Exercise intolerance in retrievers - Veterinary Medicine
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Exercise intolerance in retrievers
When a dog from one of the popular retriever breeds is brought to you because it collapsed while exercising or seems to tire easily, you need to sift through the many potential underlying causes. It could be anything from an out-of-shape weekend athlete to a dog with an inherited metabolic myopathy. Here are some of the conditions to consider.


Cardiovascular disorders

Dogs may be presented for acquired exercise intolerance caused by cardiovascular dysfunction. Dogs with heart failure from congenital anomalies, acquired valvular heart disease, or cardiomyopathy will be unable to exercise because of poor perfusion and tissue hypoxia. These dogs will typically exhibit physical evidence of cardiac failure at rest, including tachycardia, cough, weak femoral pulses, crackles on lung auscultation from pulmonary congestion or edema, and perhaps cyanosis and a murmur.

The most common congenital heart defects in retrievers include subvalvular aortic stenosis (SAS) in golden retrievers and tricuspid valve dysplasia (TVD) in Labrador retrievers.14,15 Many golden retrievers with mild SAS can live a normal lifespan with normal exercise tolerance, but more severely affected dogs are at risk for sudden death, arrhythmias, and congestive heart failure.14 Most Labrador retrievers with TVD have a shorter than normal lifespan (mean of 2 years), with affected dogs developing exercise intolerance, right-sided heart failure, and arrhythmias.14,15

Cardiac rhythm disturbances can cause episodic weakness, which may or may not be associated with periods of exercise. Tachyarrhythmias and bradyarrhythmias can cause serious reductions in cardiac output resulting in weakness, syncope, or sudden death. Auscultation, thoracic radiographs, electrocardiography, and echocardiography will be normal at rest in many dogs with arrhythmia-associated exercise intolerance or syncope.16 Cardiac event recording is a useful tool in these dogs to determine whether a cardiac arrhythmia is the cause of their clinical signs.16 Labrador retrievers are a breed at risk for high-grade second-degree and third-degree atrioventricular (AV) block, which can lead to exercise intolerance, episodic weakness, syncope, and sudden death.17 Pacemaker implantation is recommended in affected dogs to stabilize cardiac output. Neurocardiogenic (vasovagal) bradycardia with syncope occasionally occurs in otherwise normal golden retrievers during periods of exercise and excitement.18

Dogs with pericardial effusion causing cardiac tamponade are often presented to a veterinarian for evaluation of exercise intolerance, weakness, or collapse.19 Acquired pericardial effusion sufficient to cause tamponade and exercise intolerance is usually due to neoplasia, but idiopathic effusions can also occur.20 Hemangiosarcoma arising from the right atrial appendage or the wall of the right atrium is the most common neoplastic cause in retrievers, particularly in golden retrievers, which are predisposed to this neoplasia.19

When the pericardial effusion develops quickly, even a relatively small volume of fluid can prevent diastolic filling and dramatically decrease cardiac output, but pericardial effusion that accumulates slowly must be a high volume before tamponade occurs.20 Muffled heart sounds, tachycardia, jugular venous distention, and poor pulse quality are common physical findings.19 Pulsus paradoxus may be detected, with a weak femoral arterial pulse during inspiration and a stronger pulse during expiration.19 With chronic effusions, additional consequences of right-sided congestive heart failure may develop, including ascites and pleural effusion.

Cardiac tamponade is suspected based on clinical findings, but echocardiography is required to confirm the effusions and demonstrate diastolic collapse of the right atrium or ventricle consistent with tamponade. Transthoracic two-dimensional echocardiography has an 80% sensitivity for the detection of cardiac masses causing pericardial effusion and tamponade.19

Average survival in dogs with pericardial effusion secondary to right atrial hemangiosarcoma is one to three months. Idiopathic pericardial effusion may spontaneously resolve after therapeutic pericardiocentesis or may recur, requiring pericardiectomy, but the prognosis is good, with 72% survival after 18 months.20

Respiratory disorders

Table 2. Respiratory Disorders That Cause Exercise Intolerance in Retrievers
Abnormalities of the larynx, pharynx, trachea, airways, pulmonary parenchyma, or pleural space can impair a dog's ability to ventilate normally. As the affected dog exercises, it may be unable to keep up with the oxygen demands of the tissues, resulting in signs of weakness, exercise intolerance, or collapse. Auscultation and observation of the respiratory pattern at rest and during and after exercise can be useful in localizing a problem within the respiratory tract.21 Obtain thoracic radiographs in all dogs with exercise intolerance. Also, further diagnostic tests to evaluate the respiratory system should be performed as necessary to reach a diagnosis (Table 2).

Laryngeal paralysis. Laryngeal paralysis is a common disorder in geriatric large-breed dogs, particularly Labrador retrievers.22-24 Most cases are idiopathic—with no detectable underlying cause. Affected dogs may be mildly symptomatic for months, with inspiratory noise (stridor) and dyspnea that are only apparent during exercise or when the dog is overheated. Some owners also notice a chronic mild cough and a voice change.

Acute treatment of dogs presenting with severe stridor includes oxygenation, cooling, corticosteroids (dexamethasone 0.1 mg/kg intravenously), and tranquilization (acepromazine 0.005 to 0.02 mg/kg intravenously and butorphanol 0.2 mg/kg intravenously).21 Once an affected dog is stabilized medically, it should be evaluated systemically for an underlying cause or concurrent disease.

Table 3. Anesthetic Protocol for Evaluating Laryngeal Function
Routine screening blood tests, thoracic radiography, and a complete nervous system examination should always be performed. Additional tests may be warranted to rule out hypothyroidism, myasthenia gravis, and a generalized polyneuropathy.22,25 Diagnosis of laryngeal paralysis requires laryngoscopy using an anesthetic protocol that will not abolish laryngeal movement (Table 3). The respiratory stimulant doxapram can be administered to improve respiratory efforts during evaluation.22 Treatment of idiopathic laryngeal paralysis involves unilateral arytenoid lateralization (tie-back).22,23,26

The prognosis after surgery is good for large-breed dogs with idiopathic laryngeal paralysis, with 90% of owners pleased with their dogs' improvement one year postoperatively and a median survival of one to five years.23-26 Aspiration pneumonia is a potential complication. Some Labrador retrievers with laryngeal paralysis will, over time, develop clinical evidence of a progressive generalized peripheral neuropathy that will lead to exercise intolerance and weakness in spite of successful resolution of their respiratory obstruction.25


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