Dear Editor,
This journal has recently published an intriguing article discussing the possible reasons that therapeutic hypothermia for hypoxic–ischemic brain injury is more effective in newborn infants than in older patients (Whitelaw and Thoresen, 2023). Among their hypotheses, No. 6 stated, “Body (and therefore brain) temperature can be lowered much faster in newborns, having a larger surface area-to-weight ratio than adults.” The issue of speed of cooling is quite possibly the most important of all the proposed hypotheses. Illustrating the importance of body size, 32 kg swine can be cooled to 34°C in just 9 minutes by using convective-immersion surface cooling (Janata et al., 2008), whereas adult humans cooled with this same method require 37 minutes to reach target (Howes et al., 2010).
A prior report discussing the mechanisms of hypothermic neuroprotection (Gunn and Thoresen, 2006) noted the importance of completing therapeutic hypothermia induction to 34°C as soon as possible after reperfusion. These authors cited experimental studies in near-term fetal sheep subjected to 30 minutes of cerebral ischemia. Cooling to 34°C initiated 90 minutes after reperfusion was strongly neuroprotective, whereas cooling initiated 5.5 hours postreperfusion was only about half as neuroprotective; cooling initiated 8.5 hours after reperfusion provided no neuroprotection.
The importance of speed of cooling in humans has been discussed in a review of 4091 adult patients treated with postresuscitation cooling (Schock et al., 2016). This review concluded that attainment of a temperature <34°C within 3.5 hours of resuscitation and using a cooling rate of >3°C/h appeared to improve outcomes when compared with slower cooling. The above and other studies suggest that more emphasis should be placed on improving the timing of initiation and speed of cooling in postischemic patients of all sizes.
Author Disclosure Statement
I am coinventor of the ThermoSuit System®, a convective-immersion surface cooling device that is marketed by Life Recovery Systems. I am also cofounder of the company.
Funding Information
National Heart, Lung, and Blood Institute of the National Institutes of Health, Grant No. 5R44HL072542-03.
References
- Gunn AJ, Thoresen M. Hypothermic neuroprotection. NeuroRx 2006;3(2):154–169; doi: 10.1016/j.nurx.2006.01.007. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Howes D, Ohley W, Dorian P, et al. Rapid induction of therapeutic hypothermia using convective-immersion surface cooling: Safety, efficacy and outcomes. Resuscitation 2010;81(4):388–392; doi: 10.1016/j.resuscitation.2009.12.025. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Janata A, Weihs W, Bayegan K, et al. Therapeutic hypothermia with a novel surface cooling device improves neurologic outcome after prolonged cardiac arrest in swine. Crit Care Med 2008;36(3):895–902; doi: 10.1097/CCM.0B013E318165FB33. [DOI] [PubMed] [Google Scholar]
- Schock RB, Janata A, Peacock WF, et al. Time to cooling is associated with resuscitation outcomes. Ther Hypothermia Temp Manag 2016;6(4):208–217; doi: 10.1089/ther.2016.0026. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Whitelaw A, Thoresen M. Therapeutic hypothermia for hypoxic-ischemic brain injury is more effective in newborn infants than in older patients: Review and hypotheses. Ther Hypothermia Temp Manag 2023;13(4):170–174; doi: 10.1089/ther.2023.0050. [DOI] [PubMed] [Google Scholar]