Effectiveness of forced air warming after pediatric cardiac surgery employing hypothermic circulatory arrest without cardiopulmonary bypass

STUDY OBJECTIVE: To evaluate the effectiveness of forced-air warming compared to radiant warming in pediatric cardiac surgical patients recovering from moderate hypothermia after perfusionless deep hypothermic circulatory arrest. DESIGN: Prospective unblinded study. SETIING: Teaching hospitals. PATIENTS: 24 pediatric cardiac surgical patients. INTERVENTION: Noncyanotic patients undergoing repair of atrial or ventricular septal defects were cooled by topical application of ice and rewarmed initially in the operating room by warm saline lavage of the pleural cavities. On arrival at the intensive care unit (ICU), patients were warmed by forced air (n = 13) or radiant heat (n = 11). The time, heart rate, and blood pressure at each 0.5 degrees C increase in rectal temperature were measured until normothermia (36.5 degrees C) to determine the instantaneous rewarming rate. MEASUREMENTS AND MAIN RESULTS: Baseline characteristics were not different in the two groups. The mean (+/- SD) age was 5.6 +/- 3.4 years, weight was 20 +/- 8 kg, esophageal temperature for circulatory arrest was 25.7 +/- 1.3 degrees C, and duration of circulatory arrest was 25 +/- 11 minutes. The mean core temperature on arrival at the ICU was 29.9 +/- 1.3 degrees C and ranged from 26.1 to 31.5 degrees C. The mean rewarming rate for each 0.5 degrees C was greater (p < 0.05) for forced-air (2.43 +/- 1.14 degrees C/hr) than radiant heat (2.16 +/- 1.02 degrees C/hr). At core temperatures <33 degrees C, the rewarming rate for forced-air was 2.04 +/- 0.84 degrees C/hr and radiant heat was 1.68 +/- 0.84 degrees C/hr (p < 0.05). At core temperatures > or = 33 degrees C, the rewarming rate for forced air was 2.76 +/- 1.20 degrees C/hr and radiant heat was 2.46 +/- 1.08 degrees C/min (p = 0.07). Significant determinants of the rewarming rate in a multivariate regression model were age (p < 0.001), temperature (p < 0.05), time after arrival to the intensive care unit (p < 0.05), pulse pressure (p < 0. 05) and warming device (p < 0.001). The duration of ventilatory support and ICU length of stay was not different in the two groups. CONCLUSIONS: Both forced-air and radiant heat were effective for rewarming moderately hypothermic pediatric patients. When core temperature was less than 33 degrees C, the instantaneous rewarming rate by forced air was 21% faster than by radiant heat.     

Efficacy of two methods for reducing postbypass afterdrop

BACKGROUND: Afterdrop, defined as the precipitous reduction in core temperature after cardiopulmonary bypass, results from redistribution of body heat to inadequately warmed peripheral tissues. The authors tested two methods of ameliorating afterdrop: (1) forced-air warming of peripheral tissues and (2) nitroprusside-induced vasodilation. METHODS: Patients were cooled during cardiopulmonary bypass to approximately 32 degrees C and subsequently rewarmed to a nasopharyngeal temperature near 37 degrees C and a rectal temperature near 36 degrees C. Patients in the forced-air protocol (n = 20) were assigned randomly to forced-air warming or passive insulation on the legs. Active heating started with rewarming while undergoing bypass and was continued for the remainder of surgery. Patients in the nitroprusside protocol (n = 30) were assigned randomly to either a control group or sodium nitroprusside administration. Pump flow during rewarming was maintained at 2.5 l x m(-2) x min(-1) in the control patients and at 3.0 l x m(-2) x min(-1) in those assigned to sodium nitroprusside. Sodium nitroprusside was titrated to maintain a mean arterial pressure near 60 mm Hg. In all cases, a nasopharyngeal probe evaluated core (trunk and head) temperature and heat content. Peripheral compartment (arm and leg) temperature and heat content were estimated using fourth-order regressions and integration over volume from 18 intramuscular needle thermocouples, nine skin temperatures, and "deep" hand and foot temperature. RESULTS: In patients warmed with forced air, peripheral tissue temperature was higher at the end of warming and remained higher until the end of surgery. The core temperature afterdrop was reduced from 1.2+/-0.2 degrees C to 0.5+/-0.2 degrees C by forced-air warming. The duration of afterdrop also was reduced, from 50+/-11 to 27+/-14 min. In the nitroprusside group, a rectal temperature of 36 degrees C was reached after 30+/-7 min of rewarming. This was only slightly faster than the 40+/-13 min necessary in the control group. The afterdrop was 0.8+/-0.3 degrees C with nitroprusside and lasted 34+/-10 min which was similar to the 1.1+/-0.3 degrees C afterdrop that lasted 44+/-13 min in the control group. CONCLUSIONS: Cutaneous warming reduced the core temperature afterdrop by 60%. However, heat-balance data indicate that this reduction resulted primarily because forced-air heating prevented the typical decrease in body heat content after discontinuation of bypass, rather than by reducing redistribution. Nitroprusside administration slightly increased peripheral tissue temperature and heat content at the end of rewarming. However, the core-to-peripheral temperature gradient was low in both groups. Consequently, there was little redistribution in either case.     

Efficacy of Two Methods for Reducing Postbypass Afterdrop

Background: Afterdrop, defined as the precipitous reduction in core temperature after cardiopulmonary bypass, results from redistribution of body heat to inadequately warmed peripheral tissues. The authors tested two methods of ameliorating afterdrop: (1) forced-air warming of peripheral tissues and (2) nitroprusside-induced vasodilation.

Methods: Patients were cooled during cardiopulmonary bypass to approximately 32[degrees]C and subsequently rewarmed to a nasopharyngeal temperature near 37[degrees]C and a rectal temperature near 36[degrees]C. Patients in the forced-air protocol (n = 20) were assigned randomly to forced-air warming or passive insulation on the legs. Active heating started with rewarming while undergoing bypass and was continued for the remainder of surgery. Patients in the nitroprusside protocol (n = 30) were assigned randomly to either a control group or sodium nitroprusside administration. Pump flow during rewarming was maintained at 2.5 l [middle dot] m-2 [middle dot] min-1 in the control patients and at 3.0 l [middle dot] m-2 [middle dot] min-1 in those assigned to sodium nitroprusside. Sodium nitroprusside was titrated to maintain a mean arterial pressure near 60 mmHg. In all cases, a nasopharyngeal probe evaluated core (trunk and head) temperature and heat content. Peripheral compartment (arm and leg) temperature and heat content were estimated using fourth-order regressions and integration over volume from 18 intramuscular needle thermocouples, nine skin temperatures, and "deep" hand and foot temperature.

Results: In patients warmed with forced air, peripheral tissue temperature was higher at the end of warming and remained higher until the end of surgery. The core temperature afterdrop was reduced from 1.2 +/- 0.2[degrees]C to 0.5 +/- 0.2[degrees]C by forced-air warming. The duration of afterdrop also was reduced, from 50 +/- 11 to 27 +/- 14 min. In the nitroprusside group, a rectal temperature of 36[degrees]C was reached after 30 +/- 7 min of rewarming. This was only slightly faster than the 40 +/- 13 min necessary in the control group. The afterdrop was 0.8 +/- 0.3[degrees]C with nitroprusside and lasted 34 +/- 10 min which was similar to the 1.1 +/- 0.3[degrees]C afterdrop that lasted 44 +/- 13 min in the control group.     

Efficacy of forced-air warming systems with full body blankets

PURPOSE: Postoperative hypothermia after cardiac surgery is still a common problem often treated with forced-air warming. This study was conducted to determine the heat transfer efficacy of 11 forced-air warming systems with full body blankets on a validated copper manikin. METHODS: The following systems were tested: 1) Bair Hugger 505; 2) Bair Hugger 750; 3) Life-Air 1000 S; 4) Snuggle Warm; 5) Thermacare; 6) Thermacare with reusable Optisan blanket; 7) WarmAir; 8) Warm-Gard; 9) Warm-Gard and reusable blanket; 10) WarmTouch; and 11) WarmTouch and reusable blanket. Heat transfer of forced-air warmers can be described as follows: Q = h x DeltaT x A. Where Q = heat flux (W), h = heat exchange coefficient (W x m-2 x degrees C-1), DeltaT = temperature gradient between blanket and manikin surface (degrees C), A = covered area (m2). Heat flux per unit area and surface temperature were measured with 16 heat flux transducers. Blanket temperature was measured using 16 thermocouples. The temperature gradient between blanket and surface (DeltaT) was varied and h was determined by linear regression analysis. Mean DeltaT was determined for surface temperatures between 32 degrees C and 38 degrees C. The covered area was estimated to be 1.21 m2. RESULTS: For the 11 devices, heat transfers of 30.7 W to 77.3 W were observed for surface temperatures of 32 degrees C, and between -8.8 W to 29.6 W for surface temperatures of 38 degrees C. CONCLUSION: There are clinically relevant differences between the tested forced-air warming systems with full body blankets. Several systems were unable to transfer heat to the manikin at a surface temperature of 38 degrees C.     

Comparison of two different temperature maintenance strategies during open abdominal surgery: upper body forced-air warming versus whole body water garment

BACKGROUND: A new system has been developed that circulates warm water through a whole body garment worn by the patient during surgery. In this study the authors compared two different strategies for the maintenance of intraoperative normothermia. One strategy used a new water garment warming system that permitted active warming of both the upper and lower extremities and the back. The other strategy used a single (upper body) forced-air warming system. METHODS: In this prospective, randomized study, 53 adult patients were enrolled in one of two intraoperative temperature management groups during open abdominal surgery with general anesthesia. The water-garment group (n = 25) received warming with a body temperature (rectal) set point of 36.8 degrees C. The forced-air-warmer group (n = 28) received routine warming therapy using upper body forced-air warming system (set on high). The ambient temperature in the operating room was maintained constant at approximately 20 degrees C. Rectal, distal esophageal, tympanic, forearm, and fingertip temperatures were recorded perioperatively and during 2 h after surgery. Extubated patients in both groups were assessed postoperatively for shivering, use of additional warming devices, and subjective thermal comfort. RESULTS: The mean rectal and esophageal temperatures at incision, 1 h after incision, at skin closure, and immediately postoperatively were significantly higher (0.4-0.6 degrees C) in the group that received water-garment warming when compared with the group that received upper body forced-air warming. The calculated 95% confidence intervals for the above differences in core temperatures were 0.7-0.1, 0.8-0.2, 0.8-0.2, and 0.9-0.1, retrospectively. In addition, 14 and 7% of patients in the control upper body forced-air group remained hypothermic (< 35.5 degrees C) 1 and 2 h after surgery, respectively. No core temperature less than 35.5 degrees C was observed perioperatively in any of the patients from the water-garment group. A similar frequency of the thermal stress events (shivering, use of additional warming devices, subjective thermal discomfort) was observed after extubation in both groups during the 2 h after surgery. CONCLUSIONS: The investigated water warming system, by virtue of its ability to deliver heat to a greater percentage of the body, results in better maintenance of intraoperative normothermia that does forced-air warming applied only to the upper extremities, as is common practice.     

Intraoperative patient warming using radiant warming or forced-air warming during long operations

Forced-air warming is the most commonly used and effective method of active warming. A new radiant warming device (Suntouch, Fisher and Paykel) may provide an alternative when the skin surface available for warming is limited. We conducted a randomized controlled trial to compare the efficacy of the Suntouch radiant warmer and forced-air warming. With ethics committee approval 60 surgical patients having procedures anticipated to be more than two hours in duration were recruited. Patients were randomized to either radiant warming or forced-air warming. All intravenous fluids were warmed but prewarming was not used. The final intraoperative core temperatures (degrees C) for the radiant warming and forced-air warming groups were 36.0 +/- 0.5 and 36.4 +/- 0.6 (P=0.002) respectively. No other patient variables were significantly different. The Suntouch is not as effective as the forced air warming for intraoperative warming during long surgical procedures. The device may be useful when forced-air warming is not possible.     

Comparison of Two Different Temperature Maintenance Strategies during Open Abdominal Surgery: Upper Body Forced-air Warming versus Whole Body Water Garment

Background: A new system has been developed that circulates warm water through a whole body garment worn by the patient during surgery. In this study the authors compared two different strategies for the maintenance of intraoperative normothermia. One strategy used a new water garment warming system that permitted active warming of both the upper and lower extremities and the back. The other strategy used a single (upper body) forced-air warming system.

Methods: In this prospective, randomized study, 53 adult patients were enrolled in one of two intraoperative temperature management groups during open abdominal surgery with general anesthesia. The water-garment group (n = 25) received warming with a body temperature (rectal) set point of 36.8[degrees]C. The forced-air-warmer group (n = 28) received routine warming therapy using upper body forced-air warming system (set on high). The ambient temperature in the operating room was maintained constant at approximately 20[degrees]C. Rectal, distal esophageal, tympanic, forearm, and fingertip temperatures were recorded perioperatively and during 2 h after surgery. Extubated patients in both groups were assessed postoperatively for shivering, use of additional warming devices, and subjective thermal comfort.

Results: The mean rectal and esophageal temperatures at incision, 1 h after incision, at skin closure, and immediately postoperatively were significantly higher (0.4-0.6[degrees]C) in the group that received water-garment warming when compared with the group that received upper body forced-air warming. The calculated 95% confidence intervals for the above differences in core temperatures were 0.7-0.1, 0.8-0.2, 0.8-0.2, and 0.9-0.1, retrospectively. In addition, 14 and 7% of patients in the control upper body forced-air group remained hypothermic (< 35.5[degrees]C) 1 and 2 h after surgery, respectively. No core temperature less than 35.5[degrees]C was observed perioperatively in any of the patients from the water-garment group. A similar frequency of the thermal stress events (shivering, use of additional warming devices, subjective thermal discomfort) was observed after extubation in both groups during the 2 h after surgery.     

术中保温对患者核心体温的影响

目的　探讨术中保温对患者核心体温的影响。方法　将 4 0例择期全麻下行腹部手术的患者随机分为对照组和加温组 ,各 2 0例。对照组患者术中不采用任何升温装置 ,加温组患者术中采用输液加温器及充气升温毯加温。观察两组患者术中核心体温、失血量和输血量、拔管时间及术后寒战发生率。结果　术毕核心温度 ,加温组为 ( 36 4± 0 4 )℃ ,对照组为 ( 35 3± 0 5 )℃ (t =7 5 4 7,P <0 0 1)。两组患者术中失血量和输血量差异无显著性 (P >0 0 5 )。加温组拔管时间为 ( 18± 6 )min ,短于对照组的 ( 2 6± 10 )min(t=- 3 36 4 ,P =0 0 0 2 )。对照组有 6例患者发生术后寒战 ( 30 % ) ,加温组无术后寒战发生 ( 0 % ,χ2 =7 0 5 9,P =0 0 0 8)。结论　术中采用输液加温器和温毯 ,可有效地维持患者正常体温 ( 36 0℃～ 37 0℃ ) ,从而缩短拔管时间、减少术后寒战发生。     

A comparison of patient rewarming devices after cardiac surgery

Three regimens for rewarming patients after cardiac surgery involving hypothermic cardiopulmonary bypass were studied in 30 patients. The control group (n = 10) received no active rewarming, the oesophageal group (n = 10) was warmed centrally using an oesophageal heat exchanger and the radiant group (n = 10) was warmed peripherally with an overhead radiant heater. There were no statistically significant differences between the groups apart from the higher mean skin temperatures in the peripheral group.     

Comparison of forced-air warming and radiant heating during transurethral prostatic resection under spinal anaesthesia

Forced-air warming is commonly used to warm patients intraoperatively, but may not achieve normothermia during a short procedure. Comparative trials of a new radiant warming device in general anaesthesia (Suntouch, Fisher and Paykel, Auckland, New Zealand) have had conflicting results. We conducted a randomized controlled trial to compare the efficacy and thermal comfort of the Suntouch radiant warmer and forced-air warming in patients at high risk of hypothermia during neuraxial blockade. With ethics committee approval, 60 patients having transurethral resection of the prostate under spinal were randomized to either radiant warming or forced-air warming. All intravenous and irrigation fluids were warmed but pre-warming was not used. The final intraoperative rectal temperatures for the radiant warming and forced-air warming groups were 36.1 degrees C and 36.4 degrees C respectively (P= 0.03). A large proportion of patients in both groups (46% and 33% respectively, P=0.3) were hypothermic (<36 degrees C) on arrival in the post-anaesthesia care unit. No other patient variables were significantly different. Neither warming device reliably prevented hypothermia, although forced-air warming was slightly superior.     

Intraoperative warming therapies: a comparison of three devices.

STUDY OBJECTIVE: To compare the effectiveness of three commonly used intraoperative warming devices. DESIGN: A randomized, prospective clinical trial. SETTING: The surgical suite of a university medical center. PATIENTS: Twenty adult patients undergoing kidney transplantation for end-stage renal disease. INTERVENTIONS: Patients were assigned to one of four warming therapy groups: circulating-water blanket (40 degrees C), heated humidifier (40 degrees C), forced-air warmer (43 degrees C, blanket covering legs), or control (no extra warming). Intravenous fluids were warmed (37 degrees C), and fresh gas flow was 5 L/min for all groups. No passive heat and moisture exchangers were used. MEASUREMENTS AND MAIN RESULTS: The central temperature (tympanic membrane thermocouple) decreased approximately 1 degree C during the first hour of anesthesia in all groups. After three hours of anesthesia, the decrease in the tympanic membrane temperature from baseline (preinduction) was least in the forced-air warmer group (-0.5 degrees C +/- 0.4 degrees C), intermediate in the circulating-water blanket group (-1.2 degrees C +/- 0.4 degrees C), and greatest in the heated humidifier and control groups (-2.0 degrees C +/- 0.5 degrees C and -2.0 degrees C +/- 0.7 degrees C, respectively). Total cutaneous heat loss measured with distributed thermal flux transducers was approximately 35W (watts = joules/sec) less in the forced-air warmer group than in the others. Heat gain across the back from the circulating-water blanket was approximately 7W versus a loss of approximately 3W in patients lying on a standard foam mattress. CONCLUSION: The forced-air warmer applied to only a limited skin surface area transferred more heat and was clinically more effective (at maintaining central body temperature) than were the other devices. The characteristic early decrease in central temperature observed in all groups regardless of warming therapy is consistent with the theory of anesthetic-induced heat redistribution within the body.     

Body morphology and the speed of cutaneous rewarming

BACKGROUND: Infants and children cool quickly because their surface area (and therefore heat loss) is large compared with their metabolic rate, which is mostly a function of body mass. Rewarming rate is a function of cutaneous heat transfer plus metabolic heat production divided by body mass. Therefore, the authors tested the hypothesis that the rate of forced-air rewarming is inversely related to body size. METHODS: Isoflurane, nitrous oxide, and fentanyl anesthesia were administered to infants, children, and adults scheduled to undergo hypothermic neurosurgery. All fluids were warmed to 37 degrees C and ambient temperature was maintained near 21 degrees C. Patients were covered with a full-body, forced-air cover of the appropriate size. The heater was set to low or ambient temperature to reduce core temperature to 34 degrees C in time for dural opening. Blower temperature was then adjusted to maintain core temperature at 34 degrees C for 1 h. Subsequently, the forced-air heater temperature was set to high (approximately 43 degrees C). Rewarming continued for the duration of surgery and postoperatively until core temperature exceeded 36.5 degrees C. The rewarming rate in individual patients was determined by linear regression. RESULTS: Rewarming rates were highly linear over time, with correlations coefficients (r2) averaging 0.98+/-0.02. There was a linear relation between rewarming rate (degrees C/h) and body surface area (BSA; m2): Rate (degrees C/h) = -0.59 x BSA (m2) + 1.9, r2 = 0.74. Halving BSA thus nearly doubled the rewarming rate. CONCLUSIONS: Infants and children rewarm two to three times faster than adults, thus rapidly recovering from accidental or therapeutic hypothermia.     

Effect of postoperative skin-surface warming on oxygen consumption and the shivering threshold

Cutaneous warming is reportedly an effective treatment for shivering during epidural and after general anaesthesia. We quantified the efficacy of cutaneous warming as a treatment for shivering. Unwarmed surgical patients (final intra-operative core temperatures approximately 35 degrees C) were randomly assigned to be covered with a blanket (n = 9) or full-body forced-air cover (n = 9). Shivering was evaluated clinically and by oxygen consumption. Forced-air heating increased mean-skin temperature (mean (SD) 35.7 degrees C (0.4) vs. 33.2 degrees C (0.8); p < 0.0001) and lowered core temperature at the shivering threshold (35.7 degrees C (0.2) vs. 36.4 degrees C(0.2); p < 0.0001). Active warming improved thermal comfort and significantly reduced oxygen consumption from 9.7 (4.4) ml x min(-1) x kg(-1) to 5.6 (1.9) ml x min(-1) x kg(-1) (p = 0.038). However, the duration of shivering was similar in the unwarmed (37 min (11)) and active warming (36 min (10)) groups. Core temperature contributed about four times as much as skin temperature to control of shivering. Cutaneous warming improved thermal comfort and reduced metabolic stress in postoperative patients, but did not quickly obliterate shivering.     

Effects of a Circulating-water Garment and Forced-air Warming on Body Heat Content and Core Temperature

Background: Forced-air warming is sometimes unable to maintain perioperative normothermia. Therefore, the authors compared heat transfer, regional heat distribution, and core rewarming of forced-air warming with a novel circulating-water garment.

Methods: Nine volunteers were each evaluated on two randomly ordered study days. They were anesthetized and cooled to a core temperature near 34[degrees]C. The volunteers were subsequently warmed for 2.5 h with either a circulating-water garment or a forced-air cover. Overall, heat balance was determined from the difference between cutaneous heat loss (thermal flux transducers) and metabolic heat production (oxygen consumption). Average arm and leg (peripheral) tissue temperatures were determined from 18 intramuscular needle thermocouples, 15 skin thermal flux transducers, and "deep" hand and foot thermometers.

Results: Heat production (approximately 60 kcal/h) and loss (approximately 45 kcal/h) were similar with each treatment before warming. The increases in heat transfer across anterior portions of the skin surface were similar with each warming system (approximately 65 kcal/h). Forced-air warming had no effect on posterior heat transfer, whereas circulating-water transferred 21 +/- 9 kcal/h through the posterior skin surface after a half hour of warming. Over 2.5 h, circulating water thus increased body heat content 56% more than forced air. Core temperatures thus increased faster than with circulating water than forced air, especially during the first hour, with the result that core temperature was 1.1[degrees] +/- 0.7[degrees]C greater after 2.5 h (P < 0.001). Peripheral tissue heat content increased twice as much as core heat content with each device, but the core-to-peripheral tissue temperature gradient remained positive throughout the study.     

Heating efficacy of external heat supply during and after open-heart surgery with hypothermia

Heat balance after cardiac surgery with hypothermic cardiopulmonary bypass (CPB) was studied in 156 patients. In spite of rewarming during CPB there was residual hypothermia at the end of operation. This heat deficit could not be prevented by intraoperative use of a heating mattress at 38 degrees C and/or heated (39 degrees C) humidified inspired gases. Postoperatively, in four groups of patients, the core and finger skin temperatures were recorded and the mean skin and mean body temperatures and heat balance were calculated. Heating of humidified inspired gases (n = 22) gave little improvement in the time course of the temperatures and heat balance as compared with that in a control group with no external warming postoperatively (n = 49). A thermal ceiling (a low-temperature radiator suspended above the bed and providing radiant heat (n = 35] significantly increased the measured temperatures, which were restored to normal earlier than those in the controls. Also, with this radiant heat postoperative shivering was almost abolished. With a combination of radiant heat and heated, humidified gases (n = 50), the postoperative heat balance was improved somewhat further. For patients treated with radiant heat, postoperative rewarming was accomplished in a shorter time and almost without active endogenous muscular thermogenesis, as was evident by the great reduction in postoperative shivering.     

Body Morphology and the Speed of Cutaneous Rewarming

Background: Infants and children cool quickly because their surface area (and therefore heat loss) is large compared with their metabolic rate, which is mostly a function of body mass. Rewarming rate is a function of cutaneous heat transfer plus metabolic heat production divided by body mass. Therefore, the authors tested the hypothesis that the rate of forced-air rewarming is inversely related to body size.

Methods: Isoflurane, nitrous oxide, and fentanyl anesthesia were administered to infants, children, and adults scheduled to undergo hypothermic neurosurgery. All fluids were warmed to 37[degrees]C and ambient temperature was maintained near 21[degrees]C. Patients were covered with a full-body, forced-air cover of the appropriate size. The heater was set to low or ambient temperature to reduce core temperature to 34[degrees]C in time for dural opening. Blower temperature was then adjusted to maintain core temperature at 34[degrees]C for 1 h. Subsequently, the forced-air heater temperature was set to high ([almost equal to] 43[degrees]C). Rewarming continued for the duration of surgery and postoperatively until core temperature exceeded 36.5[degrees]C. The rewarming rate in individual patients was determined by linear regression.

Results: Rewarming rates were highly linear over time, with correlations coefficients (r2) averaging 0.98 +/- 0.02. There was a linear relation between rewarming rate ([degrees]C/h) and body surface area (BSA; m2): Rate ([degrees]C/h) = -0.59 [middle dot] BSA (m2) + 1.9, r2 = 0.74. Halving BSA thus nearly doubled the rewarming rate.     

Effects of a circulating-water garment and forced-air warming on body heat content and core temperature

BACKGROUND: Forced-air warming is sometimes unable to maintain perioperative normothermia. Therefore, the authors compared heat transfer, regional heat distribution, and core rewarming of forced-air warming with a novel circulating-water garment. METHODS: Nine volunteers were each evaluated on two randomly ordered study days. They were anesthetized and cooled to a core temperature near 34 degrees C. The volunteers were subsequently warmed for 2.5 h with either a circulating-water garment or a forced-air cover. Overall, heat balance was determined from the difference between cutaneous heat loss (thermal flux transducers) and metabolic heat production (oxygen consumption). Average arm and leg (peripheral) tissue temperatures were determined from 18 intramuscular needle thermocouples, 15 skin thermal flux transducers, and "deep" hand and foot thermometers. RESULTS: Heat production (approximately 60 kcal/h) and loss (approximately 45 kcal/h) were similar with each treatment before warming. The increases in heat transfer across anterior portions of the skin surface were similar with each warming system (approximately 65 kcal/h). Forced-air warming had no effect on posterior heat transfer, whereas circulating-water transferred 21+/-9 kcal/h through the posterior skin surface after a half hour of warming. Over 2.5 h, circulating water thus increased body heat content 56% more than forced air. Core temperatures thus increased faster than with circulating water than forced air, especially during the first hour, with the result that core temperature was 1.1 degrees +/- 0.7 degrees C greater after 2.5 h (P < 0.001). Peripheral tissue heat content increased twice as much as core heat content with each device, but the core-to-peripheral tissue temperature gradient remained positive throughout the study. CONCLUSIONS: The circulating-water system transferred more heat than forced air, with the difference resulting largely from posterior heating. Circulating water rewarmed patients 0.4 degrees C/h faster than forced air. A substantial peripheral-to-core tissue temperature gradient with each device indicated that peripheral tissues insulated the core, thus slowing heat transfer.     

Efficacy of intraoperative heat administration by ventilation with warm humidified gases and an oesophageal warming system

We measured changes in body temperature in 12 hypothermic (mean aural temperature 34.4 (SD 1.0) degrees C) pigs during general anaesthesia with an open abdominal cavity and the effect of two warming systems: heating of inspired gases to 39 degrees C (intratracheal temperature) and oesophageal warming to 39 degrees C by a water perfused oesophageal heat exchanger. Each animal underwent both treatments and the control period in random sequence. Each condition was studied over 1 h. No additional protection against heat loss (drapes, blankets, i.v. fluids warming, etc.) was used. Anaesthesia, room temperature and relative humidity, amount and temperature of infusions and extension of exposed visceral surfaces were standardized. Mean decrease in body temperature was 1.0 (0.7) degree C (P < 0.005) without warming and 0.6 (0.2) degree C (P < 0.005) with heated inspired gases: this difference was not statistically significant. Oesophageal warming was very efficient as mean body temperature did not change significantly (-0.1 (0.2) degree C; ns).      

Effects of sympathetic blockade on the efficiency of forced-air warming during combined spinal-epidural anesthesia for total hip arthroplasty.

STUDY OBJECTIVE: To evaluate if active cutaneous warming of the two upper limbs with reflex vasoconstriction is less effective in maintaining intraoperative normothermia than warming the vasodilated unoperated lower limb during combined spinal-epidural anesthesia (CSE). DESIGN: Prospective, randomized study. SETTING: Inpatient anesthesia at university departments of orthopedic surgery. PATIENTS: 48 ASA physical status I, II, and III patients, who were scheduled for elective total hip arthroplasty. INTERVENTIONS: Patients received CSE with intrathecal injection of 15 mg of 0.5% hyperbaric bupivacaine. All procedures started 8 to 10 AM, and operating room temperature was maintained between 21 degrees and 23 degrees C, with relative humidity ranging between 40% and 45%. For warming therapy, patients received active forced-air warming of either the two upper limbs (Group Upper body, n = 24), or the unoperated lower limb (Group Lower extremity, n = 24). Core temperature was measured before CSE placement (baseline), and then every 30 minutes until completion of surgery. Time for fulfillment of clinical discharging criteria from the recovery area was evaluated by a blinded observer. MEASUREMENTS AND MAIN RESULTS: Demographic data, duration of surgery, intraoperative blood losses, crystalloid infusion, and hemodynamic variables were similar in the two groups. Core temperature slightly decreased in both groups, but at the end of surgery the mean core temperature was 36.2 degrees +/- 0.5 degree C in Group Upper body and 36.3 +/- 0.5 in Group Lower extremity (NS). At recovery room arrival, seven patients in Group Upper body (29%) and three patients in Group Lower extremity (12.5%) had a core temperature less than 36 degrees C (NS). Shivering was observed in one patient in Group Upper body and in two patients in Group Lower extremity (NS). Clinical discharging criteria were fulfilled after 37 +/- 16 minutes in Group Upper body and 30 +/- 32 minutes in Group Lower extremity (NS). CONCLUSIONS: Forced-air cutaneous warming allows the anesthesiologist to maintain normothermia during CSE for total hip replacement even if the convective blanket is placed on a relatively small skin surface with reflex vasoconstriction. Placing the forced-air warming system on the vasodilated unoperated lower limb may be troublesome to the surgeons and does not offer clinically relevant advantages in warming efficiency.     

A prospective comparison of three heat preservation methods for patients undergoing hypothermic cardiopulmonary bypass

OBJECTIVE: To prospectively compare 3 methods of body heat preservation in patients undergoing surgery requiring the use of hypothermic cardiopulmonary bypass (CPB). DESIGN: Prospective, randomized, and nonblinded. SETTING: University teaching hospital. PARTICIPANTS: Adult cardiac surgery patients (n = 101). INTERVENTIONS: Subjects were randomly assigned to 1 of 3 treatment groups: Group 1 (n = 33) used a fluid-filled warming blanket, group 2 (n = 31) used a heated and humidified breathing circuit, and group 3 (n = 37) used intravenous fluid warmers for the administration of all fluids. Treatments started on separation from CPB and concluded at the end of the intraoperative experience. Anesthetic technique, minute ventilation, conduct of CPB, and room temperature in the operating room were standardized. MEASUREMENTS AND MAIN RESULTS: Blood temperature was measured at its nadir on CPB, on separation from CPB, and just before departure from the operating room. No differences were found among groups for CPB duration, coldest venous temperature on CPB, rewarming time, rate of rewarming, room temperature, or blood temperature on separation from CPB. There were no significant differences found in post-CPB temperature afterdrop among groups. CONCLUSIONS: This study suggests that there is no statistically significant disparity in the effectiveness of these 3 intraoperative heat preservation methods. Ease of use and cost-effectiveness should guide the choice of warming method post-CPB.