The sinus node is the pacemaker of the heart. It sits high in the atria and sends out a regular signal for the heart to beat. This signal is controlled by neurological and hormonal triggers that make the heart speed up and slow down as needed. There is a delay switch between the atria and ventricle that is called the AV node. Arrhythmias occur when the heart is either beating too fast (tachycardia) or too slow (bradycardia). The most common arrhythmias consist of isolated and premature atrial contractions (PACs), and extra beats, called premature ventricular contractions (PVCs). These are normal. Some people are quite sensitive and can feel the heart skip or flip in their chest, followed by a brief pause before the heart returns to its normal rhythm. Most people notice this when sitting quietly or lying in bed. Triggers include smoking, alcohol, coffee, tea, chocolate, or other stimulants. These are not life threatening and do not require treatment.
PVCs are also associated with CHF. While frequent PVCs may be a sign of increased risk in patients with ischemic heart disease and congestive heart failure, treating these with antiarrhythmic medication has been proven to increase mortality. Treatment is therefore reserved only for symptomatic and sustained ventricular tachycardia (VT), which causes symptoms of weakness, lightheadedness, or syncope. Sustained VT can cause sudden death and requires defibrillation with electrical paddles. This is commonly depicted in television and movies, with survival rates on television of approximately 75 percent. In reality, survival rates are generally less than 20 percent.
Unfortunately, medications that have been used to treat ventricular tachycardia have had modest success at best. Newer automatic implantable cardiac defibrillators (AICD) are special pacemakers, and they can be programmed to give a shock to restore normal rhythm when VT is detected. These devices are very expensive, but very effective.
Other common arrhythmias in the upper chamber of the heart include supraventricular (SVT) or atrial tachycardias. If prolonged, these can cause palpitations, shortness of breath, fatigue, weak spells, and even syncope.
Atrial fibrillation is an irregular SVT that can occur intermittently or continuously. Increasing age is a major risk factor for atrial fibrillation, which occurs in 5 percent of people over sixty-five and as many as 10 percent of people over the age of eighty. Patients who have infrequent atrial fibrillation lasting only a few minutes may not require any antiarrhythmic medication. If atrial fibrillation causes weakness, shortness of breath, angina, or heart failure, treatment with medication is warranted.
If patients do not convert (from abnormal to normal rhythm) with medication or are unstable, they may require electrical cardioversion, in which patients are sedated ,or asleep, and then shocked with external paddles to restore normal rhythm.
Frequently, patients are not symptomatic with atrial fibrillation, and it may be picked up incidentally. This can happen when a patient undergoes an electrocardiogram (ECG) as part of a routine or presurgical check-up and atrial fibrillation is discovered as a result. In this situation, there is no clear benefit to trying to restore sinus rhythm, as many antiarrhythmic medications carry significant side effects. Beta blockers or calcium channel blockers may be used to control heart rates with atrial fibrillation. The other important risk is stroke. Patients who have atrial fibrillation and no valvular disease, as well as no other risk factors, have a risk of stroke of 1 percent per year. Risk factors include age greater than seventy-five, prior stroke or transient ischemic attack (TIA), diabetes, and hypertension. These risks increase the annual stroke rate to 4 to 5 percent per year. Anticoagulation using warfarin reduces the risk of stroke by 70 to 80 percent. The major risk associated with using this medication is an increased risk of bleeding. If warfarin is deemed unsafe, aspirin reduces the risk by 35 percent.
Bradycardia, or slow heart rate, is caused by sinus node disease, AV node disease, or heart block (which means the electrical impulse fails to reach the ventricle; heart block is caused by AV node disease). It also increases in frequency with increasing age. Symptoms include weakness or lightheadedness, fatigue, shortness of breath, or syncope. Bradyarrhythmias can be diagnosed by an ECG at the time of symptoms. Additionally, a holter monitor (a small device worn for twenty-four to forty-eight hours to record all heart beats) can detect and record arrhythmias.
Several cardiac medications, such as beta blockers, calcium channel blockers, and digoxin, can cause bradyarrhythmias and may need to be stopped. A pacemaker is used to treat bradycardia, and is generally inserted in the operating room or in a cardiac care unit. Pacemakers have a single wire in the ventricle and sometimes a second one in the atria. Modern pacemakers are no longer affected by interference caused by micro-waves, metal detectors, or store security systems, though there has been some interactions noted with cellular phones. As the world becomes more electronically busy, the potential for interference with pacemakers changes, and pacemaker manufacturers must continue to strive to keep ahead of new potential hazards.
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