Nonpharmacologic approach to minimizing shivering during surface cooling: a proof of principle study

Abstract

Purpose: This study had 2 objectives: (1) to quantify the metabolic response to physical cooling in febrile patients with systemic inflammatory response syndrome (SIRS) and (2) to provide proof for the hypothesis that the efficiency of external cooling and the subsequent shivering response are influenced by site and temperature of surface cooling pads.

Methods: To quantify shivering thermogenesis during surface cooling for fever, we monitored oxygen consumption (VO(2)) in 6 febrile patients with SIRS during conventional cooling with cooling blankets and ice packs. To begin to determine how location and temperature of surface cooling influence shivering, we compared 5 cooling protocols for inducing mild hypothermia in 6 healthy volunteers.

Results: In the patients with SIRS, core temperature decreased 0.67 °C per hour, all patients shivered, VO(2) increased 57.6%, and blood pressure increased 15% during cooling. In healthy subjects, cooling with the 10 °C vest was most comfortable and removed heat most efficiently without shivering or VO(2) increase. Cooling with combined vest and thigh pads stimulated the most shivering and highest VO(2) and increased core temperature. Reducing vest temperature from 10 °C to 5 °C failed to increase heat removal secondary to cutaneous vasoconstriction. Capsaicin, an agonist for the transient receptor potential cation channel subfamily V member 1 (TRPV1) warm-sensing channels, partially reversed this effect in 5 subjects.

Conclusions: Our results identify the hazards of surface cooling in febrile critically ill patients and support the concept that optimization of cooling pad temperature and position may improve cooling efficiency and reduce shivering.

Conflict of interest statement

The authors have no conflicts of interest to declare. The study sponsors had no role in collection or analysis of the data presented.

Copyright © 2012 Elsevier Inc. All rights reserved.

Figures

Figure 1. Effect of external cooling on core temperature and VO2 in febrile patients with SIRS

Six patients with SIRS and core temperature ≥38.8°C despite receiving acetaminophen were subjected to cooling using cooling blankets and ice packs for 90 min and core temperature (A) and VO2 (B) were measured every 15 min. Mean ± SE. Differences were tested by repeated measures ANOVA and p-values are indicated.

Figure 2. Effect of different cooling protocols on heat removal, shivering, and oxygen consumption in normal subjects

Six normal volunteers were subjected to 30 min cooling with the indicated pad positioning and temperature without or with 0.15% capsaicin applied beneath the pad. The same symbol represents each subject in all panels in figures 2 and 3. A. Total heat removed (calories) was estimated from the temperature of the circulating refrigerant entering and leaving the cooling device and the flow rate recorded every min during each 30 min cooling period is shown (see Methods). B. Shivering was measured objectively every 5 min during cooling using a 4-point bedside shivering assessment scale . The sum the shivering scores is shown. C. VO2 was measured every 5 min during cooling and AUC for the 30 min cooling period calculated using the trapezoidal rule . D. The ratio of heat removed to VO2 AUC for each 30 min cooling period was calculated. Individual subject data and mean ± SE are shown. *, †, §, and ¶ denote p < 0.05 vs. 10°C vest, 5°C vest, 5°C thigh pads, and 5°C vest with capsaicin, respectively.

Figure 3. Effect of different cooling protocols on core temperature in normal subjects

Six normal volunteers were subjected to 30 min cooling with the indicated pad positioning and temperature without or with 0.15% capsaicin applied beneath the pad. Core temperature was recorded from esophageal probes just before and every 5 min during each 30 min cooling period. A. The mean temperature for 30 min cooling period was calculated from the six individual measurements. B. The starting temperature for each cooling period is shown C. The change in core temperature during cooling was calculated as the difference between the core temperatures at the end and beginning of each 30 min cooling period. Individual subject data and mean ± SE are shown. *, †, and ¶ denote p