SENSOR FOR RIGHT VENTRICULAR VOLUMES USING THE TRAILING EDGE VOLTAGE OF A PULSE-GENERATOR OUTPUT

Cardiac and thoracic volume (Vol) signals are useful for diagnosis and adaptive rate pacing. Present systems need an AC or pulsed constant current carrier to measure conductivity (proportional to Vol), causing high battery drain and requiring complex detection algorithms or special leads. The aim of this study was to propose a new and simple sensor for cardiac volumes using standard pacing leads and no AC carrier signal. "Constant voltage" pulse generators (PG) deliver a square pulse to the lead via a capacitor (Cap). The signal resulting from the interaction between the PG and the tissues and blood is trapezoidal, with a leading edge voltage determined by PG output and a trailing edge voltage (TEV) dependent on electrode surface and Cap value (device constants), and patient load. The hypothesis that right ventricular (RV) and chest Vol variations could produce load-related TEV changes was tested. Four PGs were connected to bipolar (Bip) leads within containers with 5-80 cc of saline, and TEV was measured with every 5 cc Vol change. Fifteen patients with previously implanted Bip PGs were studied during VOO pacing, scanning the intrinsic cardiac cycle. TEV was noninvasively measured as a function of time from onset of QRS. Results: (1) There was a nonlinear relationship between Vol and TEV, best expressed by exponential, power, or hyperbolic equations; (2) Cardiac cycle scanning revealed smaller TEV at end-systole than at end-diastole (mean difference = 88 mV [P < 0.001]); (3) Reconstruction of RV Vol curve by scanning-rendered waveforms very similar to those obtained by the conventional impedance technique, end-diastolic Vol, end-systolic Vol, stroke Vol, and ejection fraction could thus be derived. Conclusion: TEV sensor provides a simple, reliable signal for hemodynamic diagnosis and for adaptive rate pacing in implantable devices, using conventional bipolar leads.