An Analysis of the Factors Limiting Maximal Oxygen Consumption in Healthy Subjects

Abstract

This article discusses and supports the hypothesis that the limits to whole body Vo₂max are multifactorial. At least 4 resistances to maximal O₂ flow of physiologic relevance are identified: (1) convective and diffusive transfer of O₂ from environment to arterial blood; (2) convective O₂ transport by the circulation; (3) peripheral O₂ transfer from capillaries to mitochondria; and (4) O₂ utilization in the mitochondria. The relative role of these resistances in healthy subjects at sea level amounts to about 0.38, 0.50, 0.06, and 0.06, respectively. However, because of the shape of the O₂ dissociation curve of blood, changes of resistance (1) lead to only negligible modifications of the overall O₂ flow. Thus the role of convective and diffusive transfer of O₂ from environment to arterial blood as limiting factor is negligible. Downstream from the lung, the greatest role (0.70 to 0.80) is played by resistance (2), ie, by the convective transport of O₂ by the circulation, the remaining 2 resistances accounting for about 0.10 to 0.15 each. Finally, it must be pointed out again that the above analysis and calculations were based on data on healthy subjects at sea level. It would be interesting to test if and to what extent the above model, can be useful for evaluating the transfer of O₂ in pathologic conditions such as those encountered in chronic respiratory diseases and possibly extended to analyze also CO₂ output.