Dynamic baroreflex sensitivity in anesthetized horses, maintained at 1.25 to 1.3 minimal alveolar concentration of halothane.

The researchers investigated how anesthesia affects the heart rate of horses in response to increasing arterial pressure. They found that anesthesia significantly influences the heart rate and arterial pressure relationship during rapid pressure changes.

Objective of the research

• The authors aimed to quantitatively assess the effects of anesthesia on the relationship between heart rate and arterial pressure during rapid pressure changes in horses. They specifically studied this phenomenon via dynamic baroreflex sensitivity for increasing arterial pressure (DBSI).

Methodology

• Anesthesia was induced in the horses using intravenous (IV) xylazine and ketamine, and then maintained using halothane at a constant end-tidal concentration of 1.1 to 1.2%.
• The systolic arterial pressure (SAP) was increased by a minimum of 30 mm of Hg as a result of an IV bolus injection of phenylephrine HCl.
• Through linear regression, the researchers determined the slope of the R-R interval/SAP relationship, that is, the relationship between the time interval between successive R-waves of the QRS complex on the EKG (R-R interval) and the SAP.

Key findings

• Some of the most significant findings were observed when horses were anesthetized: during dynamic increases in SAP, a strong correlation between the R-R interval and SAP was seen in 8 out of 8 horses. The correlation coefficients were greater than 0.80 in 5 out of 8 horses.
• On the other hand, in awake horses, a significant correlation between the R-R interval and the SAP was found in only 3 out of 6 horses during the same SAP increases.
• The researchers concluded that rapidly increasing SAP using an IV bolus of phenylephrine is not a reliable method for evaluating changes in heart rate, mediated by baroreceptors, in awake horses. This is because the reflex slowing of heart rate in response to increased arterial pressure was overshadowed by the effects of excitement.

Meaning of the study

• This study provides valuable insights about the impact of anesthesia on the relationship between heart rate and arterial pressure during rapid pressure changes in horses. These findings could have significant implications on veterinary practices, particularly in scenarios where horses have to undergo anesthesia.