In dogs, as in humans and other animals, the heart beats ceaselessly throughout the animal's life to maintain blood circulation. If the heart comes to a standstill, death occurs within just a few minutes. Ventricular standstill is a condition that causes periodic brief irregular heart rhythms in which the ventricles of the heart can stop beating normally for a few seconds. Dogs afflicted with this syndrome often faint or collapse during these spells, and if the heart does not return to a normal rhythm quickly, the animal can die.
Ventricular standstill has several underlying causes, but often reflects problems with the sino-atrial (SA) node or the atrioventricular (AV) nodes within the heart. These nodes serve as "pacemakers" for the heart, keeping it beating normally.
Treating ventricular standstill can be difficult, depending on the underlying cause. Some dogs require an implanted pacemaker to keep their heart beating normally. Unfortunately, even with treatment, the prognosis for many dogs with this condition is poor.
What Is Ventricular Standstill?
Each beat of a heart, whether in a dog, human, or other animal, is a complex process that relies on electrical signals traveling unobstructed through the cells of the heart. To understand ventricular standstill, it helps to understand the basics of this process.
In the normal heart, blood that's finished its journey around the body enters a small chamber towards the top of the heart called the right atrium. From there, it enters a larger lower chamber called the right ventricle, which contracts and sends the blood to the lungs where it gathers oxygen.
Freshly oxygenated blood from the lungs travels back to the heart, filling a small upper chamber called the left atrium. It then flows into the lower left ventricle and finally is pumped out to circulate through the body once again. This complex cycle is controlled by two electrical pacemakers called the sino-atrial (SA) node and the atrioventricular (AV) node, both of which are located in the right atrium.
When something goes wrong with the SA node or AV node, they cannot effectively transmit electrical signals that keep the heart beating normally. Often, this causes the heart to beat more slowly than usual or to develop abnormal rhythms. At times, it can lead to a ventricular standstill.
Ventricular standstill is also known as asystole, meaning lack of heartbeat. It occurs when the ventricles don't receive normal signals from the SA and AV nodes and stop working properly. This attack can last several seconds, during which the dog might collapse or faint. Often, the heart regains fairly normal rhythm quickly, and the dog regains consciousness. However, if the heart does not regain an effective pumping rhythm within a few minutes, the dog will suffer brain damage or death.
Symptoms of Ventricular Standstill in Dogs
For many dogs with various types of disease in their SA node or AV node, a slower than normal heartbeat leads to long-term weakness or lethargy. If the disorder becomes severe enough, the dog might suffer from fainting spells, also called syncope. Syncope is a primary symptom of ventricular standstill, although your dog can faint for many other reasons.
Dogs with heart disease are often less energetic than normal and may not enjoy or be able to tolerate walks or other exercise. This is because an ineffective heart leads to lowered amounts of oxygen within the dog's cells.
If your dog's heart disease leads to ventricular standstill, your dog will faint or collapse. Often, it will recover quickly and appear normal right away, however, as the heart regains a fairly normal rhythm. This can help differentiate syncope from a seizure, in which a dog also may collapse. With a seizure, recovery takes quite a bit longer.
During an episode of ventricular standstill, you might not feel any heartbeat if you place your hand on your dog's chest. This is an emergency situation that calls for cardiopulmonary resuscitation and an immediate trip to the veterinarian's office.
Causes of Ventricular Standstill
There are a few causes of ventricular standstill in dogs. These are some of the most common.
AV or SA Node Degeneration
The four chambers of the heart work in concert to pump blood through the body. Each chamber has valves that open and close to keep blood flowing in the right direction. Electrical impulses from the sino-atrial (SA) node and atrioventricular (AV) node tell those valves when to open and close.
When one of these nodes degenerates due to age, disease, or unknown causes, signals to trigger the opening and closing of the heart valves can become erratic or absent, leading to ineffective or insufficient blood flow. In some dogs, that will lead to a ventricular standstill. There are several types of AV node and SA node degeneration, but the most common that are of concern in dogs include second-degree and third-degree AV block and sick sinus syndrome, which is a degeneration of the SA node.
Referred to medically as hyperkalemia, extremely high potassium levels in the blood from under active adrenal glands, obstructed or burst urinary bladders and urinary tracts, or other problems can cause ventricular standstill due to an AV block.
Many types of heart disease can cause abnormal heart rhythms, called arrhythmias. Arrhythmias, when severe or persistent, can lead to collapse, syncope, or death from ventricular standstill.
Some severe systemic illnesses, including cancer, metabolic disease, sepsis (infection in the blood), and Lyme disease can cause heart block that leads to ventricular standstill. Trauma can do the same.
Diagnosing Ventricular Standstill in Dogs
When a dog has frequent fainting spells or collapse that suggest ventricular standstill or an issue with the SA or AV nodes, your veterinarian will run an electrocardiogram (EKG), which traces the electrical impulses running through the heart and pinpoints various abnormalities of rhythm or electrical conduction. An EKG can be used to diagnose heart block, sick sinus syndrome, and other heart diseases.
One important reading on the electrocardiogram is referred to as a QRS complex. This is the visual representation of the electrical impulse going through the ventricles. It is seen as the main spike on an EKG tracing line. When ventricular standstill occurs, a flat line is seen on an EKG in place of the QRS complex, because there are no electrical impulses occurring in the heart ventricles. However, unless your dog experiences a ventricular standstill while actually hooked to the EKG monitor, your vet will not see this occur.
Other tests your vet will probably order include blood tests to check for high potassium or other electrolyte imbalances. A general chemistry panel to look at the dog's overall organ function is usually performed, as well as a complete blood count to assess the possibility of infection or anemia. Your vet might check your dog's urine as well, which can reveal a range of health problems.
Treatment & Prevention
If your dog has collapsed due to ventricular standstill, cardiopulmonary resuscitation (CPR) may be called for. This involves applying rhythmic pressure to your dog's chest to force blood flow from the heart, as well as breaths into the dog's snout to provide oxygen until the dog revives.
However, for dogs not in ventricular standstill currently, but suffering from periodic attacks of collapse or syncope due to heart block or other heart disease, the recommended treatment involves surgical implantation of a cardiac pacemaker, which is an electronic device that stimulates the heart when the pulse rate drops too low.
For dogs with ventricular standstill caused by electrolyte imbalance or other health issues beyond heart block and similar conduction disorders, your veterinarian might prescribe medications or therapies to relieve that health problem, whether it's cancer, infection, or another systemic illness.
Because ventricular standstill is usually caused by an underlying problem with the heart's electrical conduction system, and these problems are most often due to aging or to unknown causes, there is no specific prevention for ventricular standstill.
Prognosis for Dogs with Ventricular Standstill
Unfortunately, the prognosis for dogs that experience ventricular standstill is poor, as many die suddenly before any treatment can be started. However, if your dog has a pacemaker implanted, then it may have a good quality of life for many years. You will need to have frequent visits to the veterinary cardiologist, however, and follow any special medication routines, dietary or exercise precautions that are recommended.
Atrioventricular (AV) Block. Petcardia Veterinary Cardiology.
Sick Sinus Syndrome. East Coast Veterinary Cardiology.
Atrioventricular (AV) Block. Gulf Coast Veterinary Cardiology.
Ventricular Arrhythmias. Petcardia Veterinary Cardiology.
Syncope. East Coast Veterinary Cardiology.
Arrhythmias (Abnormal Rhythms) in Dogs. Cornell University College of Veterinary Medicine.
Heart Block. East Coast Veterinary Cardiology.