Arterial hypotension and the development of postanesthetic myopathy in halothane-anesthetized horses.

The research studied the connection between low blood pressure induced by halothane anesthesia and the development of postanesthetic muscle disorder in horses. The findings suggest that a reduction in arterial blood pressure during halothane anesthesia leads to significant muscle dysfunction.

Study Set Up

• Six healthy adult horses were tested in this study, where each underwent two anesthetic sessions with halothane (a type of anesthesia).
• The first session maintained a normal blood pressure (normotension) through regulation of the halothane concentration, while in the second session, the concentration was adjusted to create a low blood pressure state (hypotension).
• The anesthetic duration for each session was 3.5 hours, with carbon dioxide levels maintained at 45 to 55 mm of Hg using intermittent positive-pressure ventilation.

Findings

• After the normotensive anesthesia, all horses recovered without complications. However, post-hypotensive anesthesia, every horse exhibited some degree of muscle dysfunction.
• Three horses suffered from distinct lameness and discomfort in more than one limb, leading to immediate euthanasia. The remaining three also showed lameness, while one had swollen areas on the hip, rib, and face which were touching the surgical table during the procedure, and another presented signs of facial nerve paralysis.
• Analysis of blood tests showed significantly greater levels of aspartate transaminase and creatinine after hypotensive anesthesia compared to after normotensive anesthesia. Aspartate transaminase, total bilirubin, and creatinine levels were also significantly elevated in comparison to the pre-anesthetic state. Additionally, a massive surge in creatine kinase (an enzyme related to muscle damage) was observed.
• One day after the completion of hypotensive anesthesia, the three surviving horses showed significantly higher creatine kinase and lactate dehydrogenase levels compared to their post normotensive anesthesia values. This reflected further muscle damage.

Conclusion

• The study concluded that halothane-induced hypotension contributes to postanesthetic myopathy development in horses. This condition was quantitatively evident through the biochemical markers of muscle damage such as creatine kinase and aspartate transaminase and qualitatively through clinical presentations including lameness and swollen patches on the body.