Introduction
Perioperative hypothermia (PH), defined as a core temperature below 36°C, is one of the most common phenomena seen among surgical patients (1). Hypothermia during surgery can be generated by various factors such as: exposure to surgical environment, thermoregulatory dysfunction during general or regional anesthesia and medications. PH leads to intraoperative blood loss, cardiac events, coagulopathy, an increase in hospital stay and associated costs (1, 2). Even mild hypothermia (approximately 2°C below normal temperature) can triple the incidence of wound infection and prolong hospitalization by about 20% (2). Passive and active warming methods are used in the operating room (OR) to prevent the incidence of PH. Blanket use to provide insulation and radiant heat loss prevention (passive method), warming intravenous fluids, and patient warming devices (active methods) are well known (3).
Currently, a plethora of patient warming devices are utilized to reduce the incidence of intraoperative hypothermia among surgical patients (2, 3) (Table (Table1).1). We focus our review on two most commonly used and widely studied methods—forced air warming (FAW) and resistive heating (RH)—based on recent publications associated with their use (3).
FAW system employs air convection to transfer heat, utilizing a heat generator with a temperature management unit, a blower to circulate heated air and a temperature control system equipped with various sensors (4-6). The air enters through a 0.2 μm rated intake filter (4-6). The blower unit is connected, via rubber hose, to a disposable perforated air blanket (6). The warm air passes through this blanket and helps maintain patient’s surface temperature within a physiological range (6).
RH system converts electrical energy into heat and warms the patient through conduction (2). Effective warming is delivered by non-disposable carbon polymer fiber fabric strips (7, 8).
Recently, the debate between FAW and RH has intensified. There is a concern that FAW device can increase the incidence of surgical site infections by disrupting the OR laminar air flow, (9-12) and by transporting contaminants from the floor to the sterile surgical field (13). Laminar flow ventilation is described as an entire body of highly filtered air (>99.997%) within a designated space (operating room) moving with uniform velocity (0.3 to 0.5 m/s) in unidirectional downward parallel flow (2, 9). Laminar flow ventilation system reduces the number of contaminants in the OR air by generating a continuous flow of low bacteria containing air and sweeping the contaminants away from the surgical area (9, 14). Some authors stressed the importance to check the compatibility of the devices used in the operating room (OR) with the laminar airflow system (13). The heat from FAW devices creates hot air convection current and disrupts the OR laminar air flow with a potential compromise of surgical site sterility and increased incidence of infections (2, 9, 15, 16). McGovern argued that FAW devices do not disrupt the laminar flow and the effect of OR laminar flow on the incidence of surgical site infections is questionable (9). Recently, several cohort studies (2, 17, 18) reported no significant benefit of laminar air flow in reducing the incidence of surgical site infections.
Furthermore, the difference in warming efficacy of these two patient warming devices is debatable. Some studies (19, 20) conclude that FAW is more effective than RH. Others (21) consider the RH being more effective or report (8, 13, 15, 22) the same efficacy (Tables (Tables22-4).
Recently, the concerned manufacturers have commissioned scientists to conduct independent studies and refute publications that criticized their sponsored devices. Hence we decided to perform a review of all the available published literature in order to get a scientific insight of this present controversy.