July 2005 - The Hong Kong College of Anaesthesiologists

Bull HK Coll Anaesthesiol Volume 14, Number 2 July, 2005The aim of this study is to evaluate therelative effectiveness of forced air warming inpreventing intraoperative hypothermia, whenapplied to the upper or lower body in patientsundergoing prolonged open abdominal surgery.Materials and MethodsA prospective, randomized, single‐blindedstudy was conducted to compare the relativeeffectiveness of upper and lower body forced airwarming in patients undergoing prolongedopen abdominal surgery of more than 2 hoursduration.Power analysis was based on a type I errorof 0.05 and a type II error of 0.2 using the Powerand Precision computer software (Biostat, Inc.,Englewood, NJ, USA). According to the resultsof previous studies 13,14 , we calculated that atleast 42 patients per group is required todemonstrate a mean difference in core bodytemperature of 0.5 o C with a standard deviationof 0.8 o C.Approval by the hospital ethics committeeof the study was obtained. Eighty seven adultpatients, older than 18 years, American Societyof Anesthesiologists physical status I to III,undergoing open abdominal surgery with anexpected duration of more than 2 hours, wererecruited in this study. Simple randomisation ofpatients into two groups of either the upper orlower body warming was performed. Exclusioncriteria included pre‐existing hypothermia (coretemperature < 36 o C) or hyperthermia (> 38 o C),patients undergoing abdominal aortic surgery,burn injury or multiple traumatic injuries, andsurgery in lithotomy position.Preoperatively, demographic data wererecorded and tympanic membrane temperaturesof patients were recorded by infrared probe(ThermoScan Pro 1, Braun AG, Germany).All patients received general anaesthesiawith tracheal intubation. Nasopharyngeal andrectal thermistor temperature probes (Datexmodel 16561, Helsinki, Finland) were insertedafter induction of anesthesia. Thenasopharyngeal and rectal temperatures ofpatients were continuously monitored duringsurgery using the temperature‐monitoringmodule (model M‐ESTPR 00‐02) and displayedon a Datex AS/3 anesthesia monitor. The initialreadings and further readings at 15‐minuteintervals were recorded after induction ofanesthesia until the end of surgery.After baseline recordings, forced airwarming using Warm Touch model 501‐5800(Mallinckrodt Medical, St Louis, MO, USA) wasapplied to all patients for the wholeintraoperative period. For the upper body group,an “upper body blanket” (Warm Touch model503‐0820) was applied to cover the upper trunkand limbs. For the lower body group, a “lowerbody blanket” (Warm Touch model 503‐0830)was applied to cover the lower limbs up to themiddle of the thighs. The forced air warmerswere initially set to the “high” setting for bothgroups. The warmer settings were reduced tomedium or low if the core temperature exceeded37.5 o C.All intravenous and irrigation fluids usedwere warmed in fluid warming cabinet prior touse. Circle anesthetic circuits with soda‐limeabsorbers were used. The use of additionalwarming devices such as heat and moistureexchangers, intravenous fluid warming coils orcirculating water mattresses were recorded. Theambient temperature in the operating theatrewas set to 21 o C, and the temperature at the startand end of the surgery was recorded using anordinary alcohol thermometer.Statistical analysis of temperature readingswas performed using two‐tailed factorialanalysis of variance with repeated measures.Analysis of demographic data was performedwith χ 2 test for categorical variables andStudent’s t‐test for continuous variables.ResultsThe 87 patients enrolled in the study wererandomized into 45 patients in the upper bodygroup and 42 patients in the lower body group.The summary of patient characteristics is shownin Table 1. No difference was found betweenthe groups in demographic characteristics,preoperative temperature, type of surgery,ambient operating theatre temperature and72