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 determinations made by the carbon monoxide method, using a ventilator-driven administration system.


METHODS
Prospective within-patient comparison, using simultaneous measurements by two methods to determine accuracy. Prospective laboratory investigation in animals to estimate repeatability.


METHODS
For accuracy: Nineteen ventilated critically ill patients in a university hospital intensive care unit. For repeatability: Six anesthetized, mechanically ventilated normovolemic pigs because this is impossible to perform in humans.


METHODS
In the accuracy study, a small mass of carbon monoxide was administered via a closed breathing system and arterial blood samples were taken from existing cannulas. In the repeatability study, an intramuscular sedative was given, followed by an inhalational anesthetic induction and mechanical ventilation via a tracheal tube. Left axillary artery and external jugular vein cannulas were sited. Anesthesia was maintained using an intravenous infusion. Five sequential circulating hemoglobin and blood volume estimations were made using the carbon monoxide method.


RESULTS
The small carboxyhemoglobin increase produced by uptake of a small, known mass of carbon monoxide was used to estimate the circulating blood volume. Simultaneous measurement, using 51Cr-labeled red blood cells, was performed. Twenty measurements were made in 19 patients. The bias (mean difference between blood volume measurements by the two methods) was 397 mL (5.53 mL x kg(-1)) +/-415 mL (+/-5.95 mL x kg(-1)); the limits of agreement (mean difference +/-2 SD) were -433 mL and 1227 mL (-6.36 mL x kg(-1) and 17.42 mL x kg(-1)). Therefore, 95% of expected differences will lie between these limits. The mean blood volume was 75.8 mL x kg(-1) in the animals. The coefficient of variation of repeated estimates was 9.49%. Mean circulating hemoglobin mass was 7.31 mmol with a coefficient of variation of 10.18%. The mean hemoglobin concentration, by co-oximetry, was 5.014 mmol x L(-1), coefficient of variation, 2.99%.


CONCLUSIONS
This arrangement is a potential bedside method of estimating blood volume and circulating hemoglobin mass. We have rendered the technique more acceptable clinically by creating a ventilator-driven administration system.
Dingley
J
J
Department of Anesthetics, University Hospital of Wales, South Glamorgan, UK.
Foëx
B A
BA
Swart
M
M
Findlay
G
G
DeSouza
P R
PR
Wardrop
C
C
Willis
N
N
Smithies
M
M
Little
R A
RA
D000280:Administration, Inhalation; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001810:Blood Volume; D001811:Blood Volume Determination / Q000379:methods*; D002248:Carbon Monoxide / Q000008:administration & dosage; D002263:Carboxyhemoglobin / Q000378:metabolism; D016638:Critical Illness; D004195:Disease Models, Animal; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D007385:Intermittent Positive-Pressure Ventilation; D007442:Intubation, Intratracheal; D008875:Middle Aged; D009102:Multiple Organ Failure / Q000097:blood / Q000503:physiopathology* / Q000628:therapy; D015588:Observer Variation; D019095:Point-of-Care Systems; D011446:Prospective Studies; D015203:Reproducibility of Results; D013552:Swine
D006454:Hemoglobins; D002248:Carbon Monoxide; D002263:Carboxyhemoglobin