Presented at the Neonatal Society 2002 Spring Meeting.
Tooley JR1, Satas S1, Silver IA2, Thoresen M1
1 Dept of Child Health, St Michaels Hospital, Southwell St, Bristol, UK
2 Dept of Anatomy, School of Veterinary Science, University of Bristol, UK
Background: Hypothermia (HT) has been shown to be neuroprotective in animal models of HI brain injury. It is proposed that one mechanism by which HT mediates its protection is by reduction in metabolic rate. Metabolism increases with rising temperature but also with increased stress. Elevated plasma cortisol and accelerated heart rate are good markers of stress in many species. In our previous study (1) where piglets were not anaesthetised, we found no neuroprotection by posthypoxic HT. This contrasts with results in our current study on anaesthetised animals. We propose that the stress of HT may negate the neuroprotective effect of HT.
Objective: To determine whether general anaesthesia compared to no anaesthesia, influences physiological and biochemical markers of stress during HT in our piglet model of HI.
Design/Methods: 21 <1d old pigs underwent a 45 min global HI insult followed by a 24 h period of either a) head cooling with anaesthesia (anaesHT, n=8) (Propofol 4-12mg/kg/hr and Fentanyl 10mcg/kg/hr), b) normothermia with anaesthesia (anaesNT, n=10) (Propofol 4-12mg/kg/hr and Fentanyl 10mcg/kg/hr) or c) total body cooling with light analgesia only (analgHT, n=3). Heart rate was recorded and samples of blood taken at baseline, 24(end of cooling), 48 and 72 hours.
Results: Baseline cortisol levels were similar in all three groups (207nmol/l ± 116 (mean ± SD)). After 24 hours of HT cortisol levels in the non anaesthetised animals were significantly higher than those in the anaesHT or anaesNT groups (455 ± 200 vs 106 ± 96 and 65 ± 37 nmol/l respectively). Forty eight hours after the insult cortisol levels had fallen in the non anaesthetised animals and were similar to those in the other 2 groups (148 ± 98). Baseline heart rates were similar in all 3 groups (147 ±16), but during HT the rates in the anaesHT group were significantly lower than in either the anaesNT or analgHT group (116±22 vs 196±30 vs 212±48). Forty eight hours after the insult heart rates were similar in all three groups (167 ±24).
Conclusions: Cortisol levels and heart rates are significantly higher in piglets during HT without deep anaesthesia than in anaesthetised animals, which may indicate a higher stress response in the former. This raises the question whether individuals who are cooled as a posthypoxic intervention should be anaesthetised or at least heavily sedated.
References1. Thoresen et al. Pediatr Res. 2001;50:405-11