Developmental pattern of muscle fiber types in human ventilatory muscles.

Premature infants tolerate respiratory loads poorly. This may reflect incomplete development of the ventilatory muscles (VM) causing poor resistance to fatigue. To study the developmental pattern of human VM, 31 postmortem specimens of diaphragm and intercostal muscles were obtained. Individual muscle fibers were classified as type I (slow-twitch, high-oxidative) or type II (fast-twich, low-oxidative) using histochemical staining methods for myofibrillar adenosine triphosphatase (M-ATPase) (pH 10.30) and nicotinamide adenine dinucleotide (NADH) tetrazolium reductase. In the diaphragm, premature infants (less than 37 wk gestation) had only 9.7 +/- 1.3% type I fibers, full-term newborns 25.0 +/- 1.1%, and older subjects (greater than 2 yr of age) 54.9 +/- 1.3%. There was no further increase after 8 mo postpartum. In the intercostal muscles, premature infants had only 19.0 +/- 4.8% type I fibers, full-term newborns 45.7 +/- 1.3%, and older subjects 65.2 +/- 2.6%. There was no further increase after 2 mo postpartum. These findings suggest the ventilatory muscles of newborn infants are more susceptible to fatigue than those of older subjects. This may contribute significantly to respiratory problems in the neonate.