Respiratory muscle strength can be assessed by measurement of maximal inspi
ratory (P-IMAX) and maximal expiratory pressure (P-EMAX) during crying. The
re are, however, relatively few data on P-IMAX and P-EMAX in infancy, parti
cularly fr om those born preterm. Our aim was to investigate which factors
influenced P-IMAX and P-EMAX in preterm and term infants. Forty infants, me
dian gestational age 37 weeks (range 26-43) and birthweight 2.579 kg (range
0.956-5.180) were studied at a postconceptional age (PCA) of 38 weeks (ran
ge 32-44). None had respiratory problems. A facemask was placed firmly over
the infant's mouth and nose and the infant studied during spontaneous cryi
ng. A pneumotachograph fitted snugly into the facemask and from a sideport
airway pressure changes were measured. During crying, the distal end of the
pneumotachograph was occluded for five breaths and at least three separate
occlusions were made. The highest P-EMAX value sustained for at least 1 s
and the highest peak inspiratory pressure P-IMAX were recorded. The mean P-
IMAX and P-EMAX were higher in the term compared to the preterm infants (70
cmH(2)O +/-S.D. 19 versus 58 cmH(2)O +/-S.D. 17 P-IMAX and 53 cmH(2)O +/-S
.D. 13 versus 44 cmH(2)O +/-S.D. 19 P-EMAX, P < 0.05). Both P-IMAX and P-EM
AX related significantly with postconceptional age, gestational age and wei
ght, but not postnatal age. Stepwise regression analysis demonstrated P-IMA
X related independently with PCA and P-EMAX with weight. These results sugg
est respiratory muscle strength is influenced by maturation at birth. (C) 2
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