EFFECTS OF SEGMENTAL THORACIC EXTRADURAL ANALGESIA ON SYMPATHETIC BLOCK IN CONSCIOUS DOGS

A study [1] based on temperature measurements has suggested that the extent of sympathetic block after spinal anaesthesia is greater than assumed previously and this has renewed interest in this area [2]. Based on loss of cold sensation, the traditional view is that the rostral level of sympathetic block extends two or more segments above the level of insensitivity to pinprick stimu-lation with spinal [3], but not with extradural anaesthesia [4]. This concept is based on the assumption that cold afferent fibres have a diameter and sensitivity to local anaesthetics similar to those of sympathetic efferent fibres so that the dermatomes with loss of cold sensation should be consistent with the area of sympathetic block [1,3]. In this study we have examined the spread of sympathetic block with segmental extradural anaesthesia by multiple temperature measure-ments. Changes in skin temperature can reflect only changes in skin blood flow, and thus sympathetic tone, when both ambient and body temperature are constant [5]—conditions not achie-ved easily in an operating room. This and ethical constraints against human volunteer studies led us to assess the effects of segmental extradural block at upper thoracic, mid thoracic and lumbar levels under conditions of constant environmental temperature in trained conscious dogs. MATERIALS AND METHODS Investigations were carried out in six trained short fur dogs (German boxers; weight 19-25 kg