Method and apparatus for measuring blood volume

Abstract

In one aspect, a conductance catheter is provided for measuring the volume of a fluid. The conductance catheter comprises a series of electrodes and a circuit to compensate for variations in sensitivity of the electrodes in the catheter. In another aspect, a resistivity sensor is provided for determining the resistivity of a fluid. The sensor comprises a series of electrodes spaced such that the total distance between endmost electrodes does not exceed the diameter of the catheter deploying the sensor.

1 . A sensing tip for measuring the volume of a fluid comprising one or more electrodes for measuring said volume and a resistivity sensor disposed on said tip in communication with said fluid to incorporate a current measurement of resistivity during measurement of said volume. 
     
     
         2 . The sensing tip according to  claim 1  wherein said resistivity sensor comprises a series of resistivity electrodes spaced from each other such that a total distance between endmost ones of said resistivity electrodes does not exceed the diameter of said sensing tip. 
     
     
         3 . The sensing tip according to  claim 2  wherein said resistivity electrodes are diametrically spaced. 
     
     
         4 . The sensing tip according to  claim 2  wherein said resistivity electrodes are axially spaced. 
     
     
         5 . The sensing tip according to  claim 1  configured for continuously obtaining said measurement of resistivity. 
     
     
         6 . A system for measuring volume of a fluid comprising a sensing tip having a plurality of pairs of electrodes, each pair of electrodes being connected to a circuit to compensate for variations in sensitivity of respective pairs of electrodes according to the positioning of said respective pairs along said sensing tip. 
     
     
         7 . The system according to  claim 6  wherein said circuit comprises individual gain and offset circuits to adjust the strength of a respective signal. 
     
     
         8 . The system according to  claim 7  comprising an output for displaying said respective signal to facilitate adjustment of said gain and offset circuits. 
     
     
         9 . The system according to  claim 6  comprising a control box for calibrating said sensing tip, said control box including a gain and offset circuit for each pair of electrodes to adjust the strength of a respective signal; and a monitor for displaying said respective signal to facilitate adjustment of said gain and offset circuits. 
     
     
         10 . The system according to  claim 9  wherein said control box obtains a resistivity measurement from said sensing tip and incorporates a value of resistivity into a calibration measurement. 
     
     
         11 . The system according to  claim 9  comprising a plurality of cuvettes for containing fluids of differing properties. 
     
     
         12 . A method for calibrating a sensing tip used for measuring volume of a fluid comprising inserting said sensing tip into a plurality of cuvettes containing fluids having differing properties, and when in each well, said method comprises: obtaining a plurality of conductance signals using a plurality of electrodes on said sensing tip; adjusting said conductance signals to compensate for variations is sensitivity of said pairs of electrodes due to the positioning of said electrodes along said sensing tip; obtaining a measurement of resistivity; and using said conductance signals and said measurement of resistivity to calibrate said sensing tip. 
     
     
         13 . The method according to  claim 12  wherein said differing properties comprises differing volumes and said measurement of resistivity is obtained using a resistivity measurement on said sensing tip. 
     
     
         14 . The method according to  claim 12  wherein volume measurements obtained by said sensing tip are calibrated according to a linear relationship. 
     
     
         15 . The method according to  claim 12  wherein said differing properties comprises differing known values of conductance and said method comprises providing a segmental output that is proportional to the conductance of the fluid in the respective cuvette. 
     
     
         16 . A method for positioning a sensing tip disposed in a ventricle comprising obtaining an excitation waveform generated by one pair of electrodes disposed on said sensing tip; obtaining a conductance waveform sensed by another pair of electrodes disposed on said sensing tip; comparing said waveforms to determine a phase shift between said waveforms; and adjusting said positioning until said phase shift is deemed acceptable. 
     
     
         17 . The method according to  claim 16  wherein said comparing comprises superimposing said waveforms to provide a visual indication of said phase shift. 
     
     
         18 . The method according to  claim 16  wherein said phase shift is determined by comparing zero-crossings of said waveforms. 
     
     
         19 . The method according to  claim 16  performed in real time while said sensing tip is measuring volume of fluid in said ventricle. 
     
     
         20 . The method according to  claim 16  performed during a calibration operation.