Heart Block, Sinus Bradycardia: A Slow Heart Rate

Bradycardia describes the condition where the heart is beating at a rate lower than the normal resting heart rate of 60 to 100 beats per minute. While mild bradycardia is not cause for much concern, very low heart rates may result in insufficient levels of oxygen being delivered to vital organs, resulting in a various health complications.

 

Bradycardia can affect anyone, from young children to the elderly. Bradycardia generally affects the elderly, though.

Term You Should Know: Sinus Bradycardia Bradycardia describes a lower-than-normal heart rate. Sinus bradycardia is a more specific term that describes a lower heart rate as a result of the sinoatrial node (otherwise known as the sinus node).

Bradycardia may be caused by a number of things. Heart attacks, hypothermia (low body temperature), and hypothyroidism (underactive thyroid) may all cause bradycardia. Use of certain medications, such as beta blockers and calcium channel blockers, may also result in bradycardia.

 

A common cause of sinus bradycardia is a malfunction in the sinoatrial node (SA node). The electrical activity in the sinoatrial node may not be working properly, resulting in a lower heartbeat. Sick sinus syndrome is one of the culprits of a malfunctioning sinoatrial node.

 

Sometimes bradycardia occurs, but the sinoatrial node is working properly. The atria contract at a regular rate, but the ventricles do not. This type of bradycardia is caused by problems in the electrical pathway of the heart—called heart block. The electrical signal originating from the sinoatrial node must travel through a specific path to get from the atria to the ventricles. In order to get from the atria to the ventricles, the signal must go through the atrioventricular node (AV node), but if the AV node is not functioning properly, the ventricles may receive an impaired signal, or no signal at all. This will result in a lower heart rate.

 

Heart block describes a malfunctioning AV node that results in a delayed electrical signal delivered to the ventricles, or no signal at all. Heart block is classified according to the level of impairment—1st degree, 2nd degree and 3rd degree. Each is treated differently.

 

Or course bradycardia may not be caused by a medical condition at all. People who are sleeping have lower heart rates, and extremely fit athletes may have a resting heart rate of less then 60 beats per minute as well. For example, ex-professional tennis player Björn Borg had a resting heart rate of 35 beats per minute during his athletic peak.

People with mild bradycardia generally display no symptoms, but those with heart rates of less than fifty beats per minutes have very little energy and complain about:

Since the elderly are most likely to be affected by bradycardia, they generally attribute these symptoms to "getting older".

Your doctor will most likely ask you about symptoms you've been experiencing before using an electrocardiogram (EKG) test. With an EKG, your doctor will be able to determine whether you are suffering from bradycardia.

Most people with bradycardia do not need treatment. Depending on the cause (i.e. medications), dealing with the cause (i.e. stopping use of medication) will treat the resulting bradycardia.

 

In some instances, however, a pacemaker is necessary to treat bradycardic conditions—heart block and sick sinus syndrome. A pacemaker is a battery-operated device that is surgically implanted near the heart. The pacemaker produces an electric current that restores the heart's normal rhythm in the event of electrical problems.

 

Evolution of the Pacemaker: The pacemaker was developed in the 1960s. The first pacemakers were large and cumbersome. Over time, however, new technology led to a lighter, smaller device. Today, a pacemaker weighs less than an ounce and is surgically implanted beneath the skin over the heart.

 

The pacemaker battery is long lasting, but will run down eventually. When this occurs, the pacemaker must be replaced surgically. The pacemaker battery is checked at routine intervals, usually during regular medical appointments.

Depending on patient need, the pacemaker may run constantly or only when it detects electrical problems. A pacemaker that can detect electrical problems is called a demand pacemaker, and includes sensors that detect changes in the heart's electrical current.

Not all cases of heart block require a pacemaker—1st degree heart block rarely does. Depending on the severity of the block, 2nd degree heart block may require a pacemaker. Third degree heart block, on the other hand, is considered a medical emergency and a pacemaker is implanted immediately.

Because the sinus node acts as the heart's natural pacemaker, when it malfunctions the heart beats erratically, sometimes too slow (bradycardia) and sometimes too fast (tachycardia). Because sick sinus syndrome occurs mostly in the elderly and can cause dangerously low blood pressure, treatment usually entails implantation of a pacemaker.

 

Resources

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Beers, M.H. & Berkow, R. (ed). Arrhythmias: Anatomy. The Merck Manual of Diagnosis and Therapy, 17th Edition. Merck Research Laboratories, NJ, 1999.

Beers, M.H. & Jones, T. (ed). Sinus bradycardia and sick sinus syndrome. The Merck Manual of Health and Aging. Merck Research Laboratories, NJ, 2004.

Fauci, A., Braunwald, E., Isselbacher, K., Wilson, J., Martin, J., Kasper, D., Hauser, S. & Longo, D. (ed). Harrison's Principles of Internal Medicine, 14th Edition. McGraw-Hill, New York, 1998.

Guidant Corporation. (updated 2005). Bradycardia.

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Mayo Foundation for Medical Education and Research. (updated 2005). Treatment of bradycardia at Mayo Clinic.

MedicineNet, Inc. (updated 2002). Sinus bradycardia.

National Library of Medicine. (updated 2003). Arrhythmias. MedlinePlus Medical Encyclopedia.

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National Library of Medicine. (updated 2004). Pacemaker. MedlinePlus Medical Encyclopedia.