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 dose of intravenous morphine on postoperative pain in extremely premature neonates after thoracotomy.

DESIGN

Descriptive correlational study.

PATIENTS: Twenty-four critically ill mechanically ventilated premature neonates 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 before, and 20 and 60 minutes after the administration of the first postoperative dose of morphine (0.1 mg/kg).

RESULTS

One-way repeated-measures ANOVA revealed no significant change in plasma NE levels from baseline levels (df[2,32] = 2.40, p = 0.11). Pre- and postmorphine VTI values were significantly lower than preoperative values (df[3,60] = 6.04, p = 0.0012), but no significant differences were found between pre- and postmorphine VTI values. Neonates (n = 10) who had a flexor reflex response during the postoperative period demonstrated no significant differences in the force required to elicit a flexor reflex across the four measurements (df[3,27] = 0.76, p = 0.53); however, the flexor reflex responses were significantly less vigorous during the postoperative period than at baseline.

CONCLUSIONS

Findings from this pilot study suggest that the dose of morphine commonly used to treat postoperative pain in premature neonates does not affect NE, VTI, and flexor reflex threshold values within 1 hour of administration.