Effects of hypohydration on lung functions in humans

We studied lung function tests during euhydration (Days 1 and 2), hypohydration (Days 3 and 4), and rehydration (Days 5, 6, and 7) in 6 normal subjects. Hypohydration was induced by administering chlorthalidone, and this resulted in a 4.5% loss of body weight. During hypohydration, lung volumes increased significantly. Interestingly, ventilatory lung function tests including peak expiratory flow rate, FEV1, maximal voluntary ventilation, and flow rates at low lung volumes also increased significantly and returned to normal upon rehydration. Diffusing capacity for carbon monoxide remained unchanged. The improvement in ventilatory lung function tests during hypohydration was surprising, and it is suggested that this was related to loss of water within and/or around the airways. This mechanism is opposite to that which occurs in pulmonary edema with excess lung water where reduction in flow rates occurs.