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, enf lurane, and desflurane in desiccated carbon dioxide absorbents. The inspira tory CO concentration depends on the dryness and identity of the absorbent and anesthetic. The adaptation of existing mathematical models to a rebreat hing circuit allows identification of patient factors that predispose to mo re severe exposures, as identified by carboxyhemoglobin concentration. Methods: From our companion study, the authors used quantitative in vitro C O production data for 60 min at 7.5% desflurane or 1.5% isoflurane at 1 1/m in fresh gas flow. The carboxyhemoglobin concentration was calculated by it eratively solving the Coburn Forster Kane equation modified for a rebreathi ng system that incorporates the removal of CO by patient absorption, Demons trating good fit of predicted carboxyhemoglobin concentrations to published data from animal and human exposures validated the model. Carboxyhemoglobi n 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 mod el. These equations indicate the severity of CO poisoning is inversely rela ted to the hemoglobin quantity of a subject. Fraction of inspired oxygen ha d 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 quantiti es mill have more severe CO exposures based on the attainment of a higher c arboxyhemoglobin concentration. This includes patients of small size (pedia tric population) and patients with anemia.