Background

Inadvertent perioperative hypothermia (IPH) is a common occurrence in the perioperative period.1 Multiple studies suggest that there is a significant drop in core body temperature during the first hour post-anesthesia induction leading to IPH in 48% to 70% of surgical patients undergoing general anesthesia.1-4 The National Institute for Health and Clinical Excellence (NICE) defines normal core body temperature of adult patients to be in the range of 36.5[degrees]C and 37.5[degrees]C.5,6 Subsequently, hypothermia is defined as core body temperature of less than 36[degrees]C.6 In addition, the severity of hypothermia is categorized into mild hypothermia: core temperature of 35.0[degrees]C to 35.9[degrees]C; moderate hypothermia: core temperature of 34.0[degrees]C to 34.9[degrees]C; severe hypothermia: core temperature less than or equal to 33.9[degrees]C.6

Cutaneous heat loss in a low temperature environment is mainly due to radiation and convection, which accounts for 85% of total heat loss.7,8 Patients with no underlying comorbidities can effectively compensate for this heat loss primarily via hypothalamic activities, which compare thermal inputs of different body tissues with the threshold temperature.9 Once core body temperature falls below the threshold temperature, thermoregulatory response triggers vasoconstriction, shivering and nonshivering thermogenesis to restore temperature within the interthreshold range (0.2-0.3[degrees]C in humans).9-11 General anesthesia obtunds thermoregulatory control, causing cold-response to be markedly reduced. Clinically observed core body temperature decreases between 0.5[degrees]C to 1.5[degrees]C during the first hour post induction of anesthesia.12 This is caused by redistribution hypothermia, which accounts for 81% reduction in the core body temperature.12-13 It is therefore imperative to make every attempt to prevent core body temperature drop during this time. There is significant evidence showing that the patients pre-warmed under forced-air warming maintained a higher core body temperature within the first hour post-anesthesia induction.12,15,19-21

Hypothermia can inadvertently induce many undesirable surgical outcomes resulting in coagulation, cardiac, metabolic and immunologic problems.13-15 The ability of the body to regulate coagulation cascade is an important property of normal homeostasis. Platelet dysfunction starts to appear when temperature drops below 35[degrees]C.16 Production of clotting enzymes and plasminogen activator inhibitors becomes impaired in a severe hypothermic state, increasing the risk of bleeding.16 Inhalation anesthesia reduces the metabolic rate by 20% to 30%.9 Cardiac output and oxygen extraction, which is largely dependent on the metabolic rate, can also be affected during hypothermia.17 Surgical site infection is a severe complication of anesthesia and surgery. Hypothermia increases surgical site infection by impairing the normal immunologic function. This is primarily due to reduction in oxygen delivery to tissues at the surgical site.9 Post-anesthetic shivering is yet another common complication of surgery which can be caused by intraoperative hypothermia, redistribution of core body temperatures secondary to anesthetic agents, as well as activation of inflammatory responses and cytokine release from surgical stimulation.18 Evidence shows that shivering can increase oxygen consumption by as much as 400% to 500%.13 This can cause an imbalance in myocardial oxygen supply-demand, significantly increasing the risks of ischemia in cardiovascular patients leading to a poor prognosis in some cases.

Prevention of redistribution hypothermia following induction of anesthesia is the key to maintaining normothermia throughout the perioperative period.12 Although most surgical suites focus on intraoperative warming, which can effectively increase core body temperature during the immediate postoperative period, it does not stop the rapid decrease in core temperatures during the first hour post-anesthesia induction.9,12,15,19,20 Anesthesia-induced peripheral vasodilation causes massive heat flow from the core-to-periphery, which requires at least one hour of intraoperative warming after redistributive hypothermia has already occurred.11 For an anesthesia duration lasting less than one hour, intraoperative warming alone may not be adequate to compensate for the initial heat loss. Therefore, preoperative warming is important in attenuating IPH by increasing the initial body heat content.12 Arms and legs are the major peripheral thermal compartments, which subsequently become the target of warming during the preoperative period.11 Adequate heat storage in the distal portions of the arms and legs decreases the core-to-peripheral temperature gradient.11,12 As a result, less redistribution of heat will occur after induction of anesthesia.9,11

Preoperative warming can be most effectively achieved by warming the patient for at least 30 to 60 minutes using forced-air warming devices.12,15,19-21 A forced-air warming blanket is the most widely used device in the market. This technology allows direct surface-surface contact between the blanket and the patient.22 The temperature of the air is externally controlled by a warming unit, which is attached to the blanket using a hose.13 The heat transfer from the blanket to the body depends on several factors: heat exchange coefficient, the temperature gradient between the blanket and the body, as well as the homogeneity of heat distribution inside the blanket.23

Maintenance of normothermia is undoubtedly an important goal of perioperative anesthesia. In comparison to hypothermic patients, normothermic patients have shorter hospital stays (11.77 days versus 19.44 days) and lower infection rates (6.95% versus 19.07%), resulting in cost savings of $4602 and $1696, respectively.24 Although existing systematic reviews by Proveda et al.,12 Moola and Lockwood15 and Sajid et al.19 have shown that forced-air pre-warming resulted in a higher core body temperature postoperatively, there is no clear, consistent data focusing on core body temperature drop in the first hour after anesthetic induction which is the outcome temperature of interest for this review. Consistency in the timing of temperature recordings is important as the patient's core temperatures can vary significantly throughout the perioperative period. Immediate postoperative core temperature is less accurate at measuring the effectiveness of pre-warming since both preoperative and intraoperative warming can potentially increase core body temperature postoperatively. An initial search using CINAHL, The Cochrane Library, EMBASE, MEDLINE, Web of Science, and Clinical Key databases revealed no systematic review on the effect of 30 to 60 minutes of forced-air pre-warming on maintaining intraoperative core temperatures during the first hour post-anesthesia induction has been published. Given the negative outcomes of redistribution hypothermia especially in the first hour following anesthesia induction, a well-conducted systematic review will provide valuable insight into perioperative heat loss and maintenance of core body temperature during the most critical hour, contributing significantly towards positive patient outcomes.

Inclusion criteria

Types of participants

This review will consider studies that include adult patients aged 18 years and over undergoing elective surgical procedures that require general anesthesia. Patients undergoing emergency surgeries will be excluded from the review since 30 to 60 minutes of pre-warming is often not feasible. Pregnant women will be excluded because cesarean sections often involve regional anesthesia which has different anesthetic implications than general anesthesia.25 Pediatric patients will also be excluded due to their difference in thermoregulation during anesthesia.26 Studies should include patients having an American Society of Anesthesiologists (ASA) classification of I, II and/or III. General anesthesia is defined as anesthetics delivered via endotracheal intubation, nasal intubation, or with a laryngeal mask airway. A minimum of one hour of anesthesia time is required as the focus of the review is on core temperatures during the first hour post induction of anesthesia. For the purposes of this review, we will consider all settings where general anesthesia is induced, such as an operating room environment.

Types of interventions

This review will consider studies that pre-warm patients for at least 30 to 60 minutes using forced-air warming devices versus conventional cotton blanket/sheet/duvet in the preoperative area. The forced-air warming device uses an external warming unit that attaches to a blanket or a gown via a hose, delivering continuous warm air to the patient.23

A subgroup analysis on core body temperature recordings at induction plus 60 minutes will be performed among studies that implement active pre-warming only versus those that implement both active pre- and intraoperative warming. This analysis will not only help in drawing comparisons on the varying effects of pre-warming alone but also the combined effects of pre- and intra-operative warming in maintaining core body temperature at 60 minutes post induction.

Outcomes

This review will consider studies that include mean core temperature measurements of the forced-air pre-warming group and the control group at 60 minutes post induction of anesthesia. All methods of core temperature measurement will be included, such as tympanic, bladder, esophageal, rectal, pulmonary artery catheter, etc. To be included in the review, the studies should have core temperature measurements presented in Celsius. Temperature recordings presented in other units (e.g. Fahrenheit) will be converted to Celsius using the standard temperature conversion formula. Other outcomes including postoperative core temperature measurements will be excluded from the review.

Types of studies

This review will consider any experimental study design, including randomized controlled trials, non-randomized controlled trials, quasi-experimental, and before and after studies for inclusion.

Search strategy

The search strategy aims to find both published and unpublished studies. A three-step search strategy will be utilized in this review. An initial limited search of MEDLINE and CINAHL will be undertaken followed by an analysis of the text words contained in the title and abstract, and of the index terms used to describe the article. A second search using all identified keywords and index terms will then be undertaken across all included databases. Thirdly, the reference list of all identified reports and articles will be searched for additional studies.

For the purpose of this review, studies published from 1990 to 2015 will be included, as forced-air warming is a technology that has been used worldwide since 25 years ago. Our review seeks to include evidence arising during this period of time. Only studies published in English will be considered for inclusion in this review.

The databases to be searched include:

CINAHL

The Cochrane Library

EBSCO Host Health Source: Nursing/Academic Edition

EMBASE

ISI Web of Science

MEDLINE

Clinical Key

National Guideline Clearinghouse

TRIP (Translating Research Into Practice)

The search for unpublished studies will include:

New York Academy of Medicine Grey Literature Report

MEDNAR

ProQuest Dissertations and Theses

Initial keywords to be used will be: pre-warming, pre-induction warming, forced-air, warming blankets, preoperative, hypothermia, general anesthesia, adults, temperature, surgery, elective, American Society of Anesthesiology classification, experimental, RCT, non-RCT, quasi experimental, redistributive hypothermia.

Assessment of methodological quality

Papers selected for retrieval will be assessed by two independent reviewers for methodological validity prior to inclusion in the review using standardized critical appraisal instruments from the Joanna Briggs Institute Meta Analysis of Statistics Assessment and Review Instrument (JBI-MAStARI (Appendix I). Any disagreements that arise between the reviewers will be resolved through discussion.

Data extraction

Data will be extracted from papers included in the review using the standardized data extraction tool from JBI-MAStARI (Appendix II). The data extracted will include specific details about the interventions, populations, study methods and outcomes of significance to the review question and specific objectives. The authors will be contacted if necessary for any missing data.

Data synthesis

Quantitative data will, where possible, be pooled in statistical meta-analysis using JBI-MAStARI. All results will be subject to double data entry. Effect sizes expressed as odds ratio (for categorical data) and weighted mean differences (for continuous data) and their 95% confidence intervals will be calculated for analysis. Heterogeneity will be assessed statistically using the standard Chi-square. Where statistical pooling is not possible, the findings will be presented in narrative form, including tables and figures to aid in data presentation where appropriate. A subgroup analysis on core body temperatures at induction +60 minutes will again be performed among studies that prewarm patients for 30 minutes versus those prewarm for 31-60 minutes. Base on the results of subgroup analysis, we can determine whether there is a dose-dependent response between minutes of pre-warming and the maintenance of core body temperature.

Acknowledgements

Texas Christian University Harris College of Nursing and Health Sciences School of Nurse Anesthesia for support of the project.

Appendix I: Appraisal instruments

MAStARI appraisal instrument

Appendix II: Appraisal instruments

MAStARI appraisal instrument

References