A system model for closed-circuit inhalation anesthesia. I. Computer study.

Developing a custom computer program to simulate the uptake, distribution, and elimination of inhalational anesthetics allows the anesthesiologist to address specific problems, but extensive skills are required to translate the involved processes first into a set of mathematical equations and then into a satisfactory computer program. The first step is often facilitated by solutions offered in the literature. The second step demands computer proficiency that is often not available, but this problem can be obviated by means of a special-purpose simulation language (SPSL). We therefore constructed a model for closed-circuit inhalation anesthesia with the aid of the block-structured SPSL TUTSIM. Noticeable differences with previous models are that the linear, 14-compartment basic model does not assume a constant alveolar concentration and mimics circulation times through the use of blood pools. Advanced features of the SPSL were used to develop variants of the basic model to simulate feedback-controlled isoflurane administration, nitrous oxide uptake, and the impact of a nonlinearity by incorporating the effect of enflurane on cardiac output. Two variants were concatenated to form a multiple model showing the concentration and second-gas effects. The model was capable of reproducing the anesthetic uptake from previous experimental studies for nitrous oxide. After its validation for other anesthetic agents, the model can be used for clinical, teaching, and research purposes. The SPSL freed the authors from the problems associated with computer programming and allowed them to concentrate on the structure of the model.