Thermal drift correction while continuously monitoring cardiac output
First Claim
1. A method for measuring cardiac output of a heart that is compensated for a drift in a baseline temperature of blood, comprising the steps of:
- (a) producing a periodically varying input signal that is used for changing a temperature of blood within the heart, so that a temperature of the blood leaving the heart varies periodically with respect to the baseline temperature of the blood;
(b) sensing the temperature of the blood leaving the heart, producing a blood temperature signal corresponding thereto that varies periodically;
(c) filtering the blood temperature signal to determine a filtered output signal comprising an in-phase component and a quadrature component;
(d) selecting a measurement interval for the blood temperature signal relative to the input signal, so as to insure that the filtered output signal predominantly comprises the in-phase component; and
(e) determining the cardiac output of the heart as a function of the input signal and of the filtered output signal, the effect of any baseline temperature drift being minimized in this determination because the baseline temperature drift primarily comprises the quadrature component, which is substantially out of phase with the in-phase component of the filtered output signal.
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Abstract
A method for compensating the determination of cardiac output for a baseline blood temperature drift, or other long-term noise. The method is applicable to the continuous monitoring of cardiac output, which is carried out by modifying the temperature of blood within a heart (12) using a periodically varying electrical current that is applied to a heater (22) disposed on the outer surface of a catheter (14) inserted within the heart. A temperature sensor (24) disposed near a distal end (18) of the catheter monitors the temperature of blood leaving the heart, producing a blood temperature signal that periodically varies in a manner corresponding to the input signal. Alternatively, the temperature of blood within the heart can be modified by heat exchange with a fluid circulated within a heat exchanger (16) on the catheter, so that heat is transferred between the fluid and blood. Changes in the baseline blood temperature caused by a gradual warming of the patient from a chilled condition or due to other long term noise are compensated by either carefully selecting a measurement interval during which the blood temperature signal is filtered (by performing a discrete Fourier transform) so that the blood temperature signal (without the effect or drift) comprises only a real component, or by shifting the starting point of the input power signal to achieve the same result. The baseline temperature drift of the blood or other noise is thus in quadrature with the blood temperature signal (without drift) and has little effect on the determination of cardiac output. Further compensation in the measurement interval and/or starting time of the input signal can be made to compensate for other time delays, such as delays due to mixing volume, transport delay, and delays associated with the time constants of the apparatus used to modify the temperature of the blood and determine its temperature.
122 Citations
21 Claims
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1. A method for measuring cardiac output of a heart that is compensated for a drift in a baseline temperature of blood, comprising the steps of:
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(a) producing a periodically varying input signal that is used for changing a temperature of blood within the heart, so that a temperature of the blood leaving the heart varies periodically with respect to the baseline temperature of the blood; (b) sensing the temperature of the blood leaving the heart, producing a blood temperature signal corresponding thereto that varies periodically; (c) filtering the blood temperature signal to determine a filtered output signal comprising an in-phase component and a quadrature component; (d) selecting a measurement interval for the blood temperature signal relative to the input signal, so as to insure that the filtered output signal predominantly comprises the in-phase component; and (e) determining the cardiac output of the heart as a function of the input signal and of the filtered output signal, the effect of any baseline temperature drift being minimized in this determination because the baseline temperature drift primarily comprises the quadrature component, which is substantially out of phase with the in-phase component of the filtered output signal. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A method for measuring cardiac output of a heart so as to compensate for a drift in a baseline temperature of blood and for other noise, comprising the steps of:
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(a) producing a periodic input signal that changes a temperature of blood leaving the heart with respect to the baseline temperature of blood entering the heart, said input signal comprising a wave form defining a plurality of cycles, each cycle extending over a predefined period of time; (b) sensing the temperature of blood leaving the heart, producing a blood temperature signal that varies periodically in a manner corresponding to the period of the input signal, said blood temperature signal comprising a wave form defining a plurality of cycles, each cycle extending over the predefined period of time; (c) providing a selected phasal relationship between a start of the input signal and a start of a measurement interval during which the blood temperature signal is produced, so that the drift in the baseline temperature of blood and other noise add to the blood temperature signal in quadrature; and (d) determining the cardiac output of the heart as a function of the input signal and of the blood temperature signal, any effect of the baseline temperature drift and other noise being minimized in this determination because any contribution of the baseline temperature drift and noise to the blood temperature signal is substantially out of phase with changes in the blood temperature signal caused by the input signal and is therefore substantially negligible. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21)
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Specification