Estimation of inspiratory capacity in health and in subjects with respiratory muscle paralysis

¹ Southwestern Poliomyelitis Respiratory and Rehabilitation Center and Departments of Rehabilitation and Physiology, Baylor University College of Medicine, Houston, Texas

A method is described for the reproduction of the natural inspiratory capacity in healthy subjects utilizing a pressure breathing device capable of delivering high inflow rates at low pressures (1.3 l/sec. at 5 cm of water in the delivery system). A highly significant correlation was found between natural inspiratory capacity and the estimation of inspiratory capacity carried out in this manner in healthy subjects. The reliability of this method in cooperative individuals suggested applicability in patients with respiratory muscle paralysis when maximum natural inspiration is impossible. In poliomyelitis subjects with respiratory muscle paralysis the values of estimated inspiratory capacity were significantly reduced below those found in health. The degree of reduction was not directly related to the diminution of natural inspiratory capacity or to the maximum volume changes that could be produced by means of tank respirator assistance at comparable peak transthoracic pressures. Measurement of inspiratory capacity with this technique together with the determination of functional residual capacity makes it practical to estimate the total lung capacity and thus describe the lung compartments in such subjects.