Diagnostic and Therapeutic Approaches to Bradycardia
ACVIM 2008
William Rausch, DACVIM (Cardiology)
Portland, OR, USA

Bradycardia

The normal heart rate in a given patient is a moving target influenced necessarily by species, age and body size, and then further amended by a large number of dynamic physiologic factors (hydration, activity, emotional state, oxygenation, blood pressure, etc). Although the lower limits of normal heart rate has been variably reported to be between 60-70 beats per minute (bpm) in the awake canine and 100-120 bpm in the awake feline, these limits are somewhat arbitrary and heart rates below these levels are not necessarily associated with pathology. Similarly, a relative bradycardia should be suspected if the heart rate is less than clinical circumstances warrant. The term bradyarrhythmia is used to refer to an arrhythmia accompanied by a bradycardia.

Generally speaking, bradycardia may occur due to a decrease in the frequency of impulse propagation within the heart and/or impaired electrical intracardiac conduction of that impulse once propagated. These events can occur because of primary heart disease primarily affecting the conduction system or as a secondary effect of more generalized macro cardiac disease (e.g., myocarditis). Additionally, extracardiac factors can act upon a normal or a diseased heart yielding bradycardia as an appropriate response to those abnormal extracardiac factors. Vagally-mediated bradycardia (described below) would be the most common such example.

Clinical Presentation

Dogs and cats with bradycardia may be subclinical and the bradycardia can be an incidental finding on auscultation. When clinical signs are present, they are generally related to, and proportional to, the degree of associated hemodynamic impairment. Interestingly, since many of the patients we see with bradycardia are older animals, owners often attribute decreased exercise tolerance/desire and other behavioral changes to age and the patients present to us as "asymptomatic" when in fact they are limited by a bradyarrhythmia. If these rhythm disturbances can be improved, these patients have the unique ability to go from asymptomatic to improved--usually with more energy and alertness. This is worth bearing in mind, as the lack of reported symptoms is not in and of itself a reason to not consider a treatment trial in a bradycardic patient.

The hemodynamic impairment associated with a bradyarrhythmia is due to diminished cardiac output and the resultant drop in peripheral tissue oxygenation. Clinical signs can range from very mild weakness, lethargy and exercise intolerance to disorientation and syncope. Some bradyarrhythmias can be variable and intermittent which will tend to make the symptoms episodic. The symptoms can also be episodic, even when the arrhythmia is not, due to changes in the animal's activity level. A given bradyarrhythmia might provide enough cardiac output such that no clinical signs are seen at rest, but might not adequately support tissue oxygenation during a sudden sprint after a squirrel. Many of the presenting complaints that can be associated with bradycardia have differential diagnoses that often include neurologic, neuromuscular, musculoskeletal and metabolic causes. There truly are no pathognomonic presenting complaints for bradyarrhythmia. It is also important to keep in mind that not all cases of cardiogenic syncope are associated with bradycardia. Distinguishing features and differing treatments for various types of cardiogenic syncope will be discussed in lecture.

Vagally-Mediated Bradycardia

Vagally-mediated bradycardia is a common bradycardia, especially in canines. While it is not normal, the presence of a vagally-mediated bradycardia suggests an autonomic imbalance, rather than a necessarily a primary cardiac disturbance. The autonomic nervous system governs motor function to smooth muscle and cardiac muscle. The autonomic nervous system is functionally and pharmacologically subdivided into the sympathetic and parasympathetic nervous systems. Animals affected by vagally-mediated bradycardia have increased parasympathetic tone and/or decreased sympathetic tone (Parasympathetic tone is also known as vagal tone as the parasympathetic fibers innervating the heart originate from the vagus nerve--cranial nerve X). Arrhythmias that commonly occur with increased vagal tone include respiratory and non-respiratory sinus arrhythmia, sinus bradycardia, AV nodal block and sinoatrial block or sinus arrest. The most commonly seen vagally-mediated arrhythmia in small animal practice is a respiratory sinus arrhythmia (RSA). This is a non-pathologic variant recognized in dogs and much less commonly in cats. RSA is characterized by a relative tachycardia during inspiration and a relative bradycardia during exhalation. The act of inspiration stimulates inhibitory afferent impulses to the central vagal center, which decreases tonic vagal inhibition of the sinoatrial node and thereby temporarily increases heart rate. The absence of this phenomenon during exhalation brings the heart rate back down. Certain dogs have relatively high level of resting vagal tone. A RSA can be seen in many breeds--and is especially prevalent in brachycephalic breeds at rest. Diseases of the respiratory, gastrointestinal, CNS and ocular systems all have the potential to increase vagal tone and promote this arrhythmia. This arrhythmia is considered innocent in nearly all circumstances because of minimal hemodynamic significance and the fact that most animals can overcome this arrhythmia by increases in sympathetic tone when physiologically appropriate. It is important to distinguish this arrhythmia from pathologic arrhythmias.

Due to the lack of pathognomonic clinical signs for bradyarrhythmias and their intermittent nature, careful physical exam evaluation and electrocardiography (ECG) are the first necessary steps in their evaluation. The ECG is uniquely poised to give specific and critical information on heart rate, sinus rate and cardiac rhythm and many examples will be shown in lecture and the role of ambulatory ECG will be discussed. With this information serving as background, the lecture will focus on the diagnosis and management of pathologic bradyarrhythmias, including severe AV block, sick sinus syndrome, persistent atrial standstill, sinoatrial exit block and bradyarrhythmias associated with hyperkalemia, hypothermia, hypothyroidism and toxicities.

Speaker Information
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William Rausch, DACVIM (Cardiology)
Portland Cardiology Associates
Portland, OR


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