Supplemental reflective blankets and wool socks help maintain body temperature in dogs undergoing celiotomy procedures: a prospective randomized controlled clinical trial

Bridget A. O’Neil Department of Emergency and Critical Care, BluePearl Specialty and Emergency Pet Hospital, Glendale, WI

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Andrew K. J. Linklater Department of Emergency and Critical Care, BluePearl Specialty and Emergency Pet Hospital, Glendale, WI

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Abstract

OBJECTIVE

To compare results for body (esophageal) temperature measurements obtained during celiotomy in normothermic (presurgical) canine patients receiving routine heat support versus routine heat support plus supplemental support (consisting of covering the thorax in a reflective blanket and placing reflective blankets plus wool socks on 3 limbs) in a prospective randomized controlled clinical trial.

ANIMALS

44 dogs requiring celiotomy that were presented sequentially to an emergency and specialty referral veterinary hospital.

PROCEDURES

The 44 dogs undergoing celiotomy were randomly assigned to 2 groups. The control group received routine intraoperative heat support consisting of a circulating warm water blanket and either a towel or blanket. The supplemental group received the same routine heat support plus a reflective blanket covering the cervical and thoracic regions and reflective blankets wrapped around 3 limbs and covered with wool socks from the digits to the axillary or inguinal region.

RESULTS

Mean esophageal temperature for both the control and supplemental groups dropped throughout celiotomy. Esophageal temperature measurements at several time points were significantly higher for the supplemental group than for the control group. The lowest temperature measurement for the supplemental group, adjusted for initial esophageal temperature and procedure duration, was significantly higher by 0.8 °C than that for the control group.

CLINICAL RELEVANCE

Covering the cervical and thoracic regions with a reflective blanket and wrapping limbs in reflective blankets and wool socks is an affordable adjunctive method to provide passive heat support and minimize perioperative hypothermia in canine patients undergoing celiotomy.

Abstract

OBJECTIVE

To compare results for body (esophageal) temperature measurements obtained during celiotomy in normothermic (presurgical) canine patients receiving routine heat support versus routine heat support plus supplemental support (consisting of covering the thorax in a reflective blanket and placing reflective blankets plus wool socks on 3 limbs) in a prospective randomized controlled clinical trial.

ANIMALS

44 dogs requiring celiotomy that were presented sequentially to an emergency and specialty referral veterinary hospital.

PROCEDURES

The 44 dogs undergoing celiotomy were randomly assigned to 2 groups. The control group received routine intraoperative heat support consisting of a circulating warm water blanket and either a towel or blanket. The supplemental group received the same routine heat support plus a reflective blanket covering the cervical and thoracic regions and reflective blankets wrapped around 3 limbs and covered with wool socks from the digits to the axillary or inguinal region.

RESULTS

Mean esophageal temperature for both the control and supplemental groups dropped throughout celiotomy. Esophageal temperature measurements at several time points were significantly higher for the supplemental group than for the control group. The lowest temperature measurement for the supplemental group, adjusted for initial esophageal temperature and procedure duration, was significantly higher by 0.8 °C than that for the control group.

CLINICAL RELEVANCE

Covering the cervical and thoracic regions with a reflective blanket and wrapping limbs in reflective blankets and wool socks is an affordable adjunctive method to provide passive heat support and minimize perioperative hypothermia in canine patients undergoing celiotomy.

Introduction

Perioperative hypothermia, defined as a canine core body temperature < 37.5 °C, is a common occurrence in veterinary patients undergoing general anesthesia and open-cavity procedures.1,2 Several factors contribute to the development of perioperative hypothermia in patients undergoing celiotomy. Radiative and conductive heat losses may occur as veterinary patients often have a high surface area-to-body mass ratio, room-temperature IV fluids are commonly used, and patients may be in contact with cool preparation and surgical rooms, tables, and equipment. For instance, the mean core body temperature of a 70-kg human patient decreased by 0.25 °C with each liter of ambient-temperature fluids administered IV.3 Use of vasodilatory drugs, cool temperatures of lavage solutions and preparatory rinses, and other factors may all contribute to the development of perioperative hypothermia.4,5 Evaporative and convective heat losses also occur during aseptic preparation, during open-cavity procedures such as celiotomy, and due to the low humidity of inhaled gases.

Perioperative hypothermia in humans contributes to complications including hypotension that may be refractory to norepinephrine, prolonged recovery time, bradycardia that is refractory to atropine, other cardiac conduction disturbances, and clinical coagulopathies. Hypothermia can also alter drug pharmacokinetics, contribute to organ failure, impair wound healing in some populations,6 and increase patient discomfort and shivering, which can increase oxygen consumption by up to 40%.7 Although adverse effects of perioperative hypothermia are not as well documented in dogs, a drop in body temperature of 1 °C prolonged the time to extubation by approximately 5.9 minutes in 1 study.8 Pottie et al9 also demonstrated prolonged recovery time from anesthesia with decreasing esophageal temperature.

Both active and passive methods can be used to prevent perioperative hypothermia in human and veterinary patients. Active methods include temperature-controlled circulating warm water blankets, forced-air warming, radiant heat, electric temperature-controlled blankets, increasing operating room temperatures, and using heaters and humidifiers on anesthetic circuits.1014,15 Temperature-controlled circulating water blankets and resistive heating mats are commonly used in anesthesia, but perioperative hypothermia persists.1,2 Forced-air warming is more effective than passive warming11 but might increase contamination of the surgical field. Wood et al16 pointed out that forced-air warming devices create a temperature gradient that impedes downward laminar flow and can increase contamination of the surgical site. Albrecht et al17 showed that, despite a filter on a Bair Hugger hose, 5% to 7% of ambient airborne contaminants can enter and colonize a forced-air warming device itself. The cost to purchase and maintain forced-air warming devices also makes their use less common. Tjoakarfa et al18 found no difference in sublingual temperature in prewarmed human patients undergoing hip or knee arthroplasty when a reflective blanket versus a forced-air warming device was used during the procedure. Cabell et al19 showed that placing circulating warm water mattresses around available limbs was more effective in maintaining core body temperature than placing single or double circulating warm water mattresses over the trunk. However, this method may be impractical in many situations. Rodriguez-Diez et al15 demonstrated that increasing the environmental temperature in induction and operating rooms to 24 °C reduced perioperative hypothermia in dogs and cats undergoing open surgery.

Passive methods of heat support include the use of socks, reflective blankets, warm water bottles, and heated rice bags. The latter 2 options are largely avoided because of their potential to cause patient burns. Socks and reflective blankets reduce hypothermia primarily by minimizing radiative and conductive losses.20 Lee et al12 found that human patients wearing warmed socks during spinal surgery experienced less shivering and thermal discomfort and also maintained a higher core body temperature than did the control group.

Very few studies in veterinary medicine have evaluated the use of a combination of peripheral passive warming devices to avoid or limit perioperative hypothermia. Tünsmeyer et al14 found that wrapping the entire core regions of small (< 10-kg) canine patients in reflective blankets during surgery, in addition to placing the patients on warmed gel pads, resulted in a higher mean body temperature after 40 minutes, compared with canine patients on warmed gel pads alone. In that study,14 the patients were not undergoing celiotomy, so their evaporative and convective losses were minimal.

In this prospective randomized controlled clinical trial, we compared esophageal temperature measurements obtained during celiotomy in normothermic (presurgical) canine patients receiving routine heat support versus routine heat support along with supplemental passive heat support (consisting of reflective blankets covering the thorax and reflective blankets plus wool socks wrapping 3 limbs). The hypothesis was that supplemental passive heat support would minimize perioperative hypothermia in an affordable manner in canines undergoing celiotomy.

Materials and Methods

Animals

This study was reviewed and approved by the BluePearl Science Institutional Review Board and performed in compliance with humane animal care and use guidelines. Informed owner consent was obtained for each patient.

All canine patients that were eligible for inclusion in the study required elective or emergency celiotomy with no other additional procedures and were normothermic (rectal temperature, 37.5 to 39.1 °C) at the time of presentation to an emergency and specialty referral hospital between May 2020 and March 2021. Patients were excluded from the study if they were hypothermic (rectal temperature, < 37.5 °C) or hyperthermic (rectal temperature, > 39.1 °C) at the time of presentation, could not have 3 limbs wrapped in a reflective blanket and socks, or had records that lacked documentation of the rectal temperature at the admitting physical examination, environmental temperatures of the surgical preparation and operating rooms, esophageal temperature immediately after intubation in the surgical preparation room and immediately prior to undraping in the operating room, and postoperative rectal temperature. Patients were also excluded if they underwent any diagnostic or other procedures (such as CT) between induction of anesthesia and transport to the operating room, because a prolonged preoperative anesthetic period could allow for continued heat loss prior to application of the reflective blankets and socks.

Patients were assessed with a physical examination by the attending veterinarian, assigned a body condition score (on either a 5- or 9-point scale on the basis of the attending clinician’s preference), and later grouped as underconditioned (0.5/5 to 2.5/5; 1/9 to 4/9), normal (3/5; 5/9), or overconditioned (3.5/5 to 5/5; 6/9 to 9/9) for analysis.21 Prescribed diagnostic tests and treatments were performed at the discretion of the attending veterinarian to assess the individual patient’s needs for celiotomy, therapeutics, and anesthesia.

Groups and assignment

Patients were randomly assigned to 1 of 2 groups at the time of inclusion on the basis of a random-numbers table generated prior to initiation of the study (Excel version 2202; Microsoft Corp). Patients with odd numbers were allocated to receive routine heat support as the control group, and patients with even numbers were allocated to receive routine plus supplemental heat support as the supplemental group.

Routine heat support consisted of a temperature-controlled circulating warm water blanket (heat therapy pump model No. TP700; Stryker), set to 42 °C and covered by a towel, and the use of warmed lavage fluids stored in an incubator maintained at 38.9 ± 2 °C, as indicated. Routine heat support also included covering the patient with surgical drapes. Patients in the supplemental group received the same routine heat support along with reflective blankets (emergency mylar thermal blanket; Dongguan City Risen Medical Products Co) cut to size and placed over the cervical and thoracic regions and reflective blankets wrapped around 3 limbs (those without an IV catheter in place) and covered with commercially available wool-blend socks (Below Zero wool socks; J&J Hosiery) from the digits to the axillary or inguinal region. Reflective blankets were placed with the reflective surface toward the patient. Blankets and socks were placed on the patient (Figure 1) in the operating room after the patient was positioned by the surgeon. Blankets were cut to size and reused when not torn or contaminated; wool socks were washed and dried between uses.

Figure 1
Figure 1
Figure 1
Figure 1
Figure 1
Figure 1

Images of supplemental heat support used along with routine heat support for 22 client-owned dogs that were normothermic on presentation at an emergency and specialty referral veterinary hospital, required elective or emergency celiotomy between May 2020 and March 2021, and were randomly assigned to the supplemental group (vs control group; received routine heat support alone; n = 22) in a randomized controlled clinical trial. A—A reflective blanket, with the silver reflective side toward the patient, is wrapped around the cervical and thoracic regions. B—Reflective blankets are placed around 3 limbs, leaving 1 limb free for IV access. C—Wool socks are placed onto each reflective blanket–wrapped limb from the digits to the axillary or inguinal region. D—Three of the anesthetized patient’s limbs are wrapped in reflective blankets and covered with wool socks; the fourth limb has an IV catheter. E—The cervical and thoracic regions of the same anesthetized patient are covered with a reflective blanket before celiotomy.

Citation: Journal of the American Veterinary Medical Association 260, 11; 10.2460/javma.22.01.0001

Anesthetic protocol and monitoring

Anesthesia was induced with injectable formulations of medications at the discretion of the attending veterinarian, with drug and dose recorded on the patient anesthetic record. All patients underwent endotracheal intubation and maintenance of anesthesia with isoflurane delivered in oxygen and titrated to an acceptable level of anesthesia. Anesthetic monitoring included observation and management by a certified technician utilizing electrocardiography, oscillometric blood pressure measurement, pulse oximetry, and continuous side-stream capnography (Surgivet Advisor V9212AR; Smiths Medical). All patients received room-temperature, balanced, buffered, isotonic crystalloid solution (PlasmaLyte A) administered IV throughout anesthesia. The starting fluid rate was 5 mL/kg/h, and the total volume delivered was determined by the attending veterinarian and based on the individual patient’s fluid requirements.

Temperature evaluation

Rectal temperature was measured with a standard fast-read digital rectal thermometer (temperature probe models varied, but the most common model was Vicks V912 Speed Read digital thermometer; Kaz Inc) at the initial presentation of the animal to the emergency or surgery service. The environmental temperature of the surgical preparation room was recorded immediately prior to intubation. Wall clocks with built-in thermometers (atomic wall clock model No. SPC932; Sharp Electronics Corp) mounted in the surgical preparation room and operating room were used to obtain environmental temperatures. Esophageal temperature was measured by use of a standard, calibrated esophageal thermometer (Surgivet; Smiths Medical) immediately after intubation and application of anesthetic monitoring equipment. The esophageal temperature was recorded every 10 minutes during the procedure as anesthetic and surgical circumstances allowed and then immediately prior to undraping. The surgical preparation room table was equipped with a circulating warm water blanket, which was preheated prior to placement of the animal. The hair was clipped from the ventral aspect of the animal’s abdomen from the xiphoid to the pubis and laterally on either side to the flank fold. The abdomen was then scrubbed with gauze and by means of alternating room-temperature solutions of 4% chlorhexidine scrub (VetOne) and 0.9% saline (NaCl) solution (Braun Medical) in a circular manner starting at the midline overlying the future incision site and working outward. At least 3 rounds of scrubbing with both solutions were performed, and scrubbing was continued until the gauze sponges were not accumulating visible debris. The patient was then moved to the operating room, and the environmental temperature of the operating room was recorded. The operating table was also equipped with a preheated circulating warm water blanket on which the patient was placed. Supplemental heat support (reflective blankets and wool socks) was then applied to patients in the supplemental group only (Figure 1). For animals in both groups, a second aseptic surgical scrub was performed and then drapes were applied by the surgeon. After the surgical procedure was complete, the final esophageal temperature was obtained immediately prior to undraping the animal and all heat support in both groups was discontinued. No additional procedures were performed after the final esophageal temperature was obtained. Rectal temperature was again obtained immediately upon the animal’s arrival at the intensive care unit with a standard fast-read digital rectal thermometer around the time of extubation.

Statistical analysis

A priori power analysis was performed by use of anesthetic records from 50 consecutively performed celiotomies at the study institution in the preceding year with careful attention paid to include only those meeting the study’s inclusion criteria. The power analysis indicated that a minimum sample size of 20 dogs/group would yield adequate statistical power to detect a difference of 0.55 °C in the lowest body temperature between groups.

All data analyses were performed with the use of statistical software (StatView version 9.4; SAS Institute Inc). Values of P < 0.05 were considered significant. Temperature measurements were compared between groups with the Welch t tests due to the nonhomogeneous variance of the lowest esophageal temperatures. To adjust for preintervention temperature differences, a covariate of initial esophageal temperature was included in all linear models described subsequently. Quantile-quantile plots and histograms of model residuals were used to confirm normality. An ANCOVA with an independent factor of group was used to test for differences in temperature measurements between groups. Potential confounders and interactions with group were body condition (grouped into categories of underconditioned, normal condition, and overconditioned on the basis of body condition score), whether splenectomy was performed, and durations of anesthesia, surgical preparation, and surgical procedure. Individual linear models were developed for each potential confounder to predict temperature with the potential confounder as a predictor. Any potential confounders with P < 0.10 were included together with group in a linear model to adjust for their effects. To test for potential interactions, individual linear models with factors of group, predictor, and a group-by-predictor interaction were executed. For any interaction effect with P < 0.10, subset analyses were performed.

Additionally, a linear mixed model was developed to compare intraoperative (10 to 60 minutes) esophageal temperature measurements between groups. Time points after 60 minutes were excluded from analysis because fewer than 8 dogs underwent anesthesia > 60 minutes. The model had fixed factors of group, time (duration), a group-by-time interaction, a covariate of initial esophageal temperature, and a random factor to account for repeated measurements.

Initial, change, mean, and final esophageal temperature measurements, as well as esophageal temperature measurements at 10-minute intervals, were evaluated statistically.

Results

Animals

A total of 74 dogs were initially enrolled in the study. Thirty dogs were excluded on the basis of the exclusion criteria, and 44 dogs were included in the analysis. Patients consisted of 20 castrated males, 13 spayed females, 8 sexually intact males, and 3 sexually intact females. There were 11 mixed breeds, 4 Labrador Retrievers, 3 French Bulldogs, 3 pit bull–type dogs, 2 German Shepherd Dogs, 2 Labrador Retriever–Poodle crosses, and 1 each of Akita, American Staffordshire Terrier, Beagle, Boston Terrier, Bull Terrier, Doberman Pinscher, Golden Retriever, Great Dane, Ibizan Hound, Jack Russell Terrier, Plott Hound, Pomeranian, Pug, Siberian Husky, Staffordshire Bull Terrier, Standard Poodle, Greater Swiss Mountain Dog, Toy Poodle, and Vizsla. Age ranged from 6 months to 15 years, and body weight ranged from 4.5 to 66.9 kg. Of the 44 dogs, 22 (50%) were assessed to have normal body condition (11 in the control group and 11 in the supplemental group), 13 (30%) to be underconditioned (7 in the control group and 6 in the supplemental group), and 9 (20%) as overconditioned (4 in the control group and 5 in the supplemental group).

Anesthesia

All patients received a full μ-opioid receptor agonist and a benzodiazepine. The most common opioid was hydromorphone (n = 35); methadone (7) and fentanyl continuous rate infusions (2) were used less commonly. Midazolam was the most commonly selected benzodiazepine (n = 41), and diazepam was selected for 3 patients. The benzodiazepine was either administered as premedication or used as a coinduction agent with ketamine. Almost all patients had anesthesia induced with ketamine and midazolam, apart from 3 patients that received a benzodiazepine as premedication and had anesthesia induced with propofol titrated to effect. No patient received an α2-adrenergic receptor agonist at any time during the anesthetic event.

Surgery

Procedures performed included enterotomy (n = 18), gastropexy (16), gastrotomy (14), splenectomy (8), gastrointestinal biopsy (4), intestinal resection and anastomosis (3), liver biopsy (2), cholecystectomy (2), ovariohysterectomy (2), lymph node biopsy (2), cystotomy (1), and inguinal hernia repair (1), alone or in combination, with 28 patients having had 2 or more procedures performed. Surgical duration ranged between 39 and 150 minutes with a mean of 64 minutes. Descriptive statistics for body weight and durations of anesthesia, surgical preparation, and surgery were compiled for each of the 2 groups (Table 1).

Table 1

Summary statistics for 44 client-owned dogs that were normothermic on presentation at an emergency and specialty referral veterinary hospital, required elective or emergency celiotomy between May 2020 and March 2021, and were randomly assigned to receive either routine heat support (control group; n = 22) or routine heat support plus supplemental heat support with the use of reflective blankets and wool socks (supplemental group; 22) during the surgery.

Variable Control group Supplemental group
Body weight (kg)
 Mean ± SD 26.3 ± 16.2 22.3 ± 9.2
 Range 4.5–66.9 6.2–44.2
Duration of anesthesia (min) 95.2 ± 25.8 94.5 ± 17.4
Duration in surgery preparation room (min) 13.7 ± 5.9 16.9 ± 7.2
Duration of procedure (min) 67.6 ± 27.8 59.9 ± 16.5

Data reported as mean ± SD, unless otherwise indicated.

Outcome evaluation

The primary outcomes evaluated included initial esophageal temperature after induction, esophageal temperature at application of the intervention (supplemental heat support), intraoperative esophageal temperature trend, final esophageal temperature prior to undraping, and lowest esophageal temperature throughout anesthesia. The time at which the initial esophageal temperature was measured was recorded as “–15 minutes,” representing the mean duration between induction of anesthesia and movement to the operating room. The mean ± SD duration spent in the surgical preparation room was 13.7 ± 5.9 minutes for the control group, 16.9 ± 7.2 minutes for the supplemental group, and not statistically different between the 2 groups (Table 1). The time point for application of intervention was recorded as “time 0.” This was considered equivalent to the surgery start time, as it was within minutes of the abdominal incision occurring (second skin preparation and draping occurred during that period). Another esophageal temperature reading within that period was not obtained due to the small amount of time separating the 2 events.

Mean room temperature in the surgical preparation room was 18.7 ± 1.5 °C for the control group, 19.2 ± 1.3 °C for the supplemental group, and not significantly different between the 2 groups (P = 0.249; Welch t test). The mean room temperature in the operating room was 19.3 ± 2.4 °C for the control group, 19.6 ± 1.6 °C for the supplemental group, and not significantly different between the 2 groups (P = 0.657; Welch t test).

Mean esophageal temperature for both the control and supplemental groups decreased throughout anesthesia. Mean initial esophageal temperature was higher for the supplemental group (37.4 °C) than for the control group (37.1 °C). To account for this, the intraoperative temperature values were adjusted by including initial temperature as a covariate in all models. The adjusted mean final esophageal temperature for the control group (35.5 °C) was 0.4 °C lower than the adjusted mean final esophageal temperature for the supplemental group (35.9 °C), but this difference was not statistically significant (P = 0.177; Table 2).

Table 2

Adjusted mean ± SD intraoperative esophageal temperatures for the control group versus supplemental group described in Table 1 immediately after intubation (time, –15 minutes), at the onset of supplemental warming and start of surgery (time, 0 minutes), throughout surgery (times, 10, 20, 30, 40, 50, and 60 minutes), and just prior to undraping (final), as well as the lowest mean intraoperative esophageal temperature for each group.

Time (min) Control group Supplemental group
No. of data points Adjusted mean ± SD esophageal temperature (°C)a No. of data points Adjusted mean ± SD esophageal temperature (°C)a P value
Initial (–15) 22 37.1 ± 1.05 22 37.4 ± 0.66
0 18 36.3 ± 1.10 14 37.0 ± 0.66
10 16 36.0 ± 1.21 17 36.5 ± 0.72 0.038a
20 15 35.8 ± 1.10 15 36.3 ± 0.66 0.024a
30 19 35.7 ± 0.99 14 36.2 ± 0.66 0.047a
40 17 35.7 ± 0.99 14 35.9 ± 0.66 0.199a
50 15 35.6 ± 1.05 13 36.0 ± 0.61 0.037a
60 8 35.6 ± 0.66 5 35.9 ± 0.66 0.257a
Final 22 35.5 ± 1.10 22 35.9 ± 0.94 0.177b
Lowest 19 35.2 ± 1.21 17 36.0 ± 0.61 0.007b

— = Not calculated

Linear mixed model with initial temperature as a covariate.

ANCOVA with initial temperature as a covariate.

Adjusted intraoperative esophageal temperature measurements

Mean intraoperative esophageal temperature measurements, when adjusted for initial esophageal temperature measurements, were significantly (P = 0.045) higher for the supplemental group by 0.5 °C on average (95% CI, 0 to 1.8 °C). Mean intraoperative esophageal temperatures were significantly higher for the supplemental group than for the control group at 10, 20, 30, and 50 minutes but not at 40 or 60 minutes (Table 2).

Final esophageal temperature

Several potential confounding factors were investigated to determine whether they were significantly different between the control and supplemental groups and may have influenced final esophageal temperature measurements. These factors included body condition category (overconditioned, normal, or underconditioned), splenectomy performed, duration of anesthesia, duration of surgical preparation, and duration of surgical procedure. None of these factors were significantly different between the 2 groups on the basis of cutoff values of P < 0.20 for potential confounder effect and P < 0.10 for interaction effect (Table 3); therefore, no further analyses adjusting for their effects on the groups were performed.

Table 3

Results of ANCOVA to identify potential confounding factors for differences in the mean final and mean lowest esophageal temperatures for the control group versus supplemental group described in Table 2 in a linear model with group, factor, group-by-factor interaction, and initial esophageal temperature as covariates.

Potential confounding factor P value for lowest esophageal temperature (n = 36) P value for final esophageal temperature (n = 44)
Body condition 0.862 0.755
Body weight (kg) 0.542 0.455
Splenectomy 0.444 0.371
Duration of anesthesia (min) 0.405 0.576
Time in surgery preparation room (min) 0.438 0.282
Procedure duration (min) 0.084 0.231

Lowest esophageal temperature

The P values for separate linear models for each potential confounder or interaction for lowest esophageal temperature measurement were organized and evaluated (Table 3). Procedure duration was identified as a potential confounder (P = 0.084) that could have decreased the lowest temperatures and was added to the linear model. When adjusted for procedure duration in addition to the initial esophageal temperature, the adjusted mean lowest temperature for the control group (35.2 °C; n = 19) was significantly (P = 0.001; linear model) lower (0.8 °C; 95% CI, 0.4 to 2.4 °C) than that for the supplemental group (36.0 °C; 17).

Potential interaction

Time (duration) in the surgical preparation room was identified as having a possible interaction with group (P = 0.080) on the basis of a value of P < 0.10 for interaction effect. Cases were split into 2 groups according to the median preparation time of 15 minutes. For the 18 patients with < 15 minutes of preparation recorded, the mean lowest temperature for the control group when adjusted for initial temperature was 34.8 °C, which was significantly lower by 1.3 °C (95% CI, 0.6 to 3.7 °C) than that for the supplemental group, which was 36.1 °C (P = 0.010; ANCOVA). In contrast, among the 18 patients with ≥ 15 minutes of preparation recorded, the mean lowest temperature for the control group when adjusted for initial temperature was 35.7 °C, which was only 0.3 °C lower than (95% CI, 2.0 °C lower to 1.0 °C higher) and not significantly different from the final temperature for the supplemental group, which was 35.9 °C (P = 0.507; ANCOVA).

Material cost

Six pairs of wool-blend socks at a purchase price of $20 and 8 reflective blankets at a total cost of $17 were the only additional materials used in this study. The additional per-patient cost associated with the supplemental heat support was $0.50 when all originally eligible 74 dogs were considered or $0.84 when only the included 44 dogs were considered. There was material left over at the end of the study that could be used for additional patients.

Discussion

Results of the present randomized controlled study indicated that the use of supplemental heat support, consisting of reflective blankets and wool socks, maintains a significantly higher body temperature in canine patients under anesthesia at most time points. The mean lowest esophageal temperature measured during anesthesia was also significantly higher for the supplemental group, even when adjusted for the difference in initial esophageal temperature. The differences in final and mean esophageal temperature were not significant.

Further studies are needed to evaluate the clinical impact of these supplemental methods on time to recovery, the need for heat support during recovery, or intraoperative blood pressure.

Mean body temperature was higher for the supplemental group than for the control group at most of the 10-minute intervals but not at 40 or 60 minutes. The comparison at 60 minutes was thought to not be statistically significant because only a few patients were included (n = 8). We did not require the esophageal temperature to be recorded at each 10-minute time point and only required initial and final esophageal temperatures to be recorded, which was a limitation of the present study. Future investigations could require body temperature to be recorded at each 10-minute time point to more thoroughly analyze the impact of the intervention at each interval.

Because the lower mean initial esophageal temperature for the control group could have impeded an accurate evaluation of the statistical significance of the intervention, the intraoperative results were adjusted on the basis of results for initial esophageal temperature. The adjustment widened the gap between the mean lowest esophageal temperature for the control group and that for the supplemental group to a difference of 0.8 °C. Potential causes for the difference in initial temperature included differences in time elapsed between premedication and induction of anesthesia, type of premedication administered, degree of sedation prior to anesthesia, and environmental temperature in the different areas of the hospital where patients waited prior to anesthesia. These data points were not recorded or analyzed in this study and could be evaluated in future investigations. Procedure duration was identified as a potential confounder or interaction for lowest temperature measurements, which could be expected given that a patient continues to lose heat during anesthesia, especially in celiotomy in which the abdomen is open and both convective and evaporative losses play a large role. Even when adjusting for both initial esophageal temperature and procedure duration, the difference in lowest temperature between the control and supplemental groups remained significant. This result highlighted the potential of supplemental heat support to reduce perioperative hypothermia.

Among patients that spent less time (< 15 minutes) in the surgical preparation room, the mean lowest esophageal temperature significantly differed between the control and supplemental groups, but this difference was not significant in patients spending longer periods (> 15 minutes) in the surgical preparation room. The latter patients were without supplemental heat support for a longer time period and may have been subject to more prolonged radiative and conductive heat losses in the surgical preparation room. This result could indicate that passive warming methods are best applied to the patient as early as possible to minimize perioperative hypothermia. Application in the surgical preparation room may be considered in future studies, but there is concern that operating room and surgical site contamination may occur during transport and positioning of these patients as clipped hair could stick to the reflective blankets and wool socks and travel into the operating room with the patient.

The present study did not investigate whether consequences of hypothermia affected patients. Further investigations should evaluate whether and to what degree patients receiving supplemental heat support experience consequences of hypothermia. Studies evaluating the incidence of low blood pressure, time to return to clinically normal body temperature, need for and duration of postoperative heat support, time to return to normal body temperature, recovery quality, and incidence of adverse events are needed.

Patients undergoing preoperative procedures or with abnormal rectal temperatures were excluded from the present study. The impact of supplemental heat support on patients with preexisting hyperthermia or hypothermia or those undergoing prolonged preoperative procedures, such as CT, should be investigated in the future. Although the present study had some specific limitations, such as the wide range of patients and procedures, the inclusion criteria allowed comparison of results for patients with baseline rectal temperatures within reference limits, and the study population reflected a typical veterinary hospital caseload.

The agents used in anesthetic protocols may impact body temperature to varying degrees, but in the present study, the protocols were highly similar for all patients. In addition, anesthetic drug choice is more often based on clinician experience, comfort with the medication, anticipated procedure, and concurrent medical issues rather than on patient body temperature. Therefore, although this variable was not controlled, the study design reflected a realistic approach to anesthetic choice. The anesthetic breathing circuit type (rebreathing or nonrebreathing) as well as inflow rate of fresh gas were recorded but not evaluated in this study and offer another area for future investigation as either one or both of these variables may have impacted results. Rectal temperature measurements were primarily performed with 1 brand of thermometer, but equipment did vary occasionally, which could have been considered a minor confounding factor. The nature of the intervention for the supplemental group, specifically the visibility and application of the reflective blankets and socks, did not allow for the surgeon or the technician to be blinded to the patient’s group.

Finally, by performing a brief cost evaluation in this study, we determined that the supplemental heat support described in this study was affordable. The application of reflective blankets and wool socks is a simple intervention using widely available and inexpensive materials that can be applied to many practice situations, from shelter medicine to specialty hospitals. The cost associated with the time spent applying the materials was not assessed, but it was subjectively noted that, with minimal experience, these materials could be applied in 1 to 2 minutes.

In conclusion, the present study demonstrated that covering the cervical and thoracic regions with a reflective blanket and wrapping 3 limbs in reflective blankets and wool socks from the digits to the axillary or inguinal region is an affordable, adjunctive means of passive heat support that can be applied to canines undergoing celiotomy to help minimize perioperative hypothermia, supporting our hypothesis that such supplemental passive heat support would minimize perioperative hypothermia in an affordable manner in canines undergoing celiotomy.

Acknowledgments

Statistical analysis provided by Deborah A. Keys, PhD, Kaleidoscope Statistics. The statistical analysis for this work was financially supported by the BluePearl Specialty and Emergency Pet Hospital Study Design and Review Committee. Funding sources did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.

The authors declare that there were no conflicts of interest.

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