The effects of respiratory rate on the mechanical work of breathing during muscular exercise

Abstract

1. The mechanical work of breathing was measured from simultaneous records of endoesophageal pressure and tidal volume on three normal young subjects exercising on a bicycle-ergometer. The subjects were allowed to increase their ventilation with respiratory rate either spontaneous or regulated by a metronome (f ranging from 20 to 60 cycles per min). 2. At spontaneous respiratory rates the mechanical work of breathing (\(\dot W\)) was found to increase with increasing ventilation (\(\dot V\)) according to the equation proposed byOtis et al. (1950): $$\dot W = K_1 \dot V^2 + K_2 \dot V^3 .$$ 3. The values of the constantsK1 andK2 of equation 1 were found to be inversely proportional to the total lung capacity values of the subjects. 4. When tidal volume was within the range of the resting inspiratory capacity (i. e. when the end-expiratory level during exercise was equal to or above the resting end-expiratory level), the mechanical work of breathing was not affected appreciably by variations in respiratory rate or depth. When, on the other hand, the tidal volume became excessively large (i. e. when lung volumes below the resting end-expiratory level had to be utilized) the mechanical work of breathing was greatly increased. Since at a constant respiratory rate the subjects increased pulmonary ventilation by increasing the tidal volume alone, the lower the value off, the sooner were the subjects obliged to mobilize lung volumes below the resting end-expiratory level. Accordingly, mechanical work versus ventilation curves rise more steeply the lower is f. 5. When allowed to increase ventilation without any control of respiratory rate, the subjects chose spontaneously respiratory rates and levels in the range where pulmonary ventilation involved minimal effort.