Objective. To determine the effect of being in the neonatal intensive care unit (NICU) on pain responses in infants of 32 weeks' postconceptual age (PCA).

Design. A cross-sectional comparative design was used.

Setting. Two level III NICUs, each in metropolitan, university teaching hospitals.

Patients. Infants of 32 weeks' PCA born within the past 4 days (the newly born group; n = 53) were compared with infants of the same PCA who had been born 4 weeks earlier (the earlier-born group; n = 36) and had spent that time in an NICU.

Outcome Measures. Heart rate, oxygen saturation levels, and facial actions were used as outcomes in a between-group repeated measures analysis of variance across the heel stick procedure. Background variables of Apgar, weight at birth and data collection, severity of illness, age group, and total number of invasive procedures were entered into a stepwise regression.

Results. The two groups responded differently to the heel stick: the earlier-born infants had less behavioral manifestations of pain than the newly born infants. The number of invasive procedures was the primary factor that explained those behavioral differences, with Apgar as a second explanatory factor. The earlier-born infants had higher heart rates and lower oxygen saturation than the newly born infants before as well as during the procedure. These physiological differences were explained by the perinatal factors of age at birth and birth weight.

Conclusion. Preterm infants who spend PCA weeks 28 through 32 in an NICU are less mature in their pain response than newborn premature infants of 32 weeks' PCA. Greater frequency of invasive procedures is associated with behavioral immaturity, whereas birth factors are associated with physiological immaturity.

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