Failure of Rate Responsive Ventricular Pacing to Improve Physiological Performance in the Univentricular Heart

The physiological efficacy of single chamber, rate responsive ventricular pacing (VVIR) is unknown for symptomatic patients following the Fontan procedure for univentricular hearts. A total of six postoperative children, ages 6-21 years (mean 13), with symptomatic bradycardia requiring pacing therapy, underwent comparative treadmill exercise testing in randomized fixed rate (VVI) and VVIR pacing modes. In all instances, implanted activity pulse generators (Medtronic Model 8403) were programmed to identical age-appropriate low paced rates during VVI and VVIR modes with the upper rate response at 150 ppm. All studies were performed at least 2 weeks apart. Physiological values of heart rate, blood pressure, work rate (watts), oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory exchange ratio (RER) were monitored continuously during each test using a 1 minute incremental treadmill protocol. Ventilatory anaerobic threshold (VAT) was calculated from VO2, VCO2, and minute ventilation. The results demonstrated that although there was a significant increase in paced heart rate per minute throughout exercise (P < 0.01) with VVIR pacing, maximum watts, VO2, and VAT remained unchanged. These findings indicate that in spite of an improved chronotropic response to exercise, children with univentricular hearts following the Fontan procedure continue to demonstrate altered hemodynamics which negate potential benefits of VVIR pacing