title: Dynamic baroreflex sensitivity in anesthetized horses, maintained at 1.25 to 1.3 minimal alveolar concentration of halothane.
abstract: 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 (less than or equal to 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 greater than 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.
authors.affiliation: Department of Anatomy, Physiological Sciences, and Radiology, College of Veterinary Medicine, North Carolina State University, Raleigh 27606.
authors.forename: P W
authors.initials: PW
authors.lastname: Hellyer
authors.forename: J R
authors.initials: JR
authors.lastname: Dodam
authors.forename: G S
authors.initials: GS
authors.lastname: Light
mesh_terms: D000768:Anesthesia, General / Q000662:veterinary*; D000818:Animals; D001794:Blood Pressure; D006221:Halothane; D006339:Heart Rate; D006736:Horses / Q000502:physiology*; D007275:Injections, Intravenous / Q000662:veterinary; D010656:Phenylephrine / Q000008:administration & dosage / Q000494:pharmacology; D011311:Pressoreceptors / Q000187:drug effects / Q000502:physiology*; D012044:Regression Analysis
chemical_list: D010656:Phenylephrine; D006221:Halothane