Evidence of acute diaphragmatic fatigue in a "natural" condition. The diaphragm during labor.

Acute diaphragmatic fatigue has been experimentally shown to occur in normal healthy subjects and in patients with chronic obstructive pulmonary disease by asking them to modify their pattern of breathing or to breathe against high inspiratory resistances. During the expulsive period of labor women are asked periodically to make strong expulsive efforts and to sustain them isometrically for many seconds; this is likely to produce "natural" diaphragmatic fatigue. To investigate whether this was the case, six women were studied in the delivery room from the moment of the rupture of the amnion until delivery of the infant occurred. The development of diaphragmatic fatigue was assessed both by measuring the static maximal inspiratory pressure (MIP) and by analyzing the electromyographic power spectrum of the diaphragm (H/L ratio). The majority of contractions were concentrated in the expulsive period of labor. MIP, recorded in the supine position, significantly decreased from 103.2 +/- 17.2 cm H2O (at the beginning of the expulsive period) to 73.8 +/- 10.1 cm H2O (after the delivery). The H/L ratio fell progressively during the expulsive period; after the delivery, it was 59.2 +/- 15.7% of the value recorded at the beginning of the expulsive period. This study demonstrates that (1) the diaphragm is active in the expulsive efforts during labor and (2) the tension developed and the time each contraction is maintained may lead to the development of diaphragmatic fatigue. Therefore, we provide evidence of acute diaphragmatic fatigue in a natural condition.