Effect of pre-warming on perioperative hypothermia and anesthetic recovery in small breed dogs undergoing ovariohysterectomy

Abstract

This study compared perianesthetic body temperatures and times to recovery from general anesthesia in small dogs that were either warmed for 20 minutes prior to anesthesia or not warmed. Twenty-eight client-owned dogs that were presented for ovariohysterectomy were included in the study. Small (<10 kg body weight) dogs with normal circulatory status were randomly assigned to receive pre-warming for 20 minutes or no treatment. Body temperature was measured during the procedure using a calibrated rectal probe. Duration of anesthesia and surgery, time to rescue warming, time to extubation, presence and duration of shivering, and time to return to normal temperature were recorded. Temperature at the end of surgery was significantly higher in the control group than the pre-warmed group. There was no difference in time to extubation or duration of postoperative shivering between groups. Pre-warming did not result in improved temperature or recovery from anesthesia.

Introduction

Hypothermia is the most common anesthetic complication in dogs and cats, occurring in approximately 40% of anesthetized animals (1). Hypothermia can be detrimental to overall outcomes because it causes or contributes to sympathetic activation, pharmacokinetic alterations, coagulation abnormalities, blood loss, cardiac morbidity, wound infection, and shivering. Factors contributing to perioperative hypothermia are convection, conduction, radiation, evaporation, intravenous fluid administration, cold and dry inhaled gases, and patient surgical preparation using cold solutions (2). Anesthesia inhibits both heat-seeking behaviors and activities that cause heat production (movement and shivering), and some anesthetic drugs cause vasodilation, which could contribute to heat loss, although this loss is minimal compared with losses due to blood redistribution (3). Anesthetics also interfere with or inhibit thermoregulation through inhibition of vasoconstriction (2). Hypothermia develops in 3 phases: i) redistribution of core heat to body surfaces during anesthetic-induced inhibition of tonic thermoregulatory vasoconstriction causes core temperatures to decrease by 1°C to 1.5°C during the first hour of anesthesia (2–4); ii) heat loss exceeds metabolic heat production resulting in further decreases in core temperatures over the next 2 to 3 h; and iii) core temperatures plateau as metabolic heat is constrained to the body core due to peripheral vasoconstriction, typically occurring within 3 to 4 h of anesthesia (2,5,6). Hypothermia results in delayed recovery times in dogs, defined as the time from the start of anesthetic recovery to sternal recumbency (7), which may be due to decreases in the rate of drug metabolism (8). Concurrently, prolonged recovery from anesthesia may delay the return to pre-anesthetic body temperature. After surgery, residual inhalant anesthetic concentrations may decrease vasoconstriction, shifting the dose-response curve of thermoregulatory inhibition (9). Opioids, which are commonly used in surgical and post-surgical patients to treat pain, impair the shiver response and may cause generalized thermoregulatory impairment (9).

Humans report that hypothermia is an uncomfortable condition that impairs smooth anesthetic recovery (9). Small and immature patients have a greater body surface area to mass ratio than larger patients, resulting in increased heat loss (10). Pre-warming prior to general anesthesia in humans is usually accomplished using forced warm air blankets and has been associated with higher postoperative core body temperatures (11–13).

Methods to maintain body temperature in anesthetized dogs and cats have been investigated. Moderate to severe postoperative hypothermia, defined as a temperature below 36.5°C, was noted in 70% of cats when no active warming methods were used during surgery (14). Heat loss and hypothermia can be moderated in cats, however, using intraoperative warming methods (15–17). Pre-warming has been investigated in anesthetized cats and dogs not undergoing surgery and prevented peri-operative hypothermia (18,19). Pre-warming of dogs before cutaneous surgery has been shown to be ineffective, but the time period of pre-warming was not standardized and indices of recovery were not reported (20). To our knowledge, the effect of pre-warming on intraoperative and postoperative body temperatures has not been investigated in dogs undergoing abdominal surgery.

The purpose of this study was to compare body temperature in small dogs that were pre-warmed for 20 min before general anesthesia and abdominal surgery to body temperature in small dogs that were not pre-warmed. Additionally we wanted to compare recovery times in these dogs. It was hypothesized that pre-warming small breed dogs would result in higher postoperative temperatures compared to dogs that were not pre-warmed and that pre-warmed dogs would recover faster from general anesthesia.

Materials and methods

Dogs

This was a randomized, controlled prospective clinical study that was approved by the Clinical Research Advisory Committee of The Ohio State University College of Veterinary Medicine. Owner consent was obtained before enrollment in the study. The study population consisted of 28 dogs that were presented to The Ohio State University Veterinary Medical Center for ovariohysterectomy. A physical examination was performed on each dog before inclusion in the study, and packed cell volume and serum total solids were also obtained for each patient. Dogs were included in the study if they were < 10 kg body weight. Exclusion criteria included clinical dehydration (hemoconcentration, skin tenting, mucous membrane examination), cardiac disease (previous diagnosis, heart murmur greater than grade II/VI), and any condition that precluded the use of the standardized anesthetic protocol.

Dogs were randomly assigned to a study protocol consisting of the following groups: Group C — no pre-warming or Group P — pre-warming for 20 min.

Study design

Baseline body temperature was measured using a rectal thermometer probe (YSI 2100 Temperature Probe; YSI, Yellow Springs, Ohio, USA) the morning of surgery during the preoperative physical examination. The thermometer was calibrated before initiation of the study. A pre-determined pre-anesthetic, induction, and maintenance drug protocol suitable for young, healthy dogs was used in all dogs. Acepromazine (Vedco, St. Joseph, Missouri, USA), 0.05 mg/kg body weight (BW), and buprenorphine (Reckitt Benckiser, Hull, England), 0.01 mg/kg BW, were administered intramuscularly (IM). Dogs were then placed in an infant incubator (DRE Infantia NB1, Louisville, Kentucky, USA) with dimensions of 108.5 cm × 64.0 cm × 90.0 cm, with the warming source turned off. Dogs were housed in this incubator for 20 min to allow the pre-anesthetic drugs to take effect.

After 20 min, an IV catheter was placed in a cephalic vein and rectal temperature was measured and recorded. Dogs in Group C were placed in the incubator without the heating unit turned on. Dogs in Group P were placed in a heated incubator for 20 min with the internal temperature set to 43°C. A thermometer was placed in the incubator to measure ambient temperature. After 20 min the dogs were removed from the incubator.

Propofol (Abbott Laboratories, North Chicago, Illinois, USA), 4 mg/kg BW, IV, was administered within 5 min of removal from the cage. Dogs were intubated and maintained with isoflurane in 100% oxygen at the lowest vaporizer setting that provided an appropriate anesthetic depth. A non-rebreathing anesthetic circuit with the oxygen flow rate set at 200 mL/kg BW per minute was used for all dogs. Dogs received room temperature intravenous fluids (lactated Ringer’s solution) at a rate of 5 mL/kg BW per hour from the time of induction until the end of anesthesia. Heart rate was determined using a Doppler ultrasound (Parks Medical Electronics, Aloha, Oregon, USA) with the Doppler probe placed over the palmar digital artery. Heart rhythm was monitored using an electrocardiogram. Respiratory rate was determined by visualizing movement of the reservoir bag. Arterial blood pressure was estimated using the Doppler, an appropriately sized blood pressure cuff, and a sphygmomanometer. Potential intraoperative complications, such as bradycardia (defined as a heart rate of 60 beats/min or lower) or hypotension (defined as a systolic arterial blood pressure of 80 mmHg or lower) were treated as deemed appropriate by the supervising anesthesiologist. Rectal temperature was monitored continuously (YSI 2100) and recorded every 5 min. While in the surgery room, dogs were placed on a thermal warming system (Gaymar Stryker T/Pump; Stryker, Orchard Park, New York, USA) and covered with a blanket that was not operative initially. The thermal warming system was turned on to a temperature of 42°C if the rectal temperature fell to 35°C during the surgical procedure, and the time was recorded.

At the end of surgery dogs were transferred to a recovery room. If body temperature at the end of surgery was lower than 36.7°C, dogs were rewarmed following surgery using a forced warm air blanket set to the “high” setting (43°C) and covered with a medium weight pad or by placing them in a heated incubator set to 43°C.

The following were recorded:

• baseline temperature (the morning of surgery);

• incubator temperature;

• time interval from induction of anesthesia [defined as the time of propofol administration until the dog was moved into the operating room (OR)], (defined as the time of temperature probe placement in the OR);

• abdominal preparation time (clipping and scrubbing in the pre-surgical area);

• ambient temperature in the OR;

• first temperature in the OR;

• temperature at the start of surgery (defined as the time of initial skin incision);

• time interval from anesthetic induction to start of surgery;

• time interval from moving into the OR to start of surgery;

• time interval from anesthetic induction to rescue warming;

• time interval from start of surgery to rescue warming;

• duration of surgery;

• duration of anesthesia (defined as the time from start of isoflurane administration to the time the isoflurane vaporizer was turned to the off position);

• rectal temperature at the end of surgery (defined as the time at which the last suture was placed);

• time interval to extubation (defined as the time from the end of surgery to the patient swallowing and removal of endotracheal tube); and

• duration of shivering were recorded.

Pain was assessed in the immediate perioperative period and treated as deemed appropriate by the supervising anesthesiologist. Postoperative pain was treated by the attending surgeon using non-steroidal anti-inflammatory drugs and/or opioids.

Analysis of data

All data were normally distributed (D’Agostino & Pearson omnibus normality test) and were analyzed using an unpaired one-tailed t-test. Values of P < 0.05 were considered significant. All data are reported as mean ± standard deviation (SD).

Results

Sixteen breeds were represented in this study, including 5 mixed-breed dogs, 4 Jack Russell terriers, 3 toy poodles, 2 Cavalier King Charles spaniels, 2 Yorkshire terriers, 2 miniature schnauzers, and 1 dog each for 10 other breeds (French bulldog, English bulldog, Chihuahua, miniature pinscher, miniature dachshund, papillon, Shetland sheepdog, Havanese, Alaskan klee kai, and Pembroke Welsh corgi). All dogs were female and were anesthetized for ovariohysterectomy. There were 14 dogs in each group.

Dogs in group P ranged in age from 6 mo to 7 y and weighed 4.4 ± 2.2 kg, and dogs in group C ranged in age from 5 mo to 5 y and weighed 5.8 ± 2.0 kg. There was no difference in baseline rectal temperature taken on the morning of surgery. The incubator temperature for group C was 22.5°C ± 1.6°C and the incubator temperature for group P was 37.9°C ± 1.7°C.

There was no significant difference in preparation time, time from anesthetic induction to moving to the operating room, ambient temperature in the operating room, first rectal temperature in the OR, or rectal temperature at the start of surgery. The time from anesthetic induction to the start of surgery in group C was significantly shorter at 30 ± 5 min, compared with group P for which it was 37 ± 9 min (P = 0.0167), but there was no difference between groups in time from anesthetic induction or start of surgery to rescue warming or in time from moving to the operating room to start of surgery. All dogs in group P and 13 of 14 dogs in group C required rescue warming. There was no difference in duration of surgery or duration of anesthesia between groups. The temperature at the end of surgery was significantly lower in the group P dogs (34.0°C ± 1.0°C) compared with group C dogs (34.7°C ± 1.2°C) (P = 0.0273).

One dog in each group was treated for intraoperative bradycardia by administration of an anticholinergic. No other complications were noted in the pre-warmed group. In the control group, 2 dogs were sedated with acepromazine in recovery (following extubation) due to emergence delirium, and 1 dog was noted to have occasional ventricular premature complexes during anesthesia that were not treated and resolved during the anesthetic period.

The mean ± SD time to extubation was 9 ± 3 min for pre-warmed dogs and 8 ± 5 min for the control group. Shivering occurred in 13 dogs that were pre-warmed and 11 dogs in the control group. Duration of shivering was 42 ± 24 min in the pre-warmed group and 31 ± 23 min in the control group. There were no differences between groups in any of these variables.

Discussion

Pre-warming dogs before induction of general anesthesia in this study did not result in decreased incidence of perioperative hypothermia. All dogs in this study developed hypothermia regardless of treatment group. These results are consistent with those of Rigotti et al (20), who evaluated the effect of pre-warming for an unspecified duration before anesthesia for minor surgical or diagnostic procedures.

In contrast, pre-warming for 20 to 30 min has been shown to decrease incidence of perioperative hypothermia in dogs premedicated with acepromazine or dexmedetomidine (19) and pre-warming for 20 min decreased the incidence of perioperative hypothermia by 62% in adult humans (11), even though in both of these studies, intraoperative warming was not instituted until body temperature decreased to 36°C. It is possible that body surface area in our study was a limiting factor. Dogs weighing < 10 kg were used in Rigotti’s study (20), whereas the size of the dogs in Read’s study was not specified (19).

Pre-warming may have resulted in peripheral vasodilation causing increased heat loss due to redistribution hypothermia (12). Body temperature after pre-warming just prior to anesthetic induction was not checked so as not to cause excitement or anxiety before administration of anesthetic induction drugs. This is a limitation of this study as starting temperatures of these patients before movement into the OR approximately 15 to 18 min later are not known. Subjectively, dogs in the pre-warmed group were panting during the 20 min of pre-warming, but this does not necessarily equate to an overall increase in body temperature. Heat loss occurs in different phases, with the redistribution phase occurring within the first hour of general anesthesia (2–4). Vasodilation has also been demonstrated to occur in awake humans after forced warming and persists for the duration of warming (12). It is unknown what effect warming followed by withdrawal of warming has on vascular tone. Additionally, acepromazine and isoflurane, both of which cause vasodilation, may have exacerbated the peripheral vasodilatory effect of pre-warming.

There was a slightly longer time to start of surgery (defined as the time interval from anesthetic induction to start of surgery) in the pre-warmed group compared to the control group, but there was no difference in time interval from anesthetic induction to the operating room, in time interval from anesthetic induction or start of surgery to rescue warming, or in overall duration of surgery. This slightly longer time to surgery is unlikely to have affected anesthetic recovery or the incidence of perioperative hypothermia.

Rescue warming was initiated when rectal temperature dropped to 35°C, as in studies by Horn et al (21) and Rigotti et al (20). All dogs in the pre-warming group, and all but 1 in the control group required intraoperative warming with no difference in time to initiation of preoperative warming. However, dogs in the control group were significantly warmer at the end of surgery than were dogs that received pre-warming, suggesting that the dogs in the control group either had improved thermoregulation or were able to maintain core body temperature better than the pre-warmed dog group. There was no difference in extubation time or shivering between groups: dogs were extubated 8 to 9 min following cessation of inhalant administration regardless of the group.

A limitation of the present study was the use of a non-rebreathing system, which could have resulted in lower rectal temperatures due to the associated high fresh gas flow. It is possible that body temperature in both groups would have been higher if a rebreathing circuit was used; however, Kelly et al (22) demonstrated no temperature difference related to circuit type. We chose to standardize the breathing circuit in all dogs and use of non-rebreathing circuits varies with weight of the dog. There is no body weight limit for use of a re-breathing circuit, but non-rebreathing circuits have a history of being used in dogs weighing less than 5 kg (23).

Another limitation of this study is that patients didn’t receive any warming during the anesthetic period. Pre-warming before and after epidural block before induction of general anesthesia has been demonstrated to prevent perioperative hypothermia in humans (21). In this study all patients received intraoperative warming. The use of intraoperative warming did not prevent the development of perioperative hypothermia. We chose to pre-warm patients before induction, but did not warm during surgery unless the temperature fell to 35°C in order to establish the effects of 20 min of pre-warming on intraoperative temperature and recovery from anesthesia. Additional studies to determine if pre-warming along with intraoperative warming modalities would have prevented or prolonged the development of intraoperative hypothermia are warranted though studies on peripheral skin warming have indicated that warming of the extremities may be more beneficial for maintaining body heat in dogs (24).

In conclusion, pre-warming small dogs for 20 min before induction of general anesthesia did not reduce the incidence of perioperative hypothermia or improve recovery time, and may have resulted in decreased body temperatures in these dogs.

Table 1.

Temperature data from prewarmed and non-prewarmed dogs undergoing general anesthesia for abdominal surgery

Parameter	Prewarmed	Control
Baseline temperature (°C)	38.3 ± 0.5	38.4 ± 0.8
Incubator temperature (°C)	37.9 ± 1.7a	22.5 ± 1.6a
Time from induction to operating room (min)	18 ± 7	15 ± 4
Preparation time (min)	5 ± 4	4 ± 3
Ambient temperature in operating room (°C)	19.6 ± 1.2	19.8 ± 1.2
First temperature in operating room (°C)	35.5 ± 1.5	36.1 ± 1.0
Temperature at start of surgery (°C)	34.8 ± 1.3	35.5 ± 1.1
Time from induction to start of surgery (min)	37 ± 9a	30 ± 5a
Time from operating room to start of surgery (min)	19 ± 6	18 ± 5
Time from induction to rescue warming (min)	44 ± 24	45 ± 23
Time from start of surgery to rescue warming (min)	12 ± 16	16 ± 16
Duration of surgery (min)	86 ± 22	81 ± 32
Duration of anesthesia (min)	125 ± 23	114 ± 33
Temperature at end of surgery (°C)	34.0 ± 1.0a	34.7 ± 0.7a
Time to extubation (min)	9 ± 3	8 ± 5
Duration of shivering (min)	42 ± 24	31 ± 23

Data are presented as mean ± standard deviation.

^aValue is significantly different from the other treatment.