Makoto Ozaki

Heat flow and distribution during induction of general anesthesia

Background Core hypothermia after induction of general anesthesia results from an internal core‐to‐peripheral redistribution of body heat and a net loss of heat to the environment. However, the relative contributions of each mechanism remain unknown. The authors evaluated regional body heat content and the extent to which core hypothermia after induction of anesthesia resulted from altered heat balance and internal heat redistribution. Methods Six minimally clothed male volunteers in an [nearly equal] 22 degrees Celsius environment were evaluated for 2.5 control hours before induction of general anesthesia and for 3 subsequent hours. Overall heat balance was determined from the difference between cutaneous heat loss (thermal flux transducers) and metabolic heat production (oxygen consumption). Arm and leg tissue heat contents were determined from 19 intramuscular needle thermocouples, 10 skin temperatures, and “deep” foot temperature. To separate the effects of redistribution and net heat loss, we multiplied the change in overall heat balance by body weight and the specific heat of humans. The resulting change in mean body temperature was subtracted from the change in distal esophageal (core) temperature, leaving the core hypothermia specifically resulting from redistribution. Results Core temperature was nearly constant during the control period but decreased 1.6 plus/minus 0.3 degrees Celsius in the first hour of anesthesia. Redistribution contributed 81% to this initial decrease and required transfer of 46 kcal from the trunk to the extremities. During the subsequent 2 h of anesthesia, core temperature decreased an additional 1.1 plus/minus 0.3 degrees Celsius, with redistribution contributing only 43%. Thus, only 17 kcal was redistributed during the second and third hours of anesthesia. Redistribution therefore contributed 65% to the entire 2.8 plus/minus 0.5 degrees Celsius decrease in core temperature during the 3 h of anesthesia. Proximal extremity heat content decreased slightly after induction of anesthesia, but distal heat content increased markedly. The distal extremities thus contributed most to core cooling. Although the arms constituted only a fifth of extremity mass, redistribution increased arm heat content nearly as much as leg heat content. Distal extremity heat content increased [nearly equal] 40 kcal during the first hour of anesthesia and remained elevated for the duration of the study. Conclusions The arms and legs are both important components of the peripheral thermal compartment, but distal segments contribute most. Core hypothermia during the first hour after induction resulted largely from redistribution of body heat, and redistribution remained the major cause even after 3 h of anesthesia.

Rate and gender dependence of the sweating, vasoconstriction, and shivering thresholds in humans.

Background:The range of core temperatures not triggering thermoregulatory responses (“interthreshold range”) remains to be determined in humans. Although the rates at which perioperative core temperatures vary typically range from 0.5 to 2°C/h, the thermoregulatory contribution of different core cooling rates also remains unknown. In addition, sweating in women is triggered at a slightly greater core temperature than in men. However, it is unknown whether the vasoconstriction and shivering thresholds are comparably greater in women, or if women tolerate a larger range of core temperatures without triggering thermoregulatory responses. Accordingly, the authors sought to (1) define the interthreshold range; (2) test the hypothesis that, at a constant skin temperature, the vasoconstriction and shivering thresholds are greater during rapid core cooling than during slowly induced hypothermia; and (3) compare the sweating, vasoconstriction, and shivering thresholds in men and women. Methods: Eight men and eight women participated. The men participated on 2 separate days; no anesthesia or sedatives were administered. On each day, they were cutaneously warmed until sweating was induced and then were cooled by a central venous infusion of cold fluid. The cooling rates were 0.7 ± 0.1°C/h on 1 day and 1.7 ± 0.4°C/h on the other, randomly ordered. Skin temperature was maintained near 36.7°C throughout each trial. The women were studied only once, in the follicular phase of their menstrual cycles, at the greater cooling rate. Results:The interthreshold range was ≈ 0.2 °C in both men and women, but all thermoregulatory response thresholds were ≈ 0.3°C higher in women. All thresholds were virtually identical during slow and fast core cooling. Conclusions:Our findings confirm the existence of an interthreshold range and document that its magnitude is small. They also demonstrate that the interthreshold range does not differ in men and women, but that women thermoregulate at a significantly higher temperature than do men. Typical clinical rates of core cooling do not alter thermoregulatory responses.

Heat Flow and Distribution during Epidural Anesthesia

Background Core hypothermia after induction of epidural anesthesia results from both an internal core‐to‐peripheral redistribution of body heat and a net loss of heat to the environment. However, the relative contributions of each mechanism remain unknown. The authors thus evaluated regional body heat content and the extent to which core hypothermia after induction of anesthesia resulted from altered heat balance and internal heat redistribution.

Increasing mean skin temperature linearly reduces the core-temperature thresholds for vasoconstriction and shivering in humans

Background The contribution of mean skin temperature to the thresholds for sweating and active precapillary vasodilation has been evaluated in numerous human studies. In contrast, the contribution of skin temperature to the control of cold responses such as arteriovenous shunt vasoconstriction and shivering is less well established. Accordingly, the authors tested the hypothesis that mean skin and core temperatures are linearly related at the vasoconstriction and shivering thresholds in men. Because the relation between skin and core temperatures might vary by gender, the cutaneous contribution to thermoregulatory control also was determined in women. Methods In the first portion of the study, six men participated on 5 randomly ordered days, during which mean skin temperatures were maintained near 31, 34, 35, 36, and 37 degrees Celsius. Core hypothermia was induced by central venous infusion of cold lactated Ringers solution sufficient to induce peripheral vasoconstriction and shivering. The core‐temperature thresholds were then plotted against skin temperature and a linear regression fit to the values. The relative skin and core contributions to the control of each response were calculated from the slopes of the regression equations. In the second portion of the study, six women participated on three randomly ordered days, during which mean skin temperatures were maintained near 31, 35, and 37 degrees Celsius. At each designated skin temperature, core hypothermia sufficient to induce peripheral vasoconstriction and/or shivering was again induced by central venous infusion of cold lactated Ringers solution. The cutaneous contributions to control of each response were then calculated from the skin‐ and core‐temperature pairs at the vasoconstriction and shivering thresholds. Results There was a linear relation between mean skin and core temperatures at the response thresholds in the men: r = 0.90 plus/minus 0.06 for vasoconstriction and r = 0.94 plus/minus 0.07 for shivering. Skin temperature contributed 20 plus/minus 6% to vasoconstriction and 19 plus/minus 8% to shivering. Skin temperature in the women contributed to 18 plus/minus 4% to vasoconstriction and 18 plus/minus 7% to shivering, values not differing significantly from those in men. There was no apparent correlation between the cutaneous contributions to vasoconstriction and shivering in individual volunteers. Conclusions These data indicate that skin and core temperatures contribute linearly to the control of vasoconstriction and shivering in men and that the cutaneous contributions average [nearly equal] 20% in both men and women. The same coefficients thus can be used to compensate for experimental skin temperature manipulations in men and women. However, the cutaneous contributions to each response vary among volunteers; furthermore, the contributions to the two responses vary within volunteers.

Thermoregulatory thresholds during epidural and spinal anesthesia.

BackgroundThere are significant physiologic differences between spinal and epidural anesthesia. Consequently, these two types of regional anesthesia may influence thermoregulatory processing differently. Accordingly, in volunteers and in patients, we tested the null hypothesis that the core-temperature thresholds triggering thermoregulatory sweating, vasoconstriction, and shivering are similar during epidural and spinal anesthesia. MethodsSix male volunteers participated on three consecutive study days: epidural or spinal anesthesia were randomly assigned on the 1st and 3rd days (± T10 level); no anesthesia was given on the 2nd day. On each day, the volunteers were initially warmed until they started to sweat, and subsequently cooled by central venous infusion of cold fluid until they shivered. Mean skin temperature was kept constant near 36°C throughout each study. The tympanic membrane temperatures triggering a sweating rate of 40 g · m−2 · h−1, a finger flow less than 0.1 ml/min, and a marked and sustained increase in oxygen consumption (± 30%) were considered the thermoregulatory thresholds for sweating, vasoconstriction, and shivering, respectively. Twenty-one patients were randomly assigned to receive epidural (n = 10) or spinal (n = 11) anesthesia for knee and calf surgery (± T10 level). As in the volunteers, the shivering threshold was defined as the tympanic membrane temperature triggering a sustained increase in oxygen consumption. ResultsThe thresholds and ranges were similar during epidural and spinal anesthesia in the volunteers. However, the sweating-to-vasoconstriction (interthreshold) range, the vasoconstriction-to-shivering range, and the sweating-to-shivering range all were significantly increased by regional anesthesia. The shivering thresholds in patients assigned to epidural and spinal anesthesia were virtually identical. ConclusionsComparable sweating, vasoconstriction, and shivering thresholds during epidural and spinal anesthesia suggest that thermoregulatory processing is similar during each type of regional anesthesia. However, thermoregulatory control was impaired during regional anesthesia, as indicated by the significantly enlarged interthreshold and sweating-to-shivering ranges.

Resistive-heating and forced-air warming are comparably effective

Serious adverse outcomes from perioperative hypothermia are well documented. Consequently, intraoperative warming has become routine. We thus evaluated the efficacy of a novel, nondisposable carbon-fiber resistive-heating system. Twenty-four patients undergoing open abdominal surgery lasting approximately 4 h were randomly assigned to warming with 1) a full-length circulating water mattress set at 42°C, 2) a lower-body forced-air cover with the blower set on high, or 3) a three-extremity carbon-fiber resistive-heating blanket set to 42°C. Patients were anesthetized with a combination of continuous epidural and general anesthesia. All fluids were warmed to 37°C, and ambient temperature was kept near 22°C. Core (tympanic membrane) temperature changes among the groups were compared by using factorial analysis of variance and Scheffe[Combining Acute Accent] F tests; results are presented as means ± SD. Potential confounding factors did not differ significantly among the groups. In the first 2 h of surgery, core temperature decreased by 1.9°C ± 0.5°C in the circulating-water group, 1.0°C ± 0.6°C in the forced-air group, and 0.8°C ± 0.2°C in the resistive-heating group. At the end of surgery, the decreases were 2.0°C ± 0.8°C in the circulating-water group, 0.6°C ± 1.0°C in the forced-air group, and 0.5°C ± 0.4°C in the resistive-heating group. Core temperature decreases were significantly greater in the circulating-water group at all times after 150 elapsed minutes; however, temperature changes in the forced-air and resistive-heating groups never differed significantly. Even during major abdominal surgery, resistive heating maintains core temperature as effectively as forced air. IMPLICATIONS: Efficacy was similar for forced-air and resistive heating, and both maintained intraoperative core temperature far better than circulating-water mattresses. We thus conclude that even during major abdominal surgery, resistive heating maintains core temperature as effectively as forced air.

A new noninvasive method to measure blood pressure: results of a multicenter trial.

BACKGROUND Blood pressure (BP) monitoring with arterial waveform display requires an arterial cannula. We evaluated a new noninvasive device, Vasotrac (Medwave, Arden Hills, MN) that provides BP measurements approximately every 12-15 beats and displays pulse rate and a calibrated arterial waveform for each BP measurement. METHODS Surgical and critically ill patients (n = 80) served as subjects for the study. BPs, pulse waveforms, and pulse rates measured via a radial artery catheter were compared with those obtained by the Vasotrac from the opposite radial artery. Data were analyzed to determine agreement between the two systems of measurement. RESULTS Blood pressure measured noninvasively by the Vasotrac demonstrated excellent correlation (P<0.01) with BP measured via a radial arterial catheter (systolic r2 = 0.93; diastolic r2 = 0.89; mean r2 = 0.95). Differences in BP measured by the Vasotrac versus the radial arterial catheter were small. The mean+/-SD bias and precision were as follows: systolic BP 0.02+/-5.4 mm Hg and 3.9+/-3.7 mm Hg; diastolic BP -0.39+/-3.9 mm Hg and 2.7+/-2.8 mm Hg; mean BP -0.21+/-3.0 mm Hg and 2.1+/-2.2 mm Hg compared with radial artery measurements. The Vasotrac pulse rates were almost identical to those measured directly (r2 = 0.95). The Vasotrac BP waveform resembled those directly obtained radial artery pulsatile waveforms. CONCLUSIONS In surgical and critically ill patients, the Vasotrac measured BP, pulse rate, and displayed radial artery waveform, which was similar to direct radial arterial measurements. It should be a suitable device to measure BP frequently in a noninvasive fashion.

Epidural anesthesia impairs both central and peripheral thermoregulatory control during general anesthesia.

BackgroundThe authors tested the hypotheses that: (1) the vasoconstriction threshold during combined epidural/general anesthesia is less than that during general anesthesia alone; and (2) after vasoconstriction, core cooling rates during combined epidural/general anesthesia are greater than those during general anesthesia alone. Vasoconstriction thresholds and heat balance were evaluated under controlled circumstances in volunteers, whereas the clinical importance of intraoperative thermoregulatory vasoconstriction was evaluated in patients. MethodsFive volunteers were each evaluated twice. On one of the randomly ordered days, epidural anesthesia (&OV0312;T9 dermatomal level) was induced and maintained with 2-chloroprocaine. On both study days, general anesthesia was induced and maintained with isoflurane (0.7% end-tidal concentration), and core hypothermia was induced by surface cooling and continued for at least 1 h after fingertip vasoconstriction was observed. Patients undergoing colorectal surgery were randomly assigned to combined epidural/enflurane anesthesia (n = 13) or enflurane alone (n = 13). In appropriate patients, epidural anesthesia was maintained by an infusion of bupivacaine. The core temperature that triggered fingertip vasoconstriction identified the threshold. ResultsIn the volunteers, the vasoconstriction threshold was 36.0 ± 0.2° C during isoflurane anesthesia alone, but significantly less, 35.1 ± 0.7° C, during combined epidural/isoflurane anesthesia. Cutaneous heat loss and the rates of core cooling were similar 30 min before vasoconstriction with and without epidural anesthesia. In the 30 min after vasoconstriction, heat loss decreased 33 ± 13 W when the volunteers were given isoflurane alone, but only 8 ± 16 W during combined epidural/isoflurane anesthesia. Similarly, the core cooling rates in the 30 min after vasoconstriction were significantly greater during combined epidural/isoflurane anesthesia (0.8 ± 0.2° C/h) than during isoflurane alone (0.2 ± 0.1° C/h). In the patients, end-tidal enflurane concentrations were slightly, but significantly, less in the patients given combined epidural/enflurane anesthesia (0.6 ± 0.2% vs. 0.8 ± 0.2%). Nonetheless, the vasoconstriction threshold was 34.5 ± 0.6° C in the epidural/enflurane group, which was significantly less than that in the other patients, 35.6 ± 0.8° C. When the study ended after 3 h of anesthesia, patients given combined epidural/enflurane anesthesia were 1.2° C more hypothermic than those given general anesthesia alone. The rate of core cooling during the last hour of the study was 0.4 ± 0.2° C/h during combined epidural/enflurane anesthesia, but only 0.1 ± 0.3° C/h during enflurane alone. ConclusionsThese data indicate that epidural anesthesia reduces the vasoconstriction threshold during general anesthesia. Furthermore, the markedly reduced rate of core cooling during general anesthesia alone illustrates the importance of leg vasoconstriction in maintaining core temperature.

Epidural Anesthesia Increases Apparent Leg Temperature and Decreases the Shivering Threshold

BackgroundLower core temperatures than usual are required to trigger shivering during epidural and spinal anesthesia, but the etiology of this impairment remains unknown. In this investigation, we propose and test a specific mechanism by which a peripheral action of regional anesthesia might alter centrally mediated thermoregulatory responses. Conduction anesthesia blocks all thermal sensations; however, cold signals are disproportionately affected because at typical leg temperatures mostly cold receptors fire tonically. It thus seems likely that epidural and spinal anesthesia increase the leg temperature perceived by the thermoregulatory system. Because skin temperature reportedly contributes 5–20% to thermoregulatory control, increased apparent (as distinguished from actual) leg temperature would produce a complimentary decrease in the core temperature triggering thermoregulatory shivering. Accordingly, we tested the hypothesis that abnormal tolerance for hypothermia during epidural anesthesia coincides with an increase in apparent leg temperature. We defined apparent temperature as the leg-skin temperature required to induce a reduction in the shivering threshold comparable to that produced by epidural anesthesia. MethodsSix women were studied on 4 randomly ordered days: (1) leg-skin temperature near 32°C; (2) leg-skin temperature near 36°C; (3) leg-skin temperature near 38°C; and (4) epidural anesthesia without leg-warming (leg-skin temperature ± 34°C). At each designated leg temperature, core hypothermia sufficient to evoke shivering was induced by central venous infusion of cold fluid. Upper-body skin temperature was kept constant throughout. In each volunteer, linear regression was used to calculate the correlation between the shivering thresholds on the 3 nonepidural days and concurrent leg temperatures. The slope of these regression equations thus indicated the extent to which leg-warming increased thermoregulatory tolerance for core hypothermia, and was expressed as a percentage leg-skin and leg-tissue contribution to total thermal afferent input. The skin and tissue temperatures that would have been required to produce the observed shivering threshold during epidural anesthesia, the apparent temperatures, were then interpolated from the regression. ResultsThere was a good linear relation between the shivering threshold and leg-skin temperature (r2 = 0.94 ± 0.06). The contribution of leg-skin temperature to the shivering threshold was 11 ± 3% of the total thermal input. Apparent leg-skin temperature during epidural anesthesia was 37.8 ± 0.5°C, which exceeded actual leg-skin temperature by ± 4°C. The contribution of leg-tissue temperature to the shivering threshold was 19 ± 7% of the total. Apparent leg-tissue temperature during epidural anesthesia was 37.1 ± 0.4°C, which exceeded actual leg-skin temperature by ± 2°C. ConclusionsBecause leg-skin contributed ± 11% to the shivering threshold, it is unlikely that the entire skin surface contributes at much less than 20%. These data suggest that the shivering threshold during epidural anesthesia is reduced by a specific mechanism, namely that conduction block significantly increases apparent (as distinguished from actual) leg temperature.

Propofol causes a dose-dependent decrease in the thermoregulatory threshold for vasoconstriction but has little effect on sweating

BackgroundVolatile anesthetics increase the core temperature required to trigger sweating and decrease the core temperature required to trigger vasoconstriction. However, little is known about the effects of intravenous anesthetics on thermoregulation. We therefore tested the hypothesis that propofol increases the sweating threshold and decreases the vasoconstriction threshold, thereby increasing the interthreshold range (core temperatures not triggering autonomic thermoregulatory responses). The study was conducted using a new model in which thermal manipulations were restricted to insensate skin, and sensate skin temperature was controlled. MethodsSix healthy, male volunteers were studied on 3 randomly ordered days: no propofol, target propofol blood concentration 2 μg/ml, and target blood propofol concentration 4 μg/ml. Each day, epidural anesthesia (δT11 level) was induced, using 2% 2-chloroprocaine (one volunteer received bupivacaine). Thermal manipulations were confined to the legs, and we attempted to maintain upper-body (sensate) skin temperature constant. Propofol was infused by a computer-controlled infusion pump. Volunteers were heated until sweating was observed, then cooled until fingertip vasoconstriction was observed. The sweating threshold was defined as the tympanic membrane temperature triggering sustained evaporative heat loss ≥40 g. m−2. h−1. Similarly, the vasoconstriction threshold was defined as the tympanic membrane temperature triggering a sustained reduction in fingertip blood flow to <0.25 ml/min. Central venous blood was assayed for propofol blood concentration. ResultsIncreasing propofol concentration produced a linear decrease the vasoconstriction threshold (slope = −0.53 ± 0.34°C · μg−1. ml−1; R2 = 0.98 ± 0.04 [mean ± SD]), but had little effect on the sweating threshold. The interthreshold range was 0.51 ± 0.46°C during epidural anesthesia alone, and increased significantly, by 0.49 ± 0.31°C · μg−1. ml−1 during propofol administration. ConclusionsLike volatile anesthetics, propofol reduces the vasoconstriction threshold and increases the interthreshold range. However, propofol differs in leaving the sweating threshold unchanged.