ObjectivesTo study the effect of pancuronium-induced muscle relaxation on circulating plasma volume.DesignA prospective, controlled study. Consecutive infants who were paralyzed with pancuronium and a comparative group who were not paralyzed during mechanical ventilation were studied.SettingNeonatal ICU of a regional referral university-affiliated hospital.PatientsNewborn infants weighing >1700 g who required respiratory assistance within 24 hrs of birth and who were free of congenital heart disease, sepsis, or blood loss were eligible for entry into the study. Infants who received colloid infusions during the study period were excluded. A total of 17 consecutive infants (nine paralyzed and eight nonparalyzed control infants) were studied. Four paralyzed infants and one nonparalyzed infant received colloid infusions before the completion of the study and were excluded from the final analysis.MeasurementsPlasma volume was measured three times in the paralyzed infants: a) immediately before the first dose of pancuronium, b) after 12 to 24 hrs, and c) ≥12 hrs after the return of muscle activity, but before extubation.Plasma volume in the nonparalyzed, control infants was measured at the time of intubation, 12 to 24 hrs after commencing mechanical ventilation, and 12 hrs after extubation. Plasma volume was measured using the Evans blue dye dilution technique.ResultsThere were no changes in the plasma volume or blood volume in the three measurements among both the paralyzed and nonparalyzed infants.ConclusionPancuronium-induced muscle relaxation in mechanically ventilated newborn infants weighing >1700 g did not alter circulating plasma volume in 24 hrs.