Abstract: An implantable cardiac device that is adapted to periodically measure a body parameter, such as transthoracic impedance, at time periods selected so that the body parameter is primarily indicative of the respiration of the patient. In this way, a ventilation parameter, such as minute ventilation, can be reconstructed from the signals without requiring filtering of the sampled signals. In one embodiment, the implantable cardiac device measures transthoracic impedance during each quiescent period of the heart and thereby obtains a plurality of transthoracic impedance data points which are then used to reconstruct a ventilation signal. As the transthoracic impedance data points are obtained during the quiescent period, the contribution of the heart to the resulting transthoracic impedance measurement can be ignored and the resulting measurements are indicative of the action of the heart.

Abstract: Methods and apparatus are provided for measuring the impedance of a patient's body. Pulse generating circuitry within a rate-responsive pacemaker is used to generate an impedance measurement signal that is applied to the body of the patient with conventional pacemaker leads. The impedance measurement signal contains a series of multiphasic impedance measurement waveforms, which have no net DC value and zero value after second integration. The impedance measurement signal allows the impedance of the body to be measured without interfering with external cardiac monitoring equipment such as electrocardiogram machines.

Abstract: Methods and apparatus are provided for measuring the impedance of a patient's body. Pulse generating circuitry within a rate-responsive pacemaker is used to generate an impedance measurement signal that is applied to the body of the patient with conventional pacemaker leads. The impedance measurement signal contains a series of multiphasic impedance measurement waveforms, which have no net DC value and zero value after second integration. The impedance measurement signal allows the impedance of the body to be measured without interfering with external cardiac monitoring equipment such as electrocardiogram machines.

Abstract: An improved input protection circuit for use in a circuit for processing sensor signals from a piezoelectric patient activity sensor is provided. The input protection circuit includes a field-effect transistor to prevent the voltage at an input terminal of the processing circuit from rising beyond a first predetermined voltage or falling below a second predetermined voltage. The gate and drain terminals of the transistor are connected to a first of two input terminals of the processing circuit and the source and body terminals are connected to the second terminal. When the voltage at the first terminal rises or falls sufficiently, the transistor conducts current, thereby preventing the voltage from rising or falling further.

Abstract: A pyroelectric suppressor circuit for preventing undesirable thermally induced signals generated by a piezoelectric physical activity sensor from reaching processor circuitry within an implantable medical device is provided. The thermally induced signals typically have frequencies below a frequency in the range from about 0.1 mHz to about 10 mHz. The suppressor circuit provides a high-pass filter that rejects signals that have frequencies associated with thermally induced signals. Signals having frequencies greater than a frequency in the range from about 0.1 mHz to about 10 mHz, which correspond to patient activity, are passed on to processing circuitry of the implantable medical device.

Abstract: An improved cardiac pacemaker system of the kind that includes an oxygen sensor implantable into a patient's blood stream along with one or two electrical conductors for use in sensing heart beat activity and in selectively applying pacing pulses to the heart muscle. In an embodiment having just one implantable conductor, the oxygen sensor is integrated into the conductor, while in an embodiment having two implantable conductors, the oxygen sensor either is integrated into one of the conductors or is connected between the two conductors, in parallel with the heart muscle. In both cases, the oxygen sensor is selectively used without adversely affecting either heart beat sensing or pacing.

Abstract: An improved blood oxygen sensor apparatus having a special leakage compensation circuit that allows proper initialization of the oxygen sensor regardless or the value of leakage resistance which typically might be present in parallel with the oxygen sensor. During an initialization mode, in which a capacitor arranged in parallel with oxygen sensor is charged by a small initialization current, the leakage compensation circuit monitors the capacitor voltage and controllably increases the current if it is determined that the capacitor is charging at an insufficient rate, due to unspecified leakage resistance. This enables the oxygen sensor to be properly initialized such that it can thereafter accurately measure blood oxygen levels.

Abstract: A sensor for use with a rate-responsive pacemaker is disclosed which is responsive to blood oxygen content, thereby allowing the cardiac rate of the pacemaker to closely mimic the natural response pattern of the heart to changing physiological need. The sensor integrates the output from a photosensor driven by blood-reflected light from an LED, and when the integrated output reached a predetermined threshold latches the circuit, enabling the use of time to indicate the level of blood oxygen content. The sensor thus advantageously requires neither a voltage doubler in the driving circuitry, or an analog-to-digital converter in the output circuitry, reducing both complexity and power consumption of the blood oxygen sensor.