Although the normal heart beat is triggered by the SA node, any heart muscle tissue in either the atria or the ventricles can trigger cardiac contraction by emitting an electrical discharge. Ectopic beats arising from the atrium are referred to as premature atrial beats (PAB’s), and ectopics beats arising from either ventricle are referred to as premature ventricular beats (VPB’s).
PAB’s are characterized by premature P-waves, which come early or prematurely in the cardiac cycle and differ slightly in configuration from the normal P-waves. These early P-waves are usually followed by a QRS complex, resulting from ventricular contraction. Occasionally, however, the premature P-waves arrive so early that the ventricles are still recovering from the previous beat and are refractory to the premature atrial beat. This phenomenon results in a dropped beat which causes a pause in the heart rhythm. These very early premature atrial beats, not followed by ventricular contraction, are referred to as blocked premature beats. Premature atrial beats are common and are virtually always benign. Often PAB’s are asymptomatic,but occasional patients, especially individuals with thin chests, will experience each and every PAB as an early beat interrupting the normal regularly sequenced heart rhythm.
Usually patients with symptomatic PAB’s can be reassured by their physician that PAB’s are never life-threatening and do not require treatment with antiarrhythmic agents. Occasionally patients with very frequent PAB’s will develop symptoms due to the palpitations caused by multiple PAB’s and will require treatment with antiarrhythmic agents which suppress the PAB’s.
Ventricular premature beats (VPB’s) arising from the right or left ventricles are also common. VPB’s are characterized by early wide QRS complexes with a T-wave usually directed in the opposite direction from the QRS. VPB’s can be associated with any type of heart disease but are also common in patients with completely normal hearts. VPB’s are commonly seen in normal individuals who consume large quantities of caffeinated beverages or who became exhausted from lack of sleep.
VPB’s in patients with congestive heart failure are more ominous and are associated with a worse prognosis. In patients suffering an acute myocardial infarction, VPB’s are an indication that the heart may fibrillate or arrest. Studies have shown that in individuals with normal hearts, based on echocardiography and cardiac catheterization, VPB’s are harmless and are not associated with a poor cardiac prognosis. In fact a few individuals have frequent VPB’s all their lives with no adverse effects. The most important determinant of the clinical significance of VPB’s is the structural condition of the heart. Thus, VPB’s in a patient with a dilated weak heart are associated with a poor prognosis. Whereas, VPB’s in a patient with a structurally normal heart are nearly always benign. It is the cardiologist’s responsibility to detect structural heart disease in a patient with frequent VPB’s and to advise the patient regarding prognosis associated with the VPB’s. Occasionally, VPB’s result from inadequate blood flow to the heart muscle. Thus, VPB’s may be a helpful clue in the detection of coronary artery disease.
VPB’s, however, do not generate a full left ventricular stroke volume because the left ventricle is incompletely filled due to the prematurity of the beat and lack of atrial contraction preceding ventricular contraction. Thus, a weak pulse usually results from each VPB. In patients with frequent VPB’s, the heart rate determined by feeling the wrist pulse is often reduced because the weak pulses resulting from VPB’s are not detectable at the wrist. In a patient with ventricular bigeminy, each normal heart beat is followed by a VPB. Since only the normal beats produce a pulse, detection at the wrist will be one-half the patient’s actual heart rate. Weak peripheral pulses resulting from VPB’s can cause some patients with frequent VPB’s to feel fatigue and asensation of loss of energy.
Whether patients with frequent VPB’s should be started on antiarrhythmic medications, which may suppress VPB’s, is a complex issue which is best decided by a cardiologist who is familiar with the patient’s underlying structural heart disease and with the symptoms the patient experiences from the VPB’s. The antiarrhythmic medications currently available for suppressing VPB’s are effective in only slightly more than half of patients and occasionally cause adverse side effects, which may rarely be severe. Thus, the decision to treat a patientwith frequent VPB’s is best made by a cardiologist familiar with the patient’s symptoms, physical examination, and heart disease assessed by non-invasive cardiac tests, including a 12 lead EKG echocardiogram, treadmill study, nuclear stress test, and/or stress echocardiogram.