BLOOD GAS SENSOR AMPLIFIER AND TESTING SYSTEM

• US 3,710,778 A
• Filed: 03/15/1971
• Issued: 01/16/1973
• Est. Priority Date: 03/15/1971
• Status: Expired due to Term

• Current Assignee: Frank L. Cornelius
• Original Assignee: Frank L. Cornelius
• Inventors: Cornelius, Frank L.
• Primary Examiner(s): Howell, Kyle L.
• Application Number:
  US05/124,364
• Time in Patent Office: 673 Days
• Field of Search: 128/2E,2G,2R,2L,2.1E 204/195P 324/3R 73/23R
• US Class Current: 600/353
• CPC Class Codes: A61B 5/14542 for measuring blood gases A61B5/14551 takes precedence
• CPC Class Codes: A61B 5/1473 invasive, e.g. introduced into the body by a catheter A61B5/1482 takes precedence
• CPC Class Codes: A61B 5/1491 Heated applicators
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

The invention includes an amplifier for processing the output signal from an in vivo sensor for the partial pressure of gas in blood. Means are provided to protect the patient from excess currents and voltages. The first amplifying stage has a floating ground and is at substantially the same potential as the sensor. The output of this stage is chopped with a field effect transistor that derives its control voltage from a transformer which is driven at high frequency and which has high impedance or low leakage at power line frequencies. The chopped amplifier output signal is passed through another transformer which closely couples high frequencies. The amplified signal is demodulated by another field effect transistor in the secondary of this transformer after which the signal is further processed in circuitry that need not be isolated from ground. Means are provided for displaying the signal in terms of partial pressure of the gas in millimeters of mercury. The system includes means for testing the integrity of the sensor before and continuously after it is implanted in the body. Means are also provided for calibrating the sensor under known conditions which are conveniently established.

Claims

1. For use with a sensor which responds to partial pressure of a gas in a body fluid of a patient to produce a signal representative of the partial pressure and which has a predetermined electrical impedance in its normal condition, a monitor comprising:

a. terminal means for connection to the sensor b. sensor signal processing means having an input connected with said terminal means and adapted to produce a signal corresponding with partial pressure of the gas, c. test oscillator means adapted for being connected to the body of the patient to impose a substantially constant alternating voltage from the oscillator means on the sensor, d. a circuit means connected to said terminal means to develop a signal whose magnitude depends on the impedance of the sensor to the alternating voltage, e. means responsive to signal variations from said circuit means which correspond with a change in the impedance of the sensor, and f. means which respond to said last-named means to produce a signal indicative of said impedance change.

2. The monitor set forth in claim 1 including:

a. a test terminal connected to said test oscillator means, b. said test terminal being exposed for being contacted by the sensor when it is not implanted in the body, to thereby impose alternating voltage from said test oscillator means on the sensor for checking its electrical integrity.

3. The monitor set forth in claim 2 further including:

a. a variable resistance device connected in circuit with said oscillator means and said test terminal, the resistance of said device depending on its bias voltage, and b. an amplifier responsive to the signal magnitude on said circuit means to produce an output signal that is proportional to said signal magnitude and is applied to said variable resistance device to change its bias voltage and increase its resistance, whereby to limit current through the sensor if the sensor is in abnormal condition.

4. The monitor set forth in claim 3 wherein:

a. said variable resistance device is a field effect transistor having a source, a drain and a gate terminal, and b. said amplifier has its output signal supplied to said gate terminal to control the resistance of said transistor.

5. The monitor set forth in claim 1 wherein:

A. said circuit means is a tuned circuit and has a low impedance for signals produced by the sensor in response to the partial pressure of the gas and has a high impedance for the signals at said test oscillator frequency.

6. For use with a sensor which responds to partial pressure of a gas in a body fluid of a patient to produce a signal representative of the partial pressure and which has a predetermined electrical impedance in its normal condition, a monitor comprising:

a. first terminals for connection with the sensor, b. amplifier means amplifying the signal produced by the sensor and having input terminals one of which is connected with one of said first terminals, c. a device which has low impedance for unidirectional sensor signals and higher impedance for alternating signals, said device being in circuit between another of said first terminals and another input terminal of said amplifier means, d. means producing a variable control signal in response to corresponding variations of the alternating signal on said device, e. an oscillator producing an alternating test signal, f. an electrode for connection to the body of the patient, and g. a terminal in the output circuit of said oscillator connected to said electrode for applying the alternating test signal to a circuit including the sensor and said device, whereby said control signal producing means will be varied in correspondence with variations in the impedance of the sensor.

7. The monitor set forth in claim 6 further including:

a. a field effect transistor having source and drain terminals in said oscillator output circuit and having a gate terminal that is connected to receive said control signal, said control signal being effective to increase the resistance between said source and drain terminals in response to an increasing alternating test signal on said device corresponding with decreased impedance of the sensor.

8. The monitor set forth in claim 6 wherein:

a. said device is a parallel resonant tuned circuit which produces voltage variations in correspondence with variations in the impedance of the sensor to said alternating test signal.

9. The monitor set forth in claim 6 further including:

a. signal chopper means receiving an output signal from said amplifier means, b. demodulator means, c. a first transformer having a primary winding receiving the signal from the chopper means and a secondary winding delivering the signal to the demodulator means wherein said chopped signal is converted to a demodulated signal which corresponds to the partial pressure sensed by the sensor, said primary winding having a floating ground, d. a source of high frequency voltage, e. a second transformer having a primary winding connected to said source and a secondary winding having a floating ground, f. voltage from the secondary winding of the second transformer being applied to drive said chopper means at a potential referenced to floating ground and otherwise isolated from ground by said transformers.

10. The monitor set forth in claim 9 further including:

a. a full wave rectifier means having alternating voltage supply terminals and DC terminals, said alternating voltage supply terminals being connected to the secondary winding of said second transformer, b. a voltage regulator means connected to said DC terminals of said rectifier means, said regulator means having a floating ground and output terminals for supplying power to said amplifier means.

11. The monitor set forth in claim 9 further including:

a. a second amplifier means having an output terminal and input terminals and being adapted for multiple gain control and having one of its input terminals connected to receive said demodulated signal, b. a feedback circuit between said output terminal of said second amplifier means and another of its input terminals, said circuit including a resistor that is adjustable to control the gain of the amplifieR in accordance with the predetermined inherent gain of the sensor incidental to calibration thereof, c. a voltage divider having taps representative of the temperature of the sensor and means to apply a voltage appearing on a selected one of said taps to said another of said input terminals of said second amplifier means to control the amplifier bias in accordance with the sensor temperature, and d. a bias calibrating voltage divider which has a tap that is adjustable and connected to said another of said input terminals of said second amplifier means for introducing a voltage thereto which modifies the output signal of said second amplifier means to correspond with a predetermined partial pressure of the gas in the liquid in which the sensor is immersed during calibration, and e. a readout means driven by the output signal from said second amplifier means.

12. The monitor set forth in claim 6 including:

a. a comparator amplifier having one input terminal and a reference voltage connected thereto and another input terminal receiving said control signal, the output signal of said comparator amplifier being the amplified difference between the reference voltage and the control signal, b. means operable by said last-named output signal when it reaches a predetermined magnitude due to an abnormal condition of the sensor, and c. a signal means responsive to operation of said last-named means for indicating said abnormal condition.