Heart Beat Problems (Atrial Standstill) in Dogs

Atrial standstill is an abnormal heart rhythm characterized by a complete lack of P-waves on ECG readings as a direct result of the atria being unable to depolarize. This condition can be caused by imbalances in the potassium in the blood, disorders of the heart itself and some disorders of peripheral glands. Atrial standstill is usually indicative of a more serious underlying condition and should be evaluated by a veterinarian as soon as possible.

Atrial standstill is an abnormal heart rhythm distinguished by a lack of any P-waves when an ECG is evaluated. Atrial standstill is usually indicative of a serious underlying condition.

There are clinical signs that can be found in either younger or older dogs.

• Lethargy
• Slowed heart rate
• Syncope (fainting)

Symptoms that may be seen in cases of persistent atrial standstill will often mimic congestive heart failure.

• Abdominal distention
• Cough
• Cyanosis (blue tinge to skin and mucous membranes)
• Difficulty breathing
• Muscle wasting, especially of the forelegs and shoulders

Types 

Temporary

• The most common cause of temporary atrial standstill is a lower than usual amount of circulating potassium in the blood (hyperkalemia)
• The inflammatory heart disease myocarditis is another possible cause of temporary atrial standstill 

Persistent

• Persistent atrial standstill can be caused by a physical defect to the heart, as seen in atrioventricular muscle dystrophy, cardiomyopathy, and double-chambered right ventricle
• It can also have a peripheral cause such as hypothyroidism, hypoadrenocorticism, and immune-mediated polymyositis

Terminal

• Congestive heart failure with atrial standstill involved would be considered a terminal atrial standstill

There are several conditions that can cause atrial standstill to develop: 

• Addison’s disease (hypoadrenocorticism)
• Atrial myopathy
• Atrioventricular muscle dystrophy
• Cardiomyopathy
• Double-chambered right ventricle
• Drug intoxication
• Electrolyte abnormalities
• Genetic predisposition (English Springer Spaniel, German Shorthaired Pointer, Old English Sheepdog, Shih Tzu)
• Hyperkalemia
• Hypothyroidism 
• Immune-mediated polymyositis
• Myocarditis
• Oliguric renal failure
• Urethral obstruction

Your veterinarian will want to start with a verbal history of symptoms and a physical evaluation. During the physical evaluation, specific attention will be paid to the sound of the heart and the color of the skin and mucous membranes. The heart rate may be slowed to less than 60 beats a minute but will otherwise sound normal unless the sinus node is destroyed. If the sinus node has been destroyed from persistent atrial standstill your veterinarian may report a junctional escape rhythm, or delayed rhythm. A biochemistry profile, complete blood count, and urinalysis will also be requested to help determine the underlying cause of the sinus node abnormality. X-ray imaging may be used to view the chest area in some cases. The presumptive diagnosis will be made when the ECG reveals a complete absence of P waves due to the inability of the atria to be depolarized despite an impulse being produced in the sinoatrial node.

The first course of action when an atrial standstill occurs is to treat any underlying conditions. If the cause is hyperkalemia, aggressive fluid therapy is often needed to quickly lower potassium levels and increase the circulating blood volume. Theophylline may be used to increase the heart rate in muscle dystrophy instances, and medications designed to increase the pumping efficiency of the heart as well as control arrhythmias may be prescribed for dogs who have had congestive heart failure. A diet lower in salt and with exercise restrictions is often warranted in severe or chronic conditions. If these measures fail to correct the situation and the structures of the heart are sound, a pacemaker may be able to be surgically implanted to help control the heart rate. Once the pacemaker is successfully implanted into the heart the prognosis is generally quite good and patients have been known to live for several years after the procedure.

A follow-up appointment is likely to be recommended by your veterinarian to check on your dog’s progress with any of the causes of atrial standstill. A biochemistry profile, complete blood count, and urinalysis are likely to be ordered again to get updated information and medications and activity level recommendations will be re-evaluated at this time. If a pacemaker was implanted the patient will need to be seen approximately every six months to perform an evaluation of the pacemaker called an interrogation and can be done without any sedation or anesthesia and will help you and your veterinarian by getting information on both functionality and remaining battery life.