BJA Advance Access originally published online on June 18, 2009
British Journal of Anaesthesia 2009 103(2):275-282; doi:10.1093/bja/aep160
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Predicting the efficacy of convection warming in anaesthetized children
1 Department of Chemical Engineering, University of Newcastle, Newcastle, Australia
2 School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, Australia
3 John Hunter Hospital, Newcastle, Australia
4 University of Newcastle, Newcastle, Australia
* Corresponding author: 23 Veronica Street, Cardiff 2285, NSW, Australia. E-mail: jcpaed{at}iinet.net.au
Background: We previously described a convection warming technique (Cassey J, Armstrong P, Smith GE, Farrell PT. Paediatr Anaesth 2006; 16: 654–62). This study further analyses the children in that original study with three aims: (i) to investigate factors purported to influence children's heating rates, (ii) to describe the most effective usage of this warming technique, and (iii) to understand better the physiology of convection warming.
Methods: Children having anaesthesia for elective surgery lasting longer than 90 min in ambient temperature 21°C were warmed by a ‘Bair Hugger’ attached to a custom-built heat dissipation unit. Relationships between child and procedure characteristics and various thermal measures were analysed, and a thermodynamic model was evaluated.
Results: Thirty-nine children (aged 2 days to 12.5 yr) were studied. There were statistically significant correlations between a number of factors (e.g. height and weight) and heating efficacy. Our model demonstrated the impact of changing patient characteristics on temperature profiles. Neither the morphological characteristics nor our model could predict an individual's Tcore behaviour.
Conclusions: (i) Although the effectiveness of this warming technique is influenced by patient/procedure characteristics, these do not predict normothermia (uncertainty ±28 min). Effectiveness is independent of simple thermal measures. (ii) Previously described measures of vasoconstriction are not valid in children. (iii) Our model shows children's thermal properties change with their Tcore.. However, key factors are unknown for an individual and our model does not predict heating efficacy. (iv) To minimize the risk of hyperthermia, we recommend continuous measurement of Tcore during convection heating. The device air temperature should be turned to medium (38°C) as Tcore approaches 37°C.
Keywords: equipment, warming devices; temperature, body; temperature, monitoring; temperature, regulation