Prediction of shivering heat production from core and mean skin temperatures

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 degrees C. Certain recent studies of cold-water immersion have reported Tc as low as 33.25 degrees C. These data comprise measurements of Tc (esophageal) and mean skin temperature (Ts), 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 degrees 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 Ts of 35.92 (0.93) degrees C and 23.4 (8.9) degrees 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 Ts. 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 degrees C. Formulae that included a multiplicative interaction term between Tc and Ts did not predict as well. The best prediction using 37 degrees C and 33 degrees C as the Tc and Ts setpoints, respectively, was found with BF as an attenuation factor: Mshiv (W x m(-2)) = [155.5 x (37- Tc) + 47.0 x (33 - Ts) - 1.57 x (33 - Ts)2]/(%BF)(0.5).

Early postoperative complications following neurosurgical procedures

PURPOSE

To assess the incidence and characteristics of early postoperative complications in patients following neurosurgical procedures.

METHODS

All patients undergoing neurosurgery during a four month period were followed postoperatively for up to four hours in the post anesthetic care unit or intensive care unit. Patient information and all complications were documented by the investigators on a standardized form. Complications were classified as respiratory, cardiovascular, nausea and vomiting, shivering and other. Risk factors analyzed for the occurrence of complications included age, sex, ASA status, type of surgery, elective or emergency surgery and postoperative placement.

RESULTS

Four hundred eighty six adult patients were followed, but in 55 patients the trachea remained intubated during the four hour study period and they were eliminated from the analysis of postoperative complications. At least one complication occurred in 54.5% of the remaining 431 patients. Respiratory problems occurred in 2.8%, trauma to the airway in 4.4%, cardiovascular complications in 6.7%, neurological in 5.7% and nausea and/or vomiting in 38%. The highest incidence of patients with complications was during spine (65%) and vascular (66%) surgery, compared with tumour (47%) and other (43%) surgery, P < 0.05. Other risk factors included age < 70 yr for nausea and vomiting (P < 0.02), and elective surgery for spine and vascular surgery (P < 0.001).

CONCLUSION

There was a high incidence of early postoperative complications in neurosurgical patients. The most common problem was nausea and vomiting especially in the younger patient undergoing elective spine surgery.

The implications of hypothermia for early tracheal extubation following cardiac surgery

Thermoregulation is impaired during anesthesia for cardiac surgery. Redistribution of body heat and heat loss to the environment result in mild hypothermia before cardiopulmonary bypass. Maintenance of normothermia, rather than hypothermia, may facilitate early tracheal extubation. Hypothermia alters the distribution and decreases the metabolism of most drugs, including anesthetic drugs and muscle relaxants, thus prolonging recovery. Postoperative shivering increases metabolic rate and potentially leads to myocardial ischemia; prevention is therefore critical to the success of early tracheal extubation after cardiac surgery. Coagulopathies, increased incidence of surgical wound infection, and perioperative cardiac morbidity are other potential risk factors identified in noncardiac patients. Hypothermia, however, does have potential benefits to the patient, including protection from cerebral ischemia and hypoxemia. Mild core hypothermia (approximately 34 degrees C) may represent the optimal balance between risks and benefits for fast-track patients.

Side effect profile and adherence to in the treatment of multiple sclerosis with interferon beta-1a

This study examines the course of patient-reported side effects during the first 4 months of treatment for multiple sclerosis (MS) with interferon beta-1a (IFN beta-1a), and the relationship of those side effects to discontinuation of medication. Flu-like symptoms, muscle aches and chills decreased over the first 2 months of treatment but did not change over the second 2 months. Loss of feeling or numbness, tingling and depression increased over 4 months, however these side effects were generally mild. Loss of feeling or numbness and tingling at 2 month follow-up were significantly related to discontinuation of IFN beta-1a by 4 month follow-up.

Hypoxemia decreases the shivering threshold in rabbits anesthetized with 0.2 minimum alveolar anesthetic concentration isoflurane

Shivering has been proposed as an etiology of postoperative hypoxemia. The difficulty with this theory is that hypoxemia inhibits shivering in unanesthetized cats, rats, and humans. However, anesthesia inhibits many protective reflexes, including the ventilatory response to hypoxemia. We therefore tested the hypothesis that arterial hypoxemia fails to inhibit shivering in lightly anesthetized rabbits. Rabbits were intubated and instrumented during exposure to surgical concentrations of anesthesia, and anesthesia was then maintained with 0.2 minimum alveolar anesthetic concentration isoflurane. The core was cooled at a rate of 2-3 degrees C/h by perfusing water at 10 degrees C through a colonic thermode. Core temperatures were recorded from the distal esophagus. Sustained, vigorous shivering was considered physiologically significant. The core temperature that triggering significant shivering identified the thermoregulatory threshold for this response. Arterial blood was sampled for gas analysis at the shivering threshold in each rabbit. Hypoxemia linearly reduced the shivering threshold from 36.7 degrees C at 130 mm Hg to 35.4 degrees C at 50 mm Hg (threshold = PaO2.0.019 + 34.3; r2 = 0.49). We failed to confirm our hypothesis: instead, even mild hypoxemia reduced the shivering threshold >1 C. A 1 C decrease in the shivering threshold is likely to prevent or stop most postoperative shivering because it exceeds the reduction produced by many effective anti-shivering drugs. These data do not support the theory that shivering causes postoperative hypoxemia.

IMPLICATIONS

Shivering has been proposed as an etiology of postoperative hypoxemia. Our data, in contrast, show that mild hypoxemia inhibits shivering. Shivering is thus unlikely to be a cause of postoperative hypoxemia.

Age-related changes in oxygen and nutrient uptake by hindquarters in newborn pigs during cold-induced shivering

Newborn pigs rely essentially on shivering thermogenesis in the cold. In order to understand the rapid postnatal enhancement of thermogenic capacities in piglets, the oxygen and nutrient uptake of hindquarters was measured in vivo in 1- (n = 6) and 5-day-old (n = 6) animals at thermal neutrality and during cold exposure. The hindquarters were considered to represent a skeletal muscle compartment. Indirect calorimetry and arterio-venous techniques were used. The cold challenge (23 C at 1 day old and 15 C at 5 days old for 90 min) induced a similar increase (+90 %) in regulatory heat production at both ages. Hindquarters blood flow was higher at 5 days than 1 day old at thermal neutrality (26 +/- 3 vs. 17 +/- 1 ml min-1 (100 g hindquarters)-1) and its increase in the cold was much more marked (+65 % at 5 days old vs. +25 % at 1 day old). Oxygen extraction by the hindquarters rose from 30-35 % at thermal neutrality to 65-70 % in the cold at both ages. The calculated contribution of skeletal muscle to total oxygen consumption averaged 34-40 % at thermal neutrality and 50-64 % in the cold and skeletal muscle was the major contributor to regulatory thermogenesis. Based on hindquarters glucose uptake and lactate release, carbohydrate appeared to be an important fuel for shivering. However, net uptake of fatty acids increased progressively during cold exposure at 5 days old. The enhancement in muscular blood supply and fatty acid utilization during shivering is probably related to the postnatal improvement in the thermoregulatory response of the piglet.

Moderate exercise increases postexercise thresholds for vasoconstriction and shivering

The purpose of this study was to evaluate the effect of exercise on the subsequent postexercise thresholds for vasoconstriction and shivering. On two separate days, with six subjects (3 women), a whole body water-perfused suit slowly decreased mean skin temperature (approximately 7.0 degreesC/h) until thresholds for vasoconstriction and shivering were clearly established. Subjects were then rewarmed by increasing water temperature until both esophageal and mean skin temperatures returned to near-baseline values. Subjects either performed 15 min of cycle ergometry (65% maximal O2 consumption) followed by 30 min of recovery (Exercise) or remained seated with no exercise for 45 min (Control). Subjects were then cooled again. We mathematically compensated for changes in skin temperatures by using the established linear cutaneous contribution of skin to the control of vasoconstriction and shivering (20%). The calculated core temperature threshold (at a designated skin temperature of 30.0 degreesC) for vasoconstriction increased significantly from 36.64 +/- 0.20 to 36.89 +/- 0.22 degreesC postexercise (P < 0.01). Similarly, the shivering threshold increased from 35.73 +/- 0.13 to 36.13 +/- 0.12 degreesC postexercise (P < 0.01). In contrast, sequential measurements, without exercise, demonstrate a time-dependent decrease in both the vasoconstriction (0.10 degreesC) and shivering (0.12 degreesC) thresholds. These data indicate that exercise has a prolonged effect by increasing the postexercise thresholds for both cold thermoregulatory responses.

Non-thermoregulatory shivering in patients recovering from isoflurane or desflurane anesthesia

BACKGROUND

Although cold-induced shivering is an obvious source of postanesthetic tremor, other causes may contribute. Consistent with this theory, the authors had previously identified an abnormal clonic component of postoperative shivering and proposed that it might be nonthermoregulatory. A subsequent study, however, failed to identify spontaneous muscular activity in normothermic volunteers. These data suggested that the initial theory was erroneous or that a yet-to-be identified factor associated with surgery might facilitate shivering in patients after operation. Therefore, the authors tested the hypothesis that some postoperative tremor is nonthermoregulatory.

METHODS

One hundred twenty patients undergoing major orthopedic operation were observed. They were grouped randomly to receive maintenance anesthesia with nitrous oxide and isoflurane (0.8 +/- 0.4%) or desflurane (3.4 +/- 1.1%). Twenty patients in each group were allowed to become hypothermic, whereas normal body temperatures were maintained in the others (tympanic membrane temperature exceeding preinduction values). Arteriovenous shunt vasoconstriction was evaluated using forearm-minus-fingertip skin-temperature gradients; gradients less than 0 degrees C identified vasodilation. Postanesthetic shivering was graded by a blinded investigator. Tremor in patients who were normothermic and vasodilated was considered nonthermoregulatory.

RESULTS

Thermoregulatory responses were similar after isoflurane or desflurane anesthesia. Approximately 50% of the unwarmed patients shivered. Shivering was observed in 27% of the patients who were normothermic; 55% of this spontaneous muscular activity occurred in vasodilated patients. Among the normothermic patients, 15% fulfilled the authors' criteria for nonthermoregulatory tremor.

CONCLUSIONS

The incidence of postoperative shivering is inversely related to core temperature. Therefore, it was not surprising that shivering was most common among the hypothermic patients. The major findings, however, were that shivering remained common even among patients who were kept scrupulously normothermic and that many shivered while they were vasodilated. Thus, postoperative patients differ from nonsurgical volunteers in demonstrating a substantial incidence of nonthermoregulatory tremor.

Thermoregulatory responses of the newborn pig during experimentally induced hypothermia and rewarming

Exposure to a temperature of 14 degrees C was used to induce a progressive hypothermia in fourteen conscious newborn piglets. Heat production, body (rectal) and skin (between the shoulders) temperatures and shivering intensity assessed as the electromyographic activity (EMG) of longissimus thoracis muscle were measured until body temperature reached 30 degrees C and during a recovery period of 2 h at an ambient temperature of 24 degrees C (n = 7) or 34 degrees C (n = 7). During body cooling, heat production increased up to 9.67 +/- 1.28 W (kg BW)-1, but started to decrease below a body temperature threshold of 34.4 +/- 0.7 degrees C. EMG activity increased (P < 0.023) curvilinearly during body cooling; the main increase occurred between body temperatures of 38 and 33 degrees C (+142%, P < 0.001), and changes in EMG activity between 33 and 30 degrees C were not significant (+18%, P > 0.1). A marked increase in circulating levels of glucose (+312%, P < 0.001), glucagon (+76%, P < 0.05), adrenaline (+172%, P < 0.05) and noradrenaline (+113%, P < 0.05) occurred during body cooling. Insulin levels were not detectable at 2 h of life and increased during body cooling. During 2 h of rewarming at 24 degrees C, heat production and EMG activity remained elevated, changes in carbohydrate metabolism were not completely reversed and the final body temperature was only 35.6 +/- 0.9 degrees C. Rewarming of the piglets was faster at 34 degrees C. There was a net influx of heat into the animals and heat production and shivering activity decreased when body temperature reached 33.9 +/- 0.5 degrees C; the final body temperature was 37.5 +/- 0.2 degrees C. Circulating levels of lactate, glucagon and catecholamines returned to control levels. These results show that in conscious piglets exposed to a constant cold temperature there is an inverse relationship between EMG activity and body temperature during moderate hypothermia and that the thermoregulatory response and carbohydrate metabolism of the piglet are seriously impaired below a body temperature of 34 degrees C.

The effect of exercise training in cold on shivering and nonshivering thermogenesis in adult and aged C57BL/6J mice

To understand the mechanisms of improvement of cold-induced heat production in aged mice following exercise training, the relative contributions of shivering and nonshivering thermogenesis to cold-induced metabolic responses were assessed in adult and aged C57BL/6J male mice, which inhabited sedentarily at room temperature, or were subjected either to a regimen of moderate intensity exercise training at 6 degrees C, or to sedentary repeated exposures to the same temperature. The main findings were that (1) aged mice had greater cold-induced nonshivering thermogenesis, but lower shivering than adult mice; (2) exercise training in a cold environment enhanced cold-induced nonshivering thermogenesis in adult mice, but suppressed it in aged animals; (3) exercise training in a cold environment increased shivering thermogenesis in both age groups, but this increase was much greater in aged mice; (4) the increase of cold-induced shivering thermogenesis was mainly responsible for increased cold tolerance in aged mice after exercise training in a cold environment.

Thermoregulatory responses to cold transients: effects of menstrual cycle in resting women

Effects of the menstrual cycle on heat loss and heat production (M) and core and skin temperature responses to cold were studied in six unacclimatized female nonsmokers (18-29 yr of age). Each woman, resting supine, was exposed to a cold transient (ambient temperature = mean radiant temperature = 20 to -5 degrees C at -0.32 degrees C/min, relative humidity = 50 +/- 2%, wind speed = 1 m/s) in the follicular (F) phase (days 2-6) and midluteal (L) phase (days 19-23) of her menstrual cycle. Clothed in each of two ensembles with different thermal resistances, women performed multiple experiments in the F and L phases. Thermal resistance was 0.2 and 0.4 m2 . K . W-1 for ensembles A and B, respectively. Esophageal temperature (Tes), mean weighted skin temperature (Tsk), finger temperature (Tfing), and area-weighted heat flux were recorded continuously. Rate of heat debt (-S) and integrated mean body temperature (Tb,i) were calculated by partitional calorimetry throughout the cold ramp. Extensive peripheral vasoconstriction in the F phase during early periods of the ramp elevated Tes above thermoneutral levels. Shivering thermogenesis (DeltaM = M - Mbasal, W /m2) was highly correlated with declines in Tsk and Tfing (P <0.0001). There was a reduced slope in M as a function of Tb,i in the L phase with ensembles A (P < 0.02) and B (P < 0.01). Heat flux was higher and -S was less in the L phases with ensemble A (P < 0.05). An analytic model revealed that Tsk and Tes contribute as additive inputs and Tfing has a multiplicative effect on the total control of DeltaM during cold transients (R2 = 0.9). Endogenous hormonal levels at each menstrual cycle phase, core temperature and Tsk inputs, vascular responses, and variations in body heat balance must be considered in quantifying thermoregulatory responses in women during cold stress.

Preventing hypothermia: convective and intravenous fluid warming versus convective warming alone

STUDY OBJECTIVE

To test the hypothesis that warming intravenous (i.v.) fluids in conjunction with convective warming results in less intraoperative hypothermia (core temperature < 36.0 degrees C) than that seen with convective warming alone.

DESIGN

Prospective, randomized study.

SETTING

University affiliated tertiary care teaching hospital.

PATIENTS

61 ASA physical status, I, II, and III adults undergoing major surgery and general anesthesia with isoflurane.

INTERVENTIONS

All patients received convective warming. Group 1 patients received warmed fluids (setpoint 42 degrees C). Group 2 patients received room temperature fluids (approximately 21 degrees C).

MEASUREMENTS AND MAIN RESULTS

Lowest and final intraoperative distal esophageal temperatures were higher (p < 0.05) in Group 1 (mean +/- SEM: 35.8 +/- 0.1 degrees C and 36.6 +/- 0.1 degrees C) versus Group 2 (35.4 +/- 0.1 degrees C and 36.1 +/- 0.1 degrees C, respectively). Compared with Group 1, more Group 2 patients were hypothermic at the end of anesthesia (10 of 26 patients, or 38.5% vs. 4 of 30 patients, or 13%; p < 0.05). After 30 minutes in the recovery room, there were no differences in temperature between groups (36.7 +/- 0.1 degrees C and 36.5 +/- 0.1 degrees C in Groups 1 and 2, respectively). Intraoperative cessation of convective warming because of core temperature greater than 37 degrees C was required in 33% of Group 1 patients (vs. 11.5% in Group 2; p = 0.052).

CONCLUSIONS

The combination of convective and fluid warming was associated with a decreased likelihood of patients leaving the operating room hypothermic. However, average final temperatures were greater than 36 degrees C in both groups, and intergroup differences were small. Care must be taken to avoid overheating the patient when both warming modalities are employed together.

Human thermoregulatory responses during serial cold-water immersions

This study examined whether serial cold-water immersions over a 10-h period would lead to fatigue of shivering and vasoconstriction. Eight men were immersed (2 h) in 20 degrees C water three times (0700, 1100, and 1500) in 1 day (Repeat). This trial was compared with single immersions (Control) conducted at the same times of day. Before Repeat exposures at 1100 and 1500, rewarming was employed to standardize initial rectal temperature. The following observations were made in the Repeat relative to the Control trial: 1) rectal temperature was lower and heat debt was higher (P < 0.05) at 1100; 2) metabolic heat production was lower (P < 0.05) at 1100 and 1500; 3) subjects perceived the Repeat trial as warmer at 1100. These data suggest that repeated cold exposures may impair the ability to maintain normal body temperature because of a blunting of metabolic heat production, perhaps reflecting a fatigue mechanism. An alternative explanation is that shivering habituation develops rapidly during serially repeated cold exposures.

Thermoregulatory effects of spinal and epidural anesthesia during cesarean delivery

BACKGROUND AND OBJECTIVES

Hypothermia is likely to develop faster during spinal anesthesia than epidural anesthesia. A natural consequence of the rapid temperature decrease during spinal anesthesia is that the shivering threshold will be reached sooner and that more shivering will be required to prevent further hypothermia. We tested the hypotheses that the onset of hypothermia is more rapid and the onset and intensity of shivering earlier during spinal than epidural anesthesia.

METHODS

Patients undergoing cesarean delivery were randomly assigned to spinal anesthesia or epidural anesthesia. Spinal anesthesia was induced by injecting 2 mL 0.5% dibucaine into the L4-L5 interspace. Epidural anesthesia was induced with 20 mL 2% mepivacaine injected into the L2-L3 interspace. Thermal comfort and shivering were scored by a blinded observer.

RESULTS

Fifteen patients given each type of anesthesia had upper sensory levels > or =T4 dermatome. Sensation was entirely absent from the leg during spinal anesthesia, but lower block levels were near S5 during epidural anesthesia. Tympanic membrane temperatures initially decreased faster during spinal anesthesia, but subsequently decreased at a rate of 0.5 degrees C/h in both groups. The onset and incidence of shivering (detected qualitatively) did not differ significantly between the two groups, but shivering intensity was significantly reduced during spinal anesthesia. Furthermore, the shivering thresholds were 36.4+/-0.3 degrees C (mean+/-SD) during spinal anesthesia versus 37.1+/-0.4 degrees C in those given epidural anesthesia (P=.006). There were no clinically important differences in thermal comfort with the two kinds of neuraxial anesthesia.

CONCLUSIONS

We failed to confirm our hypothesis, but for an unexpected reason: Thermoregulation was impaired more by spinal anesthesia than epidural anesthesia. It seems likely that in our patients spinal anesthesia inhibited thermoregulatory control more than epidural anesthesia because it better blocked sensory input from the legs.

Antitumor activity of DAB389IL-2 fusion toxin in mycosis fungoides

BACKGROUND

DAB389IL-2 is a novel fusion toxin that retargets the cytotoxic A-chain of diphtheria toxin to interleukin-2 (IL-2) receptor-expressing tumors.

OBJECTIVE

The purpose of this phase I trial was to study the toxicity, maximum tolerated dose, and clinical efficacy of DAB389IL-2 in IL-2 receptor expressing lymphoproliferative malignancies, including cutaneous T-cell lymphoma.

METHODS

DAB389IL-2 was administered intravenously daily for 5 days every 3 weeks. Dose escalation occurred between patient groups. Patients were monitored for laboratory and clinical toxicity, kinetics, immune response, and clinical efficacy.

RESULTS

Thirty-five patients with cutaneous T-cell lymphoma (including 30 patients with mycosis fungoides) were treated. Previously, conventional therapy had not worked for 34 of the patients. Thirteen patients (37%) achieved an objective response, including a complete response in five patients (14%). Complete response was achieved in patients with extensive erythroderma and tumor stage mycosis fungoides. Adverse events consisted of reversible fever/chills, hypotension, nausea/vomiting, and elevation of hepatic transaminase. Doses of less than 31 microg/kg per day were well tolerated. Clinical responses were observed at all dose levels.

CONCLUSION

DAB389IL-2 is well tolerated at doses of less than 31 microg/kg per day, and it induced clinical responses in previously treated mycosis fungoides, providing evidence for the antitumor activity of this molecule.

Influence of cold shivering on fine motor control in the upper limb

The aim of the investigation was to determine the effects of cold shivering on the accuracy of force output in distal, middle and proximal muscles of the upper limb. Test of hand grip strength, elbow flexion and shoulder flexion (each done at 10% maximal voluntary contraction for 15 s) were done under three conditions: (1) thermoneutral air (27 degrees C), a condition of thermal comfort; (2) cold air (10 degrees C), a condition eliciting an increase in tonic muscle activity; (3) and cold air (10 degrees C) with a cold drink (8 degrees C), a condition that causes visible shivering. The averaged (root mean square) electromyogram (AEMG) and mean power frequency (MPF) were measured from proximal, middle and distal arm muscles during the tests and compared. The control of force output was highly effective at thermoneutral condition for all motor tasks. During the cold air condition, all muscles were tonically active but there was no effect on accuracy of test performance. However, AEMG increased approximately 20% (P < 0.05) with respect to test performance in thermoneutral condition. During the cold air/cold drink condition, all muscles were shivering to a different extend. AEMG during test performance increased 30-150% in comparison to thermoneutral condition (P < 0.05). In this case, hand grip and elbow flexion were not adversely affected (these tests require middle and distal muscles) by cold shivering. However, the accuracy of performance of shoulder flexion was adversely affected. This is consistent with the fact that proximal muscles are more active during cold shivering.

Epidural anesthesia reduces the gain and maximum intensity of shivering

BACKGROUND

Shivering can be characterized by its threshold (triggering core temperature), gain (incremental intensity increase), and maximum intensity. The gain of shivering might be preserved during epidural or spinal anesthesia if control mechanisms compensate for lower-body paralysis by augmenting the activity of upper-body muscles. Conversely, gain will be reduced approximately by half if the thermoregulatory system fails to compensate. Similarly, appropriate regulatory feedback might maintain maximum shivering intensity during regional anesthesia. Accordingly, the gain and maximum intensity of shivering during epidural anesthesia were determined.

METHODS

Seven volunteers participated on two randomly ordered study days. On one day (control), no anesthesia was administered; on the other, epidural anesthesia was maintained at a T8 sensory level. Shivering, at a mean skin temperature near 33 degrees C, was provoked by central-venous infusion of cold fluid; core cooling continued until shivering intensity no longer increased. Shivering was evaluated by systemic oxygen consumption and electromyography of two upper-body and two lower-body muscles. The core temperature triggering an increase in oxygen consumption identified the shivering threshold. The slopes of the oxygen consumption versus core temperature and electromyographic intensity versus core temperature regressions identified systemic and regional shivering gains, respectively.

RESULTS

The shivering threshold was reduced by epidural anesthesia by approximately 0.4 degrees C, from 36.7 +/- 0.6 to 36.3 +/- 0.5 degrees C (means +/- SD; P < 0.05). Systemic gain, as determined by oxygen consumption, was reduced from -581 +/- 186 to -215 +/- 154 ml x min(-1) x degrees C(-1) (P < 0.01). Lower-body gain, as determined electromyographically, was essentially obliterated by paralysis during epidural anesthesia, decreasing from -0.73 +/- 0.85 to -0.04 +/- 0.06 intensity units/degrees C (P < 0.01). However, upper-body gain had no compensatory increase: -1.3 +/- 1.1 units/degrees C control versus 2.0 +/- 2.1 units/degrees C epidural. Maximum oxygen consumption was decreased by one third during epidural anesthesia: 607 +/- 82 versus 412 +/- 50 ml/min (P < 0.05).

CONCLUSIONS

These results confirm that regional anesthesia reduces the shivering threshold. Epidural anesthesia reduced the gain of shivering by 63% because upper-body muscles failed to compensate for lower-body paralysis. The thermoregulatory system thus fails to recognize that regional anesthesia reduces metabolic heat production, instead responding as if lower-body muscular activity remained intact.

Meperidine and alfentanil do not reduce the gain or maximum intensity of shivering

BACKGROUND

Thermoregulatory shivering can be characterized by its threshold (triggering core temperature), gain (incremental intensity increase with further core temperature deviation), and maximum intensity. Meperidine (a combined mu- and kappa-agonist) treats shivering better than equianalgesic doses of pure mu-opioid agonists. Meperidine's special antishivering action is mediated, at least in part, by a disproportionate decrease in the shivering threshold. That is, meperidine decreases the shivering threshold twice as much as the vasoconstriction threshold, whereas alfentanil (a pure mu-agonist) decreases the vasoconstriction and shivering thresholds comparably. However, reductions in the gain or maximum shivering intensity might also contribute to the clinical efficacy of meperidine. Accordingly, we tested the hypothesis that meperidine reduces the gain and maximum intensity of shivering much more than alfentanil does.

METHODS

Ten volunteers were each studied on three separate days: (1) control (no drug); (2) a target total plasma meperidine concentration of 1.2 microg/ml; and (3) a target plasma alfentanil concentration of 0.2 microg/ml. Skin temperatures were maintained near 31 degrees C, and core temperatures were decreased by central-venous infusion of cold lactated Ringer's solution until maximum shivering intensity was observed. Shivering was evaluated using oxygen consumption and electromyography. A sustained increase in oxygen consumption identified the shivering threshold. The gain of shivering was calculated as the slope of the oxygen consumption versus core temperature regression, and as the slope of electromyographic intensity versus core temperature regression.

RESULTS

Meperidine and alfentanil administration significantly decreased the shivering thresholds. However, neither meperidine nor alfentanil reduced the gain of shivering, as determined by either oxygen consumption or electromyography. Opioid administration also failed to significantly decrease the maximum intensity of shivering.

CONCLUSIONS

The authors could not confirm the hypothesis that meperidine reduces the gain or maximum intensity of shivering more than alfentanil does. These results suggest that meperidine's special antishivering effect is primarily mediated by a disproportionate reduction in the shivering threshold.

Isoflurane alters shivering patterns and reduces maximum shivering intensity

BACKGROUND

Shivering can be characterized by its threshold (triggering core temperature), gain (incremental intensity increase with further core hypothermia), and maximum response intensity. Isoflurane produces a clonic muscular activity that is not a component of normal shivering. To the extent that clonic activity is superimposed on normal thermoregulatory shivering, the gain of shivering might be increased during isoflurane anesthesia. Conversely, volatile anesthetics decrease systemic oxygen consumption and peripherally inhibit skeletal muscle strength, which might limit maximum intensity despite central activation. The purpose of the present study was, therefore, to evaluate the effect of isoflurane shivering patterns and the gain and maximum intensity of shivering.

METHODS

Ten volunteers were each studied in two separate protocols: (1) control (no drug) and (2) 0.7% end-tidal isoflurane. On each day, the mean skin temperature was maintained at 31 degrees C. Core temperature was then reduced by infusion of cold fluid until shivering intensity no longer increased. The core temperature triggering the initial increase in oxygen consumption defined the shivering threshold. The gain of shivering was defined by the slope of the core temperature versus oxygen consumption regression. Pectoralis and quadriceps electromyography was used to evaluate anesthetic-induced facilitation of clonic (5-7 Hz) muscular activity.

RESULTS

Isoflurane significantly decreased the shivering threshold from 36.4 +/- 0.3 to 34.2 +/- 0.8 degrees C. The increase in oxygen consumption was linear on the control day and was followed by sustained high-intensity activity. During isoflurane administration, shivering was characterized by bursts of intense shivering separated by quiescent periods. Isoflurane significantly increased the gain of shivering (as calculated from the initial increase), from -684 +/- 266 to -1483 +/- 752 ml x min(-1) x degrees C(-1). However, isoflurane significantly decreased the maximum intensity of shivering, from 706 +/- 144 to 489 +/- 80 ml/min. Relative electromyographic power in frequencies associated with clonus increased significantly when the volunteers were given isoflurane.

CONCLUSIONS

These data indicate that isoflurane anesthesia markedly changes the overall pattern of shivering during progressive hypothermia from a linear increase to an unusual saw-tooth pattern. They further suggest that clonic muscular activity combines with shivering to increase the initial gain of shivering during isoflurane anesthesia, but that isoflurane peripherally inhibits the maximum expression of shivering.

Perioperative fever

Unlike normal thermoregulatory control, which is largely neuronally mediated, fever is activated by circulating pyrogens. Pyrogens are triggered by either infectious or non-infectious etiologies, all of which may be present in patients undergoing ambulatory surgery. Fever is a regulated elevation in the setpoint temperature for all thermoregulatory responses (warm and cold defenses). To increase core temperature according to the newly elevated setpoint, cold defenses such as vasoconstriction and shivering are activated. In contrast, anesthesia widens the interthreshold range, thus resulting in hypothermia. As a result, general anesthesia impairs the febrile response to pyrogenic stimulation. However, the precise nature of the interaction between fever and anesthesia has yet to be determined. Postoperative fever continues to be a major problem. Wound infections are responsible for many such fevers, although numerous other etiologies contribute. Initial diagnosis should thus focus on determining the etiology of fever. Once that is established, treatment can focus on the specific cause.

The convective afterdrop component during hypothermic exercise decreases with delayed exercise onset

HYPOTHESIS

Following cold water immersion, the post-cooling decrease in esophageal temperature (Tes) (i.e., afterdrop) is 3 times greater during exercise than during shivering, presumably due to increased muscular blood flow and convective core-to-periphery heat loss with exercise (J. Appl. Physiol. 63:2375, 1987). We felt that if exercise were to commence once the afterdrop period during shivering is complete, the threat of a further decrease in Tes (i.e., a second afterdrop) during the subsequent exercise would be minimized because much of the convective capacity for core cooling would already be dissipated.

METHODS

Six subjects were each cooled three times in 8 degrees C water, until Tes decreased to 35.3 +/- 0.7 degrees C, and rewarmed by either shivering alone, exercise, or exercise commencing once a shivering afterdrop period was complete.

RESULTS

The initial afterdrop was greater during Exercise only (1.1 +/- 0.4 degrees C) than Shivering only (0.35 +/- 0.3 degrees C) and Shivering-Exercise (0.45 +/- 0.2 degrees C) (p < 0.05). In contrast, exercise caused a secondary afterdrop of only 0.38 +/- 0.3 degrees C during Shivering-Exercise (p < 0.05). The initial rewarming rate during Exercise only (3.45 degrees C.h-1) was greater than the initial (2.7 degrees C.h-1) and second (2.4 degrees C.h-1) rewarming rates during Shivering-Exercise (p < 0.05), but not significantly greater than during Shivering only (2.99 degrees C.h-1) (p < 0.1).

DISCUSSION

It is likely that during the Shivering-Exercise protocol, continued blood flow to shivering muscles: a) contributes to the initial afterdrop, and thus b) diminishes the convective capacity (or heat sink) available for further cooling during subsequent exercise.