Systemic distribution of blood flow in ponies during 1.45%, 1.96%, and 2.39% end-tidal isoflurane-O2 anesthesia.

This study investigates the effects of different concentrations of isoflurane-O2 anesthesia on blood flow to various organs in ponies. The findings reveal the anesthetic’s influence on the cardiovascular system and the extent of its impact on vital organs such as the brain, adrenal glands, and kidneys.

Study Design and Methodology

• The research was conducted on nine healthy, adult ponies under isocapnic conditions (a state where the carbon dioxide level in the blood is within normal range).
• Three different minimal alveolar concentration (MAC) levels of isoflurane-O2 anesthesia were studied: 1.1, 1.5, and 1.8 MAC, which are equivalent to 1.45%, 1.96%, and 2.39% end-tidal isoflurane, respectively.
• Blood flow to several organs—brain, thyroid glands, adrenal glands, kidneys, and splanchnic organs—was examined under these anesthesia levels.
• To monitor tissue blood flows, the researchers used 15-micron diameter radionuclide-labeled microspheres. These were injected into the left ventricle of the ponies’ hearts.
• The findings under the different levels of anesthesia were then compared with data from ponies in a conscious state.

Findings

• Isoflurane anesthesia led to a dose-related reduction in cardiac output and arterial blood pressure, but it did not significantly alter total peripheral resistance.
• In the brain, vasodilation (widening of blood vessels) occurred with exposure to isoflurane that peaked at 1.5 MAC.
• This vasodilation was more pronounced in certain areas of the brain (like the cerebellum, pons, and medulla) than in others (the cerebrum).
• The anesthetic caused intense vasoconstriction (tightening of blood vessels) in the thyroid glands and pancreas, leading to a drastic reduction in blood flow to these organs.
• However, adrenal gland blood flow remained unchanged during all three isoflurane anesthesia levels, owing to vasodilation.
• Meanwhile, renal blood flow saw dose-dependent reductions under isoflurane-O2 anesthesia, but renal vasoconstriction only occurred at the highest level of anesthesia (1.8 MAC).
• Despite blood flow in the small intestines and colon decreasing with each concentration of isoflurane, the splenic blood flow remained unaffected.

Conclusions

• The study presents important insights into the cardiovascular effects of isoflurane anesthesia in ponies. It points to the importance of carefully monitoring and controlling the dosage of anesthesia, as different concentrations have distinct impacts on blood flow to various organs.
• The findings are significant for veterinary medicine, providing a basis for safer anesthesia administration practices in ponies and potentially other large animals.