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Cardiovascular Physiology Concepts

Richard E. Klabunde, PhD

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Click here for information on Cardiovascular Physiology Concepts, 2nd edition, a textbook published by Lippincott Williams & Wilkins (2011)


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Click here for information on Normal and Abnormal Blood Pressure, a textbook published by Richard E. Klabunde (2013)



Arrhythmias

What is an arrhythmia?

The rhythm of the heart is normally generated and regulated by pacemaker cells within the sinoatrial (SA) node, which is located within the wall of the right atrium.  SA nodal pacemaker activity normally governs the rhythm of the atria and ventricles.  Normal rhythm is very regular, with minimal cyclical fluctuation.  Furthermore, atrial contraction is always followed by ventricular contraction in the normal heart. When this rhythm becomes irregular, too fast (tachycardia) or too slow (bradycardia), or the frequency of the atrial and ventricular beats are different, this is called an arrhythmia.  The term "dysrhythmia" is sometimes used and has a similar meaning.

How common are arrhythmias?

About 14 million people in the USA have arrhythmias (5% of the population).  The most common disorders are atrial fibrillation and flutter.  The incidence is highly related to age and the presence of underlying heart disease; the incidence approaches 30% following open heart surgery.

What are the clinical symptoms?

Patients may describe an arrhythmia as a palpitation or fluttering sensation in the chest.  For some types of arrhythmias, a skipped beat might be sensed because the subsequent beat produces a more forceful contraction and a thumping sensation in the chest.  A "racing" heart is another description.  Proper diagnosis of arrhythmias requires an electrocardiogram, which is used to evaluate the electrical activity of the heart.

Depending on the severity of the arrhythmia, patients may experience dyspnea (shortness of breath), syncope (fainting), fatigue, heart failure symptoms, chest pain or cardiac arrest.

What causes arrhythmias?

A frequent cause of arrhythmia is coronary artery disease because this condition results in myocardial ischemia or infarction.  When cardiac cells lack oxygen, they become depolarized, which leads to altered impulse formation and/or altered impulse conduction. The former concerns changes in rhythm that are caused by changes in the automaticity (spontaneous activity) of pacemaker cells or by abnormal generation of action potentials at sites other than the SA node (termed ectopic foci).  Altered impulse conduction is usually associated with complete or partial block of electrical conduction within the heart.  Altered impulse conduction commonly results in reentry, which can lead to tachyarrhythmias.  Changes in cardiac structure that accompany heart failure (e.g., dilated or hypertrophied cardiac chambers), can also precipitate arrhythmias.  Finally, many different types of drugs (including antiarrhythmic drugs) as well as electrolyte disturbances (primarily K+ and Ca++) can precipitate arrhythmias.

What are the consequences of arrhythmias?

Arrhythmias can be either benign or more serious in nature depending on the hemodynamic consequence of the arrhythmia and the possibility of evolving into a lethal arrhythmia.  Occasional premature ventricular complexes (PVCs), while annoying to a patient, are generally considered benign because they have little hemodynamic effect.  Consequently, PVCs if not too frequent, are generally not treated.  In contrast, ventricular tachycardia is a serious condition that can lead to heart failure, or evolve into ventricular fibrillation and cause death. 

How are arrhythmias treated?

When arrhythmias require treatment, they are are treated with drugs that suppress the arrhythmia. These drugs are called antiarrhythmic drugs. There are many different types of antiarrhythmic drugs and many different mechanisms of action. Most of the drugs affect ion channels that are involved in the movement of sodium, calcium and potassium ions in and out of the cell. These drugs include mechanistic classes such as sodium-channel blockers, calcium-channel blockers and potassium-channel blockers. By altering the movement of these important ions, the electrical activity of the cardiac cells (both pacemaker and non-pacemaker cells) is altered, hopefully in a manner that suppresses arrhythmias. Other drugs affect autonomic influences on the the heart, which may be stimulating or aggravating arrhythmias. Among these drugs are beta-blockers. More details on drug therapy and specific drugs can be obtained by clicking here.

VIDEO LINK: An excellent animated video produced by Medical Media (Netherlands) that describes arrhythmias in lay terms can be found on YouTube at http://www.youtube.com/watch?v=nq7B08suZJw&feature=youtu.be.

Revised 10/11/12

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.