Regional variations in skin perfusion and skin thickness may contribute to varying efficacy of topical, local anaesthetics in neonates

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Abstract
Summary: The national Swedish screening programme for inborn errors of metabolism includes blood sampling from all neonates. Heel lancing has hitherto been the method of choice for these screening tests. Studies have recently been done to find out whether the use of EMLA can alleviate pain caused by heel lancing. EMLA had little, if any (1, 2) effect when applied to the heel of full term babies. Similar results were reported concerning preterm infants (3, 4). Other authors have found that differences in skin thickness or in skin blood perfusion may influence the effect of EMLA (5, 6). The aim of this study was to establish whether differences in skin thickness or in skin blood perfusion were present in three regions: the forehead, the dorsum of the hand and the heel. Ten healthy full term neonates were included in the study on skin perfusion. Measurements were made with a laser Doppler flux meter. Twenty‐seven healthy full term neonates were included for measurements of skin thickness using a high frequency B‐mode ultrasonic meter. The heel skin perfusion was two to three times higher than the skin perfusion at the dorsum of the hand and at the forehead but there was no significant difference when comparing skin thickness of the heel with the dorsum of the hand and the forehead. A rapid clearance of a topically applied local anaesthetic, due to high cutaneous blood flow, may explain why EMLA seems to be of little value when it is applied to the neonatal heel.