Use of Air-activated Thermal Devices during Recovery after Surgery in Mice

Abstract

Laboratory mice (Mus musculus) are susceptible to hypothermia, especially during anesthetic events, disease states, and exposure to environmental stressors. Thermal support devices for small mammals are numerous, but often require a power source and may be impractical to use for cages on a rack. Air-activated thermal devices (AATD) are mixtures of chemicals that cause an exothermic reaction. In this study, we examined the environmental effects of AATD on internal cage temperatures without the use of additional equipment as well as the physiologic effects of AATD as postoperative thermal support in mice. For environmental experiments, temperatures measured inside the cage and above the AATD peaked at 35.6 ± 2.5 °C (13.4 °C higher than control cages). We also demonstrated that the amount of heat produced by AATD and its temporal distribution are dependent on cage and rack types. For physiologic experiments, mice were surgically implanted with an intraperitoneal temperature telemetry device in a static cage setting. Recovery times and final body temperature at 5 h postoperatively did not differ significantly between mice with and without AATD. During the first 0 to 3 h after mice returned to their home cages, body temperature dropped markedly in mice without AATD but not in mice with AATD. Based on this result the physiologic results of our study support that AATD can be useful in providing extended thermal support for mice housed in static microisolation cages to help maintain body temperature postsurgically. Environmental results of our studies demonstrated that AATD provide local clinically relevant thermal support for 2.5 to 6 h, depending on cage set-up.

Figure 1.

(A) On each study day, 4 cages were set up in this fashion, with the IVC group placed in the recirculating rack and the static cages placed on a cart. The cart pictured here is stainless steel. (B) Two hygrothemometers are inside each cage. One probe sits directly on the bedding over the area of the AATD (or lack of AATD for control cages), and the other probe sits at the opposite end of the cage. Hygrothermometer screens are angled so that temperature and humidity readings are visible without opening the cage. Arrows indicate the area of the hygrothermometer that detects temperature and humidity. (C) Cage with bedding removed to show the location and placement of an AATD on the bottom of the cage.

Figure 2.

Schematic diagram of zones associated with the placement of AATD for environmental and physiologic experiments. Zone 1 is directly over the adhered AATD. Zone 2 is adjacent to the area over the AATD. Zone 3 is in the last third of the cage on the opposite side from the AATD. The × represent the locations of the hygrothermometer probes for environmental studies. The inner red box indicates the AATD under the cage. Front indicates the user-facing side of the cage.

Figure 3.

(A) Boxplots for cage temperature in environmental conditions in zone 1. (B) Boxplots for cage temperature in environmental conditions in zone 3.

Figure 4.

(A) Cage temperature in environmental conditions over time in zone 1. Individual experiments are shown as dashed lines, and the solid line indicates the mean for all experiments. (B) Cage temperature in environmental conditions over time in zone 3. Individual experiments are shown as dashed lines, and the solid line represents the mean for all experiments.

Figure 5.

Boxplots of change in postoperative body temperature.

Figure 6.

Postoperative body temperature change over time compared between mice with and without AATD. (n = 8 mice per group). Individual mice are as dashed lines, and the solid line indicates the mean for all mice.

Figure 7.

Average time mice spent in different cage zones after regaining an ambulatory state.