Endogenous opioid effect on breathing during normoxia and hypoxia in developing swine

In normoxia, the opioid antagonist naltrexone (3 mg/kg i.v.) increased respiratory drive (dITP/dt), frequency (f) and ventilation (V) and decreased tidal volume (Vt) in 1- to 4-day-old anesthetized piglets; it increased dITP/dt in 10- to 18-day-old piglets. During progressive and steady state hypoxia, naltrexone increased dITP/dt, f and V within each age was abolished by naltrexone. Thus, it appears that (1) endogenous opioids modulate breathing in young more than in older piglets during normoxia; (2) during hypoxia, opiates modulate breathing of both young and older piglets, and (3) the increase in hypoxic respiratory drive with age may be related to enhanced respiratory suppression by opioids during hypoxia in the youngest animals. Lastly, respiratory drive is a sensitive measure of subtle differences between two closely related age groups.