Clinical observations surrounding an increased incidence of postanesthetic myopathy in halothane-anesthetized horses.

The research observed an increased incidence of muscle damage (postanesthetic myopathy) in horses after anesthesia, and investigated potential ways to reduce its severity. The new method identified includes maintaining higher mean arterial pressure using high-volume fluid therapy and the drug dobutamine during anesthesia.

Research Context

• Over an 8-year period, clinical observations noticed an increased occurrence of postanesthetic myopathy (PAM) in horses that had undergone anesthesia.
• The condition, PAM, is an adverse side-effect of anesthesia which results in muscle damage.
• To probe into this, an independent anesthesiologist was engaged to identify possible causes for this observed increase.
• The anesthesiologist compared the traditional methods of anesthesia with new methods that were introduced in the study.

Methods

• Observations were conducted from October 1999 to April 2000.
• Anesthesia was induced in many horses by various drugs including xylazine, detomidine, diazepam or guaifenesin followed by ketamine. It was then maintained with a combination of halothane and oxygen, and administered with mechanical ventilation.
• Two groups of horses were formed. Group A, with 17 horses, was observed between October to December 1999. Hypotension was defined as mean arterial pressure (MAP) less than 60 mmHg and managed via low-volume crystalloid fluid therapy, etilefrine, or dopamine.
• Group B comprised of another 18 horses. For them, hypotension was redefined as MAP less than 70 mmHg, and the condition was managed through high-volume fluid therapy and the drug dobutamine.
• Muscle enzyme levels in these horses were monitored before induction, and 1 and 25 hours after recovery and standing. The occurrence of PAM was defined as the concentration of specific muscle enzymes (CK and/or AST) exceeding 1000 IU L(-1).

Results

• The findings suggest that five horses in Group A exhibited increased muscle enzymes, which can indicate muscle damage. Among these, two horses displayed clinical symptoms.
• Only one horse in Group B showed a similar increase in muscle enzymes, indicating potentially lesser muscle damage.
• While the difference between the groups was not statistically significant (p=0.088), these findings suggest that increasing the MAP to above 70 mmHg using high-volume fluids and dobutamine could reduce PAM severity.

Conclusion

• Maintaining the mean arterial pressure to levels above 70 mmHg with high-volume fluid therapy and the drug dobutamine may help to reduce the severity of myopathy in horses after anesthesia.
• While more research is necessary to conclusively define this relationship, the study sets a base for further investigations into the matter.