Respiratory water loss as a function of ventilatory or environmental factors

Since expired gas is not water-saturated (Ferrus et al., 1980, Respir. Physiol. 39: 367-381), its water content should depend on biological or environmental factors other than expired gas temperature. In order to verify this hypothesis, multiple linear regression relationships between MEH2O, the mass of water expired per litre of BTPS ventilated gas and respiratory frequency (f) or period (TR), tidal volume (VT), ventilation (V), temperature of inspired gas (TI), density of inspired gas mixture (rho I), partial pressure of water in inspired gas (PIH2O) were computed from 345 experiments performed on 7 subjects. This analysis shows that MEH2O is positively and significantly correlated to TI (0.22 mg . dm-3 . degrees C-1), to PIH2O (0.14 mg . dm-3 . Torr-1), and to TR (0.87 mg . dm3 . s-1). MEH2O is negatively and significantly correlated to f (-0.27 mg . dm-3(cy . min-1)-1) to rho I (-0.06 mg . dm-3(g . dm-3)-1) and to V (-0.09 mg . dm-3(dm3 . min-1)-1). There is no statistical correlation between MEH2O and VT. It is concluded that the respiratory water loss depends to a large degree on respiratory or environmental conditions. This dependence supports the previously published results suggesting that expired gas is not water saturated.