Background: Previous studies show that the age of an infant affects morphine clearance but that ventilatory effects from morphine infusions are similar at the same morphine steady-state concentration in infants and children after surgery. The presence of cyanotic heart disease in infants receiving postoperative morphine infusions was studied for its potential effect on ventilatory effort.
Methods: Twenty infants under 90 days of age (10 cyanotic, 10 acyanotic) received morphine intravenous infusions following thoracotomy. PaCO2 during spontaneous breathing, CO2 rebreathing trials during morphine infusions and time to wean from assisted mechanical ventilation after decreasing infusion rates were compared. Analgesia scoring in the first postoperative 24 h was also compared between cyanotic and acyanotic infants.
Results: PaCO2 during spontaneous breathing and CO2 response curve slopes were not different between groups. The time to wean from assisted mechanical ventilation after decreasing morphine infusion rates was not statistically different (median 6.9 h versus 5.8 h in cyanotic and acyanotic, respectively). Median morphine concentrations were higher in cyanotic infants at the time of ventilatory testing (32 versus 20.5 ng x ml-1 in cyanotic versus acyanotic infants, respectively, P = NS). Both groups had good pain relief. Steady-state morphine clearance rates showed two- to three-fold variability between infants. They were not statistically different in cyanotic versus acyanotic infants but tended to be lower in the cyanotic. Morphine clearance increased significantly with increasing age.
Conclusions: Morphine i.v. infusions given to infants following thoracotomy show the same ventilatory effects in infants with cyanotic heart disease as in acyanotic infants. Age is the more important determinant of morphine response by its effect on morphine clearance than cyanosis in infants aged less than 3 months.