SPECIFIC ARRHYTHMIAS
Basic concepts -- Normal heart
rates
Sinus bradycardia
Sinus Tachycardia
Premature atrial contractions
("PAC's")
Premature ventricular contractions
("PVC's")
Paroxysmal supraventricular
tachycardia ("PSVT")
Wolff-Parkinson-White Syndrome
("WPW")
Atrial fibrillation
Atrial flutter
"Sick sinus syndrome" and
"tachy-brady" syndrome
Slow heart rhythms
Bundle Branch Blocks
Basic concepts -- Normal heart rates
A "normal" heart rate is defined as anywhere from 60
to 100 beats per minute at rest. These numbers are pretty arbitrary, and many people have
slower or faster rates than this and are still "normal". The range of normal,
counting sleep and daytime activities, is around 45-95.
Slow heart rates are called "bradyarrhythmias", and fast heart rates are known as "tachycardias".
The heart rate varies widely throughout the day depending on the activity one is engaged in. There are other factors that are less well defined, such as "circadian rhythms" that change the overall rate of the body's, as well as the heart's metabolism. Appropriate variations in heart rate are actually a sign of a healthy heart.
A slow rate is particularly common among active people such as athletes. Many people are "just born with" a slow heart rate. There is not necessarily anything wrong with having a rate less than 60 beats per minute. In fact, most people's heart rate goes lower than 60, and may even go as low as 30-40 beats per minute, when they sleep. Physical activities or emotional excitement, typically cause rates greater than 100.
Sinus bradycardia
In this condition, the sinus node sets the pace for the rest of
the heart just like it usually does, but it does it more slowly than usual. As noted
above, this can be, and is usually, entirely normal even when quite exaggerated.
However, this slowing of the heart rate can also be abnormal. If the rate is too slow for a given person, then symptoms such as lightheadness or passing out ("syncope") may occur. Patients may have symptoms of congestive heart failure or may experience chest pains.
Some of the conditions associated with abnormal sinus bradycardia include the "Sick Sinus Syndrome" (which is also discussed more fully below), a condition in which different parts of the conduction system appear to simply "wear out". Slowing of the heart rate can be caused from conditions outside of the heart such as a low thyroid. Marked slowing of the heart rate during sleep is one of the symptoms of obstructive Sleep Apnea (along with loud snoring, periods of not breathing, and other findings).
Slow heart rates can be caused or worsened by medications, including:
- "Beta Blockers" of which there are many (including inderal, atenolol, lopressor and others). These have an expected effect to slow the heart to some degree, but some patients are very sensitive (or the medication may be given or taken in excessive doses).
- "Calcium Blockers" such as diltiazem (Cardizem, Dilacor, or Tiazac) or verapamil (Calan, Isoptin, Verelan) also slow the heart rate as part of their therapeutic effect. Just as in the case of the beta-blockers, these effects may be exaggerated in some patients and with some doses.
- Digoxin (Lanoxin).
- Cimetidine (Tagamet).
- Alpha-methyl dopa (Aldomet).
- Clonidine (Catapres).
- Amiodarone and other antiarrhythmic drugs.
The treatment involves correction of any underlying conditions which
may exist (such as supplementing the thyroid) and reducing or discontinuing any drugs
which may be worsening the problem. If persistent bradycardia and symptoms are present, a
pacemaker may be indicated.
Sinus Tachycardia
This is the condition where the sinus node starts the heart
going at a more rapid rate (by definition, over 100 beats per minute). It is a normal
response to increased demands on the heart, such as exercise or other stress. It can also
occur in other situations, such as:
- It may accompany infections and fevers.
- It may also be associated with conditions such as high thyroid or dehydration.
- "Anemia" (decreased amounts of oxygen-carrying cells in the blood) may be the culprit, causing the heart to beat harder and more rapidly to provide enough oxygen to the body.
- Patients with heart failure or other heart conditions may have an increased rate to make up for a lower amount of blood ejected with each beat.
- Poor physical conditioning can lead to increased resting heart rates, an particularly to a rapid increase in the heart rate when engaging in physical activities.
- Stimulants: caffeine which is found in coffee, tea (hot or iced), many carbonated beverages, and over-the-counter medications (some pain relievers, agents sold to prevent drowsiness). Chocolate contains significant amounts of stimulants.
- Nicotine and other stimulants and toxins in cigarettes, cigars, snuff and other tobacco products.
- Over the counter medications, particularly those containing "decongestants".
- Some prescription medications.
- Some people however, appear to have an imbalance in the autonomic (or "automatic") nervous system which leads to persistent rapid heart rates which are sometimes uncomfortable and difficult to treat. This has a long name (chronic non-paroxysmal sinus tachycardia), but is not ominous as all of those syllables would suggest.
Premature atrial contractions (PAC's)
PAC's are beats which originate in parts of the atrium other
than the sinus node. These impulses occur prematurely, before the sinus node depolarizes.
They are conducted through the atrium and slow down, just like a normal sinus beat, when
they reach the A-V node. They are conducted through the ventricle in the same fashion as a
normal sinus beat.
These are very common, and can be completely unknown to the person, or
perceived as a "skip" or a "pause". They may be caused by many of the
same conditions as noted for sinus tachycardia above, and are not necessarily an
indication of any disease at all. Follow the general measures for palpitations [link].
Treatment with beta-blockers, calcium-channel blockers or other agents can be useful.
Premature ventricular
contractions (PVC's)
These are also extremely common. These arise in the lower
chambers of the heart, and are associated with many of the same causes as PAC's and sinus
tachycardia. They are sometimes viewed with more concern, since multiple consecutive PVC's
(ventricular tachycardia) can be associated with serious heart problems and sudden death.
However, the vast majority are benign, and do not require treatment. Further testing may
be warranted.
Stay tuned to HeartPoint for more on ventricular arrhythmias in the
near future.
Paroxysmal supraventricular tachycardia
("PSVT")
In this situation, rapid heart beats occur which arise in
tissues above the ventricles. There are actually several specific mechanisms which cause
this disorder, most of which are associated with "short circuits" within the
heart muscle. A relatively small amount of tissue is involved which allows electricity to
go around-and-around quickly (120-250 beats per minute) and spread to the rest of the
heart muscle.
This may begin at any age. It can occur in otherwise normal hearts.
These rhythms are generally responsive to a variety of medications such as calcium channel blockers, beta blockers, digoxin, or others. Often "perfect control" is not possible, but substantial improvement can be obtained.
In cases of these arrhythmias which are unresponsive to medications, or
in the case of very fast rates, further testing with electrophysiologic studies may be
indicated.
Wolff-Parkinson-White Syndrome
("WPW")
This disorder, which is named after three prominent early
investigators, involves a "bypass tract". This bypass is not the sort associated
with blocked arteries -- rather, it is a band of tissue that penetrates directly from the
atrium to the ventricle (remember, this area that is usually electrically completely
"blocked" by fibrous tissue). Due to the presence of this bypass tract, the
electrical activity can pass directly from the atrium to the ventricle without pausing at
the AV node. Of even greater importance is the fact that a "re-entrant circuit"
is set up, where electrical activity can pass around the loop involving the atrium, the
bypass tract, the ventricle, and the AV node in such a way that very rapid tachycardias
can result.
The arrhythmias from WPW may resemble more ordinary arrhythmias, but often require different medications. Great advances have been made in the field of electrophysiology, the study of cardiac electricity, particularly in the diagnosis and treatment of WPW. The location of the bypass tract can be identified, and in many cases can be "ablated" (destroyed) using radiofrequency delivered via a relatively catheter-based technique.
Atrial fibrillation
Atrial fibrillation is a very common arrhythmia. In this
arrhythmia, the upper chambers of the heart fail to maintain a regular pattern of
depolarization. The electrical impulse moves randomly throughout the tissue, and the
mechanical contraction of the upper chambers is lost.
This arrhythmia may exist, and the patient may feel absolutely fine.
Some feel only palpitations. Others may be highly symptomatic and suffer chest pains,
lightheadedness or congestive heart failure in association with the arrhythmia. One of the
most important aspects of atrial fibrillation is that it is associated with an increased
risk of suffering a stroke, from the buildup of clot in the non-contracting atrium. We
will devote an entire section to this interesting and difficult to treat arrhythmia in the
very near future.
Atrial flutter
This arrhythmia is closely related to atrial fibrillation. In
this arrhythmia, the atria beat regularly, but at an extremely high rate, generally around
300 beats per minute. While the atria can tolerate this reasonably well, the lower
chambers (ventricle) cannot. Fortunately, the AV node is present to slow down and diminish
the number of impulses which pass through to the ventricle. There is the fortunate
production of a block at the level of the AV node, generally allowing only one of every
two or three or four of the flutter wave to be passed down to the ventricle. Other and
less regular ratios of blocked beats may occur.
Otherwise very similar to atrial fibrillation, atrial flutter by itself
is not felt to be associated with clots since there is some mechanical efficiency to keep
the blood flowing in the atrium. It is treated in a similar manner, but since clots are
not a feature, complete control of the arrhythmia is generally not as important unless the
patient is highly symptomatic. Atrial flutter can be treated by interrupting some of the
pathways using catheters that deliver radio frequency energy ("Radiofrequency
ablation"), thus minimizing or doing away with the needs for medications.
"Sick sinus syndrome" and
"tachy-brady" syndrome
A "syndrome" in medicine is a collection of
characteristics that define common findings in a group of patients. It is not a
"disease" in itself, but may reflect other disease entities. Patients with
"Sick Sinus Syndrome" (SSS) share having a variety of arrhythmias, primarily
involving the upper chambers of the heart and AV node. They may have fast or slow rhythms
involving these structures, and if they have both rapid and slow varieties, patients are
often said to have "tachy-brady syndrome". Patients with SSS may also have
atrial fibrillation, atrial flutter and other arrhythmias as well.
The initial descriptions many years ago focused on the fact that the
sinus node did not seem to be doing it's job, but as study has continued, it has become
clear that this is a disease that may involve much of the conduction system of the heart.
Many people have what appears to be problems almost entirely with fast (or
"tachy") rhythms. However, when these arrhythmias are treated with agents to
slow the heart down, it becomes apparent that they are very sensitive to these agents, and
may have dramatic slowing of the heart rate, much more than one would generally expect in
other patients. If a satisfactory balance of medications cannot be found, and this is not
uncommon, a pacemaker often needs to be placed to prevent the heart from going too slow
when given medications to prevent it from going too fast.
Other slow heart rhythms
Most of the rhythms noted above are rhythms that go too fast.
What about rhythms that are too slow? These are rhythms that occur generally for one or
both of the following reasons:
- The usual pacemaker of the heart in the sinus node will not "get with it" and beat fast enough.
- Despite the initiation of a normal electrical impulse, there is a block in its travel through the heart muscle.
These abnormalities are common, and may be caused or worsened by
certain medications, including "over-the-counter" drugs. Treatment is removal of
any offending agents or correction of any medical conditions which may be making this
worse. If this is not successful, a pacemaker may be necessary and be quite beneficial.
Bundle Branch Blocks
These really aren't arrhythmias, but are alterations in the way
the lower chambers of the heart are activated. The bundle branches (there is one on the
left and one on the right) bring the electrical impulses down from the AV Node to the
ventricles, and are illustrated and labelled on the graphic at the beginning of this
section. Bundle Branch Blocks simply result in the electrical activity spreading from cell
to cell. By itself, this method of electrical activity is not of any particular
consequence.
Another variation of this are the "hemiblocks" or "fascicular blocks". These refer to the two halves of the left bundle. There is an "anterior fascicle" and a "posterior fascicle". These may be considered the same as a left or right bundle branch block when considering their cause and basic approach to management.
Bundle Branch Blocks may be present from birth, and people can simply live a long and normal lifespan with the condition. Perhaps it is only found on an electrocardiogram done for some other or routine purpose.
It may also be associated with other conditions, and if it suddenly "appears", your doctor may very well wish to do further testing to make sure it is not associated with any other problems.
By itself, a bundle branch block doesn't imply that a pacemaker will be
needed, unless there are other associated medical or rhythm situations which will
influence such a decision.
©COPY 1997 HeartPoint
Updated October 1997
For more about Arrhythmias, also see these related subjects on
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