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:

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:

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:

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 HeartPoint:



|  Commentary  |  Food You Will Love  |  HeartPoint Gallery  |  In The News |  Health Tips  |  What's New
|  Information Center  |  Home  |

This site presents material for your information, education and entertainment. We can assume no liability for inaccuracies, errors, or omissions. Above all, material on this site should not take the place of the care you receive from a personal physician. It is simply designed to help in the understanding of the heart and heart disease, and not as a diagnostic or therapeutic aid. You should seek prompt medical care for any specific health issues. Please feel free to browse the site and download material for personal and non-commercial use. You may not however distribute, modify, transmit or reuse any of these materials for public or commercial use. You should assume that all contents of the site are copyrighted. ©COPY;1997 HeartPoint