CARDIODYNAMIC AND NEUROHORMONAL IMPORTANCE OF ATRIAL CONTRIBUTION IN RATE-RESPONSIVE PACING

To elucidate the physiologic importance of atrial contribution in rece ntly developed rate-responsive pacing, changes in cardiodynamics and n eurohormonal factors were analyzed during exercise in patients with re spiratory rate-dependent, rate-responsive atrial (AAIR; n = 6) and ven tricular (VVIR; n = 9) demand mode pacemakers implanted for sick sinus syndrome. With increasing pacing rate during bicycle ergometer exerci se, 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 sign ificant decrease in ejection fraction (p <0.05), and an increase in ca rdiac index (p <0.05) that was significantly less than in the AAIR gro up. At rest, the mean plasma concentrations of atrial natriuretic pept ide (p <0.005) and cyclic guanosine monophosphate (p <0.05) were signi ficantly greater in the VVIR group than in the AAIR group and normal s ubjects (n = 8). Atrial natriuretic peptide, norepinephrine, and cycli c adenosine and guanosine monophosphates were significantly greater (p <0.05) during exercise, and atrial natriuretic peptide was significan tly greater in the VVIR group (207.5 +/- 8.3 pg/ml) than in the AAIR g roup (116.4 +/- 51.5) and normal subjects (30.8 +/- 19.2; p <0.05); th is suggested a further increase in the nonphysiologic atrial overload with VVIR pacing, The data show both the neurohormonal and cardiodynam ic importance of atrioventricular synchrony in rate-responsive pacing,