Comparison of Three Intraoperative Patient Warming Systems

Introduction

Thermal homeostasis is important for the overall well being of humans and animals. Thermoregulation is a complex, highly integrated interaction between central and peripheral sensory input, central processing in the hypothalamus, and appropriate physiologic responses to maintain thermal steady state.¹ The thermoregulatory center moderates both behavioral and physiologic mechanisms to maintain the body at its established set point temperature.² Behavioral responses involve postural changes and conscious sensing of the internal and external environmental conditions. Autonomic thermoregulatory responses include changes in vasomotor tone, evaporative cooling (i.e., sweating and panting), and increased metabolic heat production by the activation of the sympathetic nervous system.³ Vasomotor tone determines blood flow through the subcutaneous heat exchange vasculature structures (i.e., footpads, tip of nose, and tongue of dogs) and thus the efficacy of heat transport from the thermal core to the skin surface.⁴ Heat and cold thermal receptors are widely distributed throughout the body.

Normal core body temperature for the dog ranges from 37.8°C to 39.3°C.² Hypothermia is defined as a subnormal body temperature (<37.8°C) and occurs due to increased heat loss, decreased heat production, or a combination of both.^2,4 Body heat is lost through four basic mechanisms: radiation, convection, conduction, and evaporation. Radiation and convection are the two most important mechanisms and account for approximately 80% of the total heat loss from a patient.^1,5 Anesthesia may contribute and potentiate the development of hypothermia by altering the physiologic mechanisms of thermoregulation. Preanesthesia and induction drugs can interrupt the feedback loop of thermoregulation, decrease sympathetic stimulation causing decreased metabolism and generalized vasodilation, and lower the threshold for shivering.^1,4–7 Operating room temperature can also influence hypothermia through increase heat loss by radiation and convection from the skin and by evaporation from within surgical incisions.⁵

Mild hypothermia may be beneficial in some cases. For example, intentional hypothermia can be used clinically during surgery to minimize myocardial and central nervous system ischemia by decreasing basal metabolic rate and oxygen consumption.^1,5,6,8 In contrast, severe hypothermia is associated with a host of unwanted and potentially life-threatening complications. These consequences include myocardial ischemia, coagulopathies, decreased resistance to surgical wound infections, prolonged recovery time, and shivering with resultant patient discomfort.^5,6,8–13

Because most metabolic heat is lost through the skin, cutaneous heat loss must be reduced to minimize a decrease in body temperature during surgery.^5,14 Various passive insulators and active warming systems are available for use intraoperatively. The simplest method of decreasing cutaneous heat loss is to apply passive insulation to the skin surface between the patient and metal operating table and over the patient.^1,6,15 Cotton blankets, surgical drapes, plastic sheeting, and bubble wrap are some readily available insulators.^1,6 Passive insulation alone is rarely sufficient to maintain normothermia in patients undergoing long procedures.^5,6 Active warming is often required to compensate for the relatively cool operating room environment and the heat loss associated with prolonged anesthetic periods.^1,4,6,14 Circulating water blankets^a and forced-air delivery systems^b are two active cutaneous warming systems that are commonly used in veterinary medicine.^{1,4–6,16,17} A warming method using an “even heat” distribution system^c is also now available (Figure 1). This warming unit consists of two heavy-gauge stainless steel panels that are independently heated. Each panel is also independently regulated, and the temperature can be programmed from 21.1°C to 41.3°C. The heat conductibility of the heavy-gauge stainless steel panels allows for consistent temperature from top to bottom and throughout the length of each panel. The panels are sealed so fluids cannot penetrate.

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The objective of this study was to evaluate the effect of three active warming devices used either alone or in combination (i.e., circulating water blanket with forced-air, forced-air alone, and warming panels alone) on body temperature when applied to patients undergoing prolonged surgeries. It was hypothesized that all three active warming devices would preserve body temperature over time.

Materials and Methods

Between 2005and 2008, 238 dogs were enrolled in this study. The population consisted of both client owned-dogs and dogs from the local animal shelter. Consent was obtained from pet owners, and this study was approved by the Institutional Animal Care and Use Committee. The dogs were categorized into either group A or group B. Dogs in group A (n=119) were scheduled for a celiotomy procedure (i.e., exploratory laparotomy, ovariohysterectomy) and those in group B (n=119) were scheduled for a nonceliotomy procedure (i.e., skin mass excision, orthopedic procedure, orchidectomy). To be included in the study, patients in each group underwent a procedure lasting >60 min of surgery time (incision to closure). Dogs in each group were assigned to one of three subgroups. Dogs in subgroup 1 (n=39) were placed on a circulating water blanket with the forced-air warming blanket placed over the abdomen for nonceliotomy procedures and over the thorax for celiotomy procedures. Dogs in subgroup 2 (n=40) were placed on a forced-air warming blanket only. Dogs in subgroup 3 (n=40) were placed on two warming panels.

Data collection for all patients included sex, body weight, body condition score (BCS), American Society of Anesthesiologists (ASA) physical status, and type of surgery (celiotomy or nonceliotomy). The BCS and ASA status were determined before premedication drugs were administered. The Purina body condition scoring system was used, and scoring ranged on a scale of 1–9, with 1 being emaciated and 9 being grossly obese.¹⁸

As part of the preoperative physical examination, the body temperature of each patient was recorded before induction using a digital rectal thermometer. This was defined as preoperative body temperature. Temperature was measured after induction of general anesthesia and immediately after the patient was prepared for aseptic surgery in the operating room. The temperatures were measured using an esophageal temperature probe^d that was inserted into the esophagus of each patient to the level of the eighth intercostal space. The probe was then connected to a multiparameter monitor^e to continuously record temperature throughout the surgical procedure. Two esophageal probes were used in this study and both were calibrated to within 0.1°C using a known temperature water bath. On average, the patient preparation time from induction to incision was approximately 45 min. Depending on the type of procedure and the extent of area preparation, some patients were ready for surgery within 30 min whereas others took closer to 1 hr to prepare. The duration of surgery was defined as the time from initial skin incision to placement of the last skin suture or staple. The baseline temperature (time 0) was measured at the time of skin incision. From that point, the temperature was recorded every 15 min for the first hour, then at 30 min intervals intraoperatively until the end of the procedure. The end of surgery temperature was defined as the temperature when the last suture or staple was placed. At completion of the surgical procedure, the esophageal probe was removed and temperatures were recorded every 10 min using a digital rectal thermometer. This was continued during the recovery period until the patient was extubated. The postoperative body temperature was defined as the temperature of the animal at the time of endotracheal extubation.

The ambient operating room temperature was also monitored and recorded at the beginning and end of each procedure using a commercial room thermometer^f. The room thermometers were all calibrated and tested to ensure accuracy prior to placing the thermometer in each operating room.

Three active warming devices used alone or in combination were evaluated. In subgroup 1, a forced-air 81 cm × 125 cm blanket set on “high” (∼43°C) was positioned over the patient and a 38 cm × 56 cm circulating water blanket set to ∼42°C was positioned below the patient. In subgroup 2, the forced-air blanket set on “high” (∼43°C) was positioned below the patient. In subgroup 3, two 46 cm × 122 cm warming panels set to ∼41°C were positioned in a V-configuration below the patient. All three units were calibrated. The forced-air unit was calibrated using the temperature kit^g designed for the unit. The water blanket was calibrated by folding it and placing a mercury thermometer inside when it was set to 42°C. The manufacturer of the warming panels specified that the unit has accuracy near 0.1°C. Patients were all positioned directly onto warming devices. All heating units were given time to cycle through a warm up process of 10 min prior to placement of the patient. Warming began immediately after the patient was moved to the operating table and was continued until the end of the procedure.

All patients were fasted overnight (24 hr) and underwent general inhalation anesthesia for their procedures. Anesthetic drug protocols and techniques were used at the discretion of the attending anesthesiologist. The premedication protocols used varied among patients. The most common premedication combination used was acepromazine, glycopyrolate, and hydromorphone. For all patients, anesthesia was induced with propofol and maintained with isoflurane. Drug protocols that deviated from the common protocol were equally divided among the three subgroups. All patients’ heart rates, respiratory rates, and arterial blood pressures were monitored intraoperatively using a multiparameter monitoring unit.

Results

The study was completed over a period of 20 mo. The mean and standard error of the mean values for weight, body condition, ASA status, duration of the surgery, preoperative patient temperature, patient temperature immediately after induction, patient temperature at time of sterile prepping in the operating room, patient temperature at the start and end of the surgical procedure, patient temperature at the time of endotracheal extubation, and temperature of operating room at the beginning and end of each procedure have been summarized in Table 1. There were no significant changes in each patient's physiologic parameters that affected body temperatures over time. Ambient operating room temperatures showed little variation during the study.

TABLE 1 Clinical Data Collected from 238 Dogs Before, During, and After Surgery

*

Significant change in mean temperature from induction to the beginning of surgery at a 0.05 level

SEM, standard error of the mean.

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The mean (and standard error of the mean) temperatures for group A (celiotomy) and B (nonceliotomy) with their corresponding warming device over time are shown in Table 2. Significant temperature drops occurred within 45 min from time of induction (38.1°C±0.64°C) to the start of the surgical procedures (36.7°C±0.95°C). Although body temperature was maintained once the warming units were started in all groups, there were significant differences in patient temperatures for the type of surgical procedures (celiotomy versus nonceliotomy) performed over time, except for subgroup 3 (warming panels). With the exception of time 0 for celiotomy procedures, there were no significant differences in temperatures when comparing all three groups at a given time and surgery type (Table 2).

TABLE 2 Body Temperatures of Dogs Undergoing Surgery (Celiotomy or Nonceliotomy) Using Various Warming Devices at Specific Surgical Times (0–180 min)

*

Non-Celiotomy means for the three devices at time 0 were significantly different. Two means with the same symbol are not significantly different at a 0.05 level.

†

With the exception of time 0 for celiotomy, all comparisons of groups within a given time and surgery type were not significant at 0.05 level.

§

Reflect significant differences in nonceliotomy versus celiotomy means at a 0.05 level.

Subgroup 1, circulating water blanket and forced-air; subgroup 2, forced-air; subgroup 3, warming panels.

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Discussion

The study demonstrated that all three warming devices were equally effective on body core temperature control during surgical procedures lasting >60 min. The changes in body temperature associated with the development of hypothermia followed a characteristic pattern. The most dramatic decreases in the mean body temperatures of anesthetized cats and dogs are reported to occur in the initial 20 min after anesthetic induction.⁷ Forty-five minutes after induction, the dogs in all three study groups experienced a drop in body temperature in the current study. These results are comparable to other studies in this regard.^3,4,7,9,16

The temperature drop indicates that the significant portion of hypothermia develops in the early anesthetic period. The initial decline in body temperature observed in all dogs could be due to the redistribution of heat from the warm central compartment to cooler peripheral tissues. Anesthetic-induced hypothermia is a well-recognized phenomenon, occurring in 60–90% of anesthetized human and animal patients.^4,7 Anesthesia decreases sympathetic tone, causing generalized vasodilation within the body and allowing core heat to mix with the peripheral heat. The anesthetic protocol commonly used in this study included acepromazine, hydromorphone, glycopyrolate, propofol, and isoflurane. Blockade of peripheral vasculature tone by the α-blocking action of phenothiazine tranquilizers (i.e., acepromazine) can promote peripheral vasodilation and increase surface thermal exchange.^5,7 The skin-to-environment temperature gradient is therefore increased, allowing significant heat loss. Additionally, the body's normal thermoregulatory responses to reduce heat loss (vasoconstriction) are significantly delayed.^3–5,14 Opioids (i.e., hydromorphone) and propofol lower the threshold for shivering and also cause vasodilation.¹ Central depression of hypothalamic function by barbiturates and volatile inhalants interrupt hypothalamic response to sensory input from peripheral regions.^6,7 Inhalants also cause a peripheral vasodilation that exacerbates heat loss.^7,9,14 All these drugs (alone or in combination) could have contributed to the initial decline in body temperature.

Significant temperature differences were observed between the types of surgical procedures conducted (i.e., celiotomy and nonceliotomy) for subgroups 1 and 2. Although the temperature differences between the two types of procedures were not as significant for subgroup 3, the celiotomy patients still had lower temperatures than nonceliotomy patients. This temperature difference could be due to clipping and preparing a greater surface area (complete ventral abdomen compared with an extremity in an orthopedic case, for example). Heat loss by evaporation is enhanced with the use of surgical antiseptic solutions used on a greater surface area. Patients were anesthetized, clipped, and prepared for surgery on thermally conductive surfaces (stainless steel tables) without the benefit of supplemental heat, which could have contributed to early heat loss during surgical preparation. In addition, thermal conductivity of water exceeds that of air. So during the surgical scrub, the rate of heat loss from moist body surfaces is approximately 25 times that from dry body surfaces at the same temperature.⁷ Preparation of surgical sites with room temperature surgical solutions allow greater heat transfer away from the superficial thermal zone, especially when wetting the animal's hair outside the clipped area. Temperature differences could also be due to the accentuated loss of body heat by exposure of the large surface area of abdominal organs during celiotomies. Due to greater surface area exposure when the abdominal cavity is opened, heat loss from radiation, convection, and evaporation is increased.^15,19,20 Even effective heat transfer through the body cannot compensate for the large heat losses that occur when a celiotomy is performed. Comparing the length of an incision for an exploratory laparotomy versus ovariohysterectomy and how much it affects heat loss and hypothermia were not evaluated in this study. A smaller celiotomy incision allowing less exposure of intra-abdominal surface area would intuitively result in less heat loss. Further studies comparing size of incision to the amount of heat loss and hypothermia would be helpful.

At time 0, the mean body temperature of subgroup 2 (forced-air) for the celiotomy procedure was significantly lower than subgroup 3 (warming panels). Several factors could have contributed to the lower temperature at time 0. These include longer induction to incision time, anesthetic protocols, clipping and preparing, and preoperative room temperature. Although there were no significant changes in each patient's physiologic parameters that affected body temperatures, the patient's weight, BCS, and ASA status could have contributed some. As mentioned earlier, anesthesia causes a significant drop in body temperature during the early anesthetic period. The average 45 min induction to incision time frame included clipping and preparing patients without supplemental heat. Decreasing the time to clip and prepare patients and providing heat would benefit the patient. The temperature of the preparation room was not measured. Although not likely, it is possible that the preparation room could have been colder than the operating room.

With the exception of time 0 for celiotomy procedures, there were no statistically significant differences in patient temperatures at any given time or surgery type for all three subgroups in this study. Although all three warming methods helped to preserve body temperature over time, the warming systems were unable to treat the initial decrease in body temperature. The warming units were not started until the redistribution phase was well underway. Redistribution hypothermia is difficult to treat except through prevention by active cutaneous warming before anesthesia, thus reducing the core-to-periphery temperature gradient. Preinduction skin warming has been shown to be effective in human patients.²¹ It would be less feasible with veterinary patients because it would require them to lie still with an active warming system covering their body for 30 min or more.²¹ Failure to provide active heating during this period results in redistribution hypothermia and loss of body heat, which is not easily replenished.

Environmental temperatures remain an important factor in determining heat balance in anesthetized patients.²² Raising room temperature can reduce the thermal gradient between patients and their environment, thus minimizing heat loss. Human adults require ambient temperatures of at least 22°C to maintain normal body temperatures during general anesthesia.²³ Anesthetized infants and neonates require much higher room temperatures (as high as 26°C) to prevent significant hypothermia.²² Optimal environmental temperatures for maintaining normothermia in anesthetized small animals have not been established. In this study, the temperatures did not vary significantly between each operating room at the beginning and end of each procedure.

Circulating water blankets are a classic active intraoperative warming system and have been used for years. The system consists of a power unit incorporating an electric heater and a water reserve to generate warm water. The water is delivered downstream to a plastic blanket placed against the animal and returned to the reserve unit for rewarming. Studies evaluating the efficacy of circulating water blankets used in cats, dogs, and humans have shown it to be effective in decreasing cutaneous heat loss.^4,7,14,17 The efficacy of circulating water blankets is limited by their position underneath the animal. Alternative strategies for increasing the effectiveness of circulating water blankets include positioning the blanket over the patient and wrapping the blanket around the animal, but both methods could interfere with the surgical field.⁴ The forced-air system has a power unit that incorporates an electric heater and fan to generate warm airflow, which is delivered through tubing to a quilt-like blanket. The warm air inflates the blanket and then exits toward the patient through slits, thus providing a shell of warm air around the patient.¹⁵ The blankets consist of some combination of fabric, plastic, or paper. Most are disposable and designed for single patient use. Studies documenting the success of using forced-air warming to minimize heat loss in anesthetized humans were first published in 1989.^15,24 The forced-air warming system has been evaluated in numerous other studies since that time and has consistently shown it can maintain normothermia during long operations in both humans and animals.^9,16,23–26 Advantages of the forced-air warming system include an adjustable thermostat, an unobstructed surgical field, and minimal risk of thermal injury. There have been many studies to prove the effectiveness of the forced-air warming system over other methods of patient warming.^9,16,23–26 This is the first study that compared the surgical warming panels to the forced-air warming system alone or in combination with the circulating water blanket.

A primary safety concern with any active warming method is burn prevention. The probability of burns depends on temperature and duration of patient contact with the heat source. Even after prolonged application of forced-air warming, circulating water blanket, and the conductive heating panels at the high temperature settings, none of the patients in this study sustained any immediate postoperative burn injuries or other adverse events. There were no procedures that lasted >180 min (Table 1). It is not known if a long procedure (>180 min) using the maximum temperature setting on the warming panels would cause thermal burns. Although there was no follow-up as to whether any of the animals experienced thermal burns in this study, there were no complaints from the owners at any time after surgery. To the authors’ knowledge, there are no known published reports on warming panel burns. There have been several reported cases of thermal burns in humans from the forced-air warming system and in animals from circulating water blankets.^17,27,24 Most of these cases can be attributed to improper use of the equipment.

There were several limitations to this study. The accurate measurement of temperature without the invasive implantation of thermistors and without compromising sterility of the surgical field was the first limitation. Temperature measurements were obtained using esophageal probes and rectal thermometers. Esophageal and rectal temperature measurements can vary slightly from measurements taken via thermistors implanted in a central vein.¹⁶ This variation can be attributed to factors such as inconsistent placement of the temperature probes, the sensitivity of the instrument used, and the presence of fecal material with the rectum. The different anesthetic protocols used in this study made it difficult to analyze why one subgroup had lower temperatures for a specific type of surgery over time. Having the same anesthetic protocol for all subgroups would eliminate that variable. Additionally, this study did not truly compare active warming systems. Rather than adding a circulating water blanket to another active warming unit, comparing each warming unit separately (forced-air versus circulating water blanket versus conductive heating panels) would be more useful and relevant to the veterinary field. Additionally, there was the lack of a negative control group. This also made it difficult to truly compare each subgroup. Adding a negative control group (no heat) and rescuing them by providing heat once they have reached a critical hypothermia set point (i.e., <35°C) would have provided valuable information.

Conclusion

This study demonstrated that both conductive warming panels and forced-air devices (with and without the circulating water blanket) are equally effective in minimizing mild hypothermia during general anesthesia. All warming systems in this study are able to preserve body temperatures. Therefore, the choice of warming systems is dependent on other factors such as ease of operation, safety, initial cost of unit, and the cost of disposable blankets. To reduce the on-going cost of disposable blankets, reusable cloth blankets are readily available. Further research is required to compare warming devices for specific procedures (e.g., celiotomy, thoracotomy, orthopedic) with the devices set at various temperatures.