Intraoperative Warming: Comparison of Two Temperature Management Systems

Mild perioperative hypothermia (core temperature 34-36ºC) occurs commonly during anesthesia and surgery as a result of a combination of factors: anesthetic induced impairment of thermoregulatory controls, internal redistribution of heat within the body, surgical site skin preparation and subsequent exposure to the 'cold' OR environment. Induction of general anesthesia causes marked changes in the distribution of body heat between the patient's core and periphery. An initial drop in core temperature of between 0.5 to 1.5ºC after induction is the result of internal core-to-periphery redistribution of body heat caused by a combination of anesthesia induced vasodilation and decreased thermoregulatory vasoconstriction. Continued thermal imbalance during anesthesia results from inability of metabolic heat production to overcome the heat loss.

The complications related to mild perioperative hypothermia have been documented in numerous studies and summarized in a 1999 article, "Maintaining intraoperative normothermia: a meta-analysis of outcomes with costs". Complications include reduced resistance to surgical wound infection, reduced platelet function, increased intraoperative loss of blood, increased incidence of ventricular tachycardia and morbid cardiac events, Additionally, the duration of postoperative recovery has also been shown to be prolonged by mild hypothermia.

Various warming methods, such as warm blankets, forced-air warmers and circulating water mattresses, are currently used to prevent and treat mild perioperative hypothermia. All are cutaneous approaches that rely on heating the peripheral tissues in order to increase the thermal core temperature. However, during certain surgical procedures such as laparatomy, a large section of the skin surface (ie., abdomen and part of the patient in contact with the OR table) is unavailable. This can limit the efficacy of methods such as forced-air warming which rely on a large skin surface for heat transfer. Thus, one criterion for an alternative non-invasive warming device is that the heat generated by that device reach its desired target; in the case of perioperative hypothermia, that target is the patient's thermal core.Grahn, et al determined that it would be feasible to combine subatmospheric pressure with a thermal load (heat) and then apply this combination to the body's own specialized heat transfer areas - the ArterioVenous Anastamoses (AVAs) -- found in the hands and the feet. By mechanically distending these specialized heat exchange vessels (the AVAs) through the application of negative pressure, heat can be transferred more efficiently to the body core.

AVAcore Technologies's warmUP (WU-20) Temperature Management System was developed based on work with the Aquarius Medical Acrotherm device (reference 510(k) number: K003368) and prior human hypothermic subject data showing a rewarming rate of ~1ºC in 5 minutes when heat and vacuum application was tested. AVAcore Technologies received FDA acceptance of its Section 510(k) premarket notification for the warmUP (vH1) Temperature Management System in 2005 (reference 510(k) number: K040911). This product was used on over 100 patients in the US in 2005-2006 timeframe.

The warmUP (WU-20) Temperature Management System proposed for use in this study is a minor modification to the vH1 System. The warmUP (WU-20) Temperature Management System is a released product covered with a Letter to File per an assessment based on FDA Guidance documents. The Arizant Healthcare Bair Hugger® Temperature Management System proposed for use in this study is a 'forced air' system providing convective warming via the circulation of warmed air through a specially designed blanket which is placed over a portion of the patient's body. This system has FDA acceptance (reference 510(k) number: K021473.) This product has been in use in the US for over 20 years in a majority of hospitals.

The primary objective is to determine if the AVAcore Technologies warmUP (WU-20) Temperature Management System is as effective as forcedair warming (Bair Hugger™, Arizant Healthcare, Eden Prairie, MN) for maintenance of intraoperative body temperature in patients undergoing abdominal surgery under general anesthesia. The critical endpoints to be evaluated in making this determination are 1) % of subjects with an average intraoperative esophageal temperature of ≥ 36º C and 2) % of subjects with an initial PACU sublingual temperature of ≥ 36º C.

Secondary objectives include 1) comparison of the core body temperatures

@ 60 minutes post anesthesia induction, 2) comparison of temperature trends during surgery and 3) comparison of the subjects' PACU temperature trends and hypothermic symptoms such as shivering.