Influence of expiratory flow-limitation during exercise on systemic oxygen delivery in humans

Abstract

To determine the effects of exercise with expiratory flow-limitation (EFL) on systemic O₂ delivery, seven normal subjects performed incremental exercise with and without EFL at ~0.8 l s⁻¹ (imposed by a Starling resistor in the expiratory line) to determine maximal power output under control (W′_max,c) and EFL (W′_max,e) conditions. W′_max,e was 62.5% of W′_max,c, and EFL exercise caused a significant fall in the ventilatory threshold. In a third test, after exercising at W′_max,e without EFL for 4 min, EFL was imposed; exercise continued for 4 more minutes or until exhaustion. O₂ consumption $(V'_{{\text{O}}_{2}})$ was measured breath-by-breath for the last 90 s of control, and for the first 90 s of EFL exercise. Assuming that the arterio-mixed venous O₂ content remained constant immediately after EFL imposition, we used $V'_{{\text{O}}_{2}}$ as a measure of cardiac output (Q′_c). Q′_c was also calculated by the pulse contour method with blood pressure measured continuously by a photo-plethysmographic device. Both sets of data showed a decrease of Q′_c due to a decrease in stroke volume by 10% $(p<0.001\;\hbox{for}\;V'_{{\text{O}}_{2}})$ with EFL and remained decreased for the full 90 s. Concurrently, arterial O₂ saturation decreased by 5%, abdominal, pleural and alveolar pressures increased, and duty cycle decreased by 43%. We conclude that this combination of events led to a decrease in venous return secondary to high expiratory pressures, and a decreased duty cycle which decreased O₂ delivery to working muscles by ~15%.