Gordon G. Giesbrecht

Comparison of forced-air patient warming systems for perioperative use

BackgroundPerianesthetic hypothermia is common and produces several complications, including postoperative shivering, decreased drug metabolism and clearance, and impaired wound healing. Forced-air warming transfers more than 50 W to the body and is an efficient method for either preventing or reversing decreases in core temperature. MethodsThe authors compared the efficacy of four complete forced-air warming systems: (1) Bair Hugger 250/PACU Patient Warming System with 300 Warming Cover (Augustine Medical, Eden Prairie, MN); (2) Thermacare TC1000 Power Unit with TC1050 Comfort Quilt (Gaymar Industries, Orchard Park, NY); (3) WarmAir 130 Hypothermia System with 140 Warming Tube (Cincinnati Sub-Zero Products, Cincinnati, OH); and (4) WarmTouch 5000 Patient Warming System and 503–0810 CareQuilt (with the connecting hose compressed [short] and extended [long]) (Mallinckrodt Medical, St. Louis, MO). Six minimally clothed male volunteers were studied supine in a 24.5°C environment. Cutaneous heat flux and skin temperature was measured at 14 area-weighted sites using thermal flux transducers. After 20-min control periods, volunteers were warmed for 40 min in each condition. A cotton blanket was placed over each cover. Power units were placed at the foot end of the bed, started cold, and set at maximum temperature and flow settings. All units reached maximum efficiency within 20 min. ResultsTotal heat transfer with the Bair Hugger system (95 ± 7 W) was greater (P < 0.05) than with WarmTouch (short hose 81 ± 6 W and long hose 68 ± 8 W), Thermacare (61 ± 5 W), and WarmAir (38 ± 6 W) systems. Each cover also was tested on a common power unit (Bair Hugger 200). Total heat transfer was greater (P < 0.05) with the Warming Cover (Bair Hugger) (88 ± 8 W), followed by the Comfort Quilt (Thermacare) (56 ± 6 W), CareQuilt (WarmTouch) (50 ± 7 W), and the Warming Tube (WarmAir) (43 ± 6 W). ConclusionsThe advantages of the Bair Hugger system and Warming Cover are evident in areas that are important for heat transfer from the periphery to the body core (chest, axilla, abdomen, and upper legs).

Measurement and prediction of peak shivering intensity in humans

Abstract Prediction equations of shivering metabolism are critical to the development of models of thermoregulation during cold exposure. Although the intensity of maximal shivering has not yet been predicted, a peak shivering metabolic rate (Shivpeak) of five times the resting metabolic rate has been reported. A group of 15 subjects (including 4 women) [mean age 24.7 (SD 6) years, mean body mass 72.1 (SD 12) kg, mean height 1.76 (SD 0.1) m, mean body fat 22.3 (SD 7)% and mean maximal oxygen uptake (V˙O2max) 53.2 (SD 9) ml O2 · kg−1 · min−1] participated in the present study to measure and predict Shivpeak. The subjects were initially immersed in water at 8°C for up to 70 min. Water temperature was then gradually increased at 0.8 °C · min−1 to a value of 20 °C, which it was expected would increase shivering heat production based on the knowledge that peripheral cold receptors fire maximally at approximately this temperature. This, in combination with the relatively low core temperature at the time this water temperature was reached, was hypothesized would stimulate Shivpeak. Prior to warming the water from 8 to 20 °C, the oxygen consumption was 15.1 (SD 5.5) ml · kg−1 · min−1 at core temperatures of approximately 35 °C. After the water temperature had risen to 20 °C, the observed Shivpeak was 22.1 (SD 4.2) ml O2 · kg−1 · min−1 at core and mean skin temperatures of 35.2 (SD 0.9) and 22.1 (SD 2.2) °C, respectively. The Shivpeak corresponded to 4.9 (SD 0.8) times the resting metabolism and 41.7 (SD 5.1)% of V˙O2max. The best fit equation predicting Shivpeak was Shivpeak (ml O2 · kg−1 · min−1)=30.5 + 0.348 ×V˙O2max (ml O2 · kg−1 · min−1) − 0.909 × body mass index (kg · m−2) − 0.233 × age (years); (P=0.0001; r2=0.872).

Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Frostbite

The Wilderness Medical Society convened an expert panel to develop a set of evidence-based guidelines for the prevention and treatment of frostbite. We present a review of pertinent pathophysiology. We then discuss primary and secondary prevention measures and therapeutic management. Recommendations are made regarding each treatment and its role in management. These recommendations are graded based on the quality of supporting evidence and balance between the benefits and risks/burdens for each modality according to methodology stipulated by the American College of Chest Physicians.

Wilderness Medical Society Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2014 Update

To provide guidance to clinicians, the Wilderness Medical Society (WMS) convened an expert panel to develop evidence-based guidelines for the out-of-hospital evaluation and treatment of victims of accidental hypothermia. The guidelines present the main diagnostic and therapeutic modalities and provide recommendations for the management of hypothermic patients. The panel graded the recommendations based on the quality of supporting evidence and the balance between benefits and risks/burdens according the criteria published by the American College of Chest Physicians. The guidelines also provide suggested general approaches to the evaluation and treatment of accidental hypothermia that incorporate specific recommendations.

Prediction of shivering heat production from core and mean skin temperatures

Abstract Prediction formulae of shivering metabolism (Mshiv) are critical to the development of models of thermoregulation for cold exposure, especially when the extrapolation of survival times is required. Many such formulae, however, have been calibrated with data that are limited in their range of core temperatures (Tc), seldom involving values of less than 36°C. Certain recent studies of cold-water immersion have reported Tc as low as 33.25°C. These data comprise measurements of Tc (esophageal) and mean skin temperature (T¯s), and metabolism from 14 males [mean (SD); age = 28 (5) years; height = 1.78 (0.06) m; body mass = 77.7 (6.9) kg; body fat (BF) = 18.4 (4.5)%] during immersion in water as cold as 8°C for up to 1 h and subsequent self-rewarming via shivering under dry blanketed conditions. The data contain 3343 observations with mean (SD) Tc and T¯s of 35.92 (0.93)°C and 23.4 (8.9)°C, respectively, and have been used to re-examine the prediction of Mshiv. Rates of changes of these temperatures were not used in the analysis. The best fit of the formulae, which are essentially algebraic constructs with and without setpoints, are those with a quadratic expression involving T¯s. This is consistent with the findings of Benzinger (1969) who demonstrated that the thermosensitivity of skin is parabolic downwards with temperature peaking near a value of 20°C. Formulae that included a multiplicative interaction term between Tc and T¯s did not predict as well. The best prediction using 37°C and 33°C as the Tc and Ts setpoints, respectively, was found with BF as an attenuation factor: Mshiv (W · m−2) = [155.5 · (37 − Tc) + 47.0 · (33 − T¯s) − 1.57 · (33 − T¯s)2] (%BF)0.5.

Prehospital treatment of hypothermia

This article considers several issues regarding cold stress, development of hypothermia, and prehospital care of the hypothermic patient. Advice is given on the use of clinical impressions and functional characteristics to determine the level of hypothermia. Response to cold water immersion is characterized as short-term (cold shock response), midterm (loss of performance), and long-term (development of hypothermia). Circum-rescue collapse is the dramatic worsening condition of the patient just before, during, or after rescue from cold stress. After rescue, the treatment priorities are to arrest the fall in core temperature, establish a steady, safe rewarming rate while maintaining the stability of the cardiorespiratory system, and provide sufficient physiological support.

Clonidine decreases vasoconstriction and shivering thresholds, without affecting the sweating threshold

PurposeThis study was conducted to test the hypothesis that clonidine produces a dose-dependent increase in the sweating threshold and dose-dependent decreases in vasoconstriction and shivering thresholds.MethodsSix healthy subjects (two female) were studied on four days after taking clonidine in oral doses of either 0 (control). 3. 6 or 9 μg · kg. The order followed a balanced design in a double-blind fashion. Oesophageal temperature and mean skin temperature (from 12 sites) were measured. Subjects were seated in 37°C water which was gradually warmed until sweating occurred (sweat rate increased above 50 g · m 2 · h−1). The water was then cooled gradually until thresholds for vasoconstnction (onset of sustained decrease in fingertip blood flow) and shivering (sustained elevation m metabolism) were determined. Thresholds were then referred to as the core temperature, adjusted to a designated mean skin temperature of 33°C.ResultsHigh dose clonidine similarly decreased the adjusted core temperature thresholds for vasoconstriction by 1. 16 ± 0.30°C and for shivenng by 1.63 ± 0.23°C (P< 0.01). The dose response effects were linear for both cold responses with vasoconstriction and shivenng thresholds decreasing by 0.13 ± 0.05 and 0.19 ± 0.09°C · μg−1 respectively (P < 0.0001). The sweating threshold was unaffected by clonidine, however the interthreshold range between sweating and vasoconstnction thresholds increased from control (0.19 ± 0.48°C) to high dose donidine (1.31 ±0.54°C).ConclusionThe decreases in core temperature thresholds for cold responses and increased interthreshold range are consistent with the effects of several anaesthetic agents and opioids and is indicative of central thermoregulatory inhibition.RésuméObjectifVérifier si la clonidine provoque une augmentation du seuil de sudation et une diminution des seuls de vasoconstnction et de fnsson proportionnellement à la dose.MéthodesSix patients en bonne santé (dont deux femmes) qui avaient reçu des doses orales de clonidine de 0 (contrôle). 3. 6 et 9 μg· kg ont été étudiés pendant quatre jours. Létude suivait un plan équilibré et en double insu. Les températures moyennes oesophagiennes et cutanées (à 12 endroits) ont été mesurées. Les sujets étaient assis dans l’eau à 37°C réchauffée graduellement jusqu’à l’apparition de la sudation (un taux de sudation à50 g · m2 · h−1). Leau était par la suite refroidie progressivement jusqu’aux seuils de vasoconstnction (début de la diminution du flux sanguin à l’extrémité des doigts) et de frissonnement. Ces seuils ont été reconnus comme la température centrale ajustée à une température cutanée désignée de 33°C.RésultatsLes hautes doses de clonodine abaissent également les seuils de vasoconstriction ajustés à la température centrale de 1. 16 ± 0.30°C et du frissonnement de 1,63 ± 0.23C (P < 0,01). Les effets dose-réponse sont linéaires pour les deux réponses au froid avec des seuils de vasoconstnction et de frissonnement diminuant respectivement de 0,13 ± 0,05 et de 0.19 ± 0.09°C μg−1 (P < 0,0001). Le seuil de sudation n’est pas affecté par le donodine; toutefois l’écart entre les seuils de sudation et de vasoconstriction s’élargit entre le contrôle (0,19 ± 0,48°C) et la clonodine à haute dose (1.31 ± 0.54°C).ConclusionLa baisse des seuils de la température centrale pour les réponses au froid et l’augmentation de l’écart entre les seuils sont consistants avec les effets de plusieurs agents anesthésiques et morphmiques et démontrent une inhibition de la thermorégulation centrale.

Treatment of mild immersion hypothermia by forced-air warming.

Forced-air warming is used for prevention or reversal of hypothermia in surgical patients. In the present study, the efficacy of this system for treatment of immersion hypothermia was evaluated. Six men and two women were twice immersed in 8 degrees C water until hypothermic. They were then rewarmed by either: 1) shivering-only inside a sleeping bag; or 2) forced-air warming. Esophageal and skin temperature, cutaneous heat flux and metabolism were measured. Afterdrop (+/- SD) during forced-air warming (0.43 +/- 0.26 degrees C) was approximately 30% less than during shivering (0.61 +/- 0.26 degrees C) (p < 0.001). Rewarming rate during forced-air warming (3.26 +/- 1.8 degrees C.h-1) was not significantly different from shivering (3.02 +/- 1.2 degrees C.h-1). Skin temperature was higher during forced-air warming by 3.7 degrees C early and 4.5 degrees C after 35 min of warming. Heat production increased by 77 W over the initial 20 min of shivering, and subsequently declined, compared to an immediate decrease with forced-air warming. During shivering heat flux ranged from 30 W early in rewarming, to 50 W after 35 min, compared to -237 W and -163 W respectively, for forced-air warming. Forced-air warming attenuated afterdrop and the metabolic stress of shivering while maintaining an average rate of rewarming comparable to shivering. Forced-air warming is a safe, simple, noninvasive treatment and could be used effectively in an emergency medical facility, and possibly in some rescue/emergency vehicles or marine vessels.

Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Frostbite: 2014 Update

The Wilderness Medical Society convened an expert panel to develop a set of evidence-based guidelines for the prevention and treatment of frostbite. We present a review of pertinent pathophysiology. We then discuss primary and secondary prevention measures and therapeutic management. Recommendations are made regarding each treatment and its role in management. These recommendations are graded on the basis of the quality of supporting evidence and balance between the benefits and risks or burdens for each modality according to methodology stipulated by the American College of Chest Physicians. This is an updated version of the original guidelines published in Wilderness & Environmental Medicine 2011;22(2):156-166.

Recovery of a 62-year-old Man From Prolonged Cold Water Submersion

Recovery from prolonged cold water submersion is well documented in children but rare in adults. In the few adult cases reported, significant body cooling occurred (rectal temperature ranging from 22 degrees to 32 degrees C) and the victims were relatively young (< 40 years). We report a case of a 62-year-old man who was submersed in 2 degrees to 3 degrees C water for 15 minutes (time from initial submersion to intubation = 22 minutes). At the time of rescue, he had no vital signs, received prehospital Advanced Life Support, and was transported to hospital. On arrival at hospital, the patient remained in full cardiopulmonary arrest with an agonal ECG rhythm and had an initial pH of 6.77. Initial rectal temperature was near normal (36 degrees C) but subsequently dropped to 33 degrees C. The patient was resuscitated, rewarmed by forced-air warming, and treated for acute myocardial infarction, pulmonary edema, and generalized seizures. He was discharged after 27 days with minor neurologic abnormalities. Given the near-normal initial rectal temperature, preferential brain cooling may have been at least partially responsible for the positive neurologic outcome.