Sinus bradycardia simply means that the sinus node, which normally controls the overall heart rate, is generating electrical signals that are slower than they normally should be. By definition, sinus bradycardia is present if the sinus node is producing less than 60 electrical signals per minute. Sinus bradycardia is not necessarily abnormal, though. Young, healthy people often have heart rates of less than 60 per minute, especially when resting or sleeping. Athletic people will usually have heart rates less than 60 because their hearts become very efficient at pumping (unlike people who are out of shape). Sinus bradycardia is often seen in the elderly. This arrhythmia can even be the result of medications taken for high blood pressure, angina, or tachycardia. If there are no symptoms due to a sinus bradycardia (particularly when it occurs for the above reasons), then nothing needs to be done about it. However, when sinus bradycardia occurs because of a physical deterioration of the sinus node, it can cause serious problems (see the next chapter on “Sick Sinus Syndrome”).
Sick Sinus Syndrome
Sick Sinus Syndrome (“SSS”) is the name given to the medical condition in which disease of the sinus node causes severe slowing of the heart beat associated with significant symptoms. Sometimes the sinus node stops firing temporarily, and long pauses in the heart rhythm may result. This condition causes symptoms of fatigue, lightheadedness, shortness of breath, and even fainting. It can occur at any age over 30, although it is rare in patients under the age of 50 years. There can be “intrinsic” disease of the sinus node from aging, ischemia, scarring, or physical damage. There can also be “extrinsic” sinus node problems due to medications, hormonal conditions (such as an underactive thyroid gland) or neurological imbalances. Sick Sinus Syndrome is usually treated with a Permanent Pacemaker, which is a small electronic device that is designed to keep the heart beating from beating too slowly. Using local anesthesia, the pacemaker is implanted under the skin in a simple operation. The pacemaker keeps the heart beating by supplying a small electrical signal to replace the one normally provided by the Sinus Node. A pacemaker can eliminate all the symptoms of the Sick Sinus Syndrome.
As its name implies, sinus tachycardia is a condition in which the sinus node fires too rapidly. It is usually due to some other condition, such as anemia, fever, an overactive thyroid gland, or as a side effect of certain medications. Also, the sinus node is very sensitive to physical and emotional cues (mainly through the effects of adrenaline), so sinus tachycardia can often be due to anxiety. Sinus tachycardia may cause symptoms of palpitations, lightheadedness, or fatigue. It may also cause no symptoms at all. Your physician will usually try to find some hidden cause for sinus tachycardia. Generally, treatment is aimed at the underlying cause, but occasionally no other cause is found. In these cases, a diagnosis of “Inappropriate Sinus Tachycardia” (a disorder of impulse generation) is made. In symptomatic patients, medications are available to slow the sinus node and relieve symptoms. If sinus tachycardia is due to another condition, the arrhythmia should resolve after that condition is treated.
Over the last 10-15 years, a condition known as “Postural Tachycardia Syndrome” or POTS has been recognized more and more. This is a disturbance in the part of the nervous system known as the Autonomic Nervous System, which controls heart rate and blood pressure. POTS primarily affects young women, but it can occur in men also. It causes symptoms of palpitations, chronic fatigue, lightheadedness, among others, but most of the symptoms only occur with standing up. There is a thorough discussion of POTS in the section on Dysautonomia on this website.
AV Block, or Heart Block, is a condition which causes bradycardia. It has nothing to do with “blockages” in the coronary arteries due to atherosclerosis. Instead, it is an electrical block, like a “loose connection,” in the wires of the heart. AV block occurs when the AV Node fails to properly conduct the impulses from the atria to the ventricles. The severity or “degree” of AV block varies from mild to life-threatening. First degree AV block simply means that there is an excessive delay in the conduction of the impulse from the atria to the ventricles, but all the impulses are still getting through. By definition, first degree AV block is present if the time it takes for the electrical signal to pass from the atria to the ventricles (the PR interval) is greater than 0.2 seconds. This is a benign condition that does not require treatment. It may be due to disease in the AV node or it may be caused by certain medications (particularly calcium-channel blockers or beta blockers). Second degree AV blockoccurs when some impulses that originate in the atrium fail to pass through the AV node, so the ventricle doesn’t receive the electrical “cue” to beat. This can cause an irregular heart beat and frequent pauses in the heart rhythm. Depending on the actual location of the block (i.e. within the AV node itself vs. below the AV node in the His bundle), this condition may be benign or quite serious. Sometimes AV block causes symptoms of fatigue and lightheadedness because of the relative bradycardia that results. In patients with symptoms from second degree AV block, a permanent pacemaker is indicated for the relief of symptoms. Sometimes (particularly when the location of the block is in the His bundle) a permanent pacemaker must be implanted even though there are no symptoms in order to prevent future problems. Third degree AV block, also called Complete Heart Block, is a condition in which none of the atrial impulses can get through the AV node because of severe disease in cardiac conduction system. This usually results in severe bradycardia, and occasionally the heart can actually stop beating. Complete heart block can be a medical emergency requiring the insertion of a temporary pacemaker wire. This temporary wire is inserted into a large vein in the neck or below the collarbone and is advanced into the right ventricle to keep the heart beating until a permanent pacemaker can be implanted.
In the diagram above, Third Degree AV block is present. The p-waves (atrial activity) “march through” at a faster rate than the QRS complexes (ventricular activity). The ventricles are beating slowly and regularly because of an “escape” rhythm that often arises from the lower part of the AV node (below the level of the block).
Atrial Premature Beats
Atrial Premature Beats, or APBs (some doctors call them APCs or PACs for premature atrial contractions) are premature or early beats that result from abnormal electrical signals arising from somewhere in the atrium. An APB disturbs the rhythm of the heart, and it is often perceived as a “flutter” in the chest. The sinus node usually gets “reset” after an APB, so the heart has to wait a second or so until the sinus node fires again. This often results in a pause in the heart rhythm. Since the heart is filling in between beats, a pause results in an increase in filling of the ventricles, and that extra blood that enters the heart is generally ejected with the next beat, so the heart beat following a pause is generally stronger than normal. This often causes symptoms of a “heavy beat” (like the heart is going to jump out of the chest) following the pause. Interestingly, there is a broad variation in people’s perception in irregularities of the heart rhythm. Some people can have thousands of premature beats all day long and never feel a thing, while other people are exquisitely sensitive to the inner workings of their body and feel every single premature beat. The people who do not notice arrhythmias may in fact be blessed, since otherwise benign arrhythmias like APBs can be a tremendous source of anxiety to those who feel every one. Everyone normally has a few APBs, but APBs usually occur more frequently in older people. As mentioned above, APBs are benign by their very nature. They can occur in the setting of heart disease (like coronary blockages, valvular heart disease) but they are not necessarily a sign of bad problems. Most often they are normal. Usually, no treatment is necessary. Caffeine, cigarettes, alcohol, and stress may increase APBs, so people who are plagued by symptoms of APBs should avoid these. If the palpitations are severe, medications can be used to decrease the frequency of the APBs. However, since ABPs are totally benign, most electrophysiologists prefer not to prescribe medications to treat this condition.
Supraventricular tachycardia, or “SVT,” refers to a rapid heart beat that originates from cardiac tissues above the ventricles of the heart (“supra” means “above”). The term SVT is a very generic one, as it refers to at least a dozen different kinds of arrhythmias (including sinus tachycardia as well as atrial fibrillation / atrial flutter). When doctors say “SVT” they most often are referring to a sub-classification of arrhythmias known as Paroxysmal Supraventricular Tachycardia or PSVT (the old term for these arrhythmias was Paroxysmal Atrial Tachycardia or “PAT”). Paroxysmal simply means that the tachycardia starts suddenly and stops suddenly (a paroxysm is another term for an “episode” or temporary disturbance in the heart rhythm). The paroxysm can last anywhere from seconds to days, is usually recurrent, and generally feels like a rapid regular racing of the heartbeat. PSVT includes a large number of complicated arrhythmias that may be due different electrical abnormalities in the heart. Most commonly, there is a “short circuit” somewhere in the electrical system of the heart. This causes the electrical signal to continuously travel around in a circle, forcing the heart to beat rapidly, a situation known as Reentry (a Disorder of Impulse Conduction remember?). One example of PSVT occurs when the signal travels around the atrium, and so that arrhythmia is referred to as Intra-atrial Reentry Tachycardia. Another PSVT is due to the signal traveling around the AV nodal tissues, and is called AV Nodal Reentry Tachycardia. Some patients have an extra electrical connection between the atria and the ventricles (in addition to the AV node). This abnormal connection is called an Accessory AV Pathway or “Bypass Tract.” Bypass tracts are frequent causes of one particular tachycardia known as AV Reentry Tachycardia. If the bypass tract also happens to cause the QRS complex to appear abnormal on the electrocardiogram, then a condition known as Wolff-Parkinson-White (WPW) Syndrome is present. Sometimes bypass tracts do not show themselves on the ECG. These hidden short circuits are referred to as “concealed accessory pathways.” PSVT can cause severe symptoms of palpitations, lightheadedness, shortness of breath, sweating, and even chest pain. Some people may even pass out at the onset of a rapid PSVT. When PSVT occurs, it usually makes a person want to stop what they are doing and lay down until it stops. When symptoms such as these occur frequently, medication can be effective in reducing the frequency and severity of PSVT. Usually, PSVT is not a life-threatening condition, although the WPW Syndrome has rarely been associated with sudden death in young people. A relatively new treatment known as Radiofrequency Catheter Ablation can be used to permanently eliminate PSVT in most cases. This procedure involves locating the abnormal electrical pathway in the heart and cauterizing (i.e. burning) it using a wire through a vein. It is done while the patient is awake but sedated and has a very high success rate with an acceptably low risk of complications. PSVT is one of the few cardiac disorders that can now be “cured.”
Rarely, PSVT can be caused by a Disorder of Impulse Generation in the atrium or the AV node. “Automatic Atrial Tachycardia” is the name for one of these arrhythmias. This can also cause a rapid heart beat, but not because of a short circuit. Rather, an area in the atrium is sending out rapid signals that are faster than the sinus node and so “take over” as the primary pacemaker in the heart. Often, the arrhythmia occurs in short bursts (known as “salvos”) but sustained tachycardia can also occur. Different medications are used to treat these arrhythmias, since those used to treat reentry PSVT are often ineffective. Catheter ablation can also be used to eliminate the abnormal area and cure the problem.
Atrial fibrillation (also known as A Fib or AF) is an example of a supraventricular tachycardia that is not considered to be “PSVT.” It is a condition in which the electrical activity of the atrium becomes very rapid and disorganized. Instead of the sinus node providing the normal electrical signals to the atrium, rapid circulating waves of abnormal electrical signals continuously stimulate the atrium. The atrial rate can exceed 400 beats per minute! Atrial fibrillation can occur intermittently, which is known as Paroxysmal Atrial Fibrillation (a paroxysm is a self-terminating episode), or it can be Persistent (meaning it doesn’t stop on its own). When atrial fibrillation has been present for more than 6 months and there’s little hope of restoring a normal rhythm, it is known as Permanent AF (previously known as “Chronic AF”). During atrial fibrillation, electrical signals from the atrium constantly bombard the AV node. The AV node usually passes a large number of these rapid signals to the ventricles, which often beat rapidly and irregularly. In fact, the overall rate of the ventricles varies tremendously, depending on the age of the patient, the health of the AV node, and whether medications to slow AV conduction (such as calcium-channel blockers or beta blockers) are present. Sometimes atrial fibrillation is caused by other conditions such as an overactive thyroid or excessive alcohol use. Sometimes it is associated with cardiac conditions such as coronary artery disease, high blood pressure, congestive heart failure, or valvular heart disease. Occasionally, it occurs in the absence of other cardiac conditions and is then called “lone” atrial fibrillation. Usually medicines are required to control the heart rate, and eventually restore a normal heart rhythm. Sometimes, the only way to stop the disorganized activity of the atrium is to get a large portion of all the atrial tissue to fire at the same time. This is done by sending an electrical shock across the chest in a procedure known as “electrical cardioversion.” The shock stops the rapid disorganized atrial activity and allows the normal sinus rhythm to resume. Electrical cardioversion is usually done with deep sedation so that the patient doesn’t remember anything. It is generally a safe procedure and is usually effective unless the atrial fibrillation has been present for months or years or unless the underlying heart disease is very severe.
Patients who have atrial fibrillation may be at increased risk of developing a blood clot within the nooks and crannies found in the atrial chambers. If such a blood clot breaks off and travels in the circulatory system, it can lead to a stroke. Strokes due to atrial fibrillation are very serious, and make up about 15% of all strokes. Most people who have atrial fibrillation must take some kind of blood thinner to decrease the risk of clots formation within the atria.
Atrial flutter is a condition in which the atrium beats 250 to 350 times per minute. This causes rapid, regular or irregular heart rhythms similar to atrial fibrillation. Atrial flutter is different from atrial fibrillation because the electrical activity of the atrium is not disorganized; it is just very fast. One can actually see rapid “flutter waves” on the ECG that resemble a sawtooth pattern. Atrial flutter occurs when an abnormal electrical signal gets stuck rapidly circulating around the atrial muscle tissue. Most cases of atrial flutter occur in patients with underlying heart disease, but “lone” atrial flutter can also occur. The treatment of atrial flutter is similar to atrial fibrillation with medications first to slow AV conduction and control the ventricular rate, followed by medications to convert the flutter back to a normal sinus rhythm and prevent its recurrence. Electrical cardioversion is usually required, however, to restore a normal rhythm, after which anti-arrhythmic drugs can be used to maintain a normal sinus rhythm. Atrial flutter is not as common as atrial fibrillation, but fortunately we have learned a tremendous amount about it. Most atrial flutter is curable with Catheter Ablation, a highly successful means of permanently eliminating the flutter circuit. Catheter ablation in involves passing a small wire up from the vein at the top of the leg. The flutter circuit is “mapped” to determine where it is located, and then the wire is used to destroy a small portion of the circuit to prevent recurrence. Comparative studies have shown that the success rate of catheter ablation is much better than medical therapy for atrial flutter, and ablation is now considered the best treatment for this arrhythmia.
Patients who have atrial flutter may be at increased risk of developing a blood clot in the atrial chambers, similar to patients with atrial fibrillation. This can lead to a stroke. Strokes due to atrial flutter and fibrillation are very serious, and most people who have atrial flutter must take some kind of blood thinner to decrease the risk of clots formation within the atria, unless the flutter can be permanently eliminated with catheter ablation.
Ventricular Premature Beats
Ventricular premature beats, or VPBs (also called VPCs or PVCs for premature ventricular contractions), are a common cause of palpitations. They are similar to atrial premature beats since they disrupt the heart rhythm and result in an irregular pulse. However, APBs arise from the atria, while VPBs arise from the ventricles. Since the abnormal heart beat arises from the bottom chambers of the heart it is accompanied by reversal of the normal timing of the heart chambers. In other words, instead of the atria contracting first and the ventricles following, the ventricles contract first. Often, the atrial contraction follows, but because the AV valves are in a closed position during ventricular contraction the blood stored in the atrium has no where to go except backwards into the veins. This can cause a fullness in the neck and chest (often described as “my heart is in my throat”). For you trivia fanatics, the jugular venous pulsation in the neck often shoots up briefly when this happens and the medical term for this is a “cannon A-wave” (because it shoots up like a cannon). The compensatory pause that occurs after a VPB may be longer than after an ABP and so the overfilling of the ventricles and the resultant “heavy” beat following the pause can be exaggerated. Ventricular premature beats occur normally in most people, and they usually increase with age. Sometimes VPBs do not cause symptoms (those patients should consider themselves lucky!). If they cause severe palpitations, medical therapy can be used to suppress most of them. However, VPBs are usually benign, and they don’t require therapy unless the symptoms are intolerable. Sometimes VPBs are associated with heart disease such as coronary artery blockages or heart muscle problems, so it is reasonable for a physician to send the patient with VPBs for further cardiac testing. Generally, if there is no other heart disease, VPBs should not be treated unless the symptoms are severe. If significant heart disease is present, VPBs can be a marker of increased risk of more severe rhythm problems such as ventricular tachycardia (see the next chapter), and further testing may be beneficial to see if that patient could get in trouble in the future.
Ventricular tachycardia (sometimes referred to as “VT” or “V-Tach”) is a potentially life-threatening rhythm disturbance. It occurs when rapid, abnormal electrical impulses arise from the ventricles. It can result in heart rates of up to 300 beats per minute! When VT occurs for only a few seconds and then stops on its own, it is called “non-sustained” VT. Non-sustained VT can cause palpitations, lightheadedness, and even fainting. “Sustained” VT is ventricular tachycardia that does not stop without medical intervention. It is most often caused by reentry within the ventricular heart muscle, and is almost always associated with underlying heart disease. Most commonly, a patient with VT has had a heart attack in the past and the tachycardia is arises from the edge of the scar tissue left behind from the heart attack. The continuous rapid heart beat that results prevents the ventricles from filling properly, and so effective pumping stops (even though the ventricles are contracting rapidly). This causes the cardiac output (and the blood pressure) to bottom out, which can in some cases cause collapse and death. Sustained VT is the most common cause of sudden cardiac death. Rarely, sustained VT will be slow enough for the heart to maintain an adequate cardiac output, and will only result in weakness, lightheadedness, and palpitations. Patients with VT are generally treated by paramedics or brought to an emergency room to be treated. If a person with VT remains awake, medications can be given intravenously to stop the arrhythmia. However, most often patients with sustained VT quickly go into Cardiac Arrest (a life-threatening condition of unconsciousness associated with no detectable pulse or blood pressure). In this case, electrical cardioversion (a shock across the chest) is necessary to stop the abnormal electrical signals and rapidly restore a normal rhythm to save the person’s life. Intensive medical treatment is usually necessary to prevent recurrent sustained VT. Often, medical therapy is not effective, and another approach is required. The surgical treatment of sustained ventricular tachycardia usually involves the insertion of an Implantable Cardioverter-Defibrillator or “ICD.” An ICD is an electrical device that can automatically shock the heart back into a normal rhythm if ventricular tachycardia occurs. This device has been in use since 1985 and has saved the lives of many people who would otherwise have been sent home to die. Recent advances in ICD technology have made it relatively safe to insert. In addition, sustained VT can now be stopped by the ICD with the use of “anti-tachycardia pacing,” a rapid, painless stimulation of the ventricles that avoids the need for an uncomfortable shock. Medical research has demonstrated that ICDs are more effective than medical therapy in nearly all cases of life-threatening ventricular arrhythmias. Invasive testing of the electrical system of the heart by a cardiac electrophysiologist (an Electrophysiologic Study) is usually performed in cases of sustained VT.
Rarely, VT can occur in the absence of underlying heart disease. This condition is often called “Idiopathic Ventricular Tachycardia.” It includes several unusual electrical disturbances that, in general, are less severe than the VT that occurs in the setting of underlying heart disease. Idiopathic VT can be exercise-induced, or it may occur anytime. This arrhythmia may require medical therapy to prevent recurrent symptoms, but usually is not life-threatening. Often, idiopathic ventricular tachycardia can be cured with catheter ablation.
Ventricular fibrillation (also known as “VF” or “V-Fib”) is a rapid, disorganized rhythm disorder of the ventricles. It causes immediate collapse of the cardiovascular system and is a frequent cause of sudden cardiac death. It almost always occurs in the presence of significant heart disease. If someone is lucky enough to be resuscitated from cardiac arrest due to ventricular fibrillation or ventricular tachycardia, he or she is considered to be at very high risk of recurrence. Ventricular fibrillation is treated the same way as ventricular tachycardia. A complete electrophysiologic evaluation is necessary, along with testing for coronary insufficiency. If ventricular fibrillation is caused by blockages in the coronary arteries, fixing those blockages with bypass surgery or with balloon angioplasty can decrease the risk of recurrence. However, ventricular tachycardia or ventricular fibrillation is usually caused by a short circuit in the heart and is usually not corrected by either of these procedures. Therefore, these arrhythmias need to be treated separate from the coronary blockages. An implantable cardioverter-defibrillator or ICD can rescue a patient from recurrent ventricular fibrillation and thus save his or her life. Medical studies have demonstrated improved survival in patients with VF who received and ICD compared with patients placed on medical therapy.