Sinus arrhythmia in the dog

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Abstract
Sinus arrhythmia persists in dogs anesthetized with morphine and chloralose. Beginning about 1 sec before and continuing during inspiration, heart rate increased, dp/dt max. of left ventricular pressure increased, duration of left ventricular ejection decreased, systemic arterial systolic and diastolic pressures increased, and the second heart sound split-all occurring in the absence of observable significant alterations in left ventricular end-diastolic pressure. During expiration all variables returned gradually to the preinspiratory level following a precipitous fall in heart rate during the first 0.5 sec of expiration. Sinus arrhythmia persisted in the absence of breathing movements and following beta adrenergic blockade, but was abolished by bilateral cervical vagotomy or atropine. It is concluded that, under the conditions of this study, cardiovascular changes manifested as sinus arrhythmia may result solely from waxing and waning of vagal efferent activity.

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