Hemodynamic effects of carbon dioxide during intermittent positive-pressure ventilation in horses.

The research article explores the impacts of high arterial carbon dioxide pressure (PaCO2) on the hemodynamic status of horses during anesthesia. Increased levels of PaCO2 were found to significantly affect cardiovascular function in horses.

Study Design and Procedure

In this study, eight horses were anesthetized using a combination of drugs – xylazine, guaifenesin, and thiamylal, followed by an administration of halothane in oxygen.

• The baseline data were collected while the horses were breathing normally.
• Then the horses were placed under intermittent positive-pressure ventilation, leading to different levels of capnia – normocapnia (PaCO2 between 35 to 45 mm of Hg), moderate hypercapnia (PaCO2 between 60 to 70 mm of Hg), and severe hypercapnia (PaCO2 between 75 to 85 mm of Hg).
• The hypercapnic conditions were achieved by adding carbon dioxide to the gas that the horses were breathing.

Findings

The study recorded significant hemodynamic changes in the horses during moderate and severe hypercapnia.

• There were increases in aortic blood pressure, left ventricular systolic pressure, cardiac output, stroke volume, maximal rate of increase and decrease in left ventricular pressure, and median arterial blood flow.
• The time constant for ventricular relaxation decreased.
• These changes were accompanied by increased levels of plasma epinephrine and norepinephrine concentrations – hormones that are responsible for the body’s fight or flight response.

Impact of Beta-Blocking Drug

On administering the beta-blocking drug, propranolol hydrochloride, the hypercapnic response observed in the horses was considerably dampened.

Conclusion

The research confirmed that in horses, high arterial carbon dioxide pressure, or hypercapnia, is associated with enhanced cardiovascular function. This information is crucial for assessing the overall impact of anesthetic procedures on horses and can guide veterinarians in determining optimal anesthesia practices.