Studies in fetal lambs suggested that the minimum alveolar concentration (MAC) in preterm neonates may be less than that in full-term neonates and older infants. To determine the MAC of isoflurane in preterm neonates, 20 patients < 32 weeks gestation at birth and 16 patients 32–37 weeks gestation at birth, all less than 1 month post-natal age, were studied. Following tracheal intubation, the neonates were anesthetized with a predetermined end-tidal concentration of isoflurane in oxygen and air. The move-no move responses to skin incision were recorded, and MAC was determined using the “up-and-down” technique. Heart rate and systolic arterial pressure were recorded awake, before skin incision, and after skin incision. MAC (mean ± SD) of isoflurane in preterm neonates < 32 weeks gestation was 1.28 ± 0.17%, and MAC in neonates 32–37 weeks gestation was 1.41 ± 0.18% (P < 0.05). Although heart rate did not decrease significantly in either group during the study, systolic arterial pressure decreased between 20 and 30% below awake values both before and after skin incision in both age groups (P < 0.01). We conclude that the MAC of isoflurane in preterm neonates < 32 weeks gestation is significantly less than that in pretern neonates 32–37 weeks gestation, and that systolic arterial pressure decreases to a similar extent at ∼1 MAC isoflurane in both age groups.

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