Introduction
Hypothermia is a common complication during the perianesthetic period in dogs1 and has been associated with multiple postoperative complications, including prolonged recovery, increased susceptibility to infection, delayed wound healing, hypotension, and coagulopathies.2–5 Many active warming devices, such as electrical warming pads, circulating warm water blankets (WWBs), and forced warm air blowers, are regularly used in anesthetized veterinary patients to prevent and treat hypothermia. These methods do not completely prevent heat loss, and their use can still result in hypothermia when used alone in anesthetized dogs.6–8
Inhalation of cold, dry gasses may compound hypothermia during general anesthesia.3,9 The use of passive heat and moisture exchangers has been investigated in dogs and resulted in no significant difference in body temperature.10,11 Alternatively, heated humidified breathing circuits (HHBCs) are active warming devices that use a reservoir of heated water to deliver warm and humidified air through the inspiratory limb of the breathing circuit. In humans, HHBCs are associated with a higher body temperature when compared with conventional breathing circuits and heat and moisture exchangers.12 In dogs, limited data are available on the use of HHBCs.13,14 Due to the availability of new technology with higher maximum temperature settings and the lack of more recent studies, further investigation is warranted.
This study evaluated the use of a new HHBC (Heated and Humidified Anesthesia System Intro Kit; Jorgensen Laboratories Inc) in preventing hypothermia in healthy dogs under general anesthesia. The objective of the study was to evaluate the effect of an HHBC heated to 45 °C on rectal temperature in dogs undergoing general anesthesia for elective ovariohysterectomies. We hypothesized that dogs provided with both an HHBC and a circulating WWB would be warmer at extubation and would have a lower incidence of hypothermia than dogs provided with a WWB alone.
Materials and Methods
Study animals
The study was approved by the Institutional Animal Care and Use Committee at the University of Georgia (Protocol #A2021 03-016). Twenty-nine healthy bitches undergoing ovariohysterectomy were included in this study. Ages were unknown, and dogs’ weights ranged from 10.8 to 30.3 kg. Inclusion criteria were healthy dogs classified as American Society of Anesthesiologists status I based on the results of physical examination and hematocrit, total solids, BUN, and urine specific gravity performed prior to anesthesia (n = 35).
Study design
As the number of animals in the teaching laboratory was predetermined, a compromise sample size power analysis was performed with the expected number of animals per group (between 8 and 24 control dogs and 8 HHBC dogs) using G*Power 3.1.15 Data collected from a neuter lab at the same institution were used to calculate a 2-tailed t test for 2 independent means. The dogs in the neuter laboratory lost 2.6 ± 0.6 °C over 60 minutes. Our target for the HHBC intervention was to lose 1.6 ± 0.6 °C. The implied power obtained was between 91.9% and 96.5%, with an α of 3.4% to 8.1%.
Animals were enrolled in a prospective study. Six dogs were excluded from this study due to aggressive behavior (n = 2), diarrhea (1), a rectal mass (1), feces present in the rectum during monitoring (1), and having already been spayed (1). On each day of the study, the dog with the median weight was selected for the HHBC group (n = 8), and the remaining dogs were selected for the control group (21). Data collection was conducted in the same temperature-controlled room throughout the study.
All dogs received the same anesthetic drug protocol. Acepromazine (0.02 mg/kg) and hydromorphone (0.1 mg/kg) were administered IM for premedication. An IV catheter was placed in a peripheral vein, and general anesthesia was induced with propofol (4.4 mg/kg, IV, to effect). After endotracheal intubation, dogs were maintained on isoflurane in 100% oxygen at a flow rate of approximately 2 L/min. All subjects were monitored during anesthesia with electrocardiography, oscillometric blood pressure, pulse oximetry, and capnography. Intravenous fluids (lactated Ringer solution) kept at room temperature were administered to all dogs at a rate of 5 mL/kg/h during anesthesia, and carprofen at 4.4 mg/kg was administered SC intraoperatively. An additional 0.05 mg/kg of hydromorphone was administered IV in recovery if the dogs were considered painful by the supervising anesthesiologist.
Body temperature was measured with rectal temperature probes (VetTRENDS) before premedication (baseline), 20 minutes after premedication, at induction, after transportation to the operating room (OR), every 15 minutes during maintenance of anesthesia, and at extubation. For each dog, the same probe was used at each time point after application of a plastic cover and lubricant. The temperature reading was determined and recorded when there was no change in the reading for at least 30 seconds.
Twenty milliliters of sterile water was added to the HHBC and prewarmed at 45 °C before data collection began. The HHBC was started upon connection of the endotracheal tube to the breathing circuit. Dogs in the control group were connected to a conventional rebreathing circuit. After transportation to the OR, all dogs were placed onto a circulating WWB (T/Pump; Stryker Corp) that was prewarmed at 42 °C. In the experimental group, when the rectal temperature reached 37.2 °C, the HHBC temperature setting was decreased to 40 °C. When the rectal temperature reached 37.5 °C, the WWB was turned off in either group, and the HHBC was turned off in the experimental group. After extubation, the HHBC was disassembled and allowed to dry for use during the next procedure, which occurred 24 to 72 hours later (Figure 1).
Heated humidified breathing circuit (HHBC) disassembled (A) and mounted (B). Sterile water is added to the heating unit, which attaches to the inspiratory limb of the breathing circuit. A cotton wick within the inspiratory limb allows for saturation of heated water within the circuit to provide warm humidified air.
Citation: Journal of the American Veterinary Medical Association 261, 8; 10.2460/javma.23.01.0040
The total time of general anesthesia was recorded as time in hours from induction to extubation. Change in temperature was calculated as the difference between each dog’s rectal temperature at extubation and their baseline rectal temperature. Hypothermia and hyperthermia were defined as rectal temperatures < 37 °C and > 39.5 °C, respectively.
Statistical analysis
Data were tested for normality using the Shapiro-Wilk test. Differences in weight, time of general anesthesia, and rectal temperature between the control and HHBC groups were analyzed using unpaired 2-tailed t tests. Differences in rectal temperature over time and between groups were analyzed using mixed-effect ANOVA for multiple comparisons. These data were tested for effects of treatment, time, and the interaction of treatment and time, and each animal was considered a random effect. The incidence of hypothermia between groups was analyzed using the Fisher exact test. Statistical significance was defined as P < .05. All statistical analyses were performed using commercial software (Prism, version 9.3.1 for MacOS; GraphPad).
Results
There was no significant difference in body weight, anesthesia time, or rectal temperature measured at baseline, after premedication, at induction, or after transportation to the OR (Table 1).
Mean ± SD of the weight, anesthesia time, and rectal temperature at baseline, premedication, induction, and transfer to the operating room (OR) for all dogs in the control and heated humidified breathing circuit (HHBC) groups. There was no significant difference in these measurements between groups.
| Variable | Control mean ± SD | HHBC mean ± SD | P value |
|---|---|---|---|
| Weight (kg) | 20.7 ± 5.3 | 19.9 ± 2.4 | .659 |
| Anesthesia time (h) | 3.7 ± 0.6 | 3.5 ± 0.5 | .592 |
| Baseline temperature (°C) | 38.2 ± 0.5 | 38.1 ± 0.2 | .595 |
| Premedication temperature (°C) | 37.9 ± 0.3 | 37.8 ± 0.3 | .386 |
| Induction temperature (°C) | 37.5 ± 0.5 | 37.3 ± 0.3 | .438 |
| Transfer-to-OR temperature (°C) | 37.0 ± 0.7 | 37.1 ± 0.6 | .814 |
The mean rectal temperature at extubation was significantly higher in the HHBC group (37.7 ± 0.6 °C) compared with the control group (36.6 ± 1.0 °C; P = .006).
Dogs in both groups had a lower temperature at extubation compared with their baseline. The mean change in rectal temperature was significantly smaller in the HHBC group (–0.5 ± 0.6 °C) compared with the control group (–1.6 ± 0.9 °C; P = .002).
The overall rectal temperature was 0.5 °C higher for the HHBC group during anesthesia (P = .005). There were significant differences in rectal temperature over time regardless of treatment group (P < .001). There was no interaction between treatment and time (Figure 2).
The change in rectal temperature for all dogs over time for the control group (gray spaghetti plots) and the HHBC group (light pink spaghetti plots). The general regression of each group is represented by the bold black line (control group) and the bold red line (HHBC group). 0 °C represents the baseline temperature for each dog. Statistically significant differences were observed over time regardless of the treatment group (P < 0.001). The rectal temperature was significantly higher throughout the procedure in the HHBC group compared with the control group (P = 0.005). There was no statistically significant difference between the treatment groups over time.
Citation: Journal of the American Veterinary Medical Association 261, 8; 10.2460/javma.23.01.0040
The incidence of hypothermia at extubation was 12.5% for the HHBC group and 66.7% for the control group (P = .014). In the HHBC group, the HHBC and WWB were turned off in 3 dogs (37.5%) to avoid iatrogenic hyperthermia compared with 0 dogs in the control group (P = .015).
No apparent adverse effects related to the HHBC were observed in this study. No differences in respiratory rate or effort were noted, and no dogs were hyperthermic. All dogs recovered without complications from anesthesia and were returned to the shelter facility 1 or 2 days following the end of the study.
Discussion
This study showed that anesthetized dogs had a higher body temperature during anesthesia and a lower incidence of hypothermia when an HHBC and WWB were used, compared with when a WWB alone was used.
HHBCs have several advantages over other active warming devices. Commonly used active warming devices in veterinary medicine include circulating WWBs, forced warm air blowers, and semiconductive warming blankets. Placement of these devices is often limited due to interference with the surgical field, and they pose a potential risk of thermal burns due to direct contact with patient skin.16–19 Some studies20–25 suggest that forced warm air blowers increase the risk of bacterial contamination of the surgical site; however, this remains controversial. In the present study, no surgical site infections were observed. HHBCs can be used during transportation and surgical prepping regardless of patient positioning. In humans, HHBCs may protect the tracheobronchial ciliated epithelium from damage caused by dry gasses and can help maintain mucociliary clearance during general anesthesia.26,27 Additional research is needed to investigate these effects in animals.
In this study, there was no difference in rectal temperature between groups after transportation to the OR. At that time, the rectal temperature of all dogs had decreased approximately 1 °C from their baseline. The largest decrease in body temperature occurs within the first hour of anesthesia, likely due to a combination of peripheral vasodilation, loss of central thermoregulatory control, and surgical prepping.2 Our results suggested that the addition of an HHBC does not attenuate this initial decrease in body temperature and that it may be more useful for longer procedures.
We cannot determine from this study whether the use of an HHBC alone would lead to a similar decrease in the incidence of hypothermia, as there was no group that received an HHBC without additional heat support. Our study protocol was designed to ensure all dogs would receive a standard of care for heat support during anesthesia. Previous studies13,14 found that an HHBC alone was not sufficient for preventing hypothermia in dogs. Tan et al13 compared 4 warming techniques in anesthetized dogs, comprising an electrical heating pad, a combination of an electrical heating pad with warm water bottles and radiant heat, a forced air warming mattress, and an HHBC device. The HHBC device used in the aforementioned study was designed for humans and was set at a maximum temperature of 41 °C. Raffe et al14 compared the body temperatures in anesthetized dogs receiving no thermal support, an HHBC alone with a maximum temperature of 40 °C, and an HHBC combined with a recirculating water blanket. Similar to our results, the body temperature was better maintained with the combination of an HHBC and recirculating water blanket, although a smaller number of dogs was enrolled, the incidence of hypothermia was not determined, and no surgical procedures were performed. To the authors’ knowledge, our study was the first to investigate the combination of an HHBC and WWB on rectal temperature in dogs undergoing abdominal surgery and to use an HHBC device designed for veterinary patients with a maximum temperature setting of 45 °C. Based on the results of our study, future investigations evaluating this new HHBC as the sole source of active warming in anesthetized dogs are indicated.
The mean weight of the dogs in this study was 20 kg. Hypothermia is of greater concern in smaller dogs and in cats due to a higher ratio of surface area to body weight, which allows for more cutaneous heat loss.2 The HHBC in this study requires the use of a rebreathing circuit, and while dogs in the HHBC group did not show any apparent increased respiratory effort, the increased resistance could be of significance in smaller animals. Future studies using small-breed dogs and cats are needed to determine the efficacy of HHBCs in these populations.
The calibration of each thermometer was not confirmed before each day of the study. Nonetheless, the same model was used for all dogs, and the manufacturer adheres to International Organization for Standardization 80601-2-56:2009, with ± 0.1 °C accuracy. We did not measure ambient temperature in this study. Variation in room temperature can affect the temperature gradient between the patient and environment, resulting in different amounts of convective heat loss. However, the surgeries were conducted in the same room throughout the study, and the university regulates that the room temperature is maintained between 20 and 22.2 °C. There was also a similar number of dogs in the HHBC and control groups on each day of the study. Therefore, we believe that any differences in room temperature between days would not have significantly impacted our results. Fluid bolus administration was not accounted for in our study, and fluids were kept at room temperature so could have had a similar influence on our results.
Results of this study suggested that HHBC use combined with a WWB is more effective than a WWB alone for maintaining body temperature during general anesthesia in dogs undergoing ovariohysterectomies. Based on our results, the addition of an HHBC can decrease the incidence of postanesthetic hypothermia in healthy dogs, and while more studies are warranted, HHBC use should be considered in veterinary patients.
Acknowledgments
No third-party funding was received in connection with this study or the writing or publication of the manuscript. Veterinary Surgical Supply LLC provided a trial unit of the heated humidified breathing circuit to the Anesthesia Service at the University of Georgia without any form of financial compensation.
The authors have nothing to declare.
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