Do we need a reflex tachycardia to stand up?

Sophisticated atrio-ventricular pacing models are designed to integrate the pacemaker into cardiovascular autonomic control to react appropriately to the cardiovascular demands. Such an approach might be beneficial for patients with vasovagal responses to counterbalance the upright fall in arterial blood pressure by a pacing rate increase. We hypothesized that this approach would improve the cardiovascular response to standing in comparison with a regular pacing mode.

Two 5-minute tilt tests were performed in a random order in 5 patients with a pacemaker (CLS-INOS(2)) for sinus node disease and atrio-ventricular block. One tilt test was performed in fixed pacing rate (DDD), the other one was performed in close loop stimulation (CLS), which allowed an upright rate-rise pacing. Heart rate, systolic blood pressure, and cardiac output (modelflow) were recorded on a beat-by-beat basis.

Changes of systolic blood pressure and cardiac output in response to upright posture were not significantly different between DDD and CLS modes (2.7 +/- 13.2 vs 10.1 +/- 12.9 mmHg and -0.8 +/- 0.3 vs -1.1 +/- 0.4 L/min, respectively). But upright posture led to a tachycardia of more than 30 bpm in 3 patients in CLS mode and to a fall in systolic blood pressure greater than 20 mmHg in 3 patients in CLS mode and only in one patient in DDD mode.

Systolic blood pressure and cardiac output are not improved by the upright tachycardia and upright blood pressure response is actually worsened when an upright rate-rise pacing is used. Thus, it appears that tachycardia alone cannot compensate for an upright fall in blood pressure.