PRECISION OF A NEW BEDSIDE METHOD FOR ESTIMATION OF THE CIRCULATING BLOOD-VOLUME

The present study is a theoretical and experimental evaluation of a modification of the carbon monoxide method for estimation of the circulating blood volume (CBV) with respect to the precision of the method. The CBV was determined from measurements of the CO-saturation of hemoglobin before and after ventilation with a gas mixture containing 20-50 ml of CO for a period of 10-15 min. A special Water's to and fro system was designed in order to avoid any leakage when measuring during intermittent positive pressure ventilation (IPPV). Blood samples were taken before and immediately after ventilation with the CO gas mixture. The amount of CO administered during each determination of CBV resulted in an increase in the CO saturation of hemoglobin of 2.1%-3.9%. A theoretical noise propagation analysis was performed by means of the Monte Carlo method. The analysis showed that a CO dose corresponding to an increase of less than 2% will result in an unacceptable coefficient of variation of repeated estimates. In the experimental study the coefficient of variation of repeated estimates of CBV was determined from duplicate measurements of CBV in nine healthy subjects and in nine intensive care patients. The coefficients of variation were 6.2% and 4.7% in healthy and diseased subjects, respectively. Furthermore, the day-to-day variation of the method with respect to the total amount of circulating hemoglobin nHb) and CBV was determined from duplicate estimates separated by 24-48 h. In conclusion, determination of CBV can be performed with an amount of CO that gives rise to a harmless increase in the carboxyhemoglobin concentration. The precision of the method is sufficient for clinical purposes.