Cardiodynamic and neurohormonal importance of atrial contribution in rate-responsive pacing.

To elucidate the physiologic importance of atrial contribution in recently developed rate-responsive pacing, changes in cardiodynamics and neurohormonal factors were analyzed during exercise in patients with respiratory rate-dependent, rate-responsive atrial (AAIR; n = 6) and ventricular (VVIR; n = 9) demand mode pacemakers implanted for sick sinus syndrome. With increasing pacing rate during bicycle ergometer exercise, the AAIR group had significant increases in cardiac index (p < 0.05), left ventricular ejection fraction (p < 0.05), and ejection (p < 0.05) and peak filling (p < 0.05) rates; however, the VVIR group had a significant decrease in ejection fraction (p < 0.05), and an increase in cardiac index (p < 0.05) that was significantly less than in the AAIR group. At rest, the mean plasma concentrations of atrial natriuretic peptide (p < 0.005) and cyclic guanosine monophosphate (p < 0.05) were significantly greater in the VVIR group than in the AAIR group and normal subjects (n = 8). Atrial natriuretic peptide, norepinephrine, and cyclic adenosine and guanosine monophosphates were significantly greater (p < 0.05) during exercise, and atrial natriuretic peptide was significantly greater in the VVIR group (207.5 +/- 8.3 pg/ml) than in the AAIR group (116.4 +/- 51.5) and normal subjects (30.8 +/- 19.2; p < 0.05); this suggested a further increase in the nonphysiologic atrial overload with VVIR pacing. The data show both the neurohormonal and cardiodynamic importance of atrioventricular synchrony in rate-responsive pacing.