Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCA008

Poster Communications

Adequate oxygen uptake in exercise requires low pulmonary flow heterogeneity

T. K. Roy1, T. W. Secomb2

1. Department of Anesthesiology, Division of Critical Care, Mayo Clinic, Rochester, Minnesota, United States. 2. Departments of Physiology and Mathematics, University of Arizona, Tucson, Arizona, United States.

Under resting conditions, the amount of atmospheric oxygen necessary to satisfy tissue needs is readily acquired, and pulmonary oxygen uptake remains adequate even in the face of increased pulmonary flow heterogeneity and/or decreased diffusing capacity. This high degree of reserve in the healthy lung is nevertheless sometimes exhausted under extreme conditions such as in critical illness. Under conditions of exercise, however, oxygen transport and utilization are contingent upon adequate lung function with relatively little reserve. The purpose of this study is to investigate the impact of pulmonary flow heterogeneity on oxygen delivery in exercise. A theoretical model of pulmonary oxygen uptake is used to simulate varying degrees of heterogeneity as characterized by the coefficient of variation (CV) of pulmonary capillary blood flow, assuming prescribed tissue oxygen utilization. Uniform alveolar ventilation is assumed, and flow heterogeneity is therefore equivalent to heterogeneity in ventilation-perfusion matching. Under resting conditions (245 ml O2/min), the observed level of arterial oxygen tension is consistent with a high degree of perfusion heterogeneity (CV ≈ 3.0). When moderate or severe exercise conditions with values of cardiac output and arteriovenous oxygen content obtained from the literature [Roca J et al. (1989) AJP 67:291] are considered, it is found that this degree of heterogeneity would lead to an inability to maintain arterial PO2 and sustain levels of oxygen demand corresponding to experimental measurements. For conditions of moderate exercise (2750 ml O2/min corresponding to 60% VO2max) and extreme exercise (4460 ml O2/min corresponding to VO2max), and with normal values of lung diffusing capacity, the model implies that the CV has to be much lower, in the range of 0.5 to 1, to maintain adequate tissue oxygen supply and to support the observed arteriovenous saturation differences (ΔS = 0.68 and 0.75 respectively). In conclusion, this model shows that although a substantial degree of pulmonary flow heterogeneity does not significantly impair oxygen uptake at rest, in exercise a much lower degree of heterogeneity can be tolerated in order to achieve sufficient pulmonary oxygen uptake to prevent tissue hypoxia. The principal mechanism for minimizing heterogeneity in ventilation-perfusion matching is local regulation of pulmonary blood flow incorporating mechanisms such as hypoxic pulmonary vasoconstriction (HPV), which would be active at the low venous oxygen levels found in exercise. Our results strongly suggest that effective HPV is needed to achieve adequate oxygen transport under conditions of increased oxygen demand in exercise.

Where applicable, experiments conform with Society ethical requirements