Factors that influence the behavioral pain responses of premature infants.

The responses of preterm neonates to acute tissue-damaging stimuli have been described. However, factors which influence these responses have received little attention. In this study, we observed 124 premature infants before, during and after a routine heel lance and determined how two contextual variables (severity of illness and behavioral state) influenced their behavioral responses. Significant changes in facial actions occurred between baseline and the most invasive phase of the heel lance procedure, stick. The fundamental frequency, harmonic structure and peak spectral energy of the infant's cry were also significantly increased during the stick phase. Behavioral state was found to influence the facial action variables and severity of illness modified the acoustic cry variables. Accurate identification of pain in premature infants requires consideration of factors that influence their response.