Phrenic activity, respiratory pressures, and volume changes in cats.

In eight anesthetized cats we measured the integrated ("moving time average") phrenic activity [using phrenic electroneurogram (EPHR)] and the active transdiaphragmatic pressure [Pdi(mus)] during room air breathing, hypoxia, and hypercapnia. The relationship between Pdi(mus) and EPHR was unaffected by either hypoxic or hypercapnic stimulation of breathing, suggesting that in spontaneously breathing cats the pressure losses are negligible. In all cats, however, there was a substantial volume-related decrease in Pdi(mus), indicating that with increasing lung volume the effectiveness of the diaphragm as a pressure generator decreases. In addition, we have developed a model that allows prediction of the time course changes in lung volume for different morphology of inspiratory driving pressure. This model explains many of the features of control of breathing found experimentally in our cats.