Lung compliance and its transient elevations measured with pulse-flow method

We describe a pulse-flow method of measuring static lung compliance (CL) that is sensitive to rapid transients in CL. CL is measured by blowing air at a constant flow into the mouth and lungs for 2 s and calculated by dividing airflow in 1/s by the change in transpulmonary pressure in CMH2O/s. Pulse and static inspiratory CL was measured in five normals, four obstructives, five obese, and two patients with pulmonary fibrosis, Pulse CL after tidal breathing was correlated with static CL measured after deep breaths (r = 0.96). Pulse CL after deep breaths was higher than pulse CL after tidal breathing (p less than 0.01) and then static CL after deep breaths (p less than 0.05). In all subjects the lower the forced expiratory volume in 1 s, expressed as a percentage of vital capacity (FEV1/FVC), the greater the increase in pulse CL after a deep breath will be (r = 0.93). After deep breaths pulse CL fell from maximum CL to base-line CL at a rate related to 1/t2 where t equals the time in seconds from the last deep breath. We conclude that the increase in CL after a deep breath is related to the degree of airway obstruction and that the subsequent fall in CL is related to 1/t2.