Mathematical modeling of carbon monoxide exposures from anesthetic breakdown: effect of subject size, hematocrit, fraction of inspired oxygen, and quantity of carbon monoxide.
BACKGROUND
Carbon monoxide (CO) is produced by reaction of isoflurane, enflurane, and desflurane in desiccated carbon dioxide absorbents. The inspiratory CO concentration depends on the dryness and identity of the absorbent and anesthetic. The adaptation of existing mathematical models to a rebreathing circuit allows identification of patient factors that predispose to more severe exposures, as identified by carboxyhemoglobin concentration.


METHODS
From our companion study, the authors used quantitative in vitro CO production data for 60 min at 7.5% desflurane or 1.5% isoflurane at 1 l/min fresh gas flow. The carboxyhemoglobin concentration was calculated by iteratively solving the Coburn Forster Kane equation modified for a rebreathing system that incorporates the removal of CO by patient absorption. Demonstrating good fit of predicted carboxyhemoglobin concentrations to published data from animal and human exposures validated the model. Carboxyhemoglobin concentrations were predicted for exposures of various severity, patients of different sizes, hematocrit, and fraction of inspired oxygen.


RESULTS
The calculated carboxyhemoglobin concentrations closely predicted the experimental results of other investigators, thereby validating the model. These equations indicate the severity of CO poisoning is inversely related to the hemoglobin quantity of a subject. Fraction of inspired oxygen had the greatest effect in patients of small size with low hematocrit values, where equilibrium and not the rate of uptake determined carboxyhemoglobin concentrations.


CONCLUSIONS
This model predicts that patients with low hemoglobin quantities will have more severe CO exposures based on the attainment of a higher carboxyhemoglobin concentration. This includes patients of small size (pediatric population) and patients with anemia.
Woehlck
H J
HJ
Department of Anesthesiology, Medical College of Wisconsin and Froedtert Memorial Lutheran Hospital, Milwaukee 53226, USA. hwoehlck@mcw.edu
Mei
D
D
Dunning
M B
MB
Ruiz
F
F
D018685:Anesthetics, Inhalation / Q000737:chemistry*; D002248:Carbon Monoxide / Q000737:chemistry*; D002263:Carboxyhemoglobin / Q000737:chemistry; D000077335:Desflurane; D006400:Hematocrit; D006801:Humans; D007530:Isoflurane / Q000031:analogs & derivatives* / Q000737:chemistry*; D008962:Models, Theoretical; D012119:Respiration
D018685:Anesthetics, Inhalation; D002248:Carbon Monoxide; D002263:Carboxyhemoglobin; D000077335:Desflurane; D007530:Isoflurane