CARDIAC ARRHYTHMIAS
	The orderly repolarization and repolarization of the conduction system that directs the contraction and relaxation of the atria and ventricles does not always function perfectly, which leads to disturbances in the heart rate or rhythm which are called cardiac arrhythmias.

Diagnosis and Management of Atrial Fibrillation 
	
	The rapid irregular heart rate shortens the time available for ventricular filling in diastole and the lack of normal atrial contractions further compromises ventricular filling. The cardiac output falls, which may not be well tolerated by a person with pre-existing cardiovascular disease. Usually a normal rhythm can be restored either by an electrical cardioversion, which involves the application of an electrical shock by paddles applied to the patient’s chest to terminate the arrhythmia, or by pharmacologic therapy which uses drugs that terminate the arrhythmia by interfering with some phase in the abnormal impulse formation to “break up” the arrhythmia.  Once the normal rhythm is restored the patient may need to continue taking some medications to maintain the normal rhythm.   Management of atrial fibrillation involves several steps:
 1. Slow the Heart Rate: 
	The first treatment step is to improve the cardiac output by slowing the heart rate, which provides more time for passive ventricular filling during diastole. This can be accomplished by giving a medication that impedes the transmission of impulses through the AV node and bundle so that fewer impulses reach the ventricles. A digitalis preparation can be used (digoxin) but other pharmacologic agents are also effective. 
2. Restore Normal Heart Rhythm 
	When the heart rate has been slowed, the next step is to restore a normal heart rhythm by terminating the fibrillation. This can be accomplished either by an electrical cardioversion, which involves the application of an electrical shock  by paddles applied to the patient’s chest to terminate the arrhythmia, or by pharmacologic therapy which uses drugs that terminate the arrhythmia by interfering with some phase in the abnormal impulse formation. Once the normal rhythm is restored the patient may need to continue taking some medication to maintain the normal rhythm. 
3. Allow Fibrillation to Continue If Normal Rhythm Can’t Maintained 
	Unfortunately, it is not always possible to maintain a normal rhythm after a successful cardiioversion despite use of various drugs, and another cardioversion may be needed each time the fibrillation recurs despite taking medications to help sustain the normal rhythm. Other more radical methods of treatment are available and can be considered. However, in many instances the patient may prefer to settle for maintaining a normal heart rate by taking a drug such as digitalis or a similar pharmacologic agent and not continuing with further efforts to maintain a normal heart rhythm. This is an effective option, and the patient may be quite comfortable despite the continuing fibrillation.

4. Provide Anticoagulant Therapy to Minimize Risk of Atrial Thrombi
 	Because the atria do not contract normally, blood tends to pool in the atria instead of being ejected by systolic contractions, which predisposes to formation of blood clots (thrombi) in the atrial appendages (auricles) of the fibrillating atria. Because a thrombus dislodged from the left atrium can travel in the circulation as an embolus to block important arteries in the brain or other organs, patients with chronic atrial fibrillation need to take some medication to reduce the risk of developing atrial thrombi. Often this is a coumadin anticoagulant (“blood thinner”) that acts by reducing the concentration of blood coagulation factors, or a daily low-dose aspirin tablet (“baby aspirin”) that blocks the function of platelet so that they don’t clump together and form a blood clot. Each method has advantages and disadvantages.

 The following two cases of patients with atrial fibrillation illustrate the two different clinical presentations and responses to treatment. 

Case 13-1. Acute episode of atrial fibrillation
A healthy 67 year old man with mild asymptomatic coronary artery disease was reading the newspaper at home after dinner when he suddenly became aware that his heart was beating very rapidly. He checked his pulse and discovered that his rate was 140. He called his physician who advised him come to the emergency room at his local hospital for further evaluation and treatment. 
In the emergency room he did not appear to be in any distress despite the rapid heart rate. Blood pressure was 136/72.The electrocardiogram revealed atrial fibrillation with a rapid ventricular rate (Figure 13-7A). Blood tests to detect evidence of heart muscle injury resulting from the rapid heart rate were all normal. The various treatment options were discussed with the patient, who preferred to attempt termination of the fibrillation by pharmacologic therapy rather than by electrical cardioversion. An intravenous infusion of a medication designed to disrupt the fibrillation (procainamide intravenously at the rate of 20 mg per minute) was administered while the ECG was continually monitoring his heart rate and rhythm. After he had received at total dose of 575 mg the fibrillation abruptly ceased and the ECG revealed a completely normal tracing (Figure 13-7B). He has been followed periodically by his own physician over a 10 year period and never had another episode of atrial fibrillation 

Figure 13-7. A. Atrial fibrillation with ventricular rate of 140. Absence of P waves indicates atrial fibrillation .B. Post cardioversion.  Normal P waves (arrows) indicate normal atrial contractions. Cardiac rate and rhythm are normal.

Case 13-2.Chronic atrial fibrillation	

	A 43 year old woman experienced an initial episode of atrial fibrillation with a rapid ventricular rate and was converted pharmacologically to a normal rhythm which persisted for several weeks. Then she had another episode of atrial fibrillation which was again converted to a normal rhythm by medications. These fibrillation-conversion episodes continued many more times despite taking medications to help sustain the normal rhythm. Each conversion became harder to achieve, lasted for a shorter time, and the medications taken to maintain a normal rhythm were associated with uncomfortable side effects.  Eventually she declined further attempts to maintain a normal cardiac rhythm and decided instead to be satisfied with controlling her heart rate.  Under her physician’s supervision she takes a digitalis preparation that maintains her ventricular rate at between 60 to 70 beats per minute, and she is able to carry out her normal activities without difficulty even though the atrial fibrillation continues. She also has chosen to reduce the risk of forming thrombi in the fibrillating atria by taking a daily low-dose aspirin tablet (a “baby aspirin”) instead of a coumadin anticoagulant.

Ventricular Fibrillation 
In contrast to atrial fibrillation which disturbs normal cardiac function but can be tolerated, ventricular fibrillation is incompatible with life because the ventricles are unable to contract normally and the circulation ceases. Ventricular fibrillation sometimes occurs following a heart attack.  If recognized promptly, it is often possible to stop the fibrillation by delivering an electric shock to the heart by means of electrodes applied to the chest.  The procedure usually causes the ventricles to resume normal contractions.

Heart Block 
Heart block is a delay or complete interruption of impulse transmission from the atria to the ventricles.  Usually the condition results from arteriosclerosis of the coronary arteries in which parts of the conduction system fail to receive an adequate blood supply.  In the mildest form, heart block is manifested only as a delay in the conduction of impulses from the atria to the ventricles, as indicated by a prolonged PR interval in the electrocardiogram. In a more marked degree of block (incomplete heart block), not all the SA node impulses are conducted through the damaged AV bundle.  Every second, third, or fourth impulse may fail to reach the ventricles.  In the most severe form (complete heart block) conduction of impulses through the AV bundle is completely interrupted.  Impulses originating in the SA node cause the atria to beat normally, but the impulses are not transmitted into the ventricles.  When this occurs, the conduction system distal to the block “takes over” as the source of impulses to activate the ventricles, but the impulses are generated at a much slower rate of 30 to 40 per minute. Consequently, in complete heart block the atria contract at the usual rate of about 70 beats per minute and the ventricles contract independently at about half this rate. The extremely slow ventricular rate may not provide sufficient blood flow to the brain, resulting in periodic episodes of loss of consciousness.

Complete heart block is often treated by implanting an artificial pacemaker, which consists of a small electrode that is inserted into the heart through a vein, such as the subclavian vein, and positioned in the ventricle.  The electrode is connected to a small device implanted beneath the skin of the chest which contains a battery that can be programmed to generate impulses that stimulate ventricular contractions at a predetermined rate.  Many different types of pacemakers are available, each type having specific applications.