Download Second- and Third-Degree Heart Block

Second- and Third-Degree
Heart Block
Rebecca E. Gompf, DVM, MS, DACVIM (Cardiology)
BASIC INFORMATION
Description
Second-degree heart block occurs when some of the impulses that
arise in the small chambers of the heart (atria) are blocked from
entering the lower chambers (ventricles). As a result, the heart
rhythm pauses, and the ventricles do not contract. If these pauses
are frequent, less blood is pumped by the heart, and the animal
cannot exercise and may faint.
With third-degree heart block, none of the impulses reach the
ventricles; the blockage is complete. The ventricles may take over
and produce a heartbeat by themselves, but the heart rate is very
slow and the heart does respond when the animal becomes active
(so it may faint).
Causes
The cause of these heart blocks may not be found. Second-degree
block can arise from increased vagal tone due to irritation of the
vagus nerve. Occasionally, high potassium levels (hyperkalemia)
can cause heart block, and the block is reversible once the hyperkalemia is treated. Certain heart medications (beta-blockers, calcium channel blockers, digoxin) can slow conduction between the
atria and ventricles, and second-degree block can result.
If second-degree block is advanced or if complete block is
present, then a major problem exists in the conduction system of
the heart. Tumors, infections of the heart valves, and Lyme disease
may all cause these advanced heart blocks. Most of the time, however, the cause is never found.
Clinical Signs
Animals with occasional second-degree block often have no signs.
If another disease is causing hyperkalemia, signs of that disease
are present. Drugs that interfere with conduction in the heart may
also cause vomiting, diarrhea, and a lack of appetite.
Animals with advanced second-degree block or complete
block are not able to exercise and are lethargic. They may also
faint when stressed.
Diagnostic Tests
An electrocardiogram (ECG) is needed to diagnosis these abnormal rhythms. If the ECG is normal and the animal is having fainting episodes, then a 24-hour continuous ECG (Holter monitor) or
a 2-week intermittent ECG (event monitor) may be needed to document the cause of the fainting.
Animals with second-degree block may undergo an atropine
challenge test to determine whether the block will disappear.
If it does disappear, then additional tests are usually performed
to identify why the animal has increased vagal tone. Causes of
increased vagal tone include diseases of the respiratory, digestive,
and neurologic systems.
If the second-degree block is advanced or does not disappear
with atropine, or if third-degree block is present, then additional
tests may be recommended, such as laboratory tests, chest and
abdominal x-rays, and an echocardiogram (heart ultrasound).
TREATMENT AND FOLLOW-UP
Treatment Options
If second-degree block disappears with the atropine challenge
test, no further therapy may be required except for treatment of
the underlying cause. If episodes of second-degree block are frequent and cause signs, then drugs such as propantheline bromide,
albuterol, terbutaline, or theophylline may be tried. Response
to these drugs is unpredictable, and side effects include anxiety,
excessive panting, decreased appetite, vomiting, diarrhea, and
constipation. If the drugs do not help the clinical signs, then insertion of a pacemaker may be necessary.
Animals with third-degree block do not respond to atropine
challenge tests or to any of the oral medications listed above.
These animals always require insertion of a pacemaker.
Follow-up Care
If the animal responds to the atropine challenge test and the underlying disease resolves with treatment, no further follow-up may
be needed. Periodic ECGs may be done to ensure that the seconddegree block has not recurred. Animals with second-degree block
whose clinical signs respond to oral therapy usually have monthly
recheck visits and ECGs to make sure the problem is not advancing to third-degree block.
Animals that receive pacemakers usually have a follow-up visit
with the cardiologist 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 every 3 months.
Prognosis
Animals whose second-degree block disappears after an atropine
challenge test or following resolution of their underlying disease
have an excellent prognosis. Animals with advanced seconddegree or complete heart block require pacemaker implantation.
Placement of a pacemaker usually results in years of additional
life for the patient. The pacemakers that are implanted in dogs and
cats have a battery life of about 5 years and are usually replaced
after that time.
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