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Arrhythmias and Heart Rate Variability in Dogs with Myxomatous Mitral Valve Disease Assessed by 24-hour Holter Monitoring

Caroline Elisabeth Rasmussen



Holter monitoring is a continuous electrocardiographic technique that can be used for assessment of arrhythmias and heart rate variability (HRV). The last mentioned is a measure of the variation in heart rate (HR) caused by influence of the autonomic nervous system. In dogs, the occurrence of supraventricular arrhythmias seem to increase with disease severity in dogs with the most frequently occurring canine heart disease: myxomatous mitral valve disease (MMVD). Autonomic dysfunction is suggested to be involved in the pathophysiology of MMVD since the parasympathetic component of the autonomous nervous system seem to increase with disease severity in early stages of MMVD. However, in dogs with late stages of MMVD the parasympathetic modulation of HR decreases with increasing MMVD severity. The pathophysiology of MMVD is complex and far from being completely understood. Dogs have different risk for developing MMVD and the rate of progression is unpredictable. Some dogs never develop heart failure, while others do. During the progression of MMVD some dogs develop syncope, which is a sudden, brief and transient loss of consciousness. 

The overall objective of the studies forming this PhD thesis was to clarify the association between MMVD severity and two categories of Holter derived variables evaluated over 24 hours: arrhythmia and HRV variables. Holter derived variables were compared among three breeds of small dogs all clinically healthy, but with different likelihood of developing MMVD, and among Cavalier King Charles Spaniels (CKCS) with various degrees of MMVD severity. In addition, the influence of being a CKCS and/or having a history of syncope on arrhythmia and HRV variables were evaluated in dogs with advanced stages of MMVD. 

Several arrhythmias were occasionally seen in the clinically healthy dogs of small breeds, but only the occurrence of sinus pauses varied between breeds. Clinically healthy CKCS had a higher HR and a lower HRV reflecting decreased overall HRV and parasympathetic tone compared to the two other breeds. However, it was not possible to conclude if these breed differences were related to breed per se or the likelihood of developing MMVD. 

Arrhythmias were seen in dogs of all stages of MMVD. However, supraventricular and other arrhythmias did not appear associated with MMVD severity whereas HRV variables reflecting overall HRV and parasympathetic activity decreased as MMVD severity increased especially in late stages of the disease. These changes indicate that autonomic dysfunction was not obvious in the early stages of the disease, but a sympathovagal imbalance was present in late stages of MMVD. In addition, “premature normals” (representing sinus arrhythmia) and triangular index (representing overall HRV), showed diagnostic potential for differentiating between CKCS with and without clinical signs of heart failure. In dogs with advanced stages of MMVD, the occurrence of arrhythmias was not influenced by being a CKCS (yes/no) or having a history of syncope (yes/no). Furthermore, HRV variables were not influenced by being a CKCS, but dogs with a history of syncope tended to have decreased HRV variables compared to dogs without a history of syncope. These HRV variables reflected a decrease in overall HRV and parasympathetic activity, which is analogous to changes found in clinically healthy CKCS and in CKCS with increasing MMVD severity.  

In conclusion, the findings in this PhD thesis indicate that the breed of dog should be considered when evaluating 24-hour Holter recordings. Furthermore, it is suggested that arrhythmias are neither a prevailing factor in the pathophysiology of MMVD nor in the etiology of syncope in advanced stages of the disease. Moreover, the data suggest that autonomic dysfunction, is not implicated in the MMVD pathophysiology in early stages of the disease, but that a sympathovagal imbalance is involved in late stages of the disease. Furthermore, a sympathovagal imbalance may be involved in the etiology of syncope in advanced stages of the disease. Whether HRV can be utilized as a clinical and prognostic variable remains to be studied.