Changes in alveolar mixing efficiency during exercise

The increased alveolar-arterial oxygen gradient (A-aDO2) occurring at higher levels of exercise may reflect impaired gas mixing. This latter function was investigated using the multiple breath nitrogen washout technique in 30 physical training students (15 M, 15 F). They were examined at rest and at work loads of 50, 100 and 150 W, using a cycle ergometer. Measurements included tidal volume (VT), serial dead space (VDS) and from the washout alveolar dead space (VDA), lung volume as the end-expiratory volume (EEV) and alveolar gas mixing efficiency (AME). There were expected rises in mean V1 from 641 +/- 161 to 1650 +/- 248 ml and in minute ventilation (VE) from 11.1 +/- 3.1 to 47.1 +/- 5.2 L.min-1 at 150 W but EEV changed only slightly from 2555 +/-559 to 2321 +/- 585 ml at the same workload. Despite an increase in VDS from 156 +/- 33 to 258 +/- 35 ml and VDA from 139 +/- 40 to 305 +/- 73 ml, AME rose significantly with exercise from 72.5 +/- 5.0 to 78.3 +/- 3.3%. These results are discussed in relation to the morphology of the lung, and the stationary interface theory. They suggest that gas mixing improves at the onset of exercise only, so that the widening in A-aDO2 with increasing levels of exercise cannot be explained by impaired mixing efficiency alone, and is probably due to other factors such as critically rapid transit time of blood through the pulmonary capillaries as cardiac output increases.