Download Sick Sinus Syndrome and Atrial Standstill

Sick Sinus Syndrome and
Atrial Standstill
Rebecca E. Gompf, DVM, MS, DACVIM (Cardiology)
BASIC INFORMATION
Description
Sick sinus syndrome is a condition in which pauses occur in the
heart rate and the heart does not beat. These pauses can be so long
that the animal collapses or faints. Very fast heart rates may follow
the pauses or occur at other times, and they also result in fainting. Atrial standstill occurs when the small chambers of the heart
(atria) are not beating at all, which results in a slower than normal
heart rate.
Causes
Sick sinus syndrome has been reported in the miniature schnauzer,
American cocker spaniel, West Highland white terrier, dachshund,
and other small breeds. With this syndrome, the normal pacemaker
in the heart does not fire, the backup system does not work, and
other areas of the heart that can usually generate a heartbeat do
not. During the long pauses, no heart contractions occur, so blood
is not pumped to the body and the animal collapses. These pauses
may initially be infrequent but tend to increase in frequency with
time. The reason the conduction system stops working has not
been identified.
Atrial standstill can be caused by increased potassium levels
in the body due to problems with the adrenal gland (hypoaldosteronism, Addison’s disease), a urethral obstruction, or a blood
clot to the rear legs in cats. With extremely high potassium levels
(hyperkalemia), the whole heart stops. Atrial standstill can also
arise from diseases of the atria, such as tumors. It can occur with a
scapulohumeral muscular dystrophy of English springer spaniels.
Clinical Signs
Most affected animals cannot exercise normally and have fainting episodes that increase in frequency over time. Animals with
hyperkalemia will have signs of their underlying disease. English
springer spaniels with scapulohumeral muscular dystrophy and
atrial standstill are young dogs that cannot exercise well; they
often have fainting episodes and other muscle problems that limit
their mobility.
Diagnostic Tests
An electrocardiogram (ECG) is needed to diagnose these abnormal rhythms. Dogs with fainting episodes may also need to wear a
Holter monitor, which is a 24-hour continuous ECG, to document
whether their fainting is caused by a slow or a fast heart rhythm
or by another disease. An atropine challenge test may be done in
dogs with sick sinus syndrome to determine whether it will cause
the heart rate to speed up and the pauses to disappear.
Other laboratory tests, chest and abdominal x-rays, and possibly an echocardiogram (heart ultrasound) may be recommended
to look for any underlying diseases and to rule out other disorders
that can cause similar clinical signs.
TREATMENT AND FOLLOW-UP
Treatment Options
Some cases of sick sinus syndrome may be managed for a period
of time on drugs that increase the heart rate (similar to atropine).
Examples include propantheline bromide, albuterol, terbutaline,
and theophylline. Even dogs that do not respond to an atropine
challenge test may respond to these drugs for awhile. Side effects
of these drugs include anxiety, excessive panting, lack of appetite,
vomiting, diarrhea, and constipation.
If these drugs do not work or stop working, your pet may be
referred to a veterinary specialist for insertion of a pacemaker.
Pacemakers stabilize the heart and prevent the pauses from occurring. Additional drugs (beta-blockers or calcium channel blockers)
may be needed for periods of fast heart rates.
Atrial standstill caused by other diseases usually improves or
resolves once the underlying problem is treated. English springer
spaniels and dogs with atrial standstill from heart problems require
a pacemaker to stabilize their heart rate.
Follow-up Care
When sick sinus syndrome is treated with medications, ECGs are
initially done at least monthly. If any additional fainting episodes
occur, ECGs are done more frequently. Other monitoring is based
on the underlying disease.
Following implantation of a pacemaker, recheck visits are
usually scheduled at 1 and 3 months. After the pacemaker has
received its final adjustments, you will be asked to monitor the
heart rate weekly and to have your veterinarian run ECGs about
every 3 months.
Prognosis
Animals with sick sinus syndrome that have fainting episodes
eventually need a pacemaker, because medications rarely provide
good long-term results. Animals with atrial standstill have an excellent prognosis if their underlying problem can be corrected. Dogs
with atrial standstill from heart disease have a guarded prognosis
and will require a pacemaker. Placement of a pacemaker usually
results in years of additional life for the patient. Pacemakers typically have a battery life of about 5 years and are usually replaced
after that time.
IF SPECIAL INSTRUCTIONS HAVE BEEN ADDED, THEY WILL APPEAR ON THE LAST PAGE OF THE PRINTOUT.
Copyright © 2011 by Saunders, an imprint of Elsevier Inc. All rights reserved.