Apparatus and method for measuring cardiac output
First Claim
1. A method of initially balancing and thereafter operating a circuit for measuring the temperature change of a blood-injectate mixture and with the temperature change measured using a thermistor energized by an alternating or direct current source and in which the current is held constant during measurement, including the following stepsadjusting the initial sensitivity of the circuit prior to measurement of blood-injectate temperature to produce an initial amplified thermistor voltage (GIRo) equal to a predetermined constant;
- andholding constant the circuit sensitivity during temperature change measurement of the blood-injectate mixture by the thermistorwhereby changes in the amplified thermistor voltage which occur due to the temperature changes of the blood-injectate mixture are independed of the inital thermistor resistance and dependent only on the temperature change and the material constant (B) of the thermistor.
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Abstract
A thermodilution cardiac output computer uses an autobalancing temperature-measuring circuit which adjusts the initial voltage derived from a thermistor and then holds constant the current through the thermistor as the thermistor responds to different temperatures in a blood-injectate mixture. The initial voltage is adjusted to a predetermined constant so that the response from thermistors of different initial resistance is the same. A Wheatstone bridge is not used. Before the temperature measurement, the voltage derived from the thermistor is adjusted to be equal to a predetermined constant either by varying the current through the thermistor or, with a constant current through the thermistor, by varying the gain of the amplifier sensing the voltage across the thermistor. During the temperature measurement this current or gain is held constant. The voltage change derived from the thermistor is integrated in a conventional integrator, and the integration is stopped at the tail of the decay of the temperature-time curve by an automatic timer. This termination point is calculated not as a percentage of the peak value of the curve or at a particular slope of the curve, but as a function of the time of two points high on the decay curve where the artifacts causing undulations in the curve are small compared to the temperature height of the curve. In this fashion, the triggering of the cutoff point by undulations in the curve is avoided.
27 Citations
18 Claims
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1. A method of initially balancing and thereafter operating a circuit for measuring the temperature change of a blood-injectate mixture and with the temperature change measured using a thermistor energized by an alternating or direct current source and in which the current is held constant during measurement, including the following steps
adjusting the initial sensitivity of the circuit prior to measurement of blood-injectate temperature to produce an initial amplified thermistor voltage (GIRo) equal to a predetermined constant; - and
holding constant the circuit sensitivity during temperature change measurement of the blood-injectate mixture by the thermistor whereby changes in the amplified thermistor voltage which occur due to the temperature changes of the blood-injectate mixture are independed of the inital thermistor resistance and dependent only on the temperature change and the material constant (B) of the thermistor. - View Dependent Claims (2, 3)
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4. An autobalance circuit for sensing the temperature changes of a flowing blood-injectate mixture for use in equipment for measuring cardiac output by thermal dilution, the autobalance circuit including
a thermally variable resistor for detecting the temperature of the blood-injectate mixture in accordance with the resistance of the variable resistor, a source of current coupled to the variable resistor for producing a voltage drop across the variable resistor in accordance with the resistance of the variable resistor, first means coupled to the variable resistor and responsive to the change in the voltage drop across the variable resistor for producing an output voltage in accordance with the change in the voltage drop across the variable resistor, and wherein the first means includes a source of reference voltage and a means for subtraction coupled to the source of reference voltage and to the voltage drop across the variable resistor for producing the output voltage from the first means in accordance with subtraction of the source of reference voltage from the voltage drop across the variable resistor, a feedback loop coupled to the first means and responsive to the output voltage from the first means for producing a feedback signal having a value for providing a zero output voltage from the first means to initially balance the circuit and with the feedback loop including means for decoupling the output voltage from the first means to the feedback loop after the production of te zero output voltage from the first means and with the feedback loop including a holding control for maintaining the same value for the feedback signal after the output signal from the first means is decoupled for providing after the initial balance an output signal from the first means only in accordance with changes in the voltage drop across the variable resistor.
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10. Apparatus for terminating the integration of a time-temperature curve for use in equipment for measuring cardiac output by thermal dilution and with the time-temperature curve represented by a varying voltage having an initial rising portion to a peak and a decay from the peak, the apparatus including
first means responsive to the varying voltage for detecting and holding the peak voltage, second means coupled to the first means and responsive to the peak voltage and to the varying voltage for detecting when the varying voltage decays to a first predetermined percentage of the peak voltage, third means coupled to the first means and responsive to the peak voltage and the varying voltage for detecting when the varying voltage decays to a second predetermined percentage of the peak voltage lower than the first predetermined percentage, fourth means coupled to the second and third means and responsive to the detection by the second and third means for producing a cutoff signal which is time delayed after the detection by the third means in accordance with the time interval between the detection by the second and third means; - and
fifth means coupled to the fourth means and responsive to the cutoff signal and the varying voltage for integrating the varying voltage relative to time and for terminating the integration in response to the cutoff signal. - View Dependent Claims (11, 12, 13, 14)
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15. A cutoff timer for determining the integration of a signal representative of a time-temperature dilution curve for use in equipment for measuring cardiac output by thermal dilution, the cutoff including
first means responsive to the signal representative of the time-temperature dilution curve for determining a time interval on the time-temperature dilution curve in a region of favorable signal-to-noise ratio; -
second means responsive to the time interval for determining a time delay as a function of said time interval; and third means responsive to the time delay for stopping the integration at the end of said time delay near the end of the time-temperature curve where the signal is small.
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16. The method of terminating the integration of the area under a signal representative of time-temperature dilution curve for use in the measurement of cardiac output by including the following steps,
determining a cutoff time for the integration in advance of cutoff in accordance with properties of the signal representative of the time-temperature curve in a region where the signal is large and the signal-to-noise ratio is favorable; - and
terminating the integration of the area under the signal representative of the time-temperature curve at the predetermined cutoff time, which cutoff time occurs when the signal is small and substantially all of the area under the curve has been directly integrated. - View Dependent Claims (8, 17, 18)
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Specification