Introduction Water transport and local hydration of the airways play a critical role in the lungs, with dysfunction of airway water balance commonly associated with disease states such as cystic fibrosis and exercise-induced bronchoconstriction. The bronchial circulation, which arises from the systemic circulation, is the main supplier of water to the airways; however, limited and contradictory information is currently available on the effects of systemic dehydration on lung function.

Aim To clarify the impact of systemic dehydration on lung function in healthy individuals and to determine the role of local hydration status on any observed changes.

Methods Seven healthy young adults participated in a randomised crossover study that involved spirometry and body plethysmography at baseline (euhydration), after 28 hour of fluid restriction (systemic dehydration), and after 1 hour of systemic (oral fluid intake) or local (ultrasonic nebulisation of isotonic saline) rehydration (rehydrated). Hydration status was quantified via changes in body mass and plasma osmolality. Repeated-measures ANOVA were conducted.

Results Fluid restriction induced mild dehydration, with an average body mass loss of 2.5%±0.6% (p=0.001) and an increase in plasma osmolality from 292±2 to 298±1 mOsm·kg^-1 (p<0.001). These changes were at least partly reversed by systemic, but not local rehydration (p<0.05). Lung function data are presented in table 1. Forced vital capacity (FVC) decreased by 122±64 ml following dehydration (p=0.003) and returned to baseline post-rehydration, with no difference between modes of rehydration. Neither total lung capacity (TLC) nor residual volume (RV) were affected significantly by hydration status (p>0.05); however, RV/TLC increased by 2.1%±2.5% following dehydration (p=0.010), with this change reversed by both modes of rehydration. Functional residual capacity (FRC) increased post-dehydration by 143±161 ml, but the difference reached significance only on one study day (nebuliser day: p=0.014).

Conclusions Subtle alterations in lung volumes occur following mild dehydration in healthy individuals. That local rehydration reversed the lung function changes as effectively as systemic rehydration confirms that airway water loss contributes to the observed impairments. Assessment of hydration status may be an important consideration in the management of patients with lung diseases.