Use of a Far-Infrared Active Warming Device in Guinea Pigs (Cavia porcellus)

Abstract

Small mammals have difficulty maintaining body temperature under anesthesia. This hypothermia is a potential detriment not only to the health and comfort of the animal but also to the integrity of any treatment given or data gathered during the anesthetic period. Using an external warming device to assist with temperature regulation can mitigate these effects. In this study, we investigated the ability of an advanced warming device that uses far-infrared (FIR) heating and responds to real-time core temperature monitoring to maintain a normothermic core temperature in guinea pigs. Body temperatures were measured during 30 min of ketamine–xylazine general anesthesia with and without application of the heating device. The loss of core body heat from anesthetized guinea pigs under typical (unwarmed) conditions was significant, and this loss was almost completely mitigated by application of the FIR heating pad. The significant difference between the temperatures of the actively warmed guinea pigs as compared with the control group began as early as 14 min after anesthetic administration, leading to a 2.6 °C difference at 30 min. Loss of core body temperature was not correlated with animals’ body weight; however, weight influences the efficiency of FIR warming slightly. These study results show that the FIR heating device accurately controls core body temperature in guinea pigs, therefore potentially alleviating the effects of body heat loss on animal physiology.

Guinea pigs (Cavia porcellus) are a widely used species in biomedical research, particularly in studies investigating anaphylaxis, lung function, asthma, delayed hypersensitivity, genetics, gnotobiotics, immunology, infectious disease, nutrition, otology, and pharmacology.⁶ Ketamine hydrochloride, a dissociative anesthetic, and xylazine, a nonnarcotic sedative and muscle relaxant, are often used in combination to provide anesthesia during invasive instrumentation.⁸ In combination, ketamine and xylazine have been used successfully in the anesthesia of a wide variety of domestic, feral, and laboratory animals species, with the effective dose varying widely among species.² Combinations of ketamine with pentobarbital, xylazine, fentanyl, and droperidol diazepam, and promazine have been used in guinea pigs with varying degrees of success. The fentanyl–droperidol combination has been associated with a self-mutilation syndrome, pentobarbital alone can increase mortality up to 8%, and ketamine has been criticized for producing inadequate muscle relaxation.^5,10,14 Ketamine in combination with xylazine has been shown to produce a safe predictable anesthesia in many species. This combination provides a safe, deep anesthesia, without the muscular rigidity and pressor effects seen with ketamine alone.³

Most anesthetic procedures cause a depression of the hypothalamic thermoregulatory mechanism, predisposing animals to hypothermia. Hypothermia results from anesthetic-induced inhibition of thermoregulatory control combined with exposure to a cool environment.¹¹ This problem is even greater in small laboratory species, which have a very large surface area relative to body mass and a correspondingly greater loss of body heat.¹⁰ Substantial body heat is lost to the environment from tails, ears, and feet of rodents.^10,13 In multiple studies, intraperitoneal administration of ketamine–xylazine produced dose-dependent hypothermia in the experimental animals.^1,12 The hypothermic condition causes several physiologic effects, including cardiac arrhythmias, an increased potency of inhalant anesthetics, postanesthetic tremors, prolonged recovery time from anesthesia, and death.^5,7,13 Temperature-related effects can confound experimental data and therefore are a particularly important parameter to monitor in laboratory animals.¹⁰

Active warming is required to prevent hypothermia when laboratory rodents are anesthetized with inhalant gases, shaved during surgical preparation, wiped with cold skin disinfectants, or undergo invasive procedures.^13,15 Maintenance of normothermia in research animals may help to provide more consistent results. For example, enzymatic activity and pharmaceutical agents are affected by alterations in body temperature.¹³ The easiest way to radically reduce heat loss from the body surface is to increase the ambient temperature. However, this method is impractical, given that most operating room personnel find ambient temperatures above 22 °C uncomfortably warm.¹¹ Many common methods to maintain normothermia in laboratory animals include forced-air warming systems, circulating warm-water blankets, heat lamps, warm-water bags, and infrared heat emission.

Far infrared (FIR) is one kind of electromagnetic wave, which has the properties of light and wave. FIR is an invisible portion of the electromagnetic spectrum, with a wavelength ranging from 4.0 to 14.0 µm. FIR has 3 biologic effects: radiation, resonance, and thermal effects. The energy of the electromagnetic wave is absorbed by the skin and then transferred to heat by the irradiation, resonance, and molecular collision. This process causes local vessel dilatation, increases capillary circulation, and improves tissue repair, thus increasing the patient's pain threshold. Therefore, these effects relieve surgical pain, improve wound healing, and activate physiologic functions.¹⁶ FIR heat is absorbed deeply within an animal's body to safely warm it. Internally, the energy level of water in the body is gently increased through ‘resonant absorption,’ thereby warming the body core.⁴

In this study, we sought to verify the hypothesis that application of a FIR heating device to anesthetized guinea pigs maintains a normothermic core body temperature throughout anesthesia.

Materials and Methods

Animals.

Male IAS Hairless guinea pigs (age, 3 mo to 1 y; weight, 527 to 1027 g; n = 13) were used in this study. These guinea pigs were purchased from Charles River Laboratories (Wilmington, MA) and were certified to be free of Sendai virus, pneumonia virus of mice, reovirus, adenovirus, lymphocytic choriomeningitis virus, simian paramyxovirus type 5, Streptococcus zooepidemicus, S. pneumonia, S. moniliformis, Salmonella, Encephalitozoon cuniculi, Tyzzer disease, Bordatella bronchiseptica, and cilia-associated respiratory bacillus. The guinea pigs were individually housed in solid-bottom polycarbonate cages. (Lab Products, Seaford, DE) Fresh bedding (Alpha Dri, Shepherd Specialty Papers, Quality Lab Products, Elkridge, MD) was supplied a minimum of twice weekly. Temperature in the study room was maintained at 17 to 26 °C (63 to 79 °F), and relative humidity was 30% to 70%, with 12 to 15 air changes hourly. All animals were housed on a 12:12-h light:dark cycle (lights on, 0600 to 1800). Guinea pigs were provided a commercial chow (Guinea Pig 5P18 diet, Purina Mills, St Louis, MO) and water (purified onsite by reverse osmosis) free choice. Regular dietary supplements included a daily rotation of spinach and kale. Research was conducted in compliance with the Animal Welfare Act and other federal statutes and regulations relating to animals and experiments involving animals and adheres to principals stated in the Guide for the Care and Use of Laboratory Animals.⁹ All procedures used in this study were IACUC-approved. The facility where this research was conducted is fully AAALAC-accredited.

Temperature reading of warmed compared with unwarmed.

The warming device (MouseSTAT, Kent Scientific, Torrington, CT) evaluated in this study use FIR technology to regulate body temperature according to feedback from the rectal temperature probe and the programmed desired core body temperature.

Each guinea pig was monitored twice under sedation, once without the FIR warming system and once with it, with at least 1 wk between trials and with each trial occurring in the late morning. For both warmed and unwarmed trials, guinea pigs were sedated by using ketamine ( 100 mg/mL IM; Putney, Portland, ME) and xylazine (100 mg/mL IM; Lloyd Laboratories, Decatur, IL) injected in the right hindleg. Once the guinea pigs were sedated (tested by using the hindleg pedal withdrawal reflex), they were placed on a covered table, to avoid additional cooling from the metal table. The eyes were lubricated with ophthalmic ointment (Rugby Laboratories, Livonia, MI). A rectal temperature probe was lubricated with sterile lubricant (Fougera, Novartis, Melville, NY) and placed approximately 1.25 inches into the rectum (Figure 1). Temperature readings were recorded every 2 min for 30 min. At the end of 30 min, control (unwarmed) guinea pigs were placed on the FIR warming pad to recover. Once they were able to maintain sternal recumbency, the guinea pigs were returned to their cages.

Figure 1.

Set up of FIR warming device for application to anesthetized hairless guinea pig. Black arrow, heating pad; white arrow, FIR heating monitor and controller; blue arrow, polypropylene microfilm used to prevent thermal injury. Photo was uniformly adjusted for brightness and white balance.

During the FIR warming trial, the warming pad was placed over the sedated guinea pig and loosely secured around the animal's abdomen by using a hook-and-loop strap. Initial experiments indicated that superficial thermal injuries might occur when a hairless animal was placed in direct contact with the pad, so subsequent trials used a thin layer of 100% polypropylene microfilm (The Safety Zone, Centerbrook, CT), placed between the guinea pig and the FIR warming device, to prevent such injuries (Figure 1). At the end of the 30 min, guinea pigs were allowed to recover as previously stated. Each guinea pig was weighed on a bench scale (A and D Company Limited, Tokyo, Japan) at the termination of the experiment.

Data analysis.

Data were analyzed by using the Prism 6 software package (GraphPad, La Jolla, CA). Time-course data underwent repeated-measures 2-way ANOVA and ad-hoc multiple comparisons with Sidak correction to evaluate the average body temperature of warmed compared with unwarmed guinea pigs at each time point. Correlation of weight compared with the change in temperature data was evaluated in a simple linear regression model.

Results

At time 0 (that is, when the animal was first noted to be fully sedated and the rectal probe had reached a equilibrated temperature), the average temperature among animals was 39.3 °C (range, 37.6 to 40.4 °C) when not actively warmed (that is, unwarmed controls) and 39.5 °C (range, 38.3 to 40.0 °C) when actively warmed, a nonsignificant difference of 0.2° between treatments (Figure 2, Table 1). As time progressed, both the treatment and control applications allowed animals to lose body heat, although this effect was more severe and rapid in unwarmed animals. In warmed guinea pigs, this difference became apparent at the 8- to 10-min time point, at which their average core temperature stabilized at about 39 °C for the duration of the experiment, and became statistically significant (P < 0.05) at 14 min. This stabilization is in stark contrast to the continual decline in average core temperature in unwarmed guinea pigs. By the 30-min time point, the control group had cooled to an average of 36.5 °C, whereas the warmed group maintained a relatively normothermic core temperature; their 30-min temperature was 39.1 °C (Figure 2, Table 1).

Figure 2.

Application of FIR warming device prevents heat loss in anesthetized guinea pigs. The temperatures (°C) of individual guinea pigs (n = 12) with (filled squares, dotted line) and without (filled circles, solid line) warming were measured every 2 min. Data are presented as mean ± SEM. Significant differences (*, P < 0.05; ‡, P < 0.001; §, P < 0.0001) between averages for warmed and unwarmed guinea pigs are indicated.

Table 1.

Weights and temperatures of individual guinea pigs

Animal no.	Weight (g)	Unwarmed		Warmed
		0 min (°C)	30 min (°C)	0 min (°C)	30 min (°C)
13	775	39.2	37.0	39.6	39.5
14	907	39.3	36.1	39.5	39.5
15	793	39.0	36.2	38.3	37.3
16	1027	39.6	36.6	39.6	38.5
17	906	38.8	36.3	39.9	39.4
20	901	39.3	36.6	38.7	37.5
125	527	39.4	35.6	39.5	39.4
126	534	39.8	37.0	39.2	39.5
127	533	37.6	35.5	39.6	39.7
128	549	40.4	37.3	39.5	39.4
129	563	39.8	36.6	40.0	39.6
130	575	39.8	37.1	40.0	39.7

Temperatures of unwarmed and warmed guinea pigs were taken on different days, with rest for animals between trials. A randomized mix of guinea pigs received either control or warming treatments on each trial day.

Most striking is the difference in temperature between the 0- and 30-min readings, reflecting the overall body heat loss under either treatment condition. In the unwarmed group, the total average temperature loss was 2.8 °C (range, 2.1 to 3.8 °C), yet when the same animals were warmed by using the FIR pad, the average loss was only 0.4 °C (range, 1.1 °C loss to 0.3 °C gain; Figure 2, Table 1).

In addition, we investigated the hypothesis that the FIR heating pad would be more effective for smaller guinea pigs than larger. Overall, there was no correlation between body weight and change in core temperature among unwarmed animals (Figure 3 A, Table 1). However, there was a slight negative correlation between body weight and the ability to maintain a normothermic core temperature when the FIR warming device was applied; that is smaller animals displayed greater temperature stability in response to warming than did larger animals (Figure 3 B, Table 1).

Figure 3.

Animal weight was not a determinate of heat loss but did influence warming device efficacy. Filled dots represent data taken from an individual guinea pig. Lines of best fit were determined by simple linear regression modeling. For panels B and C, the dotted line serves as reference for no change in temperature. (A) Starting temperature (°C) during warming trial was plotted against animal weight (g). Starting temperatures during control trial are similar and therefore not represented graphically but data are found in Table 1. Slope = –0.0006164 ± 0.0008333; R² = 0.05188; Pearson r = –0.2278; P = 0.4765. (B) Change in temperature (°C) from 0 to 30 min is plotted against animal weight (g) for control (unwarmed) trial. Slope = 0.0003372 ± 0.0007790; R²= 0.01839; Pearson r = 0.1356; P = 0.6743. (C) Change in temperature (°C) from 0 to 30 min is plotted against animal weight (g) for warmed trial. Slope = –0.001701 ± 0.0006293; R² = 0.4223; Pearson r = –0.6498; P = 0.0222.

Discussion

Our results show that using a FIR heating device maintains a normothermic core body temperature in anesthetized guinea pigs. We observed that the core body temperature of the control (unwarmed) guinea pigs dropped steadily throughout the experiment, whereas that of the warmed group dropped only slightly during the first 5 to 10 min and then leveled to an average of 39° for the remaining 20 to 25 min. These findings reflect the ability of the FIR device to respond to the real-time temperature. The difference in temperature between groups reached statistical significance at 14 min, suggesting that guinea pigs anesthetized for 14 mins or longer are likely to have noticeably different physiologic responses depending on whether (or not) a FIR warming device is used (Figure 2).

The initial core temperature readings showed that individual guinea pigs have different core temperatures; this characteristic affects how the device is set. Although the average starting core temperatures were 39.3 to 39.5 °C, several animals were outliers, with core temperatures as high 40.4 °C and as low as 37.6 °C. Although we staved off cooling by using a standard set temperature of 39.5 °C, faster or more stable maintenance of normothermy might be achieved by measuring the basal body temperature before applying the pad and then targeting the set temperature to the individual animal. Interestingly, the 2 guinea pigs whose core temperatures were more than 1 SD below the 39 °C average after 30 min of warming (no. 15, 38.3 °C; no. and 20, 38.5 °C) also had the lowest temperatures at time 0. We noted that their body temperatures continued to cool throughout the 30 min; the core temperature of no. 15 fell to 37.3 °C even though the temperature of the FIR device was set at 39.5 °C. During this time, the warming pad became so hot that it caused thermal burns on the skin of this hairless guinea pig. This anomaly indicates that some individual animals may not be as amenable to FIR warming methods as others and warrants further investigation into why this guinea pig's core temperature was not maintained, even though the pad clearly was operational. This episode precipitated the addition of a protective microfilm; and we suggest that other researchers adopt a similar practice when using this device, for the safety of hairless animals.

Our guinea pigs fell into 2 weight classes—those that weighed 575 g or less and those that weighed 775 g or more—due to using animals that differed in age by a few months, without any differences in overall health. We saw no correlation between how much heat was lost under normal (unwarmed) conditions, suggesting that the rates at which anesthetized guinea pigs lose body heat over a 30-min period do not differ significantly in regard to mass (in the weight range tested here). However, we did note that the smaller animals responded better to the warming device, in that they lost less heat, in some cases exhibiting even a slight increase in core temperature. This effect is likely because, although the FIR heating methodology is more penetrating than are other heating methods, it is simply easier for a surface-applied mechanism to heat the core of a small animal than a large animal. The response of the larger animals might resemble that of the smaller animals, if the time period were extended.

In light of our results demonstrating that temperature regulation by using active FIR-based warming is efficient for anesthetized guinea pigs, we feel this device is a viable choice for warming rodents and monitoring their body temperature. The continuous monitoring–warming feedback loop eliminates the possibility of human error in maintaining body temperature as it enhances animal wellbeing and experimental accuracy. On the basis of our experiments, we recommend allowing the warming pad to reach the desired temperature before anesthetizing guinea pigs (or other rodents), to avoid unwanted cooling, and placing a cloth barrier between nonhaired skin and the warming pad, to prevent thermal injury.