Computerized System for Determination of Cardiac Output

Abstract

The CO₂ rebreathing method for the determination of cardiac output has been computerized.CO₂ concentration of expired air is analyzed by a infrared CO₂ analyzer.The output voltageof the CO₂ analyzer is sampled by a micro computer through a 12-bit analog-to-digitalconverter.The values of end-tidal CO₂ percentages (FET_CO₂) during nomal respiration and thepeak values of CO₂ percentages in the lung-bag system (Fb_CO₂) during rebreathing are detectedby the computer automatically.The detemination of the partial pressure of CO₂ in mixed venous blood (Pv_CO₂) is perfomedby computer analysis.The rise in the partial pressure of CO₂ in the lung-bag system (Pb_CO₂)with time (T) is expressed by the equation [chemical formula] This equation can be transfomed logerithmically to [chemical formula] At first, Pvco₂ is assumed to be a slightly large value than the last value of Pb_CO₂ and theregression equation of ln (1-Pb_CO₂/Pv_CO₂) on T is calculated by means of the least-squaresanalysis.Then Pv_CO₂ is successively changed positively, and the regression equations arecalculated on each time.The value of Pv_CO₂ which maximize the value of coefficient ofdetermination (r²) is adopted as the very value of Pv_CO₂.The partial pressure of CO₂ in arterialblood (Pa_CO₂) is a ssumed to be the same as in end-tidal air.A comparison was made between the cardiac output determined by this computerized andthe conventional CO₂ rebreathing methods as described by Jernerus et al.(1963).The valuesobtained by the two methods were almost identical, all of 9 paired deteminations of cardiacoutput obtained during exercise were within 10% of one another and 33 of 36 values during restand recovery were within 20% of one another.The correlation coefficient between the valuesobtained by the two methods was 0.992.The computerized method is a more precise and moreconvenient technique for deteming cardiac output.