Ventilatory Efficiency during Exercise in Healthy Subjects

Abstract

When evaluating dyspnea in patients with heart or lung disease it is useful to measure the quantity of ventilation needed to eliminate metabolically produced CO₂ (i.e., the ventilatory efficiency). Mathematically, the relationship between ventilation (V̇E) and CO₂ output is determined by the arterial CO₂ pressure and the physiologic dead space–tidal volume ratio. We decided to determine how age, sex, size, fitness, and the type of ergometer influenced ventilatory efficiency in normal subjects. Three methods were compared for expressing this relationship: (1) the V̇E versus CO₂ output slope below the ventilatory compensation point, commonly used by cardiologists for estimating the severity of heart failure; (2) the V̇E/CO₂ output ratio at the anaerobic threshold, commonly used by pulmonologists; and (3) the lowest V̇E/CO₂ output ratio during exercise, the latter parameter not previously reported. We studied 474 healthy adults, between 17 and 78 years of age during incremental cycle and treadmill cardiopulmonary exercise tests at three test sites, correcting the total V̇E for the equipment dead space. The lowest V̇E/CO₂ output ratio was insignificantly different from the ratio at the anaerobic threshold, less variable than that for the slope relationship, and unaffected by the site, ergometer, and gas exchange measurement systems. The regression equation for the lowest V̇E/CO₂ output ratio was 27.94 + 0.108 × age + (0.97 = F, 0.0 = M) − 0.0376 × height, where age is in years and height is in centimeters. We conclude that the lowest V̇E/CO₂ output ratio is the preferred noninvasive method to estimate ventilatory inefficiency.