Assessment of acute pain in preterm infants by evaluation of facial expression and frequency domain analysis of heart rate variability.

In ten preterm infants (postconceptional age 27-35 weeks) facial expression and heart rate variability (HRV) were investigated during three situations: (a) the infant at rest in its crib or incubator, (b) provocation of the withdrawal flexor reflex by application of von Frey's hairs, and (c) lancing and squeezing for blood sampling. Video recordings of facial expressions, mixed at random, were categorised as the baby being either undisturbed, disturbed or in pain and a detailed scoring for each situation was performed. Mean heart rate (HR) was calculated and power spectral analysis was assessed on data from segments of 45-s duration from the three procedures. Flexor withdrawal did not evoke visual signs of pain or influence HRV, but caused a slight increase in HR. The lancing and squeezing of the heel evoked a facial expression of pain in all infants. The HR increased and a reduction in both total HRV and power in the low frequency band of the HRV spectrum was seen during blood sampling. The differentiation between painful and non-painful procedures was more apparent when principal component analysis of HRV was applied.