Prewarming Followed by Active Warming is Superior to Passive Warming in Preventing Hypothermia for Short Procedures in Adult Rats (Rattus norvegicus) Under Isoflurane Anesthesia

Abstract

General anesthesia is a common procedure in laboratory rats; however, it impairs thermoregulation, rapidly leading to hypothermia as warm core blood is distributed to the cooler periphery. The protective strategy of prewarming before the onset of anesthesia delays hypothermia, but only for a short period. This prospective, randomized, cross-over, experimental study in adult male and female SD rats (n = 8) was designed to compare passive (fleece blanket) and active (temperature controlled heating pad) warming. Initial treatment order was randomized, with a cross-over after a minimum 5 d washout period. Both groups underwent a period of prewarming in a warming box to increase core temperature by 1% (median 0.4 °C). At completion of prewarming, general anesthesia was induced and maintained for 30 min with isoflurane carried in oxygen. Core temperature was monitored for a further 30 min after anesthesia. Active warming resulted in higher core temperatures during anesthesia. During passive warming, hypothermia occurred after approximately 30 min of anesthesia and continued into recovery. In contrast, active warming prevented hypothermia. Prewarming followed by passive warming delayed hypothermia for approximately 30min, but active warming was more effective at maintaining normothermia both during and after general anesthesia.

Mouse Anesthesia: The Art and Science

“…Conscious animal core body temperature is typically tightly regulated; however, under general anesthesia, regulation is disrupted and hypothermia is a common complication. 137 Skin heat loss occurs primarily through radiation, conduction, convection, and evaporation. 138 Furthermore, because mice have a high surface area to body size, they rapidly lose heat.…”

“…139 Minimize heat loss with a supplemental heat source and provide both pre-warming and active warming during anesthesia and surgery. 137 Recommended anesthesia heating devices include a circulating hot-water blanket or microwaveable heat pads, which maintain a surface temperature of approximately 37.5°C ( Figure 4 ). 140 , 141 Additionally, insulating materials or drapes over the thorax and abdomen minimize heat loss.…”

“…Human surgical site infections positively correlate with increasing procedure duration. 289 In addition, hypothermia during rodent anesthesia is common 137 and has recently been shown to interfere with post-operative pain assessments. 290 Thus, it is widely accepted that most procedures involving rodent sedation or general anesthesia include normothermia support and the shortest duration possible.…”

Peri‐anesthetic hypothermia in rodents: A factor to consider for accurate and reproducible outcomes in orthopedic device‐related infection studies

“…Hypothermia is recognized as a potential factor affecting data variability in biomedical research, which leads to increased animal use and misinterpretation of preclinical study results. Once hypothermia is established during an anesthetic event, it is extremely difficult to reverse, 8 and has long‐lasting effects in rodents well past the return to consciousness typically associated with a very slow return to normothermia 9,10 . Decreases in core body temperature of only 1°C in humans are associated with several host side‐effects negatively affecting clinical outcomes, including surgical site infection, impaired drug metabolism, clotting dysfunction requiring blood transfusion, morbid cardiac events, and slower post‐anesthesia recovery 11,12 .…”

“…Once hypothermia is established during an anesthetic event, it is extremely difficult to reverse, 8 and has long-lasting effects in rodents well past the return to consciousness typically associated with a very slow return to normothermia. 9,10 Decreases in core body temperature of only 1°C in humans are associated with several host side-effects negatively affecting clinical outcomes, including surgical site infection, impaired drug metabolism, clotting dysfunction requiring blood transfusion, morbid cardiac events, and slower post-anesthesia recovery. 11,12 Increased risk of infection from peri-anesthetic hypothermia in humans is partially due to a decreased number of circulating white blood cells with an impaired leukocytosis and altered immune function at the surgical site.…”