Summary of abstracts
Perioperative hypothermia is a common problem in anaesthetic practice, often underestimated or conveniently neglected. Anaesthetic induced impairement in body temperature regulation and maintenance occurs during general anaesthesia as well as regional. In any case, perioperative hypothermia complicates management of the anaesthetised patients in several ways. Postoperative stress and discomfort in awakened patients causes increase in sympathetic system outflow, increase in heart rate and blood pressure. Shivering and increase in oxygen demand further compromises especially patients with known cardiovascular diseases. Other adverse clinical outcomes include poor wound healing, and increased rate of infection. This review considered two studies - one done in patients under regional anaesthesia and the other during general anaesthesia. Both of them relate to different recognised factors that will determine extent and severity of perioperative hypothermia, that can partially be modulated by anaesthetic technique. You may also wish to briefly browse our editorial comment where we ask an important question!
1. Diabetic neuropathy predisposes to intra-operative hypothermia
Core hypothermia develops after the induction of general anaesthesia but intraoperative vasoconstriction usually prevents its progression. Diabetes mellitus is often complicated by authonomic neuropathy, which impairs normal peripheral neurovascular function. In this study authors tested the hypothesis that diabetic patients experience a greater reduction in core temperature during general anaesthesia than nondiabetic patients.
Methods: They studied 36 nondiabetic patients ( control group ) and 27 diabetic patients ( diabetic group ) undergoing elective abdominal surgery. All patients were tested for autonomic neuropathy ( HR variation with deep periodic breathing, Valsalva manouvre and head-up tilt ). There was no premedication. Anaesthesia consisted of Fentanyl (2ug/kg), Propofol (1-2 mg/kg), Vecuronium (0.1 mg/kg). All patients were mechanically ventilated, and circle system was used with FGF 6 l/min and ventilatory settings maintained ETCO 2 closely to 35 mmHg. Anaesthetic was maintained with Isoflurane (ET 1-3%) in 70%N 2 O:30%O 2 . No airway heating or Humidification was used, fluids were not actively warmed and no warming devices were used. Temperature of the operating room was maintained at 23 o C. Heart rate was monitored using three-lead ECG, BP was measured oscillometrically at 5 min intervals. Respiratory gas concentrations were quantified by end-tidal gas analyzer. Temperature probe used was thermocouple and core temperature was measured at the tympanic membrane. The mean skin temperature was calculated. Relation between abnormal autonomic nervous system tests and the tympanic membrane temperature during general anaesthesia was assessed in relation to periferal vasoconstriction.
Results: 13 patients in diabetic group showed autonomic dysfunction. Changes in temperature among the groups were similar at 90 min after the induction of anaesthesia. The core temperature of the diabetic patients with autonomic dysfunction was lower from 120 min (35.1 o C) onward compared to the rest of the groups. Peripheral vasoconstriction evaluated using the forearm fingertip skin surface temperature gradient, was delayed in patients with autonomic dysfunction compared to the others.
Conclusion: These results indicate that diabetic autonomic neuropathy is associated with more severe intraoperative hypothermia. Authors have postulated that diabetic patients become more hypothermic because their peripheral neuropathy delays the onset of thermoregulatory vasoconstriction and reduces its efficacy once triggered. These patients may therefore fail to develop a normal core temperature plateau.
2. Hypothermia during spinal anaesthesia
Body temperature is often ignored during during regional anaesthesia and this study was designed to determine the predictors of core hypothermia in patients receiving spinal anaesthesia for radical retropubic prostatectomy.
Methods: 44 patients undergoing radical retropubic prostatectomy were observed. After 1-2 mg of Dormicum was given iv, lumbal spinal block was performed using 18-22 mg of Bupivacaine (0.75% in 7.5% Dext) with 20 ug Fentanyl. After 20-25 min level of the block was evaluated. No active warming measures were implemented and iv fluids were warmed to 37C. Ambient temperature was not standardised (18.7-22.9 o C). Clinical variables potentially related to decreased core (tympanic) temperature at admission to the PACU assessed were: duration of surgery, average ambient operating room temperature, body habitus, age, and the level of the spinal block. Results: The mean core temperature at admission to the PACU was 35.1+/-0.6 o C (range 33.6-36.3 o C). Duration of surgery, ambient operating room temperature, and body habitus were not predictive of hypothermia. High level of spinal block and increasing age were predictors of hypothermia. For each incremental increase in block level, core temperature decreased by 0.15C and for each increase in age core temperature decreased by 0.3C.
Conclusion: This was the first study that has shown that a higher level of blockade is associated with a greater magnitude of core hypothermia. As with general anaesthesia, advanced age is associated with more severe hypothermia during spinal anaesthesia.