AN INVESTIGATION INTO THE RELATIONSHIP BETWEEN HEART RATE AND CARDIOVASCULAR FUNCTION IN THE NESTHETIZED HORSE / Investigação da relação entre freqüência cardíaca e função cardiovascular em eqüinos anestesiados

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

This study was designed to investigate the relationship between heart rate (HR) and cardiovascular function in anesthetized horses. During the first series of experiments, a muscarinic antagonist was used to reverse xylazine-induced bradycardia in halothaneanesthetized horses. Initially, the positive chronotropic action induced by the nonselective muscarinic antagonist, glycopyrrolate, was studied (Chapter 1). Although the increase in HR induced by glycopyrrolate (5.0 to 7.5 mg/kg, IV) resulted in an improvement in cardiac output (CO), arterial blood pressure (ABP), and oxygen delivery (DO2), the nonselective muscarinic antagonist also caused prolonged intestinal hypomotility and colic. Subsequently a muscarinic type 2 (M2) antagonist, methoctramine, was studied for its ability to treat bradycardia during halothane/xylazine anesthesia without causing an additional delay in intestinal motility, as assessed by in vitro and in vivo models (Chapters 2 and 3). When compared to glycopyrrolate, methoctramine (10 to 30 mg/kg, IV) caused a similar beneficial hemodynamic effect in xylazine/halothane anesthetized horses, i.e. increased HR, CO, ABP, and DO2, without causing undesirable side effects on the gastrointestinal system. In the last series of experiments (Chapter 4), the relationship between HR and cardiovascular function was studied using a model of pacing induced tachycardia in isoflurane-anesthetized horses. In this study, as HR was increased from 35-50 beats/min to 60-70 beats/min there was a maximum increase in CO. Increases in HR above 60-70 beats/min were associated with decreased CO. The present investigation provides evidence that heart rates below 30 beats/min are associated with reduced cardiovascular performance, and the use of a muscarinic antagonist to increase HR to 35-40 beats/min results an improvement hemodynamic function. In addition, evidence is provided that the relationship between HR and CO is dependent on the HR range, and HR increases above 60-70 beats per minute may not result in further improvements in CO during anesthesia.

ASSUNTO(S)

anestesiologia animal clínica cirúrgica animal débito cardíaco receptores muscarínicos anestesia equina cardiovascular function medicina veterinaria anesthetized horses função cardíaca marcapasso

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