Plasma norepinephrine levels, vagal tone index, and flexor reflex threshold in premature neonates receiving intravenous morphine during the postoperative period: A pilot study

Objective: The goal of this study was to evaluate the effects of a single d ose of intravenous morphine on postoperative pain in extremely premature ne onates after thoracotomy. Design: Descriptive correlational study. Patients: Twenty-four critically ill mechanically ventilated premature neon ates with a mean gestational age of 26.1 +/- 2.1 (SD) weeks and a postnatal age of 13.8 +/- 8.1 (SD) days. Outcome Measures: Plasma norepinephrine (NE) levels, vagal tone index (VTI) , and flexor reflex threshold were measured preoperatively, immediately bef ore, and 20 and 60 minutes after the administration of the first postoperat ive dose of morphine (0.1 mg/kg). Results: One-way repeated-measures ANOVA revealed no significant change in plasma WE levels from baseline levels (df[2,32] = 2.40, p = 0.11). Pre- and post-morphine VTI values were significantly lower than preoperative values (fS[3,60] = 6.04, p = 0.0012), but no significant differences were found b etween pre- and post-morphine VTI values. Neonates (n = 10) who had a flexo r reflex response during the postoperative period demonstrated no significa nt differences in the force required to elicit a flexor reflex across the f our measurements (df[3,27] = 0.76, p = 0.53); however, the flexor reflex re sponses were significantly less vigorous during the post operative period t han at baseline. Conclusions: Findings from this pilot study suggest that the dose of morphi ne commonly used to treat postoperative pain in premature neonates does not affect NE, VTI, and flexor reflex threshold values within 1 hour of admini stration.