Convective Warming During Surgery

Proper preoperative, intraoperative, and postoperative warming of patients is critical for reducing incidences of perioperative hypothermia, defined as a core body temperature of less than 36°C [1]. Besides being highly uncomfortable, hypothermia can lead to significant adverse events including intraoperative blood loss, coagulation abnormalities, increased post-op infection rates, and prolonged recoveries and hospital stays [1,2]. Several methods using convective and conductive warming techniques have been developed to lower the risk of hypothermia during and after surgery.

Conductive warming techniques involve direct contact of heated materials with exposed skin surfaces. A common example of this is pre- and postoperative warming of patients using heated blankets. This method is generally easy to deploy since most well-resourced hospitals are stocked with warm towels for surgical candidates. Though easy to utilize, this method is limited by the duration for which the material remains warm and the pressure of the material on the patient’s skin. In contrast, convective warming refers to movement of gas or liquids to transfer heat energy to another object. Forms of convective heating include the popular use of forced-air warmers or IV fluid warming devices. Forced-air warmers operate by distributing heated air generated by a power unit through a specifically designed blanket [1]. IV fluid warming involves administering warm fluids through patient IV lines. For both conductive and convective warming, there is a risk of thermal injury if heated material is too hot or placed underneath body regions that create high pressure points between the skin and heated material [1].

Literature demonstrates that conductive warming is generally less efficacious than convective warming [1-4]. Emmert et al’s 2017 study compared conductive and convective warming in patients undergoing video-assisted thoracic surgery. In this study, 60 patients were either warmed using Temp° Jelly blankets and two leg blankets (conductive warming), or with forced-air warmers (convective warming) during surgery. Their baseline assessments revealed no differences in the length of surgery, duration of pre-warming and initial core temperatures between both groups. However, a significant difference was found in core body temperatures between both groups 15 minutes into the surgery and at the end. Seventy-four percent of patients in the conductive group had a core temperature below 36°C within the first 15 minutes of surgery, compared to just 24% of patients in the convective group (p < 0.001). Only 8% of patients in the convective group had a core temperature less than 36°C at the end of surgery, compared to a staggering 56.5% in the conduction group (p < 0.001).

Emmert et al’s study is one of many that highlight the superiority of convective warming over conductive warming intraoperatively. An additional comparison can be made between two forms of convection warming: IV fluids and forced-air heating. A meta-analysis by John et al. (2014), demonstrates that administering warmed IV fluid led to significant reductions in the incidence of accidental perioperative hypothermia in gynecological and abdominal surgeries. However, a clinical trial by Boayam in 2018 found that forced-air warming was clinically more effective than fluid warming at preventing hypothermia in patients undergoing gynecological surgery.

In summary, there are many ways to prevent perioperative hypothermia. Deciding which warming method to use requires careful consideration but should be a high priority in order to minimize postoperative adverse events.

References 

1. John, M., Ford, J., & Harper, M. (2014). Peri‐operative warming devices: performance and clinical application. Anaesthesia, 69(6), 623-638. doi:10.1111/anae.12626 

2. Emmert, A., Franke, R., Brandes, I. F., Hinterthaner, M., Danner, B. C., Bauer, M., & Bräuer, A. (2017). Comparison of conductive and convective warming in patients undergoing video-assisted thoracic surgery: a prospective randomized clinical trial. The Thoracic and Cardiovascular Surgeon, 65(05), 362-366. doi:10.1055/s-0036-1583766. 

3. Convective vs. Conductive Warming. 3M Health Care. (2011). https://multimedia.3m.com/mws/media/905363O/b-8-3-convective-vs-conductive-warming.pdf 

4. Hohn, L., Schweizer, A., Kalangos, A., Morel, D. R., Bednarkiewicz, M., & Licker, M. (1998). Benefits of intraoperative skin surface warming in cardiac surgical patients. British journal of anaesthesia, 80(3), 318-323. 

5. Boayam, W. (2018). Comparison between Forced Air and Intravenous Fluid Warmer in Gynecologic Laparoscopic Surgery: A Randomized Trial (Doctoral dissertation, Department of Anesthesiology Institution Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok).