Andrea Kurz

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.

Opioid-induced bowel dysfunction: pathophysiology and potential new therapies.

Opioid treatment for postoperative or chronic pain is frequently associated with adverse effects, the most common being dose-limiting and debilitating bowel dysfunction. Postoperative ileus, although attributable to surgical procedures, is often exacerbated by opioid use during and following surgery. Postoperative ileus is marked by increased inhibitory neural input, heightened inflammatory responses, decreased propulsive movements and increased fluid absorption in the gastrointestinal tract. The use of opioids for chronic pain is characterised by a constellation of symptoms including hard dry stools, straining, incomplete evacuation, bloating, abdominal distension and increased gastroesophageal reflux.The current management of opioid-induced bowel dysfunction among patients receiving opioid analgesics consists primarily of nonspecific ameliorative measures. Intensive investigations into the mode of action of opioids have characterised three opioid receptor classes — μ, δ and κ — that mediate the myriad of peripheral and central actions of opioids. Activation of μ-opioid receptors in the gastrointestinal tract is responsible for inhibition of gut motility, whereas receptors in the central nervous system mediate the analgesic actions of opioids. Blocking peripheral opioid receptors in the gut is therefore a logical therapeutic target for managing opioid-induced bowel dysfunction.Available opioid antagonists such as naloxone are of limited use because they are readily absorbed, cross the blood-brain barrier, and act at central opioid receptors to reverse analgesia and elicit opioid withdrawal. Methylnaltrexone and alvimopan are recently developed opioid antagonists with activity that is restricted to peripheral receptors. Both have recently shown the ability to reverse opioid-induced bowel dysfunction without reversing analgesia or precipitating central nervous system withdrawal signs in non-surgical patients receiving opioids for chronic pain. In addition, recent clinical studies with alvimopan suggest that it may normalise bowel function without blocking opioid analgesia in abdominal laparotomy patients with opioid-related postoperative ileus.

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.

Mild Intraoperative Hypothermia Prolongs Postanesthetic Recovery

Background: Intraoperative hypothermia is common and persists for several hours after surgery. Hypothermia may prolong immediate recovery by augmenting anesthetic potency, delaying drug metabolism, producing hemodynamic instability, or depressing cognitive function. Accordingly, the authors tested the hypothesis that intraoperative hypothermia prolongs postoperative recovery. Methods: Patients undergoing elective major abdominal surgery (n = 150) were anesthetized with isoflurane, nitrous oxide, and fentanyl. They were randomly assigned to routine thermal management (hypothermia) or extra warming (normothermia). Postoperative surgical pain was treated with patient‐controlled analgesia. Fitness for discharge from the post‐anesthesia care unit was evaluated at 20‐min intervals by investigators blinded to group assignment and postoperative core temperatures. Scoring was based on a modification of a previously published system that included activity, ventilation, consciousness, and hemodynamic responses. Patients were considered fit for discharge when they sustained a score of 80% (13 points) for at least two consecutive measurement periods. Results: Morphometric characteristics and anesthetic management were similar in each group. Final intraoperative core temperatures differed by [nearly =] 2 [degree sign] Celsius: 34.8 +/‐ 0.6 versus 36.7 +/‐ 0.6 [degree sign] Celsius (mean +/‐ SD, P < 0.001). Postoperative pain scores and postoperative use of patient‐controlled opioid were similar. Hypothermic patients required [nearly =] 40 min longer (94 +/‐ 65 vs. 53 +/‐ 36 min) to reach fitness for discharge, even when return to normothermia was not a criterion (P < 0.001). Duration of recovery in the two groups differed by [nearly =] 90 min when a core temperature > 36 [degree sign] Celsius was also required (P < 0.001). Conclusion: Maintaining core normothermia decreases the duration of postanesthetic recovery and may, therefore, reduce costs of care.

Relationship between intraoperative mean arterial pressure and clinical outcomes after noncardiac surgery: toward an empirical definition of hypotension.

Background:Intraoperative hypotension may contribute to postoperative acute kidney injury (AKI) and myocardial injury, but what blood pressures are unsafe is unclear. The authors evaluated the association between the intraoperative mean arterial pressure (MAP) and the risk of AKI and myocardial injury. Methods:The authors obtained perioperative data for 33,330 noncardiac surgeries at the Cleveland Clinic, Ohio. The authors evaluated the association between intraoperative MAP from less than 55 to 75 mmHg and postoperative AKI and myocardial injury to determine the threshold of MAP where risk is increased. The authors then evaluated the association between the duration below this threshold and their outcomes adjusting for potential confounding variables. Results:AKI and myocardial injury developed in 2,478 (7.4%) and 770 (2.3%) surgeries, respectively. The MAP threshold where the risk for both outcomes increased was less than 55 mmHg. Compared with never developing a MAP less than 55 mmHg, those with a MAP less than 55 mmHg for 1–5, 6–10, 11–20, and more than 20 min had graded increases in their risk of the two outcomes (AKI: 1.18 [95% CI, 1.06–1.31], 1.19 [1.03–1.39], 1.32 [1.11–1.56], and 1.51 [1.24–1.84], respectively; myocardial injury 1.30 [1.06–1.5], 1.47 [1.13–1.93], 1.79 [1.33–2.39], and 1.82 [1.31–2.55], respectively]. Conclusions:Even short durations of an intraoperative MAP less than 55 mmHg are associated with AKI and myocardial injury. Randomized trials are required to determine whether outcomes improve with interventions that maintain an intraoperative MAP of at least 55 mmHg.

Aspirin in patients undergoing noncardiac surgery

BACKGROUND There is substantial variability in the perioperative administration of aspirin in patients undergoing noncardiac surgery, both among patients who are already on an aspirin regimen and among those who are not. METHODS Using a 2-by-2 factorial trial design, we randomly assigned 10,010 patients who were preparing to undergo noncardiac surgery and were at risk for vascular complications to receive aspirin or placebo and clonidine or placebo. The results of the aspirin trial are reported here. The patients were stratified according to whether they had not been taking aspirin before the study (initiation stratum, with 5628 patients) or they were already on an aspirin regimen (continuation stratum, with 4382 patients). Patients started taking aspirin (at a dose of 200 mg) or placebo just before surgery and continued it daily (at a dose of 100 mg) for 30 days in the initiation stratum and for 7 days in the continuation stratum, after which patients resumed their regular aspirin regimen. The primary outcome was a composite of death or nonfatal myocardial infarction at 30 days. RESULTS The primary outcome occurred in 351 of 4998 patients (7.0%) in the aspirin group and in 355 of 5012 patients (7.1%) in the placebo group (hazard ratio in the aspirin group, 0.99; 95% confidence interval [CI], 0.86 to 1.15; P=0.92). Major bleeding was more common in the aspirin group than in the placebo group (230 patients [4.6%] vs. 188 patients [3.8%]; hazard ratio, 1.23; 95% CI, 1.01, to 1.49; P=0.04). The primary and secondary outcome results were similar in the two aspirin strata. CONCLUSIONS Administration of aspirin before surgery and throughout the early postsurgical period had no significant effect on the rate of a composite of death or nonfatal myocardial infarction but increased the risk of major bleeding. (Funded by the Canadian Institutes of Health Research and others; POISE-2 ClinicalTrials.gov number, NCT01082874.).

Propofol Linearly Reduces the Vasoconstriction and Shivering Thresholds

BackgroundSkin temperature is best kept constant when determining response thresholds because both skin and core temperatures contribute to thermoregulatory control. In practice, however, it is difficult to evaluate both warm and cold thresholds while maintaining constant cutaneous temperature. A re

Obesity decreases perioperative tissue oxygenation

BackgroundObesity is an important risk factor for surgical site infections. The incidence of surgical wound infections is directly related to tissue perfusion and oxygenation. Fat tissue mass expands without a concomitant increase in blood flow per cell, which might result in a relative hypoperfusion with decreased tissue oxygenation. Consequently, the authors tested the hypotheses that perioperative tissue oxygen tension is reduced in obese surgical patients. Furthermore, they compared the effect of supplemental oxygen administration on tissue oxygenation in obese and nonobese patients. MethodsForty-six patients undergoing major abdominal surgery were assigned to one of two groups according to their body mass index: body mass index less than 30 kg/m2 (nonobese) or 30 kg/m2 or greater (obese). Intraoperative oxygen administration was adjusted to arterial oxygen tensions of approximately 150 mmHg and approximately 300 mmHg in random order. Anesthesia technique and perioperative fluid management were standardized. Subcutaneous tissue oxygen tension was measured with a polarographic electrode positioned within a subcutaneous tonometer in the lateral upper arm during surgery, in the recovery room, and on the first postoperative day. Postoperative tissue oxygen was also measured adjacent to the wound. Data were compared with unpaired two-tailed t tests and Wilcoxon rank sum test; P < 0.05 was considered statistically significant. ResultsIntraoperative subcutaneous tissue oxygen tension was significantly less in the obese patients at baseline (36 vs. 57 mmHg; P = 0.002) and with supplemental oxygen administration (47 vs. 76 mmHg; P = 0.014). Immediate postoperative tissue oxygen tension was also significantly less in subcutaneous tissue of the upper arm (43 vs. 54 mmHg; P = 0.011) as well as near the incision (42 vs. 62 mmHg; P = 0.012) in obese patients. In contrast, tissue oxygen tension was comparable in each group on the first postoperative morning. ConclusionWound and tissue hypoxia were common in obese patients in the perioperative period and most pronounced during surgery. Even with supplemental oxygen tissue, oxygen tension in obese patients was reduced to levels that are associated with a substantial increase in infection risk.

Forced-air warming maintains intraoperative normothermia better than circulating-water mattresses

The hypothesis that forced-air warming preserves core temperature better than circulating-water mattresses was tested in: (a) 16 adults undergoing major maxillofacial surgery, including radical node resection and flap reconstruction; (b) 53 adults undergoing hip arthroplasty, having approximately 25% of their body surface area available for warming; (c) 20 infants undergoing minor maxillofacial surgery; and (d) 10 young children undergoing pelvic or femoral osteotomies. Patients having each type of surgery were randomly assigned to forced-air warming (approximately 40 degrees C) or conductive warming using a full-length circulating-water mattress at 40 degrees C. Forced-air warming was applied to the legs of the adults undergoing maxillofacial surgery and to one arm, the shoulders, and the neck in the adults undergoing hip arthroplasty; a U-shaped, tubular forced-air cover was positioned around the pediatric patients. Core temperatures increased in all patients given forced-air warming and decreased or remained constant in those without active warming. Furthermore, we needed to decrease the temperature of the warmer from high to medium (approximately 37 degrees C) in most patients assigned to forced-air warming to prevent hyperthermia. After 15 h of anesthesia, rectal temperatures in the adults undergoing maxillofacial surgery were 3.4 degrees C higher in the forced-air group (P < 0.01). After 4 h of anesthesia, esophageal temperatures had increased 0.8 +/- 0.5 degrees C in the patients warmed with forced-air and decreased 0.8 +/- 0.3 degrees C in those warmed by circulating-water mattresses (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Postoperative pain and subcutaneous oxygen tension

Surgical patients randomly assigned to standard pain control had postoperative subcutaneous oxygen partial pressures that were significantly less than patients given better pain treatment. Our data suggest that control of postoperative pain is a major determinant of surgical-wound infection and should be given the same consideration as maintaining adequate vascular volume and normothermia.