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

SUMMARY Dynamic baroreflex sensitivity for increasing arterial pressure ( dbsi ) was used to quantitatively assess the effects of anesthesia on the heart rate/arterial pressure relationship during rapid (≤ 2 minutes) pressure changes in the horse. Anesthesia was induced with iv administration of xylazine and ketamine and maintained with halothane at a constant end-tidal concentration of 1.1 to 1.2% (1.25 to 1.3 minimal alveolar concentration). Systolic arterial pressure ( sap ) was increased a minimum of 30 mm of Hg in response to an iv bolus injection of phenylephrine HCl. Linear regression was used to determine the slope of the r-r interval/ sap relationship. During dynamic increases in sap , a significant correlation between r-r interval and sap was observed in 8 of 8 halothane-anesthetized horses. Correlation coefficients between r-r interval and sap were > 0.80 in 5 of 8 horses. Mean (± sd ) dbsi was 4.8 ± 3.4 ms/mm of Hg in anesthetized horses. A significant correlation between r-r interval and sap was observed in only 3 of 6 awake horses during dynamic increases in sap . Lack of correlation between r-r interval and sap in 3 of 6 awake horses indicated that rapidly increasing sap with an iv phenylephrine bolus is a poor method to evaluate baroreceptor-mediated heart rate changes in awake horses. Reflex slowing of heart rate in response to a rising arterial pressure appeared to have been overridden by the effects of excitement. Mean (± sd ) dbsi (3 horses) was 7.3 ± 3.3 ms/mm of Hg in awake horses.