Recovery quality and antinociceptive effects following rectus abdominis sheath block in dorsally recumbent horses: an experimental study.

Overview

• This study investigated the effects of a rectus abdominis sheath (RAS) block using bupivacaine on recovery quality and pain relief (antinociception) in horses under anesthesia.
• The research aimed to determine whether this nerve block improved pain control after anesthesia without negatively impacting recovery quality or causing side effects.

Study Design and Methods

• Type of study: Randomized crossover experimental study involving six healthy adult horses.
• Procedure: Each horse underwent anesthesia twice using an isoflurane protocol. During anesthesia, horses received bilateral RAS injections under ultrasound guidance.
• Treatments: Each horse received two different injections in separate anesthetic events:
  • Bupivacaine treatment (BT): 0.2% bupivacaine solution, total dose of 1 mL/kg
  • Saline treatment (ST): 0.9% sodium chloride (placebo), also 1 mL/kg
• Timing: Horses remained anesthetized for 1 hour after the block before recovery monitoring began.
• Measurements taken:
  • Recovery quality assessed by accelerometry-derived recovery scores (RS)
  • Times recorded for:
    • First movement
    • Sternal recumbency (lying on chest)
    • Time to stand
  • Number of standing attempts
  • Mechanical nociceptive threshold (MNT): force needed to evoke a pain response, measured at baseline and 2 to 10 hours after recovery
  • Physiological parameters: heart rate (HR), respiratory rate (f), and ataxia score (AS) ranging from 0 (no ataxia) to 4 (severe ataxia)
• Data analysis: Statistical comparisons using paired t-tests, Wilcoxon tests, and generalized linear mixed models with significance level α = 0.05.

Key Results

• Recovery quality: No significant differences found between bupivacaine and saline in recovery scores, number of standing attempts, or time-related recovery variables (p > 0.366).
• Mechanical nociceptive threshold (MNT):
  • Baseline MNT values were comparable between treatments (~15 N for BT and ~14.5 N for ST).
  • After RAS block, MNT increased significantly in the bupivacaine group (22.2 N) versus saline (13.7 N), indicating better pain relief (p < 0.001).
• Heart rate and respiratory rate: Baseline HR and f were similar between groups; no significant changes were observed after the block in either group.
• Ataxia score: Baseline AS was zero for both groups; no clinical signs of motor impairment or adverse neurological effects after the block.

Interpretation and Conclusions

• The RAS block with bupivacaine provided effective postanesthetic antinociception as evidenced by increased mechanical nociceptive thresholds compared to saline.
• This improved pain control did not negatively impact recovery quality, as there were no differences in recovery times, movement attempts, or accelerometry scores.
• No adverse cardiovascular or respiratory effects or increased ataxia were observed with bupivacaine treatment compared to saline.
• The findings suggest that the RAS block is a safe and effective analgesic technique that can be used during anesthesia in horses without compromising recovery.

Significance for Veterinary Practice

• Rectus abdominis sheath blocks offer a targeted method to reduce pain after abdominal procedures in horses.
• This technique may enhance postoperative comfort without introducing risks of delayed or complicated recovery from anesthesia.
• It supports multimodal analgesia strategies that reduce systemic drug requirements and associated side effects.