SYMPTOM
Slow heart rate
A heart rate that falls below the typical range for the breed and size, detected during clinical examination.
Cardiac Conduction Disorders
The heart's electrical conduction system generates and transmits the impulses that coordinate each heartbeat. Conditions affecting the sinoatrial node, atrioventricular node, or the conducting pathways within the heart can slow the rate at which these impulses are generated or transmitted, resulting in bradycardia. Conduction disturbances can be congenital or acquired, and their severity ranges from mild slowing to complete heart block where the atria and ventricles beat independently. The clinical significance depends on how effectively the heart can maintain adequate blood flow at the reduced rate.
Hypothyroidism
Thyroid hormones play a fundamental role in regulating metabolic rate throughout the body, including the heart. When thyroid hormone levels are insufficient, as occurs in hypothyroidism, the heart rate may decrease as part of the broader metabolic slowing. Hypothyroidism is relatively common in dogs and produces a constellation of signs that may include weight gain, lethargy, coat changes, and cold intolerance alongside the reduced heart rate. The bradycardia in hypothyroidism is typically mild to moderate and develops gradually in concert with other metabolic changes.
High Vagal Tone
The vagus nerve exerts a constant slowing influence on heart rate, and conditions that increase vagal tone can produce bradycardia. Gastrointestinal disease, respiratory conditions, eye pressure, and certain neurological conditions can all stimulate vagal activity and reduce heart rate. Athletic, well-conditioned animals may also have a naturally lower resting heart rate due to physiological conditioning. In some cases, the increased vagal tone may be transient and related to specific situations or stimuli.
Medication Effects
Several categories of medications can reduce heart rate as either a primary therapeutic effect or an unintended side effect. Beta-blockers, calcium channel blockers, certain antiarrhythmic drugs, and some sedative medications can all slow the heart rate. In animals receiving these medications, bradycardia may represent an expected pharmacological effect, though the degree of heart rate reduction may sometimes exceed therapeutic intentions, particularly if doses are not carefully titrated or if other factors compound the medication's effects.
Electrolyte Imbalances
Potassium and calcium levels in the blood directly influence cardiac electrical activity, and significant imbalances in these electrolytes can alter heart rate and rhythm. Hyperkalaemia (elevated blood potassium) is a particularly well-recognised cause of bradycardia, as excess potassium depresses electrical conduction through the heart. This electrolyte-driven bradycardia may develop in the context of kidney disease, urinary obstruction, or adrenal insufficiency, and can range from mild to potentially serious depending on the degree of the imbalance.
Why timing matters
Early observation
A mildly slow heart rate may produce no observable signs in the animal, particularly if it develops gradually and the body compensates effectively. Some animals, especially fit, athletic dogs, naturally have lower resting heart rates that are entirely physiological. Early bradycardia may only be detected incidentally during routine veterinary examinations, as the animal's behaviour and energy levels may appear unchanged. However, establishing baseline heart rate measurements during health can provide a valuable reference point for detecting meaningful changes over time.
Later presentation
As bradycardia becomes more pronounced or compensation becomes inadequate, signs of reduced cardiac output may become apparent. The animal may show exercise intolerance, becoming tired more quickly than expected during normal activities. Weakness, particularly in the hind limbs, may develop during exertion. In more severe cases, fainting episodes (syncope) may occur when the heart rate drops too low to maintain adequate blood flow to the brain, particularly during excitement or physical activity. These episodes may be brief, with the animal recovering quickly, but their occurrence indicates that cardiac output is borderline for the animal's needs.
The trajectory of bradycardia depends on its underlying cause. Conduction disorders may remain stable for extended periods or may progress gradually over months to years. Hypothyroidism-related bradycardia may respond to thyroid supplementation. Medication-related bradycardia may fluctuate with dose adjustments. Electrolyte-driven bradycardia may develop acutely and resolve with correction of the underlying imbalance. Monitoring for signs of inadequate cardiac output — exercise intolerance, weakness, or fainting — provides practical indicators of whether the bradycardia is clinically significant regardless of its cause.
Conditions commonly associated
When to explore further
A consistently low heart rate detected during veterinary examinations, particularly if it represents a change from previous measurements, may warrant investigation even if the animal appears outwardly well. The heart rate at which bradycardia becomes clinically significant varies with species, size, and individual factors, but persistent rates below the expected range for the animal deserve consideration.
Episodes of weakness, wobbling, or collapse during or after physical activity in an animal with a known or suspected slow heart rate may suggest that cardiac output is insufficient to meet the demands of exercise. These exercise-related episodes can provide important information about the functional significance of the bradycardia.
Fainting or syncopal episodes — brief periods of collapse with rapid recovery — in an animal with a slow heart rate may indicate that the heart rate is dropping to levels that temporarily compromise blood flow to the brain. These episodes, while often brief, can be significant indicators of the severity of the underlying condition.
A slow heart rate that develops alongside lethargy, weight gain, coat changes, and cold intolerance may suggest hypothyroidism as a contributing factor. The constellation of these signs together can be more informative than any single observation.
A slow heart rate detected in an animal with known kidney disease or urinary problems may suggest an electrolyte component, particularly hyperkalaemia. The combination of bradycardia with renal or urinary signs can provide important diagnostic context.
Learning to feel and count the heart rate at home — either by placing a hand on the left side of the chest behind the elbow or by feeling the femoral pulse on the inner thigh — can provide valuable baseline data and help detect changes. Counting beats over 15 seconds and multiplying by four gives a reasonable estimate of rate. Recording these measurements periodically, along with any observations about exercise tolerance or episodes of weakness, creates a longitudinal record that can be valuable for veterinary discussions. Noting the circumstances surrounding any episodes of weakness or collapse — what the animal was doing, how long the episode lasted, and how quickly it recovered — provides crucial context for understanding the clinical significance of the heart rate findings.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS