External intercostal muscle activity during acute hypoxia in the kitten

The effects of acute hypoxia on the recruitment of external intercostal muscle activity were determined in 12 kittens, aged 14 to 36 days. The animals were anesthetized with 1.23 +/- 0.23% halothane and bipolar electrodes were placed in the costal and crural diaphragm and in dorsal external intercostal muscles. Acute hypoxia was induced by the animals breathing 13% oxygen; arterial gases were sampled during baseline conditions and at 1 and 5 min after induction of hypoxia. Peak-moving average (PA) and minute electromyogram (EMG) activity (PA x f) were recorded during baseline conditions and at 1 and 5 min after onset of acute hypoxia. At 1 min of acute hypoxia, PA and PA x f of the costal diaphragm, crural diaphragm, and external intercostal muscles were significantly increased above baseline values (P less than 0.01). After 5 min of acute hypoxia, PA of all three muscles remained elevated above baseline values (P less than 0.05) but PA x f returned toward baseline levels. Respiratory frequency remained unchanged during the hypoxic stimulus. These data document that the newborn is capable of increasing inspiratory external intercostal muscle EMG activity during acute hypoxia. We speculate that this phasic recruitment could be of physiologic benefit to the newborn by stabilizing the complaint chest wall and by increasing the contribution of rib cage expansion to tidal breathing.