated that if the dynamic characteristics of the sympathetic control do not match between T(AA) and T(AV), a critical condition for initiation of reentrant tachycardia would be encountered.