Chest
Volume 101, Issue 5, Supplement, May 1992, Pages 188S-191S
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An Analysis of the Factors Limiting Maximal Oxygen Consumption in Healthy Subjects

https://doi.org/10.1378/chest.101.5_Supplement.188SGet rights and content

The factors limiting maximal oxygen consumption (VO2max) in humans are analyzed according to a multifactorial model derived from the O2 conductance equation. The alveolar ventilation (VA) and lung O2 transfer (GL) are not considered to be limiting, at least at sea level in healthy subjects, because changes in VA and/or GL are not accompanied by changes in Vo2max, due to the shape of the O2 dissociation curve. Thus, the limits to Vo2 max are shared between O2 transport by the circulation and a peripheral factor, including O2 transfer from capillaries to tissue and mitochondrial O2 utilization. In untrained healthy subjects at sea level, O2 transport by the circulation is responsible for about 70% of the overall limits, the rest depending on the peripheral factors.

Section snippets

THE PATHWAY FOR OXYGEN

The O2 path from the environment to mitochondria can be viewed as a cascade of resistances in series, each resistance being overcome by a specific pressure gradient. If this is so, the O2 flow is set by the overall pressure gradient (ΔPTot) divided by the total resistance (RTot): VO2max=ΔProt/RTotSince at steady state the O2 flow through each section of the system is the same, the following must also apply: VO2max=ΔPi/Riwhere ΔPi is the pressure gradient across the ith resistance

THE LUNG AS LIMITING STEP

Apart from the above limitations, the model shows that Vo2max is not limited by a single factor, but by a series of physiologic variables whose limiting role can be quantitatively assessed on the basis of the model itself. In addition, if the system behaves linearly, and if the overall pressure gradient from environment to mitochondria is constant, any given change of the ith resistance will lead to a change of Vo2max proportional to the relative magnitude of the resistance in question (Fi).

THE CIRCULATION AND THE MUSCLES AS LIMITING STEPS

Neglecting therefore pulmonary ventilation and diffusion, in the section that follows we will attempt to partition the resistance to O2 transport downstream the lung into 2 fractions: (1) FQ' due to O2 transport and (2) Fp' due to the sum of capillary perfusion and diffusion and mitochondrial O2 utilization (Fp' = Ft' + Fm'). (Note also that whereas FQ and Fp refer to the overall O2 cascade, FQ' and Fp' refer to the portion of the cascade downstream from the lung only.)

FQ' and Fp' will be

CONCLUSIONS

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

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