IMPLANTABLE CARDIAC STIMULATOR WITH POLARITY DETECTION FOR DETECTING ECTOPIC BEATS

An implantable medical device (100) for electrically stimulating the heart to beat including a sense circuit (208, 218) for detecting cardiac electrical activity. The sense circuit includes a sense amplifier (210, 220), band pass filter (215, 225), and threshold detector (227, 240, 245). The threshold detector determines whether sensed cardiac electrical activity resulted from a normal heart beat or an ectopic beat such as a premature ventricular contraction. In a preferred embodiment latches in the threshold detector are activated by output pulses from a pair of comparators (240, 245). A positive comparator (240) produces an output pulse upon detection of cardiac electrical activity exceeding a positive threshold voltage and a negative comparator (245) produces an output pulse upon detection of cardiac electrical activity more negative than a negative threshold. Threshold logic (250) produces output signals indicative of which comparator first produced an output pulse. A logic and control unit monitors the output signals from the threshold logic and thus determines whether the associated cardiac electrical activity represented a normal heart beat or resulted from an ectopic beat so that appropriate pacing may be provided by the implantable device.