Sick Sinus Syndrome (SSS) in Dogs

Pay close attention all of you Miniature Schnauzer lovers! The breed you fancy is prone to a heart condition called Sick Sinus Syndrome (SSS). The sinus involved is not within the respiratory tract. Rather, it is a structure called the sinus node that is located within the heart.

The sinus node is responsible for electronically initiating the normal heartbeat and establishing the normal heart rate. In dogs with SSS, the sinus node has lapses in which it discharges (beats much too slowly, or not at all). As a result, there are long pauses in between heartbeats. Sometimes, an electrical impulse originating from another part of the heart will come to the rescue, particularly if the heart has stopped for several seconds. Such rescue beats can be very rapid.

In most cases, the sinus node will eventually resume its job in which case there will be periods of normal heart rate (60-100 beats per minute). Other dogs with SSS have a constant bradycardia (heart rate is too slow). Even with exercise or excitement, the heart rate remains at less than 40 beats per minute.

Cause of sick sinus syndrome in dogs
The exact cause of the sinus node malfunction is unknown. Although any breed of dog can be affected, a genetic basis is suspect because SSS primarily affects Miniature Schnauzers, Dachshunds, Cocker Spaniels, West Highland White Terriers, and Pugs. Middle-aged to older females are particularly predisposed. The mode of inheritance is unknown, and there is no genetic testing available. Nonetheless, the appearance of SSS in a breeding dog should strongly discourage future breeding.

Symptoms of sick sinus syndrome in dogs
A dog with SSS becomes symptomatic because of his subnormal heart rate. The most common symptoms include:

  • Weakness
  • Lethargy
  • Exercise intolerance
  • Collapse
  • Fainting episodes (also known as syncopal episodes)

Some dogs with severe, long-standing SSS can develop symptoms of congestive heart failure including weakness, labored breathing and coughing.

It can sometimes be difficult to differentiate between a fainting episode (syncope) and a seizure. Videotaping such an event at home to then share with the examining veterinarian can be most helpful.

Diagnosis of sick sinus syndrome in dogs
SSS is strongly suspected based on the dog’s breed, history and a thorough physical examination. Listening with a stethoscope often reveals a heart rate that is lower than normal and stays this way even when the dog is asked to exercise. Other testing that may be recommended includes:

  • An electrocardiogram (ECG)—To look for abnormalities characteristic of SSS changes.
  • Blood testing—To rule out an underlying metabolic problem. Abnormalities in blood calcium or potassium levels have the potential to mimic SSS changes.
  • Holter monitoring—Provides a 24-hour electrocardiogram (ECG) tracing. The testing equipment is housed within a vest that is worn by the dog at home. This may be necessary to determine if a dog has SSS, particularly if the heart rate is normal at the time of the physical examination.
  • An atropine response test—To differentiate dogs with SSS. Atropine is a drug that normally causes the heart rate to escalate. When atropine is given to a dog with SSS, the very low heart rate remains unchanged.
  • Chest x-rays—To look for evidence of heart failure.
  • Cardiac ultrasound (echocardiogram)—To look for changes in the appearance of the heart valves and sizes of the four chambers that can occur secondary to chronic SSS.

Treatment of sick sinus syndrome in dogs
For dogs with SSS, the therapeutic goal is to maintain a normal heart rate so as to restore a good quality of life. If SSS is caught quite early during an annual physical exam, and the dog is symptom-free, no treatment beyond careful monitoring may be required for the time being.

For dogs experiencing symptoms, two forms of therapy can be considered:

  • Vagolytic drugs—These medications are used in an attempt to maintain a normal heart rate. While it is reasonable to try such drugs, they don’t have a very consistent track record of success. Additionally, side effects are relatively common. Examples of vagolytic drugs are theophylline, terbutaline and propantheline bromide.
  • Pacemaker implantation—This is truly the treatment of choice for most dogs with symptoms caused by SSS. When properly placed and monitored, a pacemaker is capable of restoring a normal quality of life for years to come.

Veterinarians who specialize in cardiology are the masters of pacemaker implantation. Just as in people, the pacemaker can be placed without a significant surgery involved. Access to pacemaker implantation may be limited depending on where one lives and their ability to pay for such a state-of-the-art procedure.

Questions to ask your veterinarian

  • Has SSS clearly been diagnosed?
  • What are the options for treatment?
  • What is the closest access to pacemaker implantation for my dog?
  • What should I do at home if my dog collapses or faints?

If you have any questions or concerns, you should always visit or call your veterinarian -- they are your best resource to ensure the health and well-being of your pets.

Reviewed on:

Friday, February 27, 2015


Sinus arrhythmia of the dog is unique because of the pronounced alternating beat-to-beat intervals. The clustering of these short (faster rates) and long (slower rates) intervals is not just influenced by autonomic input from breathing sinus arrhythmia can persist in the panting or apneic dog. The multiplicity of central and peripheral influences on the sinus node complicates the unraveling of the mechanisms of sinus arrhythmia. Studies of the sinus node suggest that acetylcholine can slow cellular depolarization and block sinoatrial conduction. Electrocardiographic monitoring of the dog supports this notion in that abrupt bifurcation into short and long intervals develop at lower heart rates. We sought to determine whether this phenomenon could be recapitulated in canine atrial preparations perfused with acetylcholine and whether selective pharmacologic blockade of the voltage and calcium clocks could provide insight into its mechanism. Spontaneous beat to beat (A-A) intervals were obtained from monophasic action potential recordings of perfused canine right atrial preparations before and during perfusion with acetylcholine (2-5 μM). The calcium clock was blocked with ryanodine (2-3 μM). The membrane clock was blocked with diltiazem hydrochloride (ICa,L blocker 0.25 μM) and ZD7288 (If blocker 3 μM). Hyperpolarization was hindered by blockade of IK,Ado/IK,Ach with tertiapin Q (100 nM) before and during acetylcholine perfusion.

Acetylcholine resulted in beat clusters similar to those seen in sinus arrhythmia of the dog. Beat clusters were consistent with intermittent 2:1 and 3:1 sinoatrial conduction block. Tertiapin Q abolished this patterning suggesting a role of IK,Ado/IK,ACh in the mechanism of these acetylcholine-induced beat-to-beat patterns.

Outcome and survival in canine sick sinus syndrome and sinus node dysfunction: 93 cases (2002–2014)


To evaluate the clinical presentation, diagnosis, treatment, and outcomes of a group of dogs with sinoatrial node abnormalities.


Ninety-three client-owned dogs at a referral institution.

Materials and Methods

Medical records were reviewed for clinical history, diagnostic testing, and medical or permanent artificial pacemaker (PAP) treatment. Owners or veterinarians were contacted for long-term follow-up.


Sixty-one dogs were symptomatic for their bradyarrhythmia and were diagnosed with sick sinus syndrome (SSS). Thirty-two dogs were asymptomatic for their bradyarrhythmia and were diagnosed with sinus node dysfunction (SND). Miniature Schnauzers, West Highland White terriers, Cocker spaniels, and female dogs were overrepresented. Medical management with positive chronotropic drugs successfully controlled syncope long-term in 54% of SSS dogs, and acted as a bridge to PAP in 20%. Positive atropine response predicted medical treatment success. Forty-six percent of SSS dogs eventually underwent PAP implantation. Median survival time was approximately 18 months in SND and SSS dogs regardless of treatment strategy. Congestive heart failure (CHF) associated with progressive valvular heart disease occurred commonly in all groups, particularly in dogs with bradycardia–tachycardia syndrome.


Sinus node dysfunction and SSS represent a spectrum of sinoatrial node disease, which for some dogs may also involve a component of autonomic dysfunction. Dogs with SND do not require treatment. Dogs with SSS often require treatment to reduce the frequency of syncope medical management is often useful, particularly in atropine responsive dogs. Prognosis of SSS with treatment is good, though development of CHF does not appear to be mitigated by treatment.

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Sick sinus syndrome (SSS) is a disorder of the conducting tissue that controls the heart rate and rhythm. The sinoatrial (SA) node is located in the right atrium (upper chamber of the heart) and initiates an electrical impulse that spreads through specialized tissue of the heart and triggers the heart to beat. Dogs with SSS develop changes in the SA node and other conducting tissue that results in irregular heart rhythms including bradycardia (slow heart rates), pauses, or sinus arrest. Another variation, referred to as brady-tachy syndrome, involves periods of bradycardia followed by tachycardia (fast heart rates). The cause of SSS is unknown but likely involves degeneration of the conduction tissue. It likely has a genetic component since certain breeds, such as the miniature schnauzer, are predisposed to SSS. Other breeds that are commonly affected include the West Highland white terriers, cockers spaniels, dachshunds, and pugs.

Clinical signs of SSS vary depending on the frequency and severity of the arrhythmias. Brief periods of tachycardia or bradycardia may not result in any noticeable changes. However, sustained periods of bradycardia or pauses often result in signs of syncope (collapse), muscle weakness, or exercise intolerance. Work-up typically includes blood pressure measurement, electrocardiography (ECG), radiographs (x-rays), echocardiogram (ultrasound of the heart) and 24-hour Holter monitor evaluation. An atropine challenge may also be performed to test how well the sinus node is responding. In some cases, a cardiac event monitor is needed for a more definitive diagnosis.

Treatment varies on the frequency of the arrhythmias and associated clinical signs. Dogs that are asymptomatic do not usually require treatment however, SSS can progress over time and may warrant treatment at a later date. Since symptoms are usually associated with bradycardia, treatment is aimed at preventing slow heart rates or pauses. Although medical therapy is an option it is often ineffective. Therefore, an artificial pacema¬ker is usually recommended to prevent the heart rate from dropping to low in patients that are weak or collapsing from SSS.

Watch the video: SICK SINUS SYNDROME (September 2021).