Computer simulation of O2 transport and utilization mechanisms at the onset of exercise

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Abstract
During transitions in work rate, O2 uptake (VO2) kinetics at the working tissue level might be rate limited by O2 transport and/or by O2 utilization. A computer model with parallel working and non-working tissue compartments, connected to an ideal lung by a variable-sized venous blood volume, was developed to study this. The time constant for working tissue O2 demand (tau T) was set by a first-order linear metabolic response. The model attempted to replicate the VO2 response at the alveolar level of a single subject performing step transitions on a cycle ergometer from 25 to 105 W [total lag time (equivalent to 63% increase above baseline) = 40.2 s]. Measured cardiac output kinetics (total lag time = 44.1 s) were used as a model parameter. Blood flow to the nonworking tissue (QNW) was kept constant at 4.5 or 5.0 l/min. A critical PO2 of 20 Torr was set, and the Bohr effect on the O2-hemoglobin dissociation curve was included. The “best” simulation had tau T = 36 s, QNW = 4.5 l/min, and venous blood volume = 2 liters and was not O2 transport limited. The approximation to the real data was good in all but the phase 1 response, where the model underpredicted the measured response. However, when QNW was increased to 5.0 l/min, the model was O2 transport limited; yet the predicted VO2 response at the alveolar level was not notably different from the subject's data.(ABSTRACT TRUNCATED AT 250 WORDS)

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