# Influence of Circadian Rhythm on the Surgical Stress Response in Bitches Undergoing Elective Ovariohysterectomy

## TL;DR

This study shows that surgeries on dogs done at night cause more stress and slower recovery than those done in the morning.

## Contribution

The study demonstrates that circadian timing affects postoperative recovery in bitches undergoing OVH.

## Key findings

- Night surgeries caused greater homeostatic disturbances compared to morning surgeries.
- Respiratory rate, rectal temperature, and lipid peroxidation were significantly altered in night-surgery dogs.
- Cortisol levels did not show significant changes between the groups.

## Abstract

This study evaluated the influence of the circadian rhythm on the postoperative recovery of bitches undergoing elective ovariohysterectomy (OVH). Clinical and laboratorial assessments were performed after surgeries conducted at different times of the day (morning or night). The data suggest that procedures performed in the evening or at night induce greater disturbances in homeostasis than those performed in the morning. The response to surgical stress may impair postoperative recovery and is affected by anesthetic management, surgical trauma, duration, and the patient’s condition. Understanding the mechanisms that modulate postoperative physiological alterations, is essential to minimize complications associated with an exacerbated surgical stress response.

Circadian rhythm regulates several physiological functions, and influences endocrine and metabolic responses in mammals, with cortisol acting as important modulator of this mechanism. Cortisol secretion is affected by both internal and external factors and is intensified under stress conditions. The response to surgical stress is consistently observed after surgical procedures, such as ovariohysterectomy (OVH). Therefore, this study aimed to evaluate the influence of the circadian rhythm on the surgical stress response following elective OVH in healthy bitches. Twenty patients weighing between 10 and 20 kg were hospitalized 48 h before surgery and remained hospitalized for 48 h postoperative. The animals were randomly allocated into two groups and underwent OVH either in the morning (6–8 h—a.m., GAM) or at night (18–20 h—p.m., GPM). Surgical procedures were standardized with respect to the surgical team, technique applied and duration; this was carried out in order to induce a comparable level of surgical stress. Physical parameters (systolic blood pressure, heart rate, respiratory rate and rectal temperature) and laboratorial analyses (cortisol, leukogram, protein thiols, no protein thiols, vitamin C, ferric reducing ability of plasma and thiobarbituric acid reactive substances) were assessed immediately before surgery and at 2, 4, 6, 12, 24 and 48 h, as well as 14 days postoperatively. No significant changes in cortisol profile were detected. However, significant alteration in the respiratory rate, rectal temperature, time to first urine, and lipid peroxidation were observed in the GPM group, suggesting that surgeries performed at night induce greater disturbances in homeostasis than those performed in the morning.