Vasoconstriction Threshold Research Articles

Comparison of Core Body Temperatures in Patients Administered Remimazolam or Propofol during Robotic-Assisted and Laparoscopic Radical Prostatectomy.

Background and Objectives: Different types of anesthetics affect thermoregulatory mechanisms, such as the redistribution of body temperature, loss of skin heat, or inhibition of thermoregulatory vasoconstriction. Therefore, we compared remimazolam with propofol in terms of core body temperature in patients undergoing robotic-assisted and laparoscopic radical prostatectomy. Materials and methods: Ninety patients were randomly assigned to either the propofol–remifentanil (PR) group or the remimazolam–remifentanil (RR) group. The PR group (n = 45) received effect-site concentrations of 6.0 μg/mL of propofol and 4 ng/mL of remifentanil, followed by 0.9 mg/kg of 1% rocuronium and maintenance with effect-site concentrations of 2–4 μg/mL of propofol and 3 ng/mL of remifentanil. The RR group (n = 45) received remimazolam 6 mg/kg/h by continuous intravenous infusion and the effect-site concentration of 4 ng/mL of remifentanil, followed by 0.9 mg/kg of 1% rocuronium, remimazolam 1–3 mg/kg/h, and remifentanil 3 ng/mL. The primary outcome was core body temperature, and secondary outcomes included vasoconstriction threshold (°C) and time to onset of vasoconstriction (min). Results: The core body temperature in the RR group was significantly higher at 60, 80, 100, 120, 140, 160, and 180 min after induction than in the PR group (p < 0.01). The vasoconstriction threshold was significantly higher in the RR group (35.2 ± 0.4) than in the PR group (34.8 ± 0.3) (p < 0.01). The time to onset of vasoconstriction was significantly less in the RR group (150.5 ± 10.2) than in the PR group (158.5 ± 8.4) (p < 0.01). However, the incidence of intraoperative hypothermia was not significant between two groups. Conclusions: Remimazolam appears to reduce vasoconstriction threshold less than and had a faster onset of vasoconstriction, resulting in superior thermoregulatory control.

A Comparison of the Effects of Lung Protective Ventilation and Conventional Ventilation on Thermoregulation During Anaesthesia.

During prolonged surgery, hypothermia is an unwanted condition that frequently develops and increases complication rates. It has been shown that positive end-expiratory pressure (PEEP) during mechanical ventilation reduces hypothermia development by providing earlier peripheral vasoconstriction. In the present study, an investigation was made of the effect of two different ventilation models on perioperative hypothermia development. A total of 40 patients undergoing elective lumbar disc surgery were randomised to either the conventional group (Group C, n=20, tidal volume=10 mL kg-1, PEEP=0 cm H2O) or the lung protective ventilation group (Group P, n=20, tidal volume=6 mL kg-1, PEEP=5 cm H2O). Demographic data on gender, age, weight, height, preoperative-postoperative temperatures and haemodynamic values were recorded. The point where the forearm to fingertip skin temperature difference reached 0°C was determined as the peripheral vasoconstriction development. At this point, the core temperature was recorded as the thermoregulatory vasoconstriction threshold. Demographic characteristics of the patients and haemodynamic variables were similar between the groups. Preoperative and postoperative temperature gradients were not significantly different between the two groups (p=0.827). There was also no significant difference between the two groups in respect of the vasoconstriction threshold of the patients (p=0.432). The study results showed that lung protective ventilation has no advantage in preserving the perioperative core temperature compared to conventional ventilation.

Chapter 37 - Body temperature regulation and anesthesia

General anesthesia is the induction and maintenance of a state of unconsciousness with the absence of pain sensation. General anesthesia is accomplished by the administration of a combination of inhaled anesthetic gases and intravenous drugs. These medications eliminate behavioral thermoregulatory compensations, leaving only autonomic defenses to offset environmental perturbations. Anesthetics inhibit thermoregulatory control in a dose-dependent fashion over the entire clinical range. Impairment of thermoregulatory control is observed by a change in thermoregulatory thresholds with the vasoconstriction threshold being affected about three times as much as the sweating threshold. Consequently, the zone between sweating and vasoconstriction thresholds, called interthreshold range, is widened dose-dependently. Impairment of thermoregulation, triggered by general anesthesia, typically causes inadvertent hypothermia. In febrile patients, general anesthesia reduces the magnitude of perioperative fever.

The effect of body mass index on perioperative thermoregulation.

PurposeWe evaluated the effects of body mass index (BMI) on thermoregulation in obese patients scheduled to undergo laparoscopic abdominal surgery.MethodsSixty patients scheduled to undergo laparoscopic abdominal surgery with no pre-medication were included in the study. The patients were classified into 4 groups according to BMI <24.9, 25–39.9, 40–49.9, and >50. Anesthesia was provided with routine techniques. Tympanic and peripheral temperatures were recorded every 5 minutes starting with the induction of anesthesia. The mean skin temperature (MST), mean body temperature (MBT), vasoconstriction time, and vasoconstriction threshold that triggers core warming were calculated with the following formulas: MST = 0.3 (Tchest + Tarm) + 0.2 (Tthigh + Tcalf). MBT was calculated using the equation 0.64Tcore+0.36Tskin, and vasoconstriction was determined by calculating Tforearm−Tfinger.ResultsThere was no significant difference between the groups in terms of age, gender, duration of operation, and room temperature. Compared to those with BMI <24.9, the tympanic temperature was significantly higher in those with BMI =25–39.9 in the 10th, 15th, 20th, and 50th minutes. In addition, BMI =40–49.9 in the 5th, 10th, 15th, 20th, 25th, 30th, 40th, 45th, 50th, and 55th minutes and BMI >50 in the 5th, 10th, 15th, 20th, 25th, 30th, 50th, and 55th minutes were less than those with BMI <24.9 (P<0.05). There was no significant difference in terms of MST and MBT. Vasoconstriction occurred later, and that vasoconstriction threshold was significantly higher in patients with higher BMIs.ConclusionUnder anesthesia, the core temperature was protected more easily in obese patients as compared to nonobese patients. Therefore, obesity decreases the negative effects of anesthesia on thermoregulation.

How to get away from the cold: body temperature during surgery.

How to get away from the cold: body temperature during surgery.

Effect on thermoregulatory responses in patients undergoing a tympanoplasty in accordance to the anesthetic techniques during PEEP: a comparison between inhalation anesthesia with desflurane and TIVA.

BackgroundIt has been known that positive end-expiratory pressure (PEEP) increases the vasoconstriction threshold by baroreceptor unloading. We compared the effect on the thermoregulatory responses according to anesthetic techniques between an inhalation anesthesia with desflurane and a total intravenous anesthesia (TIVA) with propofol and reminfentanil when PEEP was applied in patients undergoing tympanoplasty.MethodsForty-six patients with a scheduled tympanoplasty were enrolled and the patients were divided in two study groups. Desflurane was used as an inhalation anesthetic in group 1 (n = 22), while TIVA with propofol and remifentanil was used in group 2 (n = 24). PEEP was applied by 5 cmH2O in both groups and an ambient temperature was maintained at 22-24℃ during surgery. The core temperature and the difference of skin temperature between forearm and fingertip were monitored for about 180 minutes before and after the induction of general anesthesia.ResultsThe final core temperature was significantly higher in group 2 (35.4 ± 0.7℃) than in group 1 (34.9 ± 0.5℃). Peripheral thermoregulatory vasoconstriction was found in 5 subjects (23%) in group 1 and in 21 subjects (88%) in group 2. The time taken for reaching the thermoregulatory vasoconstriction threshold was 151.4 ± 19.7 minutes in group 1 and 88.9 ± 14.4 minutes in group 2.ConclusionsWhen PEEP will be applied, anesthesia with TIVA may have more advantages in core temperature preservation than an inhalation anesthesia with desflurane.

Effects of preoperative oral carbohydrate solution intake on thermoregulation

BackgroundWe aimed to investigate the oral carbohydrate solution administered preoperatively on thermoregulation.Material/MethodsThe study included 40 female patients under general anesthesia. Patients were randomly divided into 2 groups: Group CONT (stopped oral implementation 8 h before the operation) and Group CHO (800ml oral carbohydrate fluid 8 h before the operation and 400ml oral carbohydrate fluid 2 h before the operation). Patients were monitored as standard and temperature probes were placed. Temperatures were recorded immediately before anesthetics induction, 5 min after the anesthetics induction, and in the post-anesthesia care unit (PACU) every 10 min. Mean skin temperature (Tsk), mean body temperature (Tb), and vasoconstriction threshold were estimated.ResultsIn general, we observed a decrease in tympanic temperature and Tb following anesthetic administration in groups, and increase in Tsk, and an increase in all 3 of these levels in the recovery unit. Tympanic temperature was significantly higher at 25, 55, 65, and 95 min after induction in Group CONT compared to Group CHO (p<0.05). Tsk was found to be lower in Group CONT compared to Group CHO in almost all periods. In PACU, it was found that the tympanic temperature was higher in Group CONT compared to Group CHO at 60 min (p<0.05). Postoperative shivering score was found to be significantly higher in Group C (p<0.01). Vasoconstriction threshold was higher in Group CONT than Group CHO.ConclusionsOral carbohydrate solution administered was established to have effects thought to be negative on tympanic temperature, vasoconstriction, and vasoconstriction threshold.

Shivering: scores and protocols

Shivering is both an anticipated consequence and, potentially, a major adverse effect of therapeutic hypothermia. Even mild hypothermia can elicit a vigorous thermoregulatory defense to maintain body temperature at the hypothalamic set point. In healthy humans, peripheral vasoconstriction is triggered at 36.5°C and shivering at 35.5°C. Temperature thresholds for vasoconstriction and shivering are often higher than normal in brain-injured patients; therefore, these thermoregulatory defenses may occur more vigorously and at higher temperatures in these individuals. Control of shivering is essential for effective cooling, as shivering fights the cooling process, makes attaining target temperature difficult, is extremely uncomfortable, and can trigger massive increases in systemic and cerebral energy consumption and metabolic demand. The first step in treatment is adequate tools to recognize shivering. The Bedside Shivering Assessment Scale is a simple, validated four-point scale that enables repeated quantification of shivering at the bedside. Therapy for shivering should ideally stop or suppress the central thermoregulatory reflex rather than just uncoupling this response from skeletal muscle contraction, as the latter approach does not mitigate the ongoing cerebral and systemic stress response. Analgo-sedation with opioids, α2-receptor agonists, or propofol is almost always effective as a last resort to prevent shivering. However, nonpharmacological strategies as first-line interventions for shivering minimize the risk of excessive sedation, which can make neurological examination difficult and increase the risk of complications. The Columbia Anti Shivering protocol has been developed with these strategies in mind, and we base our approach on prospectively collected cooling data on 213 patients who underwent 1,388 patient-days of temperature modulation. Eighty-nine patients underwent hypothermia and 124 patients underwent induced normothermia. In 18% of patients and 33% of the total patient-days, only none-sedating baseline interventions were needed. The first agent used was most commonly dexmeditomidine half the time, followed by opiates and increased doses of propofol. Younger patients, men, and lower body surface area were factors associated with increased number of anti-shivering interventions. As noted by this protocol, a significant proportion of patients undergoing temperature modulation can be effectively treated for shivering without oversedation and paralysis. Patients at higher risk for needing more interventions are younger men with decreased body surface area.

Effects of PEEP on the thermoregulatory responses during TIVA in patients undergoing tympanoplasty.

BackgroundDuring general anesthesia, core temperature decreases, largely due to heat loss caused by peripheral vasodilation, resulting in heat redistribution to peripheral tissues. Multiple factors contribute to body temperature regulation during general anesthesia. It was reported that baroreceptor unloading by positive end-expiratory pressure (PEEP) attenuates anesthetically-induced hypothermia. So, we evaluated the effects of PEEP on thermoregulatory responses during total intravenous anesthesia (TIVA).MethodsForty healthy patients scheduled for tympanoplasty were allocated two groups, Group ZEEP (zero end-expiratory pressure, n = 20) and Group PEEP (PEEP application of 5 cmH2O, n = 20). Ambient temperature was maintained at 22-24℃, and anesthesia was induced and maintained with propofol-remifentanil. The core temperature and the temperature difference between forearm and fingertip skin were monitored before and after the induction of general anesthesia having a duration of 180 minutes.ResultsThe core temperature gradient (Ti-Tf) was higher in patients with ZEEP than with PEEP. The core temperature was maintained at a higher level in patients with PEEP. Additionally, the vasoconstriction threshold was higher in patients with PEEP.ConclusionsIt seems that PEEP attenuates anesthetically-induced hypothermia during TIVA.

Measurement of model coefficients of skin sympathetic vasoconstriction

Many researchers have already attempted to model vasoconstriction responses, commonly using the mathematical representation proposed by Stolwijk (1971 NASA Contractor Report CR-1855 (Washington, DC: NASA)). Model makers based the parameter values in this formulation either on estimations or by attributing the difference between their passive models and measurement data fully to thermoregulation. These methods are very sensitive to errors. This study aims to present a reliable method for determining physiological values in the vasoconstriction formulation. An experimental protocol was developed that enabled us to derive the local proportional amplification coefficients of the toe, leg and arm and the transient vasoconstrictor tone. Ten subjects participated in a cooling experiment. During the experiment, core temperature, skin temperature, skin perfusion, forearm blood flow and heart rate variability were measured. The contributions to the normalized amplification coefficient for vasoconstriction of the toe, leg and arm were 84%, 11% and 5%, respectively. Comparison with relative values in the literature showed that the estimated values of Stolwijk and the values mentioned by Tanabe et al (2002 Energy Build. 34 637–46) were comparable with our measured values, but the values of Gordon (1974 The response of a human temperature regulatory system model in the cold PhD Thesis University of California, Santa Barbara) and Fiala et al (2001 Int. J. Biometeorol. 45 143159) differed significantly. With the help of regression analysis a relation was formulated between the error signal of the standardized core temperature and the vasoconstrictor tone. This relation was formulated in a general applicable way, which means that it can be used for situations where vasoconstriction thresholds are shifted, like under anesthesia or during motion sickness.

Short of air? Cool it!

Short of air? Cool it!

Thermoregulatory defense mechanisms

Core body temperature is normally tightly regulated by an effective thermoregulatory system. Thermoregulatory control is sometimes impaired by serious illness, but more typically remains intact. The primary autonomic defenses against heat are sweating and active precapillary vasodilation; the primary autonomic defenses against cold are arteriovenous shunt vasoconstriction and shivering. The core temperature triggering each response defines its activation threshold. Temperatures between the sweating and vasoconstriction thresholds define the inter-threshold range. The shivering threshold is usually a full 1 degrees C below the vasoconstriction threshold and is therefore a "last resort" response. Both vasoconstriction and shivering are associated with autonomic and hemodynamic activation; and each response is effective, thus impeding induction of therapeutic hypothermia. It is thus helpful to accompany core cooling with drugs that pharmacologically induce a degree of thermal tolerance. No perfect drug or drug combination has been identified. Anesthetics, for example, induce considerable tolerance, but are rarely suitable. Meperidine-especially in combination with buspirone-is especially effective while provoking only modest toxicity. The combination of buspirone and dexmedetomidine is comparably effective while avoiding the respiratory depression association with opioid administration.

Thermoregulatory responses of sevoflurane, desflurane, and isoflurane during gynecologic laparoscopic surgery.

Core temperature decreases rapidly after the induction of general anesthesia, because the heat is redistributed to peripheral tissues. Thermoregulatory responses of volatile anesthetics have been tested, but their effects have not been directly compared. Therefore, we evaluated the thermoregulatory responses to sevoflurane, desflurane, and isoflurane. Sixty healthy patients scheduled for laparoscopic myomectomy or radical hysterectomy were allocated into three groups; Group S (sevoflurane, n = 20), Group D (desflurane, n = 20), and Group I (isoflurane, n = 20). Anesthesia was maintained with 1 minimum alveolar concentration (MAC) of sevoflurane, desflurane, and isoflurane in a 50/50 mixture of N2O/O2. Patients were maintained in a normovolemic and normocapnic state. The core temperature and forearm minus fingertip skin-temperature gradient (an index of peripheral vasoconstriction) were monitored after the induction of general anesthesia. Each of the seven patients given sevoflurane, desflurane, and isoflurane vasoconstricted at a core temperature of 35.3 +/- 0.5degrees C, 33.6 +/- 0.4degrees C, and 35.2 +/- 0.4degrees C, respectively. The vasoconstriction threshold was the lowest in patients anesthetized with desflurane. The core temperature gradient (Ti-Tf) was significantly higher in patients that were anesthetized with desflurane than in those that were anesthetized with sevoflurane or isoflurane. The core temperature of desflurane was significantly lower than that of sevoflurane or isoflurane 15 minutes after the induction of anesthesia until 180 minutes of anesthesia. These results indicate that the core temperature is maintained at a higher level in patients that have been anesthetized with sevoflurane or isoflurane than in those that have been anesthetized with desflurane.

Pulsed electrical stimulation for control of vasculature: Temporary vasoconstriction and permanent thrombosis

A variety of medical procedures is aimed to selectively compromise or destroy vascular function. Such procedures include cancer therapies, treatments of cutaneous vascular disorders, and temporary hemostasis during surgery. Currently, technologies such as lasers, cryosurgery and radio frequency coagulation, produce significant collateral damage due to the thermal nature of these interactions and corresponding heat exchange with surrounding tissues. We describe a non-thermal method of inducing temporary vasoconstriction and permanent thrombosis using short pulse (microseconds) electrical stimulation. The current density required for vasoconstriction increases with decreasing pulse duration approximately as t(-0.25). The threshold of electroporation has a steeper dependence on pulse duration-exceeding t(-0.5). At pulse durations shorter than 5 micros, damage threshold exceeds the vasoconstriction threshold, thus allowing for temporary hemostasis without direct damage to surrounding tissue. With a pulse repetition rate of 0.1 Hz, vasoconstriction is achieved approximately 1 min after the beginning of treatment in both arteries and veins. Thrombosis occurs at higher electric fields, and its threshold increases with vessel diameter. Histology demonstrated a lack of tissue damage during vasoconstriction, but vascular endothelium was damaged during thrombosis. The temperature increase does not exceed 0.1 degrees C during these treatments.

Fructose Administration Increases Intraoperative Core Temperature by Augmenting Both Metabolic Rate and the Vasoconstriction Threshold

Fructose Administration Increases Intraoperative Core Temperature by Augmenting Both Metabolic Rate and the Vasoconstriction Threshold

Fructose Administration Increases Intraoperative Core Temperature by Augmenting Both Metabolic Rate and the Vasoconstriction Threshold

The authors tested the hypothesis that intravenous fructose ameliorates intraoperative hypothermia both by increasing metabolic rate and the vasoconstriction threshold (triggering core temperature). Forty patients scheduled to undergo open abdominal surgery were divided into two equal groups and randomly assigned to intravenous fructose infusion (0.5 g . kg(-1) . h(-1) for 4 h, starting 3 h before induction of anesthesia and continuing for 4 h) or an equal volume of saline. Each treatment group was subdivided: Esophageal core temperature, thermoregulatory vasoconstriction, and plasma concentrations were determined in half, and oxygen consumption was determined in the remainder. Patients were monitored for 3 h after induction of anesthesia. Patient characteristics, anesthetic management, and circulatory data were similar in the four groups. Mean final core temperature (3 h after induction of anesthesia) was 35.7 degrees +/- 0.4 degrees C (mean +/- SD) in the fructose group and 35.1 degrees +/- 0.4 degrees C in the saline group (P = 0.001). The vasoconstriction threshold was greater in the fructose group (36.2 degrees +/- 0.3 degrees C) than in the saline group (35.6 degrees +/- 0.3 degrees C; P < 0.001). Oxygen consumption immediately before anesthesia induction in the fructose group (214 +/- 18 ml/min) was significantly greater than in the saline group (181 +/- 8 ml/min; P < 0.001). Oxygen consumption was 4.0 l greater in the fructose patients during 3 h of anesthesia; the predicted difference in mean body temperature based only on the difference in metabolic rates was thus only 0.4 degrees C. Epinephrine, norepinephrine, and angiotensin II concentrations and plasma renin activity were similar in each treatment group. Preoperative fructose infusion helped to maintain normothermia by augmenting both metabolic heat production and increasing the vasoconstriction threshold.

Fructose maintains normothermia during anaesthesia by augmenting metabolic rate and vasoconstriction threshold,

Fructose maintains normothermia during anaesthesia by augmenting metabolic rate and vasoconstriction threshold,

Intraoperative Thermoregulation in Laparoscopic Gynecologic Surgery with Conventional and Low Insufflation: A Comparison with Open Surgery

Background: It has been suspected that laparoscopic surgery exacerbates hypothermia to a greater extent than open surgery. Thus, this study was designed to compare the intraoperative thermoregulatory profiles of three different operative techniques: open surgery, low pressure (LP: 8 mmHg) or conventional pressure (CP: 13 mmHg) laparoscopic surgery. Methods: Forty five patients who were scheduled for radical hysterectomy were allocated to three groups, 15 in each group: group O (open surgery), group LP and group CP. Anesthesia was maintained with 2.5% sevoflurane. Intraoperative core temperature and forearm minus fingertip skin temperature gradients were measured at 15-min intervals during the first three hours. Vasoconstriction threshold was defined by the esophageal temperature at which the skin temperature gradient equalled . Results: All groups were comparable in terms of the characteristics of patients and preoperative body temperatures. Core temperatures and forearm minus fingertip skin temperature gradients were not significantly different among the three groups at all measurements. Thermoregulatory vasoconstrictions were observed in 6 of group O and 6 of laparoscopic surgical patients (4 patients from group LP and 2 patients from group CP). These 12 patients were divided into open (n = 6) and laparoscopic (n = 6) surgery group. There were no significant difference between the groups with regard to the vasoconstriction threshold and threshold time. Conclusions: Laparoscopic procedures with conventional insufflation pressure have similar profiles in terms of intraoperative thermoregulation, when compared to open surgery. Lowering insufflation pressure to 8 mmHg can not reduce the risk of intraoperative hypothermia.

Doxapram Only Slightly Reduces the Shivering Threshold in Healthy Volunteers

We determined the effects of doxapram on the major autonomic thermoregulatory responses in humans. Nine healthy volunteers were studied on 2 days: control and doxapram (IV infusion to a plasma concentration of 2.4 +/- 0.8, 2.5 +/- 0.9, and 2.6 +/- 1.1 microg/mL at the sweating, vasoconstriction, and shivering thresholds, respectively). Each day, skin and core temperatures were increased to provoke sweating, then reduced to elicit peripheral vasoconstriction and shivering. We determined the sweating, vasoconstriction, and shivering thresholds with compensation for changes in skin temperature. Data were analyzed with paired t-tests and presented as mean +/- sd; P < 0.05 was considered statistically significant. Doxapram did not change the sweating (control: 37.5 degrees +/- 0.4 degrees C, doxapram: 37.3 degrees +/- 0.4 degrees C; P = 0.290) or the vasoconstriction threshold (36.8 degrees +/- 0.7 degrees C versus 36.4 degrees +/- 0.5 degrees C; P = 0.110). However, it significantly reduced the shivering threshold from 36.2 degrees +/- 0.5 degrees C to 35.7 degrees +/- 0.7 degrees C (P = 0.012). No sedation or symptoms of panic were observed on either study day. The observed reduction in the shivering threshold explains the drug's efficacy for treatment of postoperative shivering; however, a reduction of only 0.5 degrees C is unlikely to markedly facilitate induction of therapeutic hypothermia as a sole drug.

Clonidine produces a dose-dependent impairment of baroreflex-mediated thermoregulatory responses to positive end-expiratory pressure in anaesthetized humans

Perioperative hypothermia is common and results from anaesthesia-induced inhibition of thermoregulatory control. Hypothermia is blunted by baroreceptor unloading caused by positive end-expiratory pressure (PEEP), and is mediated by an increase in the vasoconstriction threshold. Premedication with clonidine impairs normal thermoregulatory control. We therefore determined the effect of clonidine on PEEP-induced hypothermia protection. Core temperature was evaluated in patients undergoing combined general and epidural anaesthesia for lower abdominal surgery. They were assigned to an end-expiratory pressure of zero (ZEEP) or 10 cm H(2)O PEEP. The PEEP group was divided into three blinded subgroups that received placebo (Cl-0), clonidine 150 microg (Cl-150) and clonidine 300 microg (Cl-300) respectively. Placebo or clonidine was given orally 30 min before surgery. We evaluated core temperature and thermoregulatory vasoconstriction. We also determined plasma epinephrine, norepinephrine, angiotensin II concentrations and plasma renin activity. Core temperature after 180 min of anaesthesia was 35.1 (0.4) degrees C in the ZEEP group. PEEP significantly increased final core temperature to 35.8 (0.5) degrees C (Cl-0 group). Clonidine produced a linear, dose-dependent impairment of PEEP-induced hypothermia protection: final core temperatures were 35.4 (0.3) degrees C in the Cl-150 group and 35.0 (0.6) degrees C in the Cl-300 group. Similarly, clonidine produced a linear and dose-dependent reduction in vasoconstriction threshold: Cl-0, 36.4 (0.3) degrees C; Cl-150, 35.8 (0.3) degrees C; Cl-300, 35.4 (0.6) degrees C. Plasma norepinephrine, angiotensin II concentrations and renin activity were consistent with the thermoregulatory responses. Baroreceptor unloading by PEEP normally moderates perioperative hypothermia. However, clonidine premedication produces a linear, dose-dependent reduction in this benefit.