Cardiac output in the conscious and anaesthetised horse.

This study explores how cardiac output changes in horses during anaesthesia with different anaesthetics. It was found that different anaesthetics have differing effects on the horse’s heart rate, and these changes were linked to other factors like blood pH and oxygen levels.

Understanding the Research

• The study began by measuring a horse’s cardiac output or the amount of blood the heart pumps per minute before undergoing anaesthesia for a period of two hours. This was done by using two different anaesthetics, thiopentone-halothane and thiopentone-diethyl ether. The measurements were then taken at predetermined times during the two-hour anaesthesia period.
• The researchers found that the volume of blood pumped from the left ventricle with each heartbeat, otherwise known as the left ventricular stroke volume, decreased similarly regardless of the anaesthetic used. However, the total blood pumped out by the heart over a unit time, or the cardiac output, decreased more during anaesthesia with halothane.
• The difference in cardiac output is explained by the differing effects of the two anaesthetics on the heart rate. Halothane induced a slight bradycardia or slowdown in heart rate, while ether caused a minor acceleration or tachycardia. The latter was thought to result from an increase in the activity of the sympathoadrenal system, linked to the body’s “fight or flight” response.

Other Factors Considered

• The study then considered other factors that might be influencing these changes in cardiac output and stroke volume. These included the arterial blood pH, or how acidic or alkaline the blood is, and the tensions of oxygen and carbon dioxide, or how much of these gases were dissolved in the blood.
• Positive correlations were found between some of these variables and cardiac function, indicating that changes in these factors could be affecting the heart’s performance during anaesthesia.
• It was also noted that the reductions in stroke volume and cardiac output were related to the duration of anaesthesia, with the most significant changes occurring in the early stages. Over time, however, the stroke volume and cardiac output tended to return towards their original, pre-anaesthesia levels. Possible reasons for this recovery are discussed in the paper.