Impedance plethysmograph

Abstract

.Iadd.Cardiac output is measured by connecting excitation electrodes at the upper and lower ends of the thorax of a mammalian subject, connecting measuring electrodes to the thorax between the excitation electrodes, applying a constant fluctuating excitation current to the excitation electrodes, measuring, with appropriate circuitry, changes in impedance within the thorax, and simultaneously measuring the beginning and end of systole, and determining cardiac output by measuring the maximum decreasing impedance slope during systole..Iaddend.

We claim: 
     
       1. A plethysmograph for measuring cardiac output comprising in combination excitation electrode means adapted to be connected to a mammalian subject at the superior and inferior ends of the thorax, a current generator means comprising an electronic oscillator conductively connected to the electrode means for supplying a fluctuating excitation current thereto, sensor means adapted to be conductively connected to said thorax for carrying a sensed electrical signal which varies as the impedance changes in the thorax between said electrode means, a control means conductively connected to the oscillator for balancing the current from the oscillator with the sensed signal, a first amplifier conductively connected to the control means, a second amplifier connected to said sensor means whereby signals of approximately equal strength are fed to said first and second amplifiers from said control and said sensor means respectively and a voltage subtracting means conductively connected to each of the amplifiers for comparing the output of each said amplifier. 
     
     
       2. The apparatus of claim 1 wherein a first rectifying means is conductively connected between the sensor means and the subtracting means and a second rectifying means is connected between the oscillator and the subtracting means. 
     
     
       3. A plethysmograph for measuring cardiac output comprising in combination a first elongated band excitation electrode adapted to be positioned to at least partially encircle the neck of a subject, a second elongated band excitation electrode adapted to be positioned to at least partially encircle the thorax of said subject at the approximate position of the xiphisternal joint, a source of a fluctuating current conductively connected across the electrodes for supplying an excitation current thereto, sensor means adapted to be conductively connected to said thorax for receiving a sensed signal, a voltage balancing means conductively connected to the oscillator, a first output lead conductively connected to the balancing means, a second output lead connected to said sensor means, and a voltage subtracting means conductively connected to each of the output leads for comparing the voltage, whereby the adjustment of the balancing means is adapted to feed signals of approximately equal strength to the subtracting means. 
     
     
       4. The apparatus according to claim 3 wherein said sensor means comprises a pair of measuring electrodes adapted to be positioned upon said thorax. 
     
     
       5. The apparatus according to claim 3 wherein said balancing means comprises a potentiometer wired to the output of said source of fluctuating current. 
     
     
       6. A plethysmograph for measuring cardiac output comprising in combination a first elongated band excitation electrode adapted to be positioned around the neck of said subject, a second elongated band excitation electrode adapted to be positioned to encircle the thorax of said subject at the approximate location of the xiphisternal joint, an electronic oscillator means conductively connected across the excitation electrodes for supplying an excitation current thereto, sensor electrodes adapted to be conductively connected to said thorax for carrying a sensed current, a balancing means conductively connected to the oscillator, a first amplifier conductively connected to the balancing means, a second amplifier connected to one of said sensor electrodes, a voltage subtracting means conductively connected to each of the amplifiers for comparing the output thereof and a means for rectifying the current received by the subtracting means from each said amplifier means, whereby the adjustment of the balancing means is adapted to feed signals of approximately equal strength to said subtracting means. 
     
     
       7. A plethysmographic method for measuring cardiac output which comprises connecting excitation electrode means at the upper and lower ends of the thorax of a mammalian subject above the upper border of the lungs and below the lower border of the heart and lungs, respectively; connecting measuring electrode means to the thorax of the subject between said excitation electrode means; applying a constant fluctuating excitation current to said excitation electrode means and through a balancing resistance; amplifying, detecting and measuring the voltage across the measuring electrode means and across the balancing resistance and adjusting the balancing resistance to equalize the voltages; measuring the changes in impedance within the thorax as sensed by said measuring electrode means; and simultaneously measuring the beginning and end of systole of the subject and determining cardiac output therefrom. 
     
     
       8. A method according to claim 7 further characterized in that said measured change in impedance and simultaneously measured beginning and end of systole are recorded to produce a single composite graphic image, a straight line is constructed tangential to the portion of the graphic image at the maximum decreasing impedance slope during systole, said line extending to intersect parallel lines denoting beginning and end of systole, measuring the height of the line thus constructed and determining cardiac output therefrom. 
     
     
       9. The method according to claim 7 wherein said excitation electrode means comprises a pair of encircling electrodes, one adapted to at least partially encircle the neck and the other adapted to at least partially encircle the thorax of the subject at the xiphisternal joint and wherein said measuring electrode means comprises a pair of electrodes adapted to be positioned between and a short distance inwardy from each of said pair of excitation electrodes. 
     
     
       10. A plethysmograph comprising in combination: excitation electrode means adapted to be connected to a mammalian subject at the superior and inferior ends of the thorax; electric generator means for supplying a fluctuating excitation current, said generator means being conductively connected to said electrode means whereby said excitation current is applied to said electrode means; measuring means; conductor means adapted to be conductively connected to the thorax and to said measuring means for carrying an electrical signal from the portion of the thorax between said electrode means to said measuring means; balancing means connected to said generator means for providing a second electrical signal approximately equal in magnitude to said first mentioned signal; means for rectifying each of said signals; and means for comparing the signals after rectification. 
     
     
       11. A plethysmographic method for measuring cardiac output which comprises the steps of: applying a substantially constant current, fluctuating voltage, excitation signal between the upper and lower ends of the thorax of a mammalian subject from above the upper border of the lung to below the lower border of the heart and lung; measuring impedance changes within the portion of the thorax carrying the signal while simultaneously measuring the beginning and the end of systole of the subject; and recording the simultaneous measurements for determining cardiac output therefrom. 
     
     
       12. A plethysmographic method for measuring cardiac output which comprises the steps of: applying a substantially constant current, fluctuating voltage, excitation signal between the upper and lower ends of the thorax of a mammalian subject from above the upper border of the lung to below the lower border of the heart and lung; and measuring impedance changes within the portion of the thorax carrying the signal relative to a period from beginning to end of systole of the subject for determining cardiac output therefrom. 
     
     
       13. A plethysmograph comprising: electrical means adapted to induce a fluctuating excitation current between the superior and inferior ends of the thorax of a mammalian subject; and means for measuring impedance changes in said thorax, in the presence of said fluctuating excitation current, and for measuring, simultaneously, the beginning and the end of systole of the subject. .Iadd. 
     
     
       14.  A plethysmograph for measuring cardiac output comprising in combination: (a) excitation electrode means adapted to be connected to a mammalian subject at the superior and inferior ends of the thorax,   (b) a current generator means comprising an electronic oscillator conductively connected to the electrode means for supplying a fluctuating excitation current thereto,   (c) sensor means adapted to be conductively connected to said thorax for carrying a sensed electrical signal which varies as the impedance changes in the thorax between said electrode means,   (d) a control means conductively connected to the oscillator for balancing the current from the oscillator with the sensed signal,   (e) a first amplifier conductively connected to the control means,   (f) a second amplifier connected to said sensor means whereby signals of approximately equal strength are fed to said first and second amplifiers from said control and sensor means respectively,   (g) a voltage subtracting means conductively connected to each of the amplifiers for comparing the output of each said amplifier,   (h) and means connected to said subtracting means for receiving an output therefrom whereby the maximum decreasing slope of said impedance changes in the thorax may be determined. .Iaddend. .Iadd.   
     
     
       15.  a plethysmograph for measuring cardiac output comprising in combination: (a) excitation electrode means adapted to be connected to a mammalian subject at the superior and inferior ends of the thorax,   (b) a current generator means comprising an electronic oscillator conductively connected to the electrode means for supplying a fluctuating excitation current thereto,   (c) sensor means adapted to be conductively connected to said thorax for carrying a sensed electrical signal which varies as the impedance changes in the thorax between said electrode means,   (d) a control means conductively connected to the oscillator for balancing the current from the oscillator with the sensed signal,   (e) a first amplifier conductively connected to the control means,   (f) a second amplifier connected to said sensor means whereby signals of approximately equal strength are fed to said first and second amplifiers from said control and sensor means respectively,   (g) a voltage subtracting means conductively connected to each of the amplifiers for comparing the output of each said amplifier,   (h) and means connected to said subtracting means for receiving an output therefrom whereby the maximum decreasing slope of said impedance changes in the thorax and ΔZ may be determined. .Iaddend. .Iadd.   
     
     
       16.  A plethysmographic method for measuring cardiac output which comprises: (a) connecting excitation electrode means at the upper and lower ends of the thorax of a mammalian subject above the upper border of the lungs and below the lower border of the heart and lungs, respectively;   (b) connecting measuring electrode means to the thorax of the subject between said excitation electrode means;   (c) applying a constant fluctuating excitation current to said excitation electrode means and through a balancing resistance;   (d) amplifying, detecting, and measuring the voltage across the measuring electrode means and across the balancing resistance and adjusting the balancing resistance to equalize the voltages;   (e) measuring the changes in impedance within the thorax as sensed by said measuring electrode means;   (f) and simultaneously measuring the beginning and end of systole of the subject, determining the maximum decreasing impedance slope during systole, and determining cardiac output therefrom. .Iaddend. .Iadd.   
     
     
       17.  A plethysmographic method for measuring cardiac output which comprises: (a) connecting excitation electrode means at the upper and lower ends of the thorax of a mammalian subject above the upper border of the lungs and below the lower border of the heart and lungs, respectively;   (b) connecting measuring electrode means to the thorax of the subject between said excitation electrode means;   (c) applying a constant fluctuating excitation current to said excitation electrode means and through a balancing resistance;   (d) amplifying, detecting, and measuring the voltage across the measuring electrode means and across the balancing resistance and adjusting the balancing resistance to equalize the voltages;   (e) measuring the changes in impedance within the thorax as sensed by said measuring electrode means;   (f) and simultaneously measuring the beginning and end of systole of the subject, determining the maximum decreasing impedance slope during systole and ΔZ, and determining cardiac output therefrom..Iaddend.