Sick Sinus Syndrome

Dear Dr. Jody:  Our six year old West Highland white terrier, Bonnie, has recently started collapsing after excitement or exercise.  This morning she was playing with our grandson in the backyard, and she suddenly fell over on her side and lay motionless for several seconds before she slowly got back up and ambled over to us.  She seemed completely normal after that.  We are very worried about her.  What could be wrong?

I can certainly understand how worried you must be.  The symptoms you are describing would be very alarming for an owner to witness.  There are several different things that could be causing these strange symptoms, and so it is critical that you take Bonnie to her veterinarian as soon as possible for an accurate diagnosis and early treatment.  Veterinarians often find it helpful to see a video of the dog during an episode, so if you have the ability to film the dog the next time she experiences these symptoms it could provide your veterinarian with very useful information.

One possible diagnosis that could explain Bonnie’s symptoms is Sick Sinus Syndrome (SSS).  SSS is a disorder of the electrical impulses that run through the heart in a synchronized fashion to coordinate the heartbeats.  In order to better understand this disease, bear with me for some Cardiology 101.  The heart is divided into four chambers:  the right atrium, the right ventricle, the left atrium, and the left ventricle.  The blood moves through each of these chambers in sequence as the heart pumps it through the lungs for oxygenation and then on to the rest of the body. 

But how does the heart “know” when and how to pump?  These rhythmic contractions of the cardiac muscle are not controlled by brain activity in the same way that contraction of biceps or other skeletal muscle are.  The heart has its own “brain” in the form of electrical nodes, or pacemakers, that fire every time a heart beat is required.  The primary pacemaker is called the sinoatrial (SA) node, or sinus node.  It is located on the right atrium, or first chamber of the heart.  When this electrical node fires, electrical impulses travel through special fibers to the rest of the heart.  These electrical impulses trigger a coordinated wave of muscle contraction through the series of heart chambers so that blood flows effectively in one direction from one chamber to the next.

In some patients, the SA node stops firing with its normal reliable rhythm.  As you can imagine, a malfunction of this pacemaker (or of its subsidiary electrical nodes) can result in the heart not beating in a coordinated fashion or sometimes not beating at all.  This will cause the dog to collapse in a faint (also called syncope).  Once the SA node regains its ability to fire off electrical impulses in a normal fashion (usually after a few seconds), the heart beats regularly again and the dog recovers from its syncopal episode and seems to feel fine. 

An effective diagnosis of SSS is best gained by performing an electrocardiogram (ECG).  This non-invasive test painlessly measures the electrical impulses through the heart, and abnormalities can be quickly detected and diagnosed.  Unfortunately, the best treatment for SSS is the surgical implantation of a synthetic pacemaker.  This procedure can be performed on domestic animals, but as you can imagine, the procedure is cost-prohibitive for most pet owners. 

If your pet is experiencing any episodes of collapse, please contact your veterinarian immediately.