Diagnosis of Heart Rhythm problems

Slow Heart Rhythms – Bradycardia

A heartbeat below 60 beats per minute is called “bradycardia”. This is really just an arbitrary definition. For many people a heart beat of 50 or even 40 beats per minute can feel entirely normal for them. Many athletes have bradycardia because they are in such good physical condition.

Often times, however, a slow heartbeat causes symptoms of shortness of breath, poor energy and fatigue, or even lightheadedness and fainting. Let’s look at the individual causes for a slow heartbeat.

The first thing to remember is how the heartbeat is electrically started and propagated through the heart. A collection of cells called the sinus node in the right atrium start the electrical impulse (at a frequency of about 60 times a minute). If the bradycardia stems from the sinus node’s incapacity of initiating the electrical impulse appropriately, it is called “sick sinus syndrome”. Many people with sick sinus syndrome can also get atrial fibrillation, which is described in the Fast Heart Rhythm – Tachycardia section.

After being generated in the sinus node, the electrical impulse travels through the AV node, which forms an electrical “relay” between the upper chamber (atrium) and lower chamber (ventricle). Slowdown in the AV node can be another cause for bradycardia, where not all the sinus node discharges can make it through to the ventricle, a form of heart block. Sometimes slowdown in the AV node is due to medications, which may therefore need to be adjusted.

After the AV node, comes other electrical relays that are called the “His – Purkinje system”. The His-Purkinje system is a very important electrical relay that is the final pathway to the tell the ventricles to contract.  A slowdown or block in this part of the conduction system may be serious and require a pacemaker as some patients can develop “complete heart block”. In complete heart block, there are no signals from the sinus node that can penetrate to the ventricles and the patient can have a dangerously slow pulse. Adjusting medications cannot fix this type of complete heart block; a pacemaker is needed.

Fast Heart Rhythms – Tachycardia

Tachycardia refers to a situation where the pulse is faster than 100 beats per minute. Sometimes this is absolutely normal. For instance, with exercise it is entirely appropriate for the pulse to rise above 100 bpm. However, when tachycardia occurs in inappropriate situations we look to see what the specific cause is.

The first question that we ask is whether the tachycardia is originating from the upper chambers (“atria”) or the lower chambers (“ventricles”). Ventricular tachycardia refers to a fast heart beat that originates from the ventricles and can be potentially dangerous – we will talk more about this situation below.

If tachycardia originates from the upper chambers it will be one of three types:

1. Atrial fibrillation
2. Atrial flutter
3. Supraventricular tachycardia (SVT for short)

Let’s define these three types of tachycardia.

Atrial fibrillation is a chaotic electrical disturbance of the atria. An electrical impulse travels in a disorganized, very rapid, chaotic fashion throughout the atrium. This electrical impulse then travels to the AV node and the rest of the conduction system to the lower chambers (ventricles) in an irregular fashion resulting in a heartbeat that is perceived as irregular and often fast. It is the most common rhythm disturbance in older Americans, affecting 2.2 million Americans, with an average age of about 75 years. Some people are very aware of their atrial fibrillation – they notice that their heart is beating very irregularly, without any pattern whatsoever and fast. Other people are completely unaware of being in atrial fibrillation. Most people with atrial fibrillation are aware of this rhythm some of the time, but not at other times, even when they are certain that they can detect it “perfectly”.

It is most important to recognize when someone with atrial fibrillation is at significant risk for a stroke. The guidelines from the American Heart Association and the American College of Cardiology advise warfarin to thin the blood for those patients at risk for stroke. Not all patients with atrial fibrillation need warfarin, and some can be treated with aspirin. However, anyone who has had a stroke, or mitral stenosis (a tightening of the mitral valve) or has an artificial heart valve should be treated with warfarin if they have atrial fibrillation. Additionally, warfarin is recommended over aspirin if there are two or more of the following four risk factors: a) high blood pressure b) diabetes c) age over 75 years or d) heart failure. If there is only one of these risk factors then one can be treated with either aspirin or warfarin. If there are none of these risk factors, then only aspirin is recommended.

Treatment of atrial fibrillation including medications and ablation therapy will be covered in the Treatment section.

Atrial flutter is a heart rhythm disturbance that is similar to atrial fibrillation, except that the electrical discharges are semi-organized. An electrical impulse will typically revolve around the tricuspid valve. Patients with atrial flutter may feel the same symptoms as those with atrial fibrillation – a fast and irregular heart beat, though maybe not quite as irregular as with atrial fibrillation. Atrial flutter also carries a potential risk for stroke and all the same indications for warfarin as outlined by the American Heart Association and American College of Cardiology still apply.

Supraventricular tachycardia (SVT) is a fast heart rhythm disturbance that can be seen at any age but most commonly in younger adults. People will describe that their heart suddenly starts racing –from a normal 60 beats per minute to as many as 200 beats per minute. Shortly after, the episode stops just as abruptly. Many of these people have figured out on their own that they can stop their racing heart rhythm by holding their breath or bearing down.

SVT can be due to abnormal electrical “wiring” in the heart that stems from an abnormal separate electrical connection between the atrium and ventricle (bypass tract). When people have these extra pathways, an SVT can form by the electrical signal traveling through the AV node in the normal fashion and then return through the abnormal connection or bypass tract. This type of SVT is given the name “AV reentrant tachycardia”. Some people with bypass tracts have an abnormal EKG even when their heart beat is normal, with a slur in each beat that is called a delta wave. This condition is also called “Wolff-Parkinson-White” syndrome, after the cardiologists who discovered it.

A second form of SVT that is common in adults is due to extra pathways within the AV node itself. An SVT can form where the electrical circuit goes down one of these pathways and then up another, all within the AV node. This is called AV nodal reentrant tachycardia.

Finally, the third form of SVT is not due to abnormal connections or pathways but to  a spot in the upper chamber (atrium) that fires independently, all by itself. This is called atrial tachycardia.

If the tachycardia originates in the ventricles, it is called Ventricular Tachycardia. We’ve saved this tachycardia for last in this section, but it is the most serious and potentially most dangerous tachycardia. Cardiac arrest is often the result of these types of serious rhythm disturbances, so making an accurate diagnosis is very important. In particular, ventricular tachycardia is related to a heart rhythm called ventricular fibrillation, which is fatal.

Ventricular tachycardia usually occurs in people who have had heart attacks and have developed scar tissue in their heart. An electrical circuit can then form around the scar, resulting in tachycardia with heartbeats anywhere from 120 beats per minute to over 200 beats per minute, causing collapse or cardiac arrest. In ventricular fibrillation, the electrical impulse becomes completely disorganized and chaotic, and the ventricles just quiver – they can no longer beat and a cardiac arrest occurs. Many patients with a risk for ventricular tachycardia or ventricular fibrillation also have heart failure, and may benefit from a device called an implantable cardiac defibrillator (ICD).

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