Frequency response analysis of the diaphragm in vivo.

Frequency response analysis was used to determine the dynamic response characteristics of cat diaphragm under isovolumetric conditions at functional residual capacity (FRC) and at lung volumes above and below FRC. In apneic cats, sinusoidally modulated pulse rate patterns were applied to both phrenic nerves. Modulation frequencies over the range of 0.05-4 Hz were used. Amplitude ratio vs. frequency plots obtained at FRC for intratracheal, intraesophageal, and intragastric pressures were essentially flat at low frequencies but decreased at higher frequencies. Intratracheal and intraesophageal pressure responses were altered by changes in resting lung volume while intragastric pressure was not. The amplitude ratio was decreased at lung volumes above FRC but increased at volumes below FRC. Thus, lung volume significantly affected the input-output relations between phrenic nerve input and diaphragm muscle output. In all preparations studied, significant phase lags were present throughout the entire modulation frequency range. However, in contrast to the effect of lung volume on amplitude ratio, phase lag was not dependent on changes in lung volume.