Interactions among Ventilation, the Circulation and the Uptake and Distribution of Halothane—Use of a Hybrid Computer Multiple Model

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Abstract
An 18 compartment hybrid computer multiple model of the uptake and distribution of halothane was described. This model uses 88 equations and 124 parameter settings. Three submodels are incorporated into the basic model: the mass transport of halothane is simulated on the digital portion of the hybrid computer; a breath-by-breath pulmonary model with 2 compartments describes air pressure-flow relations in the airway system and a beat-to-beat cardiovascular model with 15 compartments describes in detail blood pressure-flow relations. A baroreceptor-heart rate loop is included: an increase in arterial pressure casuses a decrease in heart rate. The slope of the baroreceptor response is progressively decreased by halothane until at 2% there is no response. The model of halothane uptake and distribution is separate from the blood and air pressure-flow models, but is driven by them. Myocardial contractility (stroke volume) and certain regional vascular resistances can be affected by the concentration of halothane in 1 or any proportion of any combination of 3 compartments: arterial blood (arteriolar concentration), cerebral gray matter or myocardial. These factors significantly affect the uptake and distribution of halothane. The responses to 3 steady-state concentrations and to a step change in concentration from 0-2%, were examined. Outputs (24) were recorded, including halothane concentrations in 10 compartments; myocardial contractility; left and right ventricular and right atrial pressures, cardiac output; stroke volume, R-R interval; and blood flows in 6 regions. Two variables, alveolar concentration of halothane and arterial blood pressure, were recorded during a step change of 0-5%. The model describes the appropriate steady-state and dynamic cardiovascular responses to halothane and demonstrates the complex interrelationships among cardiac output, regional blood flow distribution and the uptake and distribution of halothane. During step changes in halothane concentration, most responses occur early, a phenomenon seen in man and goats. The model is useful not only for representing organ and tissue halothane concentrations, but for gaining new insights into cardiovascular alterations produced by rapidly changing concentrations of halothane and into the complex interactions between the circulation and the uptake and distribution of halothane.

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