Gerald K. Bristow

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.

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.

Postoperative neuroleptic malignant syndrome: a case report

RésuméOn présents un cas de syndrome neuroleptique malin en période postopératoire. Un homme de 23 ans en ban état général a subi, sans incident une capsulolomie ďune épaule sous anesthésie au fentanyl, à ľhalothane, au protoxyde ďazote et àoxygène. Afin deprévenir une nausée postopératoire, on lui a donné 5 mg de dropéridol IV et 10 mg de métocolopramide IV en période intraopératoire. En période postopératoire, une instabilité autonome, une fièvre et une rigidité musculaire généralisée se manifesto chez le patient. Son niveau de conscience s’affaissa. Ces constatations étaient compatibles avec le diagnostic ďun syndrome neuroleptique malin. Le traitement de support pour le patient comprenait des mesures actives de refroidissement, de la relaxation musculaire et de la ventilation mécanique. On discute de ľhabileté des agents antidopaminergiques, y compris le métoclopramide et le droperidol, a précipiter le syndrome neuroleptique malin. On discute brièvement du traitement du syndrome neuroleptique malin.

Alcohol lowers the vasoconstriction threshold in humans without affecting core cooling rate during mild cold exposure

Elevated blood alcohol levels are often seen in hypothermia and hyperthermia related deaths, leading to the belief that alcohol renders humans poikilothermic. We examined the core temperature (TCO) thresholds for sweating, vasoconstriction and shivering as well as core cooling rates of seven subjects immersed in 28 °C water. On two separate days, subjects exercised on an underwater cycle ergometer to elevate TCO above the sweating threshold. They then rested and cooled until they shivered vigorously. Subjects drank orange juice (7 ml·kg−1) prior to immersion during the control trial and 1 ml·kg−1 absolute ethanol, added to orange juice in a 1:6 ratio, during the alcohol trial. Mean blood alcohol concentration (breath analysis) was 0.097 ± 0.010 g% at the start of cooling and 0.077 ± 0.008 g% at the end of the cooling period. Alcohol lowered the vasoconstriction threshold by 0.32 ± 0.2 °C and elevated finger tip blood flow, but had no effect on thresholds for sweating and shivering or core cooling rate. Considering these minor effects it is unlikely that moderate alcohol consumption predisposes individuals to hypothermia or hyperthermia via impaired thermoregulation, but rather likely due to behavioral factors.

Field Torso-Warming Modalities: A Comparative Study Using a Human Model

Abstrast Objective. To compare four field-appropriate torso-warming modalities that do not require alternating-current (AC) electrical power, using a human model of nonshivering hypothermia. Methods. Five subjects, serving as their own controls, were cooled four times in 8°C water for 10–30 minutes. Shivering was inhibited by buspirone (30 mg) taken orally prior to cooling and intravenous (IV) meperidine (1.25 mg/kg) at the end of immersion. Subjects were hoisted out of the water, dried, and insulated and then underwent 120 minutes of one of the following: spontaneous warming only; a charcoal heater on the chest; two flexible hot-water bags (total 4 liters of water at 55°C, replenished every 20 minutes) applied to the chest and upper back; or two chemical heating pads applied to the chest and upper back. Supplemental meperidine (maximum cumulative dose of 3.5 mg/kg) was administered as required to inhibit shivering. Results. The postcooling afterdrop (i.e., the continued decrease in body core temperature during the early period of warming), compared with spontaneous warming (2.2°C), was less for the chemical heating pads (1.5°C) and the hot-water bags (1.6°C, p < 0.05) and was 1.8°C for the charcoal heater. Subsequent core rewarming rates for the hot-water bags (0.7°C/h) and the charcoal heater (0.6°C/h) tended to be higher than that for the chemical heating pads (0.2°C/h) and were significantly higher than that for spontaneous warming rate (0.1°C/h, p < 0.05). Conclusion. In subjects with shivering suppressed, greater sources of external heat were effective in attenuating core temperature afterdrop, whereas sustained sources of external heat effectively established core rewarming. Depending on the scenario and available resources, we recommend the use of charcoal heaters, chemical heating pads, or hot-water bags as effective means for treating cold patients in the field or during transport to definitive care.