Upright-to-supine exercise transition in younger and older subjects: effects on pulmonary gas-exchange efficiency

Introduction Aging reduces pulmonary gas-exchange (PGE) efficiency which contributes to exertional dyspnea (breathlessness) and exercise limitation. Previous work has identified an increased physiologic dead space (VDphys; areas of the lung not adequately participating in PGE), secondary to reduced pulmonary perfusion, as a mechanism for reduced PGE efficiency during exercise in older individuals. Importantly, sex also impacts the ventilatory response to exercise as females have smaller lung volumes and airway diameter relative to lung size, and greater airway resistance, leading to lower PGE efficiency compared with males. However, no studies to date have examined the interaction of age and sex on PGE efficiency during exercise. The transition from upright-to-supine position, and associated redistribution of pulmonary perfusion throughout the lung, provides a model for examining this interaction. Therefore, the objective of this study was to utilize posture (upright/supine) to examine the effect of age and sex on PGE efficiency during exercise. Methods Forty-four participants (26 young (18-35 yrs) and 18 older (60-80 years), 50% Female) performed four constant work-rate exercise tests (two upright, two supine) at 50% of peak oxygen uptake to achieve ventilatory steady state. PGE was assessed by breath-by-breath volumetric capnography to measure VDphys and alveolar (VDalv) dead space. Results During rest and exercise, older compared with younger participants had higher VDphys, VDalv, and ventilation relative to CO2 production (V̇E/V̇CO2). Older participants had greater improvement in VDphys in supine vs. upright position, but dead space remained higher in both positions compared with younger individuals. No sex differences between positions were identified. Conclusion In older adults without cardiopulmonary disease, higher VDphys suggests PGE-inefficiency; however, the improvement during exercise reflects preserved capacity for pulmonary capillary recruitment. This study adds to our understanding of the normal age-related decline in PGE efficiency, providing a baseline for assessing PGE in cardiopulmonary disease.