A model for studying the distortion of muscle oxygen uptake patterns by circulation parameters

The transmission of muscle oxygen uptake (VO2) patterns to the pulmonary site is a basically nonlinear process during unsteady state exercise. We were mainly interested in three questions concerning the dynamic relationship between power input and pulmonary VO2 output: 1. To what extent can linear system analysis be applied? 2. What is the relative influence of muscle VO2 on pulmonary VO2 as compared to other parameters such as muscle perfusion kinetics? 3. To what extent does pulmonary VO2 reflect muscle VO2? Investigations were performed by means of a mathematical model including a muscle compartment and two serial, flow-varying time delays. The non-exercising parts of the body were incorporated as one term for perfusion and one for VO2. Parameters were adjusted so as to represent a reference state of aerobic exercise while monofrequent sinusoidal changes in aerobic metabolism were used as forcing signals. The following answers were derived from the simulations: 1. Non-linear distortions of the VO2 signals are negligible provided that analyses are not driven too far into the higher frequency range (periods shorter than about 1 min). 2. Variations of muscle VO2 kinetics have greater effects on pulmonary VO2 than changes of perfusion kinetics or venous volume. This finding applies irrespective of whether or not pulmonary VO2 closely reflects muscle VO2. 3. Small differences in the time constants for muscle perfusion and muscle VO2 are a major prerequisite if pulmonary VO2 kinetics are to be taken as correct estimates of muscle VO2 kinetics. High basal muscle perfusion, small perfusion changes and small venous volumes between muscle and lungs are further factors reducing dynamic distortions of the muscle VO2 signal.