Cardiac cardioverter/defibrillator with in vivo impedance estimation

Abstract

An implantable device, such as a defibrillator which may include cardioversion and pacemaker capabilities, which automatically measures the impedance of the heart prior to the delivery of a cardioverting shock. The defibrillator adjusts the voltage level on output capacitors to deliver a selected energy to the patient's heart. Insulated gate bipolar transistors, or similar devices having an inherent capacitance such that when they are turned on a small voltage gradient will exist across the transistor, are used as switches to control the application of electrical energy to the heart for therapy. When the switches are turned on, a current flows through a connected circuit path, for example through the heart and associated leads. Measurement of the current gives a measure of the impedance of the heart. An initial measurement is performed at the time of implantation of the implantable device, when the initial energy level (and output voltage) is selected. By scaling from this initial measurement to a second in vivo measurement of current, the implantable device can be programmed to adjust the voltage on or output voltage of the output capacitors.