Modern pacemaker technology renders possible the adaptation of pacing rate to hemodynamic requirements. The most ambitious approach aims at restoration of the physiological closed-loop system by utilizing the information supplied by the autonomic nervous system (ANS) and extracted from myocardial contractile performance. Measurement is accomplished by the impedance method using the stimulation electrode as the measuring electrode. The ventricular inotropic parameter (VIP) has been identified as an ANS dependent parameter. A special detection algorithm, regional effective slope quantity (RQ), with high ANS sensitivity has been developed. Rate adaptation has been achieved by using an individually adjustable inotropic index (II). The concept has been evaluated in a multicenter study using a standardized exercise protocol. The results in patients with AV block demonstrate excellent agreement between spontaneous sinus rhythm and the ANS-controlled stimulation rate during different forms of exercise. Measurement of mean arterial blood pressure (MABP) supports the physiological approach of adapting the pacing rate to various types of hemodynamic challenges.
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.
Implants are steadily increasing in importance as substitutions for body functions. With the present state of the art, the limitations of the application of cardiovascular implants are due to insufficient performance of biomaterials. Present research in this field is being concentrated on efforts to improve the thrombus resistance of conventional materials by coating with semiconducting materials to actively influence the electrochemical interaction between the condensed matter and blood proteins. Based on an electrochemical model of the interaction of fibrinogen with an artificial surface and the resulting requirements for improving hemocompatibility, a coating of amorphous hydrogenated silicon carbide deposited by plasma-enhanced chemical vapor deposition (PECVD) is presently under evaluation as a special coating material for cardiovascular prostheses and is herein described. In particular, first results are published concerning the optimum deposition parameters in the PECVD process and cell culture tests. Experimental results of comparative partial thromboplastin time studies serve the purpose of proving the validity of the electrochemical reaction model referring the hemocompatibility of implantable materials to their semiconducting surface properties. The aim of this article is to demonstrate a feasible method for an antithrombogenic surface modification based on doped amorphous silicon carbide films that is in full conformance to the above mentioned model.
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