Blood volume determination by the carbon monoxide method using a new delivery system: Accuracy in critically ill humans and precision in an animal model

Objective: To evaluate accuracy and repeatability of blood volume determina tions made by the carbon monoxide method, using a ventilator-driven adminis tration system. Design: Prospective within-patient comparison, using simultaneous measureme nts by two methods to determine accuracy. Prospective laboratory investigat ion in animals to estimate repeatability. Subjects: For accuracy: Nineteen ventilated critically ill patients in a un iversity hospital intensive care unit, For repeatability: Six anesthetized, mechanically ventilated normovolemic pigs because this is impossible to pe rform in humans. Interventions: In the accuracy study, a small mass of carbon monoxide was a dministered via a closed breathing system and arterial blood samples were t aken from existing cannulas, In the repeatability study, an intramuscular s edative was given, followed by an inhalational anesthetic induction and mec hanical ventilation via a tracheal tube, Left axillary artery and external jugular vein cannulas were sited, Anesthesia was maintained using an intrav enous infusion. Five sequential circulating hemoglobin and blood volume est imations were made using the carbon monoxide method. Measurements and Main Results: The small carboxyhemoglobin increase produce d by uptake of a small, known mass of carbon monoxide was used to estimate the circulating blood volume. Simultaneous measurement, using Cr-51-labeled red blood cells, was performed. Twenty measurements were made in 19 patients. The bias (mean difference bet ween blood volume measurements by tbe two methods) was 397 mL (5.53 mL . kg (-1)) +/- 415 mL (+/- 5.95 mL . kg(-1)); the limits of agreement (mean diff erence +/- 2 SO) were -433 mt and 1227 mL (-6.36 mL . kg(-1) and 17.42 mL . kg(-1)). Therefore, 95% of expected differences will lie between these lim its, The mean blood volume was 75.8 mL . kg(-1) in the animals. The coeffic ient of variation of repeated estimates was 9.49%, Mean circulating hemoglo bin mass was 7.31 mmol with a coefficient of variation of 10.18%. The mean hemoglobin concentration, by co-oximetry, was 5.014 mmol.L-1, coefficient o f variation, 2.99%. Conclusion: This arrangement is a potential bedside method of estimating bl ood volume and circulating hemoglobin mass, We have rendered the technique more acceptable clinically by creating a ventilator-driven administration s ystem.