Effect of maturity and infection on the rate of relaxation of the respiratory muscles in ventilated, newborn infants.

Aim: To assess the respiratory muscle time constant of relaxation (τ), an index of respiratory muscle function in ventilated newborns.

Methods: Sixty-two infants (42 born prematurely) with a median gestational age of 29 [interquartile range (IQR) 26-37] weeks were prospectively studied. Measurement of τ was taken during spontaneous breathing on endotracheal continuous positive airway pressure prior to extubation, and τ was calculated from the reciprocal of the slope of the airway pressure decline versus time. Infants were classified as having had systemic or respiratory infection (positive microbiology) if they had any positive bacterial blood or respiratory culture prior to measurement.

Results: Measurement of τ was taken at a median post-natal age of 6 (IQR 3-29) days. The median τ was higher in premature infants [17.4 (IQR 7.7-28.3) sec/cmH O] compared to term infants [6.8 (IQR 4.4-8.7) sec/cmH O, p < 0.001]. The median τ was higher in infants who had had positive microbiology [17.6 (IQR 9.9-29.1) sec/cmH O] compared to infants with negative microbiology [8.0 (IQR 6.3-17.9) sec/cmH O, p = 0.034]. τ was related to gestational age (r = -0.265, p = 0.003) and weight at measurement (r = -0.269, p = 0.002).

Conclusion: Respiratory muscle function in ventilated newborns is negatively affected by prematurity and previous systemic or respiratory infection.