Breathing pattern and CO2 response in newborn rats before and during anesthesia

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Abstract
The breathing pattern (minute ventilation, .ovrhdot.VE, tidal volume VT and respiratory rate f) was studied in newborn rats before and during barbiturate (20-30 mg/kg i.p.) or ketamine anesthesia (40-80 mg/kg i.p.). Animals were intact and prone in a flow plethysmograph in thermoneutral conditions. Before anesthesia, CO2 breathing (5 min in 5% and 5 min in 10% CO2 in O2) resulted in a substantial increase in .ovrhdot.VE (169 and 208%, respectively), which was maintained throughout the entire CO2 breathing period. This indicates that, despite the extremely large .ovrhdot.VE per kg at rest, in these small animals there is still a large reserve for a sustained increase in .ovrhdot.VE. During barbiturate, the resting .ovrhdot.VE dropped to 45% of control, due to a reduction in VT (83%) and f (59%). This latter result was due to a prolongation of the expiratory time (214%), with no significant changes in inspiratory time. CO2 response was also much depressed, to .apprx. 63% of the control. The late portion of the expiratory flow-volume curves, the slope of which represents the expiratory time constant of the system, was similar before and during anesthesia in .apprx. 50% of the animals, whereas it increased during anesthesia in the remaining animals. Although compliance of the respiratory system was generally unaltered, the increased impedance during anesthesia probably reflected an increased resistance. Qualitatively similar results were obtained during ketamine anesthesia. As observed in adult mammals, anesthesia in newborn rats has a marked depressant effect on resting breathing pattern and CO2 response, occasionally accompanied by an increase in the expiratory impedance of the respiratory system.