Sick sinus syndrome describes dysfunction of
the intrinsic pacemaker of the heart, the
sinoatrial node. As a result, the heart rhythm
becomes abnormal characterized by:
Sinus bradycardia -- slow heart rates
Tachycardias -- fast heart rates
Bradycardia-tachycardia -- alternating slow
and fast heart rhythms
Normally, the sinus node produces a steady
pace of regular electrical impulses. In sick
sinus syndrome, these signals are
abnormally paced. A person with sick sinus
syndrome may have heart rhythms that are
too fast, too slow, punctuated by long pauses
— or an alternating combination of all of
these rhythm problems.
Types and Causes:
Types of sick sinus syndrome and their causes
Sinoatrial block. Electrical signals move too slowly
through the sinus node, causing an abnormally slow
Sinus arrest. The sinus node activity pauses.
Tachycardia-bradycardia syndrome. The heart
rate alternates between abnormally fast and slow
rhythms, often with a long pause (asystole) between
Sick sinus syndrome is relatively uncommon.
It may be brought on by the use of certain drugs:
such as digitalis, calcium channel blockers, betablockers, and anti-arrhythmics.
Sick sinus syndrome usually occurs in people older
than 50, in whom the cause is often a nonspecific,
scar-like degeneration of the heart's conduction
system like amyloidosis, sarcoidosis, Chagas
disease and cardiomyopathies.
In children, a common cause of sick sinus syndrome
is heart surgery, especially on the upper chambers.
Coronary artery disease, high blood pressure, and
aortic and mitral valve diseases may be associated
with sick sinus syndrome.
Most people with sick sinus syndrome initially have few or no
symptoms. In some cases, symptoms may come and go.
When they do occur, sick sinus syndrome symptoms may
Slower than normal pulse (bradycardia)
Dizziness or lightheadedness
Fainting or near-fainting
Shortness of breath
Many of these signs and symptoms are caused by reduced blood
flow to the brain
Sick sinus syndrome can produce a variety of ECG
manifestations consisting of atrial bradyarrhythmias, atrial
tachyarrhythmias, and alternating bradyarrhythmias and
Supraventricular bradyarrhythmias may include sinus
bradycardia, sinus arrest with or without junctional escape,
ectopic atrial bradycardia, and atrial fibrillation with slow
The sino-atrial exit block that occurs in patients with sick sinus
syndrome may demonstrate a Mobitz type I block and a Mobitz
type II block. The ECG may reveal a long pause following
cardioversion of atrial tachyarrhythmias, and a greater-than
three-second pause following carotid massage. Sixty percent of
patients have tachyarrythmias.
Treatment may not be necessary if you do
not have any symptoms.
A permanent implanted pacemaker may be
needed if your symptoms are related to
bradycardia (slow heart rate).
A fast heart rate (tachycardia) may be treated
with medications. Sometimes radiofrequency
ablation is used to cure tachycardia.
Interference with the conduction process of
the heart causes the phenomena called
"heart block" or "AV block."
Heart block is classified according to the level
of impairment — first-degree heart block,
second-degree heart block or third-degree
(complete) heart block.
FIRST DEGREE HEART BLOCK
On an electrocardiogram (ECG), the PR interval is
defined as the time interval between the initial
deflection of the P wave to the start of the QRS
complex. Normally, this interval should be between
120 and 200 msec.
First-degree heart block, or first-degree
atrioventricular (AV) block, is defined as
prolongation of the PR interval on the ECG to more
than 200 msec. First-degree heart block is
considered "marked" when the PR exceeds 300
msec. While the conduction is slowed, there are no
The following are the most common causes
of first-degree atrioventricular (AV) block:
Intrinsic AV nodal disease
Enhanced vagal tone
Acute MI, particularly acute inferior wall
myocardial infarction (MI)
Electrolyte disturbances (eg, hypokalemia,
Drugs (especially those drugs that increase the
refractory time of the AV node, thereby slowing
Drugs that most commonly cause first-degree AV
block include the following:
Class Ia antiarrhythmics (eg, quinidine, procainamide,
Class Ic antiarrhythmics (eg, flecainide, encainide,
Class II antiarrhythmics (beta-blockers)
Class III antiarrhythmics (eg, amiodarone, sotalol,
Class IV antiarrhythmics (calcium channel blockers)
Digoxin or other cardiac glycosides
With first-degree atrioventricular (AV) block, every
atrial impulse is transmitted to the ventricles,
resulting in a regular ventricular rate. This type of AV
block can arise from delays in the conduction
system in the AV node itself, the His-Purkinje
system, or a combination of both. Overall,
dysfunction at the AV node is much more common
than dysfunction at the His-Purkinje system.
If the QRS complex is of normal width and
morphology on the ECG, then the conduction delay
is almost always at the level of the AV node.
If, however, the QRS demonstrates a bundle-branch
morphology, then the level of the conduction delay
is often localized to the His-Purkinje system.
It is usually an incidental finding on a routine
This is seen as a PR interval greater than
200 ms in length on the surface ECG.
Generally asymptomatic at rest
Markedly prolonged PR interval may reduce
exercise tolerance in some patients.
Syncope may result from transient high
degree AV block.
Past history of heart disease including
myocarditis or MI
Patients may be highly conditioned athletes
History of infections such as lyme disease
The intensity of the first heart sound is
decreased in patients with first degree AV
Patients may have a short, soft, blowing,
diastolic murmur heard at the cardiac apex
No specific therapy is indicated for isolated
first degree A|V block. Management usually
includes identifying and correcting electrolyte
imbalances and withholding any offending
medications. Any associated condition should
be treated appropriately