Metabolic benefits of surface counter warming during therapeutic temperature modulation.

This study introduces and validates a simple, novel method to reduce shivering among neurocritical care patients undergoing therapeutic cooling to maintain normothermia or induce hypothermia. This approach may help minimize the use of sedatives and neuromuscular blockers to abolish shivering.

A large body of literature suggests that fever may be deleterious in brain-injured patients {1}. In an effort to prevent ’secondary’ neurological injury, clinicians use various methods of surface or endovascular cooling in order to maintain normothermia. Additionally, hypothermia is sometimes induced for neuroprotection or to treat intracranial hypertension. However, artificially lowering temperature frequently leads to pronounced shivering. While the neurological implications of shivering are not well studied, it is uncomfortable, induces physiological stress (higher oxygen consumption, tachycardia, and tachypnea), interferes with achieving temperature goals, is believed to raise intracranial pressure and has even been associated with a decline in level of consciousness {2}. For these reasons, shivering is often treated with sedatives or even neuromuscular blockade. Because skin temperature is thought to contribute significantly (~ 20%) to the shivering response, the investigators studied the effects of forced-air warming in combination with therapeutic cooling. Fifty brain-injured patients (mostly with subarachnoid hemorrhage) had their temperature lowered as part of routine care primarily using a surface device (Arctic Sun) with the goal of maintaining normothermia. They were all concomitantly treated with skin counter warming. All patients underwent indirect calorimetry. Cessation of skin counter warming resulted in increased shivering in more than half of patients, as well as a marked increment in resting energy expenditure, VO2 (rate of oxygen uptake) and VCO2 (rate of elimination of carbon dioxide). All of these metabolic parameters improved again with resumption of counter warming. Importantly, this effect was most pronounced among those patients with the greatest degree of shivering at baseline. Skin counter warming is clearly not effective in all patients. Indeed, nearly three-quarters of patients were shivering to some degree at baseline despite receiving skin counter warming! Nevertheless, its simplicity, low cost, widespread availability, lack of adverse effects, and the potential to avoid sedation / neuromuscular blockade make it an attractive treatment option. Its effects could be even more pronounced with the use of endovascular cooling catheters rather than surface cooling methods. Thus, skin counter warming will likely be increasingly utilized among neurocritical care patients undergoing therapeutic temperature reduction. If clinicians wish to abolish shivering induced by therapeutic temperature reduction, they could consider attempting skin counter warming before resorting to more sedation and/or neuromuscular blockade.

References: {1} Polderman KH, Lancet 2008, 371:1955-69 [PMID:18539227]. {2} Badjatia et al. Neurocrit Care 2007, 6:186-91 [PMID:17534584].