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Asystole in Dogs

There are four chambers in the heart. The two top chambers are the atria (single: atrium), and two bottom chambers are the ventricles. Valves are provided between each atrial and ventricular pair, each on the left and right side, allowing blood to pass from the atria to the ventricles, where it is then pumped out of the heart into the body – the right ventricle pumps blood to the lungs, and the left ventricle pumps blood  to the body. The heart works with exceptional synchronization between the various atrial and ventricular structures, resulting in a consistent rhythmic pattern.

Ventricular standstill, also termed asystole, is an absence of ventricular complexes (called QRS) measured on an electrocardiogram (ECG), or absence of ventricular activity (electrical-mechanical dissociation). Electrical-mechanical dissociation is when there is a recorded ECG cardiac rhythm (P–QRS–T), but no effective cardiac output or palpable femoral pulse (the pulse of the artery in the inner thigh).

Ventricular standstill will lead to cardiac arrest and irreversible brain injury if the ventricular rhythm is not restored within 3-4 minutes. This condition can result from severe sinoatrial block or arrest (stoppage of the SA node, or pacemaker), or by third-degree atrioventricular (AV) block (which also causes blockage of the heart’s beat) without a junctional or ventricular escape rhythm (a junctional or escape rhythm would carry on the beat of the heart, saving the animal from cardiac arrest.)

Symptoms and Types

• Severe systemic illness or cardiac disease in many patients
• Other cardiac arrhythmias in some
• Syncope (fainting)
• Cardiac arrest (the heart stops momentarily)
• Collapse
• Sudden death

Causes

• A complete AV block with absence of ventricular or junctional escape rhythm
• Severe sinus arrest or block
• Hyperkalemia
• Any severe systemic illness or heart disease predisposes
• Under-active adrenal glands causing high potassium levels in the blood predisposes
• Bursting of the urinary bladder or urinary tract blockage causing high potassium levels in the blood

Diagnosis

Once the initial emergency has been managed, your veterinarian will need a thorough history of your dog's health, onset of symptoms, and possible incidents that might have led to this condition. Your doctor can then perform a complete physical exam on your dog. Initially, just an electrolyte panel may be taken to determine if your dog has high serum potassium, a condition that is known to lead to ventricular standstill. This will be followed by standard laboratory tests, including a biochemical profile, a complete blood count and a urinalysis. Systemic disease as an underlying cause of heart disease must be ruled out. Additional diagnostics will include an electrocardiogram (ECG, or EKG) recording, which can be used to examine the electrical currents in the heart muscles, and may reveal any abnormalities in cardiac electrical conduction (which underlies the heart’s ability to contract/beat).