Unfortunately, some dogs and cats may experience sudden cardiac death (SCD). This often unexpected event can lead to a lot of unanswered questions and frustration for many grieving pet owners. When a pet is otherwise not apparently ill or obviously exposed to a toxin of some kind, the heart is usually blamed as the underlying cause.
Cardiac arrhythmias are probably the most common underlying cause of true sudden cardiac death. An arrhythmia is an abnormal heart rhythm. Ventricular arrhythmias including ventricular tachycardia may predispose a pet to sudden cardiac death. Usually what happens is the heart rate becomes very fast and the rhythm disorganized until ventricular fibrillation (VF or V-fib) occurs. Without prompt defibrillation by an external defibrillator (a machine that provides an electrical shock to the heart to reset the rhythm), ventricular fibrillation is terminal. Some pets can have abrupt pauses or arrests in the heart rhythm that last long enough to result in death from low blood flow to the organs. Patients at risk for sudden cardiac death from an arrhythmia may or may not have associated symptoms. Most pets with symptoms of an arrhythmia have episodic weakness, collapse, fainting spells (syncope) or seizure-like events. An EKG (electrocardiogram or ECG) or Holter monitor (24 hour ambulatory EKG) is required to definitively diagnose a cardiac arrhythmia.
Some pets may have sudden cardiac death associated with underlying structural heart disease. Dogs with chronic mitral valvular disease (CMVDz, myxomatous valvular degeneration/MVD, “endocardiosis” – all equivalent terms) can rarely have acute congestive heart failure secondary to a rupture of a major portion of the supporting structure of the mitral valve. This is called a ruptured chordae tendinae or ruptured papillary muscle. Patients that experience this suddenly develop a large amount of leakage of blood within the heart across the mitral valve, causing abrupt increase in the blood pressure in the lungs, rapid fluid accumulation, and death may follow rather quickly as the dog drowns. Ruptured chordae tendinae in dogs is diagnosed with echocardiography (ultrasound of the heart). Some dogs may survive this event, but usually require intensive hospitalization, oxygen therapy, injectable medications, etc.
Dogs with chronic mitral valvular disease can also have sudden death from a rupture of a heart chamber. As the high-velocity jet of mitral regurgitation within the heart strikes the inside lining of the left atrium, very small tears in the muscle may occur over time. This can eventually lead to a small hole in the wall of the atrium, and bleeding may occur suddenly within the pericardial sac. If too much blood accumulates rapidly within the pericardial sac, the right side of the heart is unable to fill up properly with blood, leading to less blood getting into the lungs and oxygen depletion in the body tissues. This can lead to a rapid death. Diagnosis of a ruptured left atrium requires echocardiography. Some dogs can survive this event, but it’s generally rare, and these patients remain at high risk for further bleeding.
Tumors on the heart can start bleeding abruptly, leading to a sudden death. The most common tumor in dogs include right atrial/auricular hemangiosarcoma, which commonly bleeds. Hemorrhage into the pericardial space impairs the ability of the right side of the heart to fill just as with a ruptured heart chamber. This may lead to collapse, weakness, fainting and possibly death. Timely pericardiocentesis (“pericardial tap” – manual removal of the blood with a special catheter) is necessary in these patients.
Rarely, sudden death may occur secondary to clot formation. Cats are at an increased risk for clots to develop in the heart if they have moderate to severe heart chamber enlargement from underlying heart disease (usually hypertrophic cardiomyopathy/HCM). If a cat has a clot form inside the heart and it breaks loose, it may travel out of the heart leading to obstruction of blood flow in one or more arteries (stroke). Most often, one or more of the legs are affected, leading to limping or dragging of a limb. Rarely, a large clot may obstruct the outflow of blood of the heart itself, leading to sudden death. Diagnosis of a cardiac clot requires echocardiography.
Often, we may never find out the underlying cause of a sudden death in one of our beloved pets. Necropsy (animal autopsy) may yield some answers but is uncommonly performed. Ruptured chordae tendinae/papillary muscle with acute congestive heart failure, ruptured left atrium or cardiac tumors can be diagnosed post-mortem via necropsy. In cases of arrhythmias, underlying structural heart disease may or may not be evident if a necropsy is done. Fortunately, when sudden cardiac death does occur, it is rapid by nature, so suffering tends to be minimal.