Closed-loop stimulation using intracardiac impedance as a sensor principle: correlation of right ventricular dP/dtmax and intracardiac impedance during dobutamine stress test.

Changes of the unipolar right ventricular impedance during the cardiac cycle are related to the changing content of blood (low impedance) and tissue (high impedance) around the tip of the pacing electrode. During myocardial contraction, the impedance continuously increases reaching its maximum in late systole. This impedance increase is thought to correlate with right ventricular contractility, and thus, with the inotropic state of the heart. In the new Inos2 DDDR pacemaker, integrated information from the changing ventricular impedance (VIMP) is used for closed-loop regulation of the rate response. The aim of this study was to analyze the effect of increasing dobutamine challenge on RV contractility and the measured impedance signals. In 12 patients (10 men, 68 +/- 12 years) undergoing implantation of an Inos2 DDDR pacemaker (Biotronik), a right ventricular pigtail catheter was inserted for continuous measurements of RV-dP/dtmax and simultaneous VIMP signals during intrinsic and ventricular paced rhythm. Then, a stress test with a stepwise increase of intravenous dobutamine (5-20 micrograms/kg per min) was performed. To assess the relationship between RV contractility and measured sensor signals, normalized values of dP/dtmax and VIMP were compared by linear regression. There was a strong and highly significant correlation between dP/dtmax and VIMP for ventricular paced (r2 = 0.93) and intrinsic rhythm (r2 = 0.92), although the morphologies of the original impedance curves differed quite substantially between paced and intrinsic rhythm in the same patient. Furthermore, VIMP correlated well with sinus rate (r2 = 0.82), although there were at least four patients with documented chronotropic incompetence. We conclude, that for intrinsic and ventricular paced rhythms sensor signals derived from right ventricular unipolar impedance curves closely correlate with dP/dtmax, and thus, with a surrogate of right ventricular contractility during dobutamine stress testing. Our results suggest that "inotropy-sensing" via measurement of intracardiac impedance is highly accurate and seems to be a promising sensor principle for physiological rate adaptation in a closed-loop pacing system.