SHOULD A VVIR PACEMAKER INCREASE THE HEART-RATE WITH STANDING

To assess the usefulness of incorporating a posture sensor into a ventricular inhibited rate modulated pacemaker, the hemodynamic effects of increasing the ventricular pacing rate with standing were studied in 15 pacemaker dependent patients aged 55 +/- 3.5 years. In a randomized cross-over design, the pacing rate remained at 70 or was increased to 100 beats/min immediately prior to standing. Blood pressure was monitored continuously and forearm blood flow was measured by venous occlusion plethysmography. There was no difference in supine blood pressure (117 +/- 4/63 +/- 3 compared to 118 +/- 5/64 +/- 4 mmHg) or forearm blood flow (2.88 +/- 0.36 vs 2.94 +/- 0.32 mL/100 mL/min) before the 70 or 100 pacing rate intervention. With standing, blood pressure fell to an equivalent degree at the two pacing rates (fall in mean blood pressure at 70 beats/min 6 +/- 4 and at 100 beats/min 8 +/- 2 mmHg, P = 0.7). After 1 minute of standing differences in blood pressure were similar, but after 2.5 minutes of standing the increase in mean blood pressure was less at 70 than at 100 beats/min (increase from control 28 +/- 2 compared to 36 +/- 3 mmHg, P = 0.002). Forearm blood flow decreased after standing for 1 and 2.5 minutes but there was no difference between the 70 and 100 pacing rates (fall in forearm blood flow at 2.5 minutes 0.50 +/- 0.24 and 0.59 +/- 0.25 mL/100 mL/cm2). In six patients with postural hypotension (fall in mean blood pressure > 10 mmHg at 70 beats/min), pacing at 100 beats/min appeared to lessen the fall in blood pressure with standing (fall 19 +/- 4 mmHg compared to 6 +/- 4 mmHg respectively, P = 0.07). In contrast, the nine patients without postural hypotension at pacing rate 70 beats/min appeared to be worsened by pacing at 100 beats/min (fall in mean blood pressure, -2.5 +/- 3 compared to 9 +/- 5 mmHg, respectively, P = 0.07). These data indicate that this degree of rate acceleration with standing is unlikely to be generally beneficial in patients dependent upon rate modulated ventricular inhibited pacemakers. In selected patients with significant postural hypotension, there may be a benefit from ventricular rate acceleration with standing.