Assessment of post-laparotomy pain in laboratory mice by telemetric recording of heart rate and heart rate variability

Abstract

Background: Pain of mild to moderate grade is difficult to detect in laboratory mice because mice are prey animals that attempt to elude predators or man by hiding signs of weakness, injury or pain. In this study, we investigated the use of telemetry to identify indicators of mild-to-moderate post-laparotomy pain.

Results: Adult mice were subjected to laparotomy, either combined with pain treatment (carprofen or flunixin, 5 mg/kg s/c bid, for 1 day) or without pain relief. Controls received anesthesia and analgesics or vehicle only. Telemetrically measured locomotor activity was undisturbed in all animals, thus confirming that any pain experienced was of the intended mild level. No symptoms of pain were registered in any of the groups by scoring the animals' outer appearance or spontaneous and provoked behavior. In contrast, the group receiving no analgesic treatment after laparotomy demonstrated significant changes in telemetry electrocardiogram recordings: increased heart rate and decreased heart rate variability parameters pointed to sympathetic activation and pain lasting for 24 hours. In addition, core body temperature was elevated. Body weight and food intake were reduced for 3 and 2 days, respectively. Moreover, unstructured cage territory and destroyed nests appeared for 1-2 days in an increased number of animals in this group only. In controls these parameters were not affected.

Conclusion: In conclusion, real-time telemetric recordings of heart rate and heart rate variability were indicative of mild-to-moderate post-laparotomy pain and could define its duration in our mouse model. This level of pain cannot easily be detected by direct observation.

Figure 1

Changes in heart rate immediately following laparotomy. Changes (delta) in heart rate (HR) immediately after laparotomy and at the end of the first day after laparotomy are presented. Bars represent 3-hour means (± SEM). Asterisks indicate statistical significance (paired Student's t test with Bonferroni correction) at P ≤ 0.016.

Figure 2

Time course analysis of heart rate and heart rate variability following laparotomy. Changes (delta) in heart rate [HR, beats per minute (bpm)], interbeat interval [IBI, milliseconds (ms)], and standard deviation of interbeat interval [SDNN, milliseconds (ms)] relative to baseline (i.e., normal values taken the day before the experiment) are plotted over time. Symbols indicate 12-hour means (bars indicate ± SEM). Corresponding control experiments in which animals received anesthesia and injections only are depicted as black horizontal lines with grey columns representing ± SEM. Asterisks indicate statistical significance (n = 8, paired Student's t test with Bonferroni correction) at P ≤ 0.008. Note increased heart rate values with decreased heart rate variability parameters [IBI, SDNN] during the first light phase (12–24 h) in operated animals without pain treatment.

Figure 3

Time course of locomotor activity and core body temperature following laparotomy. Changes (delta) in locomotor activity and core body temperature relative to baseline (i.e., normal values taken the day before the experiment) are plotted against time. Symbols represent 12-hour means (bars indicate ± SEM). Corresponding control experiments in which animals received anesthesia and injections only are depicted as black horizontal lines with grey columns representing ± SEM. Asterisks indicate statistical significance (n = 8, paired Student's t test with Bonferroni correction) at P ≤ 0.008.

Figure 4

Daily body weight, food and water intake. The percentage deviation (delta) from baseline of body weight, and food and water intake is presented. Symbols represent the mean values from 8 animals, with bars indicating ± SEM. Corresponding control experiments in which animals received anesthesia and injections only are depicted as black horizontal lines with grey columns representing ± SEM. Asterisks indicate statistical significance (paired Student's t test with Bonferroni correction) at P ≤ 0.016. Note the reduction in body weight for three days and the decrease in food consumption for two days after laparotomy without pain treatment.

Figure 5

Representative pictures of cage appearance. The upper row indicates Score 0 with a well-built nest and clear structure of the cage area; feces is visible on the beddings surface (circles). The lower row illustrates Score 1 with an unstructured cage area and two nest-like resting places (arrows).