Abstract
Muscle fatigue shifts the power spectrum of the diaphragmatic electromyogram (DE) from high to low frequencies. Hypoxia and hypercapnia accompany respiratory failure in neonates, however their contribution to respiratory muscle fatigue is unknown. We tested the effects of alveolar hypoxia and hypercapnia on the DE frequency spectrum in 5 preterm infants studied at 39 ± 2 post-conceptional weeks of age while they breathed room air (control) and after 5 minutes of breathing 17% O2 or 4% CO2. We recorded DE from bipolar electrodes at the right costal margin mid-axillary line, abdominal movements with a circumferential strain gauge, skin-surface O2 (PsO2) and CO2 (PsCO2), and esophageal pressure (Pes) with a saline-filled tube. A fast Fourier transform calculated the centroid frequency (Fc) and summed amplitudes of power (SA) over 256 ms windows of DE selected at the peak of abdominal respiratory movements, free of QRS complexes. In each infant, diaphragmatic SA and Pes excursions increased during both hypoxia and hypercapnia. Fc fell with hypoxia but not with hypercapnia: (X ± SE; *p <0.05 compared to control, anova) In preterm infants, transient hypoxia but not hypercapnia causes power spectral changes consistent with diaphragmatic fatigue.
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