ABNORMAL HEART RHYTHMS
The normal heart rate is between 60 and 100 beats per min but much lower rates may be encountered and are quite normal in young adults who are physically fit. As has been mentioned, the rate also responds to exercise or inactivity and also to pain and anger. An inappropriately fast heartbeat is called tachycardia and an abnormally low rate bradycardia. Abnormal rhythms arefrequently encountered and can be regular or irregular. The contractions of the heart muscle fibers and therefore heartbeat is controlled by electrical discharges that flow through the heart along distinct pathways at controlled speeds. If disturbances occur with the flow of the electrical discharge then arrhythmias in these contractions can occur. These range from the harmless to the life threatening. For example, minor arrhythmias can be caused by excessive alcohol consumption, smoking, stress or exercise. Thyroid hormones also affect the heart rate and an over- or under-active thyroid gland may affect the rate and rhythm of the heart. Some of the drugs used to treat lung disease or high blood pressure can have similar effects.
The commonest causes of arrhythmia are heart disease, especially coronary heart disease, heart failure or abnormal valve function. In many cases patients are aware of an abnormal heartbeat and this is referred to as palpitations. This awareness may be disturbing but there are many possible causes of arrhythmias and they are often not the result of an underlying disease.
However, when they are, it is the nature and severity of the disease that is more important than the arrhythmia itself. If brought to the attention of a clinician, he or she will want to know if they are fast or slow, regular or irregular, whether they make the person feel dizzy, light-headed or even lose consciousness, whether they occur at rest or during exercise and are they accompanied by a shortness of breath or chest pain.
An ECG is, of course, helpful in diagnosis but the arrhythmias may only occur over a short period. Consequently a portable monitor may be placed on the patient to record them over 24 h. The prognosis and treatment will depend on whether the arrhythmias start in the pacemaker, the atria or the ventricles. Most arrhythmias are harmless and do not interfere with the heart’s pumping action. However, antiarrhythmic drugs are available if the patient is anxious or if the arrhythmias cause intolerable symptoms or pose a risk. Sometimes it is necessary to fit the patient with an artificial pacemaker that is programmed to replace the heart’s own pacemaker. These are usually implanted surgically under the skin of the chest and are wired to the heart. Most commonly they are used to correct abnormally slow heart rates. Sometimes an externally applied electric shock to the heart, called cardioversion, electroversion, or defibrillation, can correct an abnormal arrhythmia. Arrhythmias caused by coronary artery disease may be controlled by drugs, pacemakers or by surgery (see below). After a coronary infarction, some people have life-threatening episodes of ventricular tachycardia that may be triggered in an injured area of heart muscle that may have to be removed during open heart surgery.
Atrial fibrillation and flutter are rapid electrical discharges which make the atria contract rapidly but each contraction may not conduct to the ventricles. This causes the ventricles to contract less efficiently and irregularly, producing a condition that may be sporadic or persistent. During the fibrillation or flutter, the atrial walls simply quiver and blood is not pumped into the ventricles properly. The consequence is that inadequate amounts of blood are pumped from the heart, blood pressure falls and heart failure may occur. The diminished pumping ability may make the patient feel weak, faint and short of breath. Sometimes, especially with elderly patients, there is chest pain and heart failure. If the atria do not contract completely, blood may stagnate in the atria and clot. If pieces of clot or emboli break off they may move and block an artery elsewhere in the body. If this is in the brain it may cause an embolism or stroke.
The treatment for atrial fibrillation is to correct the disorder that causes the abnormal rhythm, restoring it to normal, and to slow the rate at which the ventricles contract so as to improve the pumping efficiency. The latter can often be achieved with digoxin but aA-blocker, such as propanolol, or other drugs may also be required. Frequently a normal rhythm has to be restored by antiarrhythmic drugs or cardioversion. In these conditions blood can pool in the ventricles and clot, hence anticoagulant drugs may be used when atrial fibrillation is present.
Sustained ventricular tachycardia, with a ventricular rate of at least 120 beats per min, occurs in various heart diseases that damage the ventricles. Most often it occurs over weeks or months after a heart attack . It is characterized by palpitations and will usually require emergency treatment because the blood pressure falls and heart failure may follow. Cardioversion is needed immediately.
Ventricular fibrillation is a form of cardiac arrest. It is similar to atrial fibrillation, but the prognosis is very serious and potentially fatal if not treated immediately. It is the product of an uncoordinated series of rapid but ineffective contractions throughout the ventricles. These, in turn, arise from multiple chaotic electrical impulses. Its commonest cause is an insufficient flow of blood to the heart muscle because of coronary heart disease or a heart attack. Given that blood is not pumped from the heart, it can lead to unconsciousness in seconds and, if untreated, the patient usually has convulsions and develops irreversible brain damage because of oxygen starvation. Ventricular fibrillation needs to be treated as a medical emergency. Cardiopulmonary resuscitation must be started within the minimum time possible, usually three minutes. This should be followed by cardioversion also as soon as possible. Subsequently drugs are needed to restore and maintain the normal heart rhythm.
Heart block describes a delay in electrical conduction through the AV node. There are various degrees of seriousness; the least may not require treatment but the most serious may require the fitting of an artificial pacemaker. First-degree heart block is common in well-trained athletes, teenagers, and young adults, but it may also be caused by rheumatic fever or by certain drugs. At the other extreme in third-degree heart block, electrical impulses from the atria to the ventricles are completely blocked. The ventricles beat very slowly and the pumping ability of the heart is compromized. Fainting, dizziness and sudden heart failure are common.