Thermoregulation and obstetric regional anaesthesia

Abstract

Neuraxial blockade is used to provide analgesia for labour and anaesthesia for caesarean section in approximately one third of deliveries in the UK (more than 200 000 per year)8. Although neuraxial blockade is considered lower risk than general anaesthesia9, it is frequently complicated by tremor and a rise in body temperature (“epidural fever”)12. Anecdotally the tremor is extremely unpleasant and epidural fever is associated with adverse neonatal neurological outcome13. Despite the harmful nature of these adverse effects little is known about either condition: the incidence of tremor is not known, the mechanisms underlying both tremor and epidural fever are unclear and their impact upon patient satisfaction has not been evaluated. In a series of experiments this PhD aimed to investigate the cause and the impact of both conditions. In the first study (chapter 3) the subjective incidence and severity of tremor and its impact upon maternal satisfaction was evaluated with a post-delivery questionnaire completed by 644 patients. Chapter 4 contains three pilot studies and a feasibility study, which prepared the ground for the main work of chapters 5 and 6. Chapter 5 investigated the hypothesis that the tremor associated with epidural extension is non-thermoregulatory shivering. In the first study of this chapter the tremor and electromyography frequency profiles of cold stress-induced shivering were defined. Tremor and surface electromyography were recorded in 40 healthy subjects as they undertook a cold stress protocol using a water-circulating mattress and blanket. In the second study tremor, surface electromyography, body temperature and perceived body temperature were recorded in 20 patients before and after epidural extension for emergency caesarean section. The tremor and electromyography frequency profiles were compared to those of cold stress-induced shivering to establish whether or not epidural extension tremor is shivering. Additionally, patients’ body temperatures (real and perceived) were compared before epidural extension and at the onset of tremor to determine whether or not the tremor is thermoregulatory. The final data chapter (chapter 6) tested the hypothesis that epidural fever is a consequence of blockade of active cutaneous vasodilation. Body temperature, cutaneous heat flux and skin blood flow were recorded before and after epidural extension in 19 patients. Changes in cutaneous vasomotor tone were inferred from the pattern of variation in skin temperature, cutaneous heat flux and skin blood flow. Chapter 6 also includes a case study of a patient who was excluded from the previous study, which documents the effects of an attempted instrumental delivery upon body temperature. The main findings are: (1) the incidence of tremor following obstetric neuraxial blockade is 48% and it is associated with reduced patient satisfaction; (2) the tremor associated with epidural extension for emergency caesarean section is non-thermoregulatory shivering generated by a perceived reduction in body temperature; and (3) following epidural extension body temperature rises due to a reduction in cutaneous heat loss, which is compatible with the blockade of active vasodilation hypothesis.