Evaluation of human diaphragm function.

When single supramaximal shocks are delivered during relaxation to both phrenic nerves simultaneously, the resulting transdiaphragmatic pressure twitch (PdiT) or mouth pressure twitch (PmT) are found to decrease linearly with increasing lung volume thereby reflecting changes in diaphragm contractility. Whereas fatigue decreases PdiT at any given lung volume, chronic lung hyperinflation tends to increase PdiT at any given lung volume. When the phrenic nerve shocks are delivered during ongoing voluntary contractions, PdiT decreases with increasing level of diaphragm activation. Its amplitude thus detects the reverse left for full activation of the diaphragm by the voluntary motor drive. The ability to maximally activate the diaphragm decreases with fatigue but is retained in patients with chronic lung hyperinflation.