Exercise‐induced arterial hypoxaemia and the mechanics of breathing in healthy young women

Abstract

Key points• By virtue of their smaller lung volumes and airway diameters, women develop more mechanical ventilatory constraints during exercise, which may result in increased vulnerability to hypoxaemia during exercise.• Hypoxaemia developed at all exercise intensities with varying patterns and was more common in aerobically trained subjects; however, some untrained women also developed hypoxaemia.• Mechanical respiratory constraints directly lead to hypoxaemia in some women and prevent adequate reversal of hypoxaemia in most women.• Experimentally reversing mechanical constraints with heliox gas partially reversed the hypoxaemia in subjects who developed expiratory flow limitation.• Due in part to increased mechanical ventilatory constraints, the respiratory system's response to exercise is less than ideal in most women. AbstractThe purpose of this study was to characterize exercise-induced arterial hypoxaemia (EIAH), pulmonary gas exchange and respiratory mechanics during exercise, in young healthy women. We defined EIAH as a >10 mmHg decrease in arterial oxygen tension (P aO 2 ) during exercise compared to rest. We used a heliox inspirate to test the hypothesis that mechanical constraints contribute to EIAH. Subjects with a spectrum of aerobic capacities (n = 30; maximal oxygen consumption (V O 2 max ) = 49 ± 1, range 28-62 ml kg −1 min −1 ) completed a stepwise treadmill test and a subset (n = 18 with EIAH) completed a constant load test (∼85%V O 2 max ) with heliox gas. Throughout exercise arterial blood gases, oxyhaemoglobin saturation (S aO 2 ), the work of breathing (WOB) and expiratory flow limitation (EFL) were assessed. Twenty of the 30 women developed EIAH with a nadir P aO 2 and S aO 2 ranging from 58 to 88 mmHg and 87 to 96%, respectively. At maximal exercise, P aO 2 was inversely related toV O 2 max (r = -0.57, P < 0.05) with notable exceptions where some subjects with low aerobic fitness levels demonstrated EIAH. Subjects with EIAH had a greaterV O 2 max (51 ± 1 vs. 43 ± 2 ml kg −1 min −1 ), lower end-exercise S aO 2 (93.2 ± 0.5 vs. 96.1 ± 0.3%) and a greater maximal energetic WOB (324 ± 19 vs. 247 ± 23 J min −1 ), but had similar resting pulmonary function compared to those without EIAH. Most subjects developed EIAH at submaximal exercise intensities, with distinct patterns of hypoxaemia. In some subjects with varying aerobic fitness levels, mechanical ventilatory constraints (i.e. EFL) were the primary mechanism associated with the hypoxaemia during the maximal test. Mechanical ventilatory constraints also prevented adequate compensatory alveolar hyperventilation in most EIAH subjects. Minimizing mechanical ventilatory constraints with heliox inspiration partially reversed EIAH in subjects who developed EFL. In conclusion, healthy women of all aerobic fitness levels can develop EIAH and begin to do so at submaximal intensities. Mechanical ventilatory constraints are a primary mechanism for EIAH in some healthy women and prevent reversal of hypoxaemia in women for whom it is not the primary mech...

Regulation of Increased Blood Flow (Hyperemia) to Muscles During Exercise: A Hierarchy of Competing Physiological Needs

“…While EIAH has not been reported in untrained male subjects, it has been reported in untrained female subjects, and it may be more prevalent in elite female endurance athletes (133,199,222,223). This hypoxemia is due to the complex interactions between the very high cardiac outputs detailed above, flow limitation at very high minute ventilations, ventilation perfusion matching, pulmonary diffusing capacity, and physiological shunts through the lung.…”

“…In retrospect, estimates of the mass of contracting muscle consuming the oxygen along with the amount of blood flow and extraction required to support this level of oxygen consumption should have been possible. Such estimates would have raised serious questions regarding the values obtained using venous occlusion plethysmography in the forearm and calf or 133 Xe washout in other muscles.…”

“…Like venous occlusion plethysmography, there are issues associated with the 133 Xe washout technique (88,92,187). In this technique, radioactive xenon is injected into a tissue and the rate of washout from the tissue is proportional to the blood flow.…”

A proportional assist ventilator to unload respiratory muscles experimentally during exercise in humans

“…As these gases have different densities (He 0.18 kg m −3 versus N 2 1.25 kg m −3 at standard temperature and pressure, dry), a He-O 2 inspirate increases the propensity for laminar flow, increases maximal flows and decreases the resistive WOB. Helium has the added benefit of being technically simple, safe and easy to implement in a variety of experimental exercise settings in healthy and clinical populations (Babb, 1997;Eves et al 2006;Dominelli et al 2013). However, there are some caveats when considering the use of He, as it reduces the WOB by variable amounts that depend on flow and is relatively expensive to use for prolonged studies.…”

Exercise-induced arterial hypoxemia is unaffected by intense physical training: a case report

“…Exercise-induced arterial hypoxemia (EIAH) is known to occur in some highly trained men and in both trained and untrained women (Dempsey et al 1984;Dominelli et al 2013). Frequently observed is the significant negative relationship between maximal oxygen consumption (V O 2max ) and end-exercise arterial oxygen tension (PaO 2 ); in other words, the most aerobically trained individuals demonstrate the most severe hypoxemia Dominelli et al 2013).…”