Relationships between maximal breathing capacity and timed expiratory capacities

Abstract

Maximal breathing capacity (MBC) and timed expiratory capacity (TEC) measurements are estimates of voluntary ventilatory capacity. The MBC embodies certain disadvantages and is dependent on coordination, endurance, learning, motivation, breathing rate and pulmonary mid-position at which breathing is performed. Correlations (R = > +0.91) were found for the relationship 0.5, 0.75 and 1.0 second TEC and the MBC in 153 normal subjects and 100 patients with ventilatory insufficiency. The MBC can be conveniently estimated from any of the TEC values with essentially equal reliability. This is only an approximation since expiratory flow rate is only one of the many variables determining the MBC. Aside from the effects of maximal inspiratory and expiratory flow rate on the determined MBC, the breathing rate must exceed 65/min. and the steepest slope of the expiratory flow curve must be repetitiously utilized, otherwise maximal ventilation is not achieved. The forced expiratory vital capacity and timed expiratory capacities are not merely convenient substitutes for the MBC, but deserve investigation as potentially valuable reflections of muscular and visco-elastic properties of the pulmonary apparatus.