Recruitment of intercostal muscle activity during hypercapnia in kittens

Little is known about the respiratory behavior of the intercostal muscles within a neonatal and developmental context. We, therefore, examined intercostal muscle electromyographic (EMG) activity in kittens (1 month old, n = 8; 2 months old, n = 7) during eupnea and heightened respiratory drive induced by hypercapnia. The kittens were anesthetized with halothane (1.25-1.50%) at comparable minimum alveolar concentrations and were studied in the prone position during an acute exposure to hyperoxic hypercapnia (8% CO2, 50% O2, balance N2) for 7 min. We recorded EMG activities from bipolar electrodes embedded in the intercostal (dorsolateral thorax, 4th-5th interspace) (EMGic) and costal diaphragm (EMGdi) muscles. Peak moving time average EMG measurements served as our index of muscle activity. Phasic inspiratory EMGic activity was present during eupnea in all animals. EMGic and EMGdi increased significantly above baseline levels during hypercapnic exposure with the increase in EMGic (133 +/- 27%) being comparable to that observed in EMGdi (141 +/- 28%) (P = 0.40). No differences in EMGic (P = 0.64) or EMGdi (P = 0.88) recruitment were noted between age groups. These data indicate that hypercapnia augments intercostal muscle EMG activity in kittens and suggest that such activity parallels costal diaphragmatic EMG recruitment. We conclude that EMGic recruitment is a frequent respiratory phenomenon associated with hypercapnically stimulated diaphragmatic muscle activity in kittens. We speculate that intercostal muscle recruitment stabilizes the compliant chest wall of the newborn and helps sustain inspiratory force generation during stimulated breathing.