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 the type of conduction disorder as well as on how well the heart maintains blood flow at the reduced rate: in dogs, complete heart block carries a risk of sudden death even when the animal appears well.
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 disease, 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. With a blocked urethra, with sudden kidney failure in which little or no urine is made, and in an Addisonian crisis, the potassium can rise to life-threatening levels. A blocked urethra shows as repeated straining with little or no urine passed, followed by vomiting and marked dullness, and in cats the time from complete obstruction to death may be less than 24 to 48 hours. An Addisonian crisis in a dog shows as severe vomiting and diarrhoea with low blood volume and low blood pressure. The potassium level and the heart rhythm cannot be judged at home: they are measured with a blood test and an ECG.
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 a quick recovery does not show that the cause is minor, and the rhythm behind an episode can only be identified on an ECG.
The trajectory of bradycardia depends on its underlying cause. Conduction disorders vary with their type and with the species. Some remain stable for extended periods, but dogs with third-degree atrioventricular block are at risk of sudden death whether or not they show clinical signs, and a pacemaker is advised for them on that basis. In cats, third-degree block often produces no clinical signs, although some cats faint. Hypothyroidism-related bradycardia may respond to thyroid hormone replacement. Medication-related bradycardia may fluctuate with dose adjustments. Electrolyte-driven bradycardia may develop acutely, can be life-threatening while the potassium stays high, and resolves only once the underlying imbalance is corrected. Signs of inadequate cardiac output (exercise intolerance, weakness, or fainting) show that the slow rate is affecting the circulation. Their absence does not rule out a serious cause, because some rhythms carry a risk of sudden death without any outward sign.
Conditions commonly associated
Hypothyroidism in Dogs
Thyroid hormone helps set the heart's baseline rate by influencing how strongly cardiac tissue responds to stimulation, so when hypothyroidism lowers circulating hormone levels, the heart rate tends to slow along with the wider drop in metabolic activity.
Addisons Disease in Dogs
In Addison's disease, insufficient aldosterone allows potassium to build up in the bloodstream, and this excess potassium interferes with the heart's electrical conduction system, causing the heart to beat more slowly than normal.
Sick Sinus Syndrome
An unusually slow or irregular heart rate reflects the faulty electrical signalling from the heart's natural pacemaker that defines this condition.
Third-Degree Atrioventricular Block
Failure of electrical signals to pass from the atria to the ventricles leaves the heart beating at a fixed, abnormally slow rate.
Urethral Obstruction
When the urethra is blocked, potassium that would normally leave in the urine builds up in the blood, and this excess potassium slows the heart and can cause life-threatening rhythm disturbances.
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. When the urinary sign is straining with little or no urine passed, the combination points to a blocked urethra with rising potassium, which is life-threatening rather than a finding to monitor.
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 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 useful context for understanding the clinical significance of the heart rate findings. A home record of this kind supports a veterinary assessment and does not stand in for one: fainting or collapse with a slow heart rate can mark a rhythm that carries a risk of sudden death, and that rhythm is identified on an ECG, not by counting the pulse.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS