Using the Oculocardiac Reflex to Characterize Autonomic Imbalance in a Naturally Occurring Canine Model of Valvular Insufficiency

Congestive heart failure (CHF) is characterized by reduced heart rate variability. Although various methods to achieve parasympathetic withdrawal and sympathetic activation have been documented, stimulation of the vagal nerve (that is, vagal maneuvers) has not often been used to assess the autonomic impairment associated with disease progression in veterinary species. In this study, we investigated cardiac autonomic control in a naturally occurring canine model of mitral insufficiency by means of individual responses to oculocardiac reflex. Indices of heart rate variability were calculated from 5-min ECG tracings obtained before and after ocular compression. After compression, significant increases in the standard deviation of RR intervals, root mean square of the successive differences in RR intervals, and vasovagal tonus index were documented in healthy control animals. In addition, these indices were increased in asymptomatic dogs with remodeled hearts, but no variation occurred in symptomatic animals. Although only the percentage change in vasovagal tonus index differed significantly between controls and diseased dogs, all other parameters showed a tendency to behave the opposite way in the symptomatic group as in the control and asymptomatic groups. Our results document CHF-dependent autonomic dysfunction in a canine model of valve insufficiency.