Epidural versus general anesthesia, ambient operating room temperature, and patient age as predictors of inadvertent hypothermia

Warmed humidified inspired oxygen accelerates postoperative rewarming

STUDY OBJECTIVE

To investigate the efficacy of warmed, humidified inspired oxygen (O(2)) for the treatment of mildly hypothermic postoperative patients.

DESIGN

Prospective, randomized, unblinded clinical trial.

SETTING

Postanesthesia care unit in a tertiary care hospital.

PATIENTS AND INTERVENTIONS

30 ASA physical status I, II, and III patients following intraabdominal surgical procedures were randomly assigned to receive either routine O(2) therapy (control group, n = 15), or warmed (42 degrees C) humidified O(2) (treatment group, n = 15) for the initial 90 postoperative minutes.

MEASUREMENTS

Core (tympanic) temperature, dry mouth score and shivering score.

MAIN RESULTS

Tympanic temperature was similar in both groups on admission ( approximately 35.8 degrees C). Rewarming rate in the first postoperative hour was greater in the treatment group (0.7 +/- 0.1 degrees C. hr(-1)) compared to the control group (0.4 +/- 0.1 degrees C. hr(-1)) (p = 0.03). Patients receiving the warmed, humidified O(2) had a lower incidence of dry mouth compared to the control group (p = 0.03). The incidence of shivering was low and similar in both groups.

CONCLUSIONS

Warming and humidifying inspired O(2) hastens recovery from hypothermia in postoperative patients.

Determinants of core temperature at the time of admission to intensive care following cardiac surgery

OBJECTIVE

To determine the predictors of core temperature on arrival in the intensive care unit (ICU) after cardiac surgery.

DESIGN

Prospective, randomized trial.

SETTING

Tertiary care medical center, operating rooms (ORs), and ICU.

PATIENTS

72 patients presenting for coronary artery bypass surgery.

INTERVENTIONS

Randomized assignment for ambient OR temperature (16-18 degrees C vs. 21-23 degrees C) and rewarming endpoint on cardiopulmonary bypass (CPB; nasopharyngeal and urinary bladder temperatures >/=36.5 degrees C and 34.0 degrees C, respectively, vs. nasopharyngeal and urinary bladder temperatures >/=37.5 degrees C and 36.0 degrees C, respectively) at the time of separation from bypass.

MEASUREMENTS AND MAIN RESULTS

The best (and only significant) predictor of core temperature on arrival in the ICU was rewarming endpoint at the time of separation from CPB (p = 0.004). Patient weight, height, body habitus, and nitroprusside administration did not significantly predict core temperature. Ambient temperature affected only body temperature when the duration of time in the OR after separation from bypass was prolonged (>90 min). A weighted average body temperature was a better predictor of complete rewarming than was any single monitoring site.

CONCLUSIONS

To reduce the incidence of hypothermia after cardiac surgery, the most important variable is rewarming endpoint achieved before separation from bypass. A warm ambient temperature (>21 degrees C) may be beneficial if the duration of time in the OR after bypass is prolonged (>90 min).

The accuracy and precision of body temperature monitoring methods during regional and general anesthesia

We tested the hypotheses that accuracy and precision of available temperature monitoring methods are different between spinal anesthesia (SA) and general anesthesia (GA), and that patients receiving SA are at equal risk for hypothermia as those receiving GA. Patients scheduled for radical retropubic prostatectomy were enrolled. Either GA (n = 16) or SA (n = 16) was given according to patient and clinician preference. Temperatures were monitored with thermocouple probes at the tympanic membrane, axilla, rectum, and forehead skin surface. Tympanic temperatures were also measured with an infrared device, and forehead skin temperatures were monitored with two brands of liquid crystal thermometer strips. Accuracy and precision of these monitoring methods were determined by using tympanic membrane temperature, measured by thermocouple, as the reference core temperature (T(c)). At the end of surgery, T(c) was similar between SA (35.0 +/- 0.1 degrees C) and GA (35.2 +/- 0.1 degrees C) (P = 0.44). Accuracy and precision of each temperature monitoring method were similar between SA and GA. Rectal temperature monitoring offered the greatest combination of accuracy and precision. All other methods underestimated T(c). These findings suggest that patients receiving SA or GA are at equal and significant risk for hypothermia, and should have their temperatures carefully monitored, recognizing that most monitoring methods underestimate T(c).

IMPLICATIONS

Body temperature should be monitored during spinal anesthesia because patients are at significant risk for hypothermia. Rectal temperature is a valid method of measuring core temperature, whereas other methods tend to underestimate true core temperature.

Age-related thermoregulatory differences in a warm operating room environment (approximately 26 degrees C)

Inadvertent hypothermia occurs frequently at typical ambient operating room (OR) temperatures, especially in elderly patients receiving general anesthesia. The aims of the current study were to 1) determine the incidence and magnitude of core hypothermia in an unusually warm OR environment, and 2) to assess age-related differences in perioperative thermoregulatory responses under these circumstances. Forty patients receiving general anesthesia for orthopedic surgical procedures (20 younger patients, 20-40 yr old) and (20 older patients, 60-75 yr old) were enrolled. Mean ambient temperature in the ORs was 25.8 degrees +/- 0.2 degrees C. Core temperature, vasoconstriction, and shivering were compared in the younger and older age groups. Mean core temperature on admission to the postanesthesia care unit was not significantly different in the younger (36.7 degrees +/- 0.1 degrees C) and older (36.4 degrees +/- 0.1 degrees C) age groups. Only 10% of patients (n = 4, 1 younger, 3 older) were admitted with a core temperature <36.0 degrees C. Only 2% of patients (n = 1, older group) had a core temperature <35.5 degrees C. This very mild degree of hypothermia was associated with postoperative vasoconstriction in 80% of the younger and 55% of the older patients (P = 0.18). Postoperative shivering occurred in 40% of the younger patients and in 10% of the older patients (P = 0.06). In summary, an ambient OR temperature near 26 degrees C (79 degrees F) is effective in preventing core hypothermia during general anesthesia regardless of patient age. Even very mild postoperative hypothermia may initiate thermoregulatory responses.

IMPLICATIONS

By increasing ambient temperature in the operating room to 26 degrees C (79 degrees F), the incidence of core hypothermia can be dramatically reduced in both younger and older patients.

Anesthesia of the sighthound

The sighthounds are an ancient group of dog breeds that have been selectively bred for high-speed pursuit of prey by sight. Probably as a consequence of this selection process, these dogs have a number of idiosyncrasies that can potentially adversely affect their anesthetic management. These include (1) nervous demeanor which can lead to stress-induced clinical complications, such as hyperthermia; (2) lean body conformation with high surface-area-to-volume ratio, which predisposes these dogs to hypothermia during anesthesia; (3) hematological differences such as a higher packed cell volume and lower serum protein compared with other dog breeds which may complicate interpretation of preanesthetic blood work; (4) Impaired biotransformation of drugs by the liver resulting in prolonged recovery from certain intravenous anesthetics, especially thiopental; and increased risks of drug interactions. Safe anesthetic management of sighthounds should include sedative premedication and appropriate use of analgesic drugs to minimize perioperative stress. Thiopental, or any other thiobarbiturate, should not be used in these dogs. Propofol, ketamine/diazepam combination, and methohexital are recommended alternative intravenous anesthetics. Avoid coadministration of agents that inhibit drug biotransformation, such as chloramphenicol. Inhalation anesthesia using isoflurane is the preferred anesthetic maintenance technique. Core body temperature should be monitored closely and techniques to minimize hypothermia should be employed both during anesthesia and into the recovery period.

Assessment of cardiovascular risks and overall risks for noncardiac surgery

Appropriate care of the elderly patient requires a concerted multi-disciplinary approach before, during, and after surgery to optimize functional outcomes, with the principal focus placed on improving quality of life and strategies for risk reduction. Perioperative physicians must be able to assess the biologic, not the chronologic, age of geriatric patients and their capacity for independent function. Physicians need to understand alterations in the physiology of elderly patients attributable to the normal aging process as well as the prevalence of concurrent pathologic conditions that necessitate special precautions. Maintaining autonomy and function as a result of an acute surgical intervention may be the most important outcome to the elderly patient. Most of the data available and guidelines promulgated do not specifically address the elderly population. It is important to collect data prospectively and use sophisticated methods for analyses to develop better management algorithms for these (often complicated) clinical issues in the elderly.

Effects of clonidine on prolonged postoperative sympathetic response

OBJECTIVE

Surgical trauma results in diffuse sympathoadrenal activation which is thought to contribute to perioperative cardiovascular complications in high-risk patients. Regional anesthetic and analgesic techniques can attenuate this "stress response" and reduce the occurrence rate of adverse perioperative events; however, their use in the postoperative period is logistically difficult and costly. The present study was undertaken to evaluate whether transdermal administration of the alpha2 adrenergic-receptor agonist, clonidine, can be used as a pharmacologic means of blunting the stress response throughout the perioperative period.

DESIGN

Double-blind, placebo-controlled clinical trial in patients undergoing pancreatico-biliary surgery.

SETTING

Operating rooms and surgical intensive care unit of a major university teaching hospital.

PATIENTS

Forty patients scheduled for major upper abdominal surgery.

INTERVENTIONS

Patients received either clonidine (0.2 mg orally and a clonidine TTS-3 patch the evening before surgery and 0.3 mg orally on call to the operating room) or matched oral and transdermal placebo.

MEASUREMENTS AND MAIN RESULTS

Heart rate, systemic arterial blood pressure, plasma catecholamine, clonidine, interleukin-6 concentrations, and 24-hr urine cortisol and nitrogen excretion were measured the day before surgery and daily thereafter for 72 hrs postoperatively. Preoperative transdermal (and oral) clonidine administration resulted in therapeutic plasma clonidine concentrations throughout the perioperative period (1.54 +/- .07 [SEM] microg/mL). Clonidine reduced preoperative epinephrine and norepinephrine concentrations by 65%. Plasma catecholamine concentrations increased in both groups following surgery but were markedly lower throughout the postoperative period in patients receiving clonidine. Patients receiving clonidine had a reduced frequency rate of postoperative hypertension. Clonidine had no effect on plasma interleukin-6 concentration, urine cortisol excretion, or urine nitrogen excretion. No adverse effects of clonidine administration were observed.

CONCLUSIONS

The combined administration of oral and transdermal clonidine effectively attenuated the catecholamine response to surgical stress throughout the postoperative study period. Clonidine administration produced specific sympatholytic effects, since other elements of the stress response were not attenuated. Undesirable side effects were not noted. The sustained sympatholytic effects we observed suggest that alpha2 adrenergic-receptor agonists may offer a pharmacologic means of modifying the sympathoadrenal response to injury, and may be useful in reducing perioperative complications.

The threshold for thermoregulatory vasoconstriction during nitrous oxide/sevoflurane anesthesia is reduced in the elderly

Elderly patients become more hypothermic during surgery, shiver less postoperatively, and take longer to rewarm than younger patients. Similarly, the vasoconstriction threshold (triggering core temperature) is reduced approximately 1 degree C in elderly patients during nitrous oxide/isoflurane anesthesia. Accordingly, we tested the hypothesis that the vasoconstriction threshold in the elderly is also reduced approximately 1 degree C during nitrous oxide and sevoflurane anesthesia. Eleven young patients aged 30-50 yr and 14 elderly patients aged 60-80 yr were anesthetized with nitrous oxide (50%) and sevoflurane (1%). Mean skin temperature was calculated from four sites. Fingertip blood flow was estimated using forearm minus fingertip skin-temperature gradients, with a gradient of 0 degree C identifying onset of vasoconstriction. The distal esophageal temperature triggering onset of vasoconstriction identified the threshold for this thermoregulatory defense. The data from five patients who did not vasoconstrict at minimum core temperatures of 33-34 degrees C were eliminated, leaving 10 patients in each group. The vasoconstriction threshold was significantly less in the elderly (35.0 +/- 0.8 degrees C) than in younger patients (35.8 +/- 0.3 degrees C), despite similar mean skin temperatures (mean +/- SD, P < 0.01, Student's t-test). Age dependence of thermoregulatory vasoconstriction during nitrous oxide/sevoflurane anesthesia is similar to that previously observed during nitrous oxide/isoflurane anesthesia.

Comparison of distal oesophageal temperature with "deep" and tracheal temperatures

PURPOSE

To compare distal oesophageal (reference) temperature with "deep-sternal," "deep-forehead," and tracheal temperatures, establishing the accuracy and precision of each.

METHODS

We studied 20 patients undergoing general anaesthesia for gynaecological surgery. Their lungs were mechanically ventilated with a circle system, at a fresh-gas flow rate of 6 L.min-1 Respiratory gases were not warmed or humidified. Tracheal temperatures were recorded from a Trachelon tube inserted approximately 21 cm. Deep-body temperatures were measured at the sternum and forehead using a Coretemp thermometer. The principle of the method is to null thermal flux through a cutaneous disk, thus obliterating thermal gradients between the sides of the disk, skin surface, and subcutaneous tissues. Distal oesophageal temperatures were measured from thermocouples incorporated into oesophageal stethoscopes. Tracheal and deep-tissue temperatures were compared with oesophageal temperature using regression and Bland and Altman analyses.

RESULTS

Tracheal, sternal, and forehead temperatures correlated similarly with distal oesophageal temperature, correlation coefficients (r2) being 0.7 in each case. The offset (oesophageal temperature minus study site) was considerably larger for tracheal temperature (0.7 degree C) than for the other sites (0.2 degree C). However, the precision was only 0.3 degree C at each site.

CONCLUSION

Our data suggest that tracheal temperatures may not be an adequate substitute for conventional core-temperature monitoring sites. In contrast, the accuracy and precision of deep-tissue temperature monitoring at the sternum and forehead was sufficient for clinical use.

Enflurane decreases the threshold for vasoconstriction more than isoflurane or halothane

Intraoperative hypothermia results largely from anesthetic-induced inhibition of tonic thermoregulatory vasoconstriction. Sufficient hypothermia, however, triggers peripheral vasoconstriction, which usually prevents further decrease in core temperature. The thermoregulatory effects of all volatile anesthetics have been tested in adults and/or children, but different anesthetics have not been directly compared. We therefore evaluated thermoregulatory responses during enflurane, isoflurane, and halothane administration. Anesthesia was maintained with 1 minimum alveolar anesthetic concentration (MAC) of halothane, isoflurane, or enflurane in 27 patients undergoing intraabdominal surgery. Patients were maintained normovolemic and normocapnic but were allowed to cool passively. A forearm minus fingertip, skin-temperature gradient of 4 degrees C identified significant vasoconstriction; the core temperature triggering vasoconstriction identified the threshold. Morphometric characteristics, initial core temperatures, ambient operating room temperatures, blood pressures, and anesthetic potencies were similar in each group. All eight patients given halothane vasoconstricted at a core temperature of 35.5 +/- 0.6 degrees C. Eight of the patients given isoflurane vasoconstricted at a core temperature of 35.2 +/- 0.5 degrees C. However, two others did not at minimum core temperatures of 34.0 and 33.8 degrees C. Only one patient given enflurane vasoconstricted at a core temperature of 34.6 degrees C. The other six patients never vasoconstricted, at minimum core temperatures of 33.6 +/- 0.4 degrees C. Our data indicate that enflurane profoundly inhibits thermoregulatory responses in children. The mechanism for this extraordinary inhibition remains unknown but does not result from any obvious anesthetic pharmacology or thermoregulatory physiology. We conclude that unwarmed pediatric patients will become colder when anesthetized with enflurane than with halothane or isoflurane.

Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. Study of Wound Infection and Temperature Group

BACKGROUND

Mild perioperative hypothermia, which is common during major surgery, may promote surgical-wound infection by triggering thermoregulatory vasoconstriction, which decreases subcutaneous oxygen tension. Reduced levels of oxygen in tissue impair oxidative killing by neutrophils and decrease the strength of the healing wound by reducing the deposition of collagen. Hypothermia also directly impairs immune function. We tested the hypothesis that hypothermia both increases susceptibility to surgical-wound infection and lengthens hospitalization.

METHODS

Two hundred patients undergoing colorectal surgery were randomly assigned to routine intraoperative thermal care (the hypothermia group) or additional warming (the normothermia group). The patient's anesthetic care was standardized, and they were all given cefamandole and metronidazole. In a double-blind protocol, their wounds were evaluated daily until discharge from the hospital and in the clinic after two weeks; wounds containing culture-positive pus were considered infected. The patients' surgeons remained unaware of the patients' group assignments.

RESULTS

The mean (+/- SD) final intraoperative core temperature was 34.7 +/- 0.6 degrees C in the hypothermia group and 36.6 +/- 0.5 degrees C in the normothermia group (P < 0.001) Surgical-wound infections were found in 18 of 96 patients assigned to hypothermia (19 percent) but in only 6 of 104 patients assigned to normothermia (6 percent, P = 0.009). The sutures were removed one day later in the patients assigned to hypothermia than in those assigned to normothermia (P = 0.002), and the duration of hospitalization was prolonged by 2.6 days (approximately 20 percent) in hypothermia group (P = 0.01).

CONCLUSIONS

Hypothermia itself may delay healing and predispose patients to wound infections. Maintaining normothermia intraoperatively is likely to decrease the incidence of infectious complications in patients undergoing colorectal resection and to shorten their hospitalizations.

Effect of continuously warmed i.v. fluids on intraoperative hypothermia

The investigators examined the effect of infusing continuously warmed (ie, 37.0 degrees C [98.6 degrees F]) i.v. fluids in two groups of middle-aged female patients undergoing laparoscopic cholecystectomy procedures. They hypothesized that increasing i.v. fluid temperature during surgery would decrease patients' risk for hypothermia. One group of patients received prewarmed i.v. fluids that cooled to room temperature during surgery. The second group received i.v. fluids that were warmed continuously by a fluid warmer during the surgical procedures. Analyses of covariance, with the first intraoperative temperature measurement treated as the covariate, revealed nonsignificant results at the P < .05 level. The results suggest that administering continuously warmed i.v. fluids intraoperatively has no significant effect on maintaining patients' body temperatures during short laparoscopic surgical procedures.

Mild hypothermia increases blood loss and transfusion requirements during total hip arthroplasty

BACKGROUND

In-vitro studies indicate that platelet function and the coagulation cascade are impaired by hypothermia. However, the extent to which perioperative hypothermia influences bleeding during surgery remains unknown. Accordingly, we tested the hypothesis that mild hypothermia increases blood loss and allogeneic transfusion requirements during hip arthroplasty.

METHODS

Blood loss and transfusion requirements were evaluated in 60 patients undergoing primary, unilateral total hip arthroplasties who were randomly assigned to normothermia (final intraoperative core temperature 36.6 [0.4] degrees C) or mild hypothermia (35.0 [0.5] degrees C). Crystalloid, colloid, scavenged red cells, and allogeneic blood were administered by strict protocol.

FINDINGS

Intra- and postoperative blood loss was significantly greater in the hypothermic patients: 2.2 (0.5) L vs 1.7 (0.3) L, p < 0.001). Eight units of allogeneic packed red cells were required in seven of the 30 hypothermic patients, whereas only one normothermic patient required a unit of allogeneic blood (p < 0.05 for administered volume). A typical decrease in core temperature in patients undergoing hip arthroplasty will thus augment blood loss by approximately 500 mL.

INTERPRETATION

The maintenance of intraoperative normothermia reduces blood loss and allogeneic blood requirements in patients undergoing total hip arthroplasty.

The etiology and management of inadvertent perioperative hypothermia

Mild perioperative hypothermia is a frequent complication of anesthesia and surgery. Core temperature should be monitored during general anesthesia and during regional anesthesia for large operations. Reliable sites of core temperature monitoring include the tympanic membrane, nasopharynx, esophagus, bladder, rectum, and pulmonary artery. The skin surface is not an acceptable site for monitoring core temperature. Anesthetic-induced vasodilation initially rapidly decreases core temperature secondary to an internal redistribution of heat rather than an increased heat loss to the environment. Both general and regional anesthetics impair thermoregulation, increasing the interthreshold range; that is, the range of core temperatures over which no autonomic response to cold or warmth occurs. Preinduction skin surface warming is the only means to prevent this initial redistribution hypothermia. Forced-air warming is the most effective method of rewarming hypothermic patients intraoperatively.

Midazolam minimally impairs thermoregulatory control

Perioperative hypothermia usually results largely from pharmacologic inhibition of normal thermoregulatory control. Midazolam is a commonly used sedative and anesthetic adjuvant whose thermoregulatory effects are unknown. We therefore tested the hypothesis that midazolam administration impairs thermoregulatory control. Eight volunteers were studied on 2 days each, once without drug and once at a target total plasma midazolam concentration of 0.3 micrograms/mL (corresponding to administration of approximately 40 mg over approximately 4 h). Each day, skin and core temperatures were increased sufficiently to provoke sweating, and then reduced to elicit peripheral vasoconstriction and shivering. We mathematically compensated for changes in skin temperature using the established linear cutaneous contributions to control of each response. From these calculated thresholds (core temperatures triggering responses at a designated skin temperature of 34 degrees C), we determined the thermoregulatory effects of midazolam. The sweating threshold was decreased approximately 0.3 degrees C by midazolam administration: 37.3 +/- 0.2 degrees C vs 37.0 +/- 0.3 degrees C (P = 0.0004, paired t-test). Midazolam decreased the core temperature that triggered vasoconstriction somewhat more: 37.1 +/- 0.2 degrees C vs 36.3 +/- 0.5 degrees C (P = 0.0002). Similarly, midazolam decreased the shivering threshold: 35.9 +/- 0.3 degrees C vs 35.3 +/- 0.6 degrees C (P = 0.03). The sweating-to-vasoconstriction (interthreshold) range, therefore, increased from 0.2 +/- 0.1 degrees C to 0.7 +/- 0.3 degrees C (P = 0.002). Although statistically significant, this relatively small increase contrasts markedly with the 3-5 degrees C interthreshold ranges produced by clinical doses of volatile anesthetics, propofol, and opioids.(ABSTRACT TRUNCATED AT 250 WORDS)

Postoperative hemodynamic and thermoregulatory consequences of intraoperative core hypothermia

STUDY OBJECTIVE

To evaluate the postoperative hemodynamic and thermoregulatory consequences of intraoperative core hypothermia.

DESIGN

Prospective, randomized clinical trial.

SETTING

Operating room and postanesthesia care unit of a university hospital.

PATIENTS

74 healthy, ASA status I, II, and III patients (average age 58 yrs) undergoing elective colon surgery.

INTERVENTIONS

Patients were randomly assigned to be kept normothermic or approximately 2.5 degrees C hypothermic during surgery. Anesthesia was maintained with isoflurane, nitrous oxide, and fentanyl. Postoperatively, surgical pain was treated with patient-controlled analgesia (PCA) opioid.

MEASUREMENTS AND MAIN RESULTS

An observer blinded to group assignment and core temperatures evaluated shivering, thermal comfort, surgical pain, heart rates (HRs), and blood pressures (BPs) during the first six postoperative hours. Morphometric characteristics, oxygen saturation, fluid balance, PCA-administered opioid, and visual analog pain scores were comparable in the two groups. Hypothermic patients felt uncomfortably cold during recovery, and their postoperative core temperatures remained significantly less than in the normothermic patients for more than four hours. Peripheral vasoconstriction and shivering were common in the hypothermic patients but rare in those kept normothermic. HRs and BPs were comparable in the two groups.

CONCLUSIONS

These data confirm that the effects of intraoperative hypothermia on postoperative HR and BP are modest in relatively young, generally healthy patients. In contrast, intraoperative hypothermia caused substantial postoperative thermal discomfort, and full recovery from hypothermia required many hours. Delayed return to care normothermia apparently resulted largely from postoperative thermoregulatory impairment.

Propofol and postanaesthetic shivering

Sixty ASA grade 1 unpremedicated patients scheduled for minor elective surgery were randomly allocated to receive general anaesthesia consisting of either propofol-nitrous oxide in oxygen or a conventional technique of thiopentone-isoflurane-nitrous oxide-oxygen. Baseline axillary temperature readings, duration of operation and intra-operative decrease in axillary temperature were similar in both groups. The patients who received propofol-nitrous oxide-oxygen anaesthesia had a significantly lower incidence of postanaesthetic shivering than the control group. A propofol-nitrous oxide-oxygen technique may be preferable when postanaesthetic shivering is deemed undesirable.

Intraoperative blood loss during radical retropubic prostatectomy: epidural versus general anesthesia

OBJECTIVES

There are conflicting reports on the influence of different anesthetic techniques, such as regional versus general anesthesia, on intraoperative blood loss. The purpose of this study was to elucidate the effects of anesthetic technique on intraoperative blood loss in men undergoing radical retropubic prostatectomy (RRP).

METHODS

One hundred patients undergoing RRP for prostate cancer were randomly assigned to receive either epidural anesthesia (EA), combined epidural and general anesthesia (EG), or general anesthesia alone (GA). Intraoperative blood loss was calculated by using a formula that accounted for the volume and hematocrit of the fluid suctioned from the surgical field, blood absorbed on surgical pads, and the patient's hematocrit.

RESULTS

Mean blood loss in the EA group (1490 +/- 90 mL; mean +/- SEM) was significantly less than mean blood loss in both the EG group (1810 +/- 100 mL) and the GA group (1940 +/- 130 mL) (P = 0.01). Blood loss was not different between the EG and the GA groups (P = 0.7). Significantly less blood was transfused during surgery in the EA group (730 +/- 50 mL) compared with the EG (960 +/- 60 mL) and GA (950 +/- 70 mL) groups (P = 0.02).

CONCLUSIONS

Similar blood loss in patients receiving general anesthesia, either alone or when combined with epidural anesthesia, implies that epidural anesthesia did not reduce bleeding, but, rather, that general anesthesia increased blood loss.

Mild hypothermia alters propofol pharmacokinetics and increases the duration of action of atracurium

Mild intraoperative hypothermia is common. We therefore studied the effects of mild hypothermia on propofol pharmacokinetics, hepatic blood flow, and atracurium duration of action in healthy volunteers. Six young volunteers were studied on two randomly assigned days, at either 34 degrees C or 37 degrees C. Anesthesia was induced with thiopental, 3 mg/kg, and maintained with 70% N2O and 0.6% isoflurane. Core hypothermia was induced by conductive and convective cooling. On the other study day, normothermia was maintained by a Bair Hugger (Augustine Medical, Inc., Eden Prairie, MN) forced-air warmer. Propofol, 1 mg/kg lean body mass (LBM), then was given, followed by a 4-h infusion at 5 mg.kg-1.h-1. After 2 h, atracurium 0.5 mg/kg was administered as an intravenous bolus. Indocyanine green was administered for estimation of hepatic blood flow. Arterial blood was assayed for propofol and indocyanine green concentration. Pharmacokinetic analysis was performed using NONMEM. Results are reported as means +/- SEM. Propofol blood concentrations averaged approximately 28% more at 34 degrees C than at 37 degrees C (P < 0.05). Hepatic blood flow decreased 23% +/- 11% in normothermic volunteers during the propofol infusion, and 33% +/- 11% in hypothermic volunteers (P = not significant). A three-compartment mamillary model fitted the data best. Inclusion of hepatic blood flow change from the prepropofol baseline as a covariate for total body clearance significantly improved the fit. The intercompartmental clearances were decreased in the presence of hypothermia.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of postoperative hypothermia after normothermic cardiopulmonary bypass

Inadvertent postoperative hypothermia in the cardiac surgical patient can have various adverse physiologic effects. Previous studies have investigated the relationship of patient, surgical, and anesthetic factors with postoperative hypothermia in patients undergoing noncardiac surgery. This study was designed to assess the relationship between postoperative hypothermia after normothermic cardiopulmonary bypass (CPB) for cardiac surgery and a variety of perioperative and patient factors. Fifty-six patients undergoing daytime elective or urgent cardiac surgery with warm (37 degrees C) CPB were studied. The following patient variables were included: age, weight, height, sex, history of previous cardiac surgery, and prebypass temperature. The following treatment factors were recorded: type of surgery, type and dose of anesthetic, use of airway humidifier, use of an intravenous (i.v.) fluid warmer, total volume of i.v. fluid administered during surgery, net fluid volume administered via CPB, total time spent on CPB, use of nitroglycerin, use of alpha-agonists during surgery, and elapsed time from end of CPB to end of surgery. Core temperature readings, as measured by a pulmonary artery catheter thermistor, were noted as follows: (1) on insertion of the pulmonary artery catheter; (2) after the patient was weaned from CPB; (3) within 30 minutes of intensive care unit (ICU) arrival; (4) 3 to 5 hours after ICU arrival; (5) 7 to 9 hours after ICU arrival; and (6) 11 to 13 hours after ICU arrival. Multiple linear regression and logistic regression for categorical variables with backward elimination were employed to determine the impact of all variables on lowest postoperative temperature. The lowest mean temperature occurred during CPB.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothermia during elective abdominal aortic aneurysm repair: the high price of avoidable morbidity

PURPOSE

Adverse outcomes apparently associated with hypothermia led us to examine patients undergoing elective abdominal aortic aneurysm (AAA) repairs to test the hypothesis that hypothermia (temperature less than 34.5 degrees C) is associated with increased morbidity and excess mortality rates.

METHODS

Two hundred sixty-two elective AAA repairs were retrospectively reviewed for preoperative and intraoperative risk factors. Core temperature, age, Acute Physiology and Chronic Health Evaluation (APACHE) II and APACHE III scores (raw and temperature-adjusted), fluid resuscitation, and perioperative organ dysfunction were recorded prospectively. Outcome measures included lengths of stay in the intensive care unit and in the hospital, and hospital mortality rates.

RESULTS

Except for a higher risk of hypothermia in women (p < 0.05), by univariate analysis, preoperative risk factors were similar in patients in the hypothermic and normothermic groups. After operation, patients with hypothermia had significantly greater APACHE scores (p < 0.0001), and patients in the hypothermic nonsurvivor group took significantly longer to rewarm (p < 0.05), suggesting marked hypoperfusion. Patients with hypothermia had significantly greater fluid (p < 0.05), transfusion (p < 0.01), vasopressor (p < 0.05), and inotrope (p < 0.05) requirements, resulting in significantly higher incidences of organ dysfunction (53.0% vs 28.7%, p < 0.01) and death (12.1% vs 1.5%, p < 0.01) and markedly prolonged lengths of stay in the unit (9.2 +/- 2.0 vs 5.3 +/- 0.6, p < 0.05) and in the hospital (24.3 +/- 2.9 vs 15.0 +/- 0.08, p < 0.01). By multivariate analysis, female gender (p = 0.004) was the only predictor of intraoperative hypothermia, whereas initial hypothermia was significantly predictive of both prolonged hypothermia and development of organ failure (p < 0.05). Organ failure (p < 0.05) and acute myocardial infarction (p < 0.01) were independent predictors of death.

CONCLUSIONS

After AAA repair, patients with hypothermia have multiple physiologic derangements associated with adverse outcomes. Although multiple etiologic factors are interacting, body temperature is one variable that should be controlled during aortic surgery.

Mild hypothermia during isoflurane anesthesia decreases resistance to E. coli dermal infection in guinea pigs

Small changes in core temperature profoundly alter cutaneous blood flow, a major factor influencing resistance to wound infection. Furthermore, when measured in vitro, various immune functions are temperature dependent in the physiological range. Accordingly, we tested the hypothesis that mild hypothermia impairs and mild hyperthermia improves resistance to dermal infections. Thirty-two guinea pigs were anesthetized for 6 h using 1.5% (1.25 MAC) inspired isoflurane. Their core temperatures were maintained at either 39 degrees C (normal for guinea pigs, n = 11), 36 degrees C (n = 12), or 41 degrees C (n = 9). One h after induction of anesthesia, 2 x 10(8) E. coli were injected intradermally with a 26-g needle at eight sites on each animal's back. Core temperatures were not controlled after recovery from anesthesia, and animals in each group were maintained in the same environment. Twenty-four h after injection, the area of induration surrounding each injection site was measured. This is a standard test of resistance to wound infection. Values were compared using one-way ANOVA and Scheffé's S tests. Results are presented as means +/- standard deviations; differences were considered significant when P < 0.05. Areas of inflammation on the hypothermic animals were significantly larger (48 +/- 10 mm2) than those on normothermic (36 +/- 10 mm2) or hyperthermic (37 +/- 6 mm2) animals. These data suggest that mild hypothermia during anesthesia significantly impairs resistance to dermal infection. In contrast, mild hyperthermia does not appear to be protective.

Effects of a forced-air system (Bair Hugger, OR-type) on intraoperative temperature in patients with open abdominal surgery

Intraoperative hypothermia is difficult to avoid and may present a significant clinical risk during the early postoperative phase. We evaluated a forced-air system [Bair Hugger, OR-type (BH)] for warming intraoperative patients with open abdominal surgery. Twenty patients received BH warming [BH(+) group] and another 20 patients, who served as controls, did not [BH(-) group]. Patients in both groups also received circulating blanket warming. Tempertures were measured at 30-min intervals throughout the operation in the rectum and on the tip of the index finger opposite the nail bed. The average operation time was 168.8±16.2 min. Rectal and fingertip temperatures in the BH(+) group were significantly higher than those in the BH(-) group, and central-peripheral temperature gradients in the BH(+) group were significantly smaller than those in the BH(-) group during the study, except at 180 min. No shivering occurred in either group. Therefore, BH is an effective warming device during open abdominal surgery.