Evaluation of respiratory depression in the newborn

Narcotic antagonists are used frequently in infants born with low Apgar Score to mothers who received Meperidine during labor. In order to determine the value of different methods used to detect respiratory depression at birth, resiratory rate, (RR) tidal volume (V(T)), minute ventilation (V(E)), end tidal CO₂ (P(A)CO₂) and ventilatory response to 4% CO₂ were determined in 24 full term neonates 20-30 minutes after delivery. The Apgar Score was recorded at 1 and 5 minutes of age. All infants were born to mothers who received only Meperidine 1.0 to 1.5 mg/kg IV within two hours prior to a normal vaginal delivery. Meperidine levels were measured in cord blood by GLC. Nine infants had a normal CO₂ response, >30 ml/kg x min x mmHg P(A)CO₂ and 15 had a decreased response, <20 ml/kg x min x mmHG P(A)CO₂ (Mean 39.5±3.3 vs 12.7 ±1.2 P <0.001). Although the two groups showed this significant difference in their response to CO₂, no significant differences were found in their RR, V(T), V(E), P(A)CO₂, or Apgar Score, all of which were within the normal range. No correlation was found between Meperidine levels in cord blood and any of these determinations. In conclusion, Meperidine given to the mother in moderately high doses does not cause respiratory depression in the newborn severe enough to be recognized by tests such as minute ventilation, end tidal CO₂ or Apgar Score but can be detected by a decreased ventilatory response to CO₂. A low Apgar Score in a newborn whose mother received meperidine 1-1.5 mg/kg is unlikely to be caused by CNS depression secondary to this medication alone. An associated factor such as fetal asphyxia should be considered in these cases. Respiratory depression was not related to the concentration of Meperidine in cold blood and therefore can be caused by Meperidine metabolites as suggested previously by others.