Effects of Cling Film Draping Material on Body Temperature of Mice During Surgery

Abstract

General anesthesia induces many systemic effects, including thermoregulatory impairment and subsequent perioperative hypothermia. Due to the animals’ small size, monitoring and maintaining body temperatures in laboratory rodents during anesthesia is important for successful surgical outcomes and prompt anesthetic recovery. Draping materials have the potential to aid in thermal support during surgical anesthesia. In this study, rectal and surface (infrared) temperatures were measured in C57BL/6 mice under isoflurane anesthesia every 5 min for the duration of a 35-min sham surgery. In addition to placement on a circulating water bath, mice (n = 6/group) were draped with commercial cling film (CF; Press'n Seal, Glad, Oakland, CA), a conventional paper drape (PD), or no drape (ND) during surgery. Results demonstrated that CF-draped animals had significantly higher rectal temperatures than nondraped animals. Furthermore, surface temperatures of CF-draped mice were considerably higher than those of both paper-draped and undraped animals. The data indicate that cling film is an effective material to help minimize hypothermia in mice and potentially in other laboratory rodents requiring general anesthesia.

Surgery and anesthesia introduce many challenges, especially in veterinary medicine, due to the diversity of species. One major challenge during general anesthesia involves changes in an animal's thermoregulatory ability.^1,14 Body temperatures in mice and rats fall significantly during anesthesia if no thermal support is provided.^29,30 Hypothermia occurs due to drug-induced alterations to the thermoregulatory center, inadequate circulation, and a loss of body heat to the environment from evaporation, radiation, conduction, and convection.⁷ Mice are particularly susceptible to hypothermia, due to their large surface area per gram of body weight, which permits significant physiologic changes in response to fluctuations in the ambient temperature.³¹ Covering the animal's body with towels, drapes, or blankets to reduce the area exposed to the environment can minimize heat loss.^6,7,13 Placing the animal on an insulated surface can limit conductive heat loss. In larger animals, warmed fluids can be given perioperatively, heated anesthetic gasses can be administered, and heated blankets and heat packs can be applied to body surfaces to provide exogenous heat.^1,7 Safer and more practical methods for rodents are circulating water heating blankets, thermal gel packs, and warming lamps, which are commonly used for thermal support during anesthesia.^5,14 Addressing all of these factors can contribute to maintaining normothermia during anesthesia.

Risk of mortality is elevated during anesthesia and in the postoperative period, including in rodents.^1,13 Hypothermia induced by anesthesia can negatively affect rodents by altering vital parameters such as heart rate and blood pressure and delaying anesthetic recovery.^3,5,12,19 These risks require careful selection of an appropriate anesthetic protocol and careful monitoring of the patient throughout anesthesia until full recovery occurs. Strict anesthetic monitoring and the use of supplemental heat devices have been shown to reduce the likelihood of complications, improve overall postoperative recovery, and reduce mortality associated with surgical procedures.^1,7,15,16 However, due to these species’ small size, monitoring equipment must be specialized and is often costly. Cost-effective and practical alternative equipment and materials would facilitate monitoring and care of rodents.

Various draping options are available for rodent surgery, and their use is vital for both sterile technique and heat retention. Traditionally, paper draping material has been a popular option, because it is relatively inexpensive and can be autoclaved together with surgical instruments.^15,16 Some institutions have adopted varying methods and types of draping, including no drape and paper draping. Commercial cling film (CF) has been used as draping due to its low cost, ease of use, and sterility straight out of the box.⁹ Our study team sought to evaluate the effects of draping material on intraoperative thermoregulation in mice by measuring rectal temperature (modified rectal probe) and surface temperature (infrared device) during a 35-min laparotomy procedure, with both temperature devices chosen for affordability and availability. We hypothesized that mice draped with CF would maintain a higher intraoperative body temperature under general anesthesia than would mice with traditional paper drapes or no drape.

Materials and Methods

Animals and facility

Male C57BL/6 mice (n = 18; weight, 25 to 30 g; age, 3 to 5 mo; The Jackson Laboratory, Bar Harbor, ME) were used in this study. All procedures were approved by Ohio State University's IACUC and were completed at the university. The facility is AAALAC-accredited, USDA-registered, and OLAW-assured. Animals were housed at 70 to 72 °F (21.1 to 22.2 °C), 30% to 70% humidity, 10 to 12 room air changes hourly, and a 12:12-h light:dark cycle with 4 or 5 mice in each polysulfone IVC (NexGen Mouse 500, Allentown Caging, Allentown, NJ) on disposable bedding (0.12-in., Bed-O-Cobs, The Andersons, Maumee, OH). Mice were fed pelleted laboratory rodent chow (Teklad LM-485 Mouse/Rat Sterilizable Diet [7912-irradiated], Envigo, Indianapolis, IN) and received water through an automatic watering system. The facility sentinel program tests for and excludes mouse parvoviruses, murine norovirus, mouse hepatitis virus, Theiler murine encephalomyelitis virus, murine rotavirus, Sendai virus, pneumonia virus of mice, reovirus types 1 through 4, Mycoplasma pulmonis, lymphocytic choriomeningitis virus, Spironucleus muris, Entamoeba, fur mites (Myobia, Myocoptes, and Radfordia spp.), pinworms (Aspiculuris and Syphacia spp.), Pneumocystis spp., Corynebacterium bovis, and mouse chapparvovirus.

Drape material

Mice were divided into 3 groups according to the draping method. Six mice were assigned randomly to each group: paper drape (PD; catalog number 89534, Sterile Half Drape, Halyard Health, Alpharetta, GA), CF (Press'n Seal, 100 ft² roll, The Glad Products Company, Oakland, CA), or no surgical drape (ND). Any draping used was sized to cover the entire animal, and fenestration was cut to match the 3 × 3 cm area of surgical preparation. The PD rested on the table around the mouse, whereas the CF drape was lightly pressed to stick it to the table on either side of the mouse.

Temperature-monitoring devices

The rectal probe was created from a skin temperature sensor device (Level 1 Skin Temperature Sensor–Adult; Smiths Medical, Minneapolis, MN) as shown in Figure 1 A. The device was connected to a vital parameter–monitoring device (Surgivet Advisor 3 Parameter Vital Signs Monitor, Smiths Medical). The fabric pad around the thermometer was removed, exposing the sensor. A small amount of sterile lubricant was applied before inserting the full length of the probe (15 mm). Prior to study initiation, a randomly selected subset of mice (n = 6) was used to verify that the modified rectal probe was giving physiologically appropriate readings by manually restraining the conscious mice and inserting the rectal probe as described.

Figure 1.

(A) Rectal temperature-monitoring device used in this study. Fabric was removed to expose the sensor (*) for rectal temperature measurement. (B) Image displays application and position of rectal sensor (arrow) in a mouse draped in CF during the procedure. The white circle indicates where the infrared device was pointed on the skin (left-lateral to incision site) for temperature readings. The white line indicates abdominal incision.

Surface temperatures of mice were monitored by using an infrared thermometer (model RAYMT4U, Raytek MiniTemp Infrared Thermometer, Fluke, Everett, WA). At each data collection point, infrared thermometers were held 6 in. (15.2 cm) away from the mouse. To ensure a consistent distance for infrared temperature measurements, a sterile cotton-tip applicator 6 in. (15.24 cm) in length was held in between the mouse and the infrared device, with one end of the applicator touching the skin and the opposite end touching the infrared device. The infrared device was pointed at a skin site 3 to 4 mm left and lateral to the abdominal incision not covered by draping material (Figure 1 B).

Surgical procedure and temperature measurements

The mice in this study were tested over 2 d, according to draping group. Mice were premedicated with buprenorphine (0.1 mg/kg SC; buprenorphine hydrochloride, injection [0.3 mg/mL], Par Pharmaceutical, Chestnut Ridge, NY) 15 to 20 min prior to anesthesia. All procedures were conducted in the same room under thermostatic control set to 72 °F (22.2 °C) and during the light phase of the photoperiod. Each mouse was placed in an induction chamber with 100% oxygen and 5% isoflurane at a flow rate of 1 L/min. After losing the righting reflex, the mouse was removed from the induction chamber and moved to the preparation table. A nose cone was applied to continue anesthesia at 2% isoflurane for the remainder of the procedure. No additional thermal support was provided. The abdomen was clipped to expose a 3 × 3 cm square of skin. A surgical scrub consisting of 6 alternating passes of chlorohexidine and isopropyl alcohol was applied to the surgical area. Lidocaine (1 mg/kg (LidoJect [lidocaine 2%], Henry Schein Animal Health, Melville, NY) diluted in sterile saline was administered by intradermal injection at the midline to provide local anesthesia. Excess surgical scrub was removed by using sterile gauze, leaving a dry surgical site for consistent surface temperature measurements. The skin preparation time averaged 4.3 min across all 3 groups.

After preparation, the mouse was moved to the surgery area and placed on a circulating water heating blanket (38 °C; model TP650, Gaymar Industries, Orchard Park, NY). The rectal probe was inserted (Figure 1 B), and the specified draping material was placed over the mouse. At this time, the first temperature measurements were taken by using the rectal probe and infrared thermometer and recorded as the 0-min time point. After 5 min, a 1-cm longitudinal surgical incision was made into the abdominal cavity to permit additional heat loss from an exposed body cavity, as occurs during a surgery. Another temperature measurement was taken at this time. Rectal and infrared temperatures were taken every 5 min after the time of surgical incision, for a total of 35 min. After the 35-min timepoint reading, mice were euthanized via exsanguination under deep anesthesia, followed by cervical dislocation.

Statistical analysis

All data were analyzed by using GraphPad Prism statistical software (Prism 7, GraphPad Software, La Jolla, CA). Two-way repeated-measures ANOVA was used for statistical comparison of temperature measurements between groups. When appropriate, posthoc analysis was performed by using the Tukey multiple-comparisons test. For all analyses, an α level of P ≤ 0.05 was considered statistically significant. Summary data are presented as mean and SEM.

Results

Surgical procedure

The preparation time for all draping groups (anesthesia induction, drug administration, and surgical preparation) averaged 4.3 min and did not differ significantly between groups. The mean total anesthesia time was 45.8 min for the PD group, 49.3 min for the CF animals, and 45.5 min for ND mice and did not significantly differ between groups. The temperature in the procedural room in which the study took place was 71.9 ± 0.2 °F (22.2 ± 0.1 ºC).

Rectal temperatures

The baseline rectal temperatures taken for rectal probe verification prior to the study had a mean of 99.2 ± 2.7°F (37.3 ± 1.7 ºC) in awake mice. The mean rectal temperatures for all groups at each time point can be seen in Figure 2. The CF group showed an increase in mean rectal temperature at each time point over the course of the procedure. In contrast, the PD and ND groups showed a decrease in mean rectal temperature during the first 10 min and began to increase thereafter.

Figure 2.

Rectal temperatures (mean ± SEM [error bars]) during anesthesia for ND (black, n = 6), PD (blue, n = 6), and CF (red, n = 6) animal groups over time (in minutes). Abdominal incision was made at timepoint 5 min (+). Rectal temperature differed significantly (P < 0.05) between CF and ND groups (red asterisk,*) and between PD and ND groups (blue asterisk, *) but not between CF and PD groups.

Examining the change in rectal temperature from 0 to 35 min revealed that the CF group had a mean increase of 2.1 ± 1.0 °F (1.2 ± 0.6 ºC), the mean change of the PD group was 0.0 ± 2.9 °F (0.0 ± 1.6 ºC), and the ND group had mean decrease of 0.1 ± 3.7 °F (0.0 ± 2.1 ºC). Overall, drape material had a significant effect (F_2,15 = 5.128, P = 0.0201) on rectal temperature. Posthoc analysis showed that CF-draped mice had significantly higher rectal temperatures than did ND mice at time points 5 min (P = 0.0336), 10 min (P = 0.0027), 15 min (P = 0.0021), 20 min (P = 0.0027), 25 min (P = 0.0035), 30 min (P = 0.0026), and 35 min (P = 0.0064). However, the mean rectal temperatures of CF-draped animals were not significantly different than those of PD animals at time points 5 min through 35 min. Rectal temperatures were significantly higher in PD mice than in ND at time points 15 min (P = 0.0404), 20 min (P = 0.0343), 25 min (P = 0.0293), and 30 min (P = 0.0204). Temperatures at all other time points were not statistically different between draping groups (P > 0.05).

Surface temperatures

The mean infrared temperatures for each group can be seen in Figure 3. As compared with the rectal temperatures, the infrared temperatures were more variable between mice within each group. Examination of the changes in surface body temperature from 0 to 35 min revealed that the CF and PD groups had mean increases of 0.8 ± 3.8 °F (0.4 ± 2.1 ºC) and 0.8 °F ± 3.8 °F (0.4 ± 2.1 ºC), respectively. The ND group had an average decrease of 0.8 ± 3.6 °F (0.5 ± 2.0 ºC) in the infrared body temperature. Drape material had a significant effect (F_2,15 = 16.77, P < 0.001) on infrared temperature. Posthoc analysis showed that CF-draped animals had significantly higher average infrared temperatures than did ND animals at 25 min (P = 0.0214), 30 min (P = 0.0214), and 35 min (P = 0.0073). In addition, CF-draped animals had significantly higher average infrared temperatures than PD animals at 5 min (P = 0.0003), 10 min (P = 0.0012), 15 min (P = 0.0018), 20 min (P = 0.0171), 25 min (P = 0.0138), 30 min (P = 0.0211), and 35 min (P = 0.0064). In contrast to rectal temperatures, infrared temperatures were significantly higher in the ND group than in the PD group at time points 5 min (P = 0.0121), 10 min (P = 0.0154), 15 min (P = 0.0097), and 20 min (P = 0.0493). Temperatures at all other time points were not statistically different between draping groups (P > 0.05).

Figure 3.

Infrared temperatures (mean ± SEM [error bars]) during anesthesia for ND (black, n = 6), PD (blue, n = 6), and CF (red, n = 6) animal groups. Abdominal incision was made at the 5-min time point (+). Surface body temperature differed significantly (P < 0.05) between CF and ND groups (red asterisk, *), PD and ND animals (blue asterisk, *), and CF and PD mice (black asterisk, *).

Discussion

Most surgical patients become hypothermic due to cold exposure (room temperature, surgical site preparation and open body cavities) and anesthetic-induced inhibition of thermoregulatory control.^1,14,20 Hypothermia is especially critical in small rodents, where even mild hypothermia may adversely affect surgical outcomes.²⁶ Therefore, providing heat support and ensuring appropriate anesthetic monitoring is vital for successful surgical outcomes and prompt anesthetic recovery. In addition to delivering exogenous heat support, the choice of draping material can influence heat retention intraoperatively. In the field of laboratory animal medicine, CF has many advantages as a rodent draping material, including availability, low cost, and transparency, which aids in intraoperative monitoring. We had previously validated CF's sterility by using ATP testing and RODAC plates,⁹ showing CF's compatibility with sterile technique. However, to our knowledge, no studies have been published to test this hypothesis that CF has a positive effect on temperature. The current study aimed to evaluate the effect of draping material on intraoperative rectal and surface body temperatures.

The normal body temperature of laboratory mice is 98.8 to 99.3 °F (37.1 to 37.4 ºC).³¹ Using the modified rectal probe, we established a baseline rectal temperature of 99.2 ± 2.7 °F (37.3 ± 1.7 ºC) in healthy, awake mice. According to both temperature-measurement devices, CF maintained the highest mean body temperatures (time point, 35 min; rectal temperature, 92.6 °F; infrared temperature, 91.8 °F) throughout the procedure. However, CF and PD mean rectal temperatures were not statistically different, perhaps warranting further investigation, possibly with a larger sample size than in the current study. Throughout the procedure, CF-draped animals did not experience a decrease in average rectal temperature early in anesthesia, as occurred with the other groups (Figure 2). Other studies have shown similar results, with drops in body temperatures in mice and rats 10 to 20 min into anesthesia.^25,30 We suspect that, compared with PD, CF provides better insulation around the animal by trapping warm air and, due to its impervious qualities, is uniquely able to reduce cutaneous heat loss.²¹ This would explain the overall higher intraoperative body temperatures with CF as compared with traditional PD. Thus, use of CF was able to ameliorate an initial dip in rectal body temperature at the beginning of anesthesia.

This study shows that using any draping material is better than using no drape at all for maintaining intraoperative rectal body temperature. When draping material was not used, the lowest mean rectal body temperature measured was 88.2 °F (31.2 ºC) and occurred 10 min into the monitoring period. Significantly, the ND group had the lowest average rectal temperatures at almost every time point, as compared with both CF and PD animals. Typical anesthetic doses increase the threshold range for body temperatures to thermoregulatory defenses.^18,20,32 The temperature at which the hypothalamus responds to hypothermia is lowered due to an anesthetic-induced dose-dependent suppression of hypothalamic activity.^8,17,21 Compensatory mechanisms to generate or preserve heat are also impaired. Anesthesia inhibits sympathetic response through cerebral suppression, resulting in decreased heart and respiratory rates in addition to the inhibition of increases in metabolic rate and heat production.¹⁴ In rodents, these physiologic changes have multiple consequences due to hypothermia during anesthesia, including delayed recovery to consciousness and decreases in heart rate and blood pressure.^3,5,12,19 Although not investigated in mice, hypothermia during anesthesia in other species, including humans, confers an increased risk of cardiac events, coagulopathies, and infection.^11,18,23,24 Therefore, maintaining normothermia through thermal support is a critical practice in mitigating hypothermia's negative effects during and after anesthetic events. Reference ranges for detrimental hypothermia have not been established in rodents. Based on institutional experience and ranges published for other species, body temperatures below 27 °C (80.6 °F) can be considered detrimental for mice,^2,5,26 however, validation of a defined temperature is needed to verify this assumption.

In comparison to findings in rectal temperatures, infrared temperature findings revealed an unexpected result. The mean infrared temperature for the ND group was higher than that for the PD group at all time points. A plausible explanation for this is that because the paper drapes were not secured to the patient or table, any movement caused the paper drape to shift or lift. This shift, even if mild, could have caused air movement over the patient and decreased temperature measured at the skin surface. Another explanation may be the method in which groups were tested. Not all mice were tested on the same day, thus introducing the possibility of the infrared device functioning differently between the 2 d. Equipment malfunction is an important consideration, especially with devices that are not ‘gold standard’ in the field for temperature monitoring in rodents. This may have played a factor in the differences in infrared temperatures.

Monitoring temperature is key to preventing severe body temperature fluctuations during anesthesia.^1,7 The need for specialized equipment for monitoring in mice and other small mammals limits the practicality of tracking body temperature throughout anesthesia and surgery. The temperature monitoring devices used in this study were chosen based on low cost, practicality, and availability. The modified rectal thermometer has important practicality because it is compatible with standard anesthetic monitoring machines found in most surgical units in laboratory animal facilities. The modified rectal device makes temperature monitoring in mice and other laboratory animal rodents feasible. Although the infrared device is relatively easy to use and noninvasive, the infrared temperatures were more variable between mice within each group as compared with the rectal temperatures. Other studies have shown variability and incongruity between infrared and rectal temperatures.^4,6,10,27,28 One study compared infrared thermometers and implanted radiofrequency identification temperature transponders in mice; infrared measurements were considerably lower throughout the experiment (5 to 6 °C lower) and variance of surface temperature was high.²² In addition, temperature measurement with the infrared thermometer can be highly dependent on user consistency and accuracy, thus introducing user-associated error. For these reasons, the rectal probe used in the current study may be more reliable than the infrared thermometer when recording body temperatures in mice. Institutions and researchers need to consider the benefits and limitations of these devices to decide which is more feasible for their work.

CF (Press'n Seal, The Glad Companies) has multiple positive attributes, including low cost, practicality, and sterility. This study demonstrates the material's superior insulating property and its use for providing thermal support for mice during anesthesia. CF maintained significantly higher mean rectal temperatures than those in nondraped mice and significantly higher mean infrared body temperatures compared with both PD and ND groups intraoperatively. The results highlight the potential of commercial cling film to reduce risk of hypothermia. However, because the material cannot be applied until after anesthesia is induced and skin preparation completed, its use still allows body temperature to fall before it is applied. Supplying supplemental exogenous heat support during surgical preparation may reduce the initial drop in body temperature recorded at time point 0, although further investigation is needed to confirm the benefits of this practice. Additional investigation with CF draping material is required to evaluate effects on anesthetic recovery times and procedural outcomes.