Heart block refers to a delay in the normal flow of electrical impulses that cause the heart to beat. They are further classified as first-, second-, or third-degree block.
The muscles of the heart contract in a rhythmic order for each heart beat, because electrical impulses travel along a specific route called the conduction system. The main junction of this system is called the atrioventricular node (AV node). Just as on a highway, there are occasionally some delays getting the impulse from one point to another. These delays are classified according to their severity.
In first-degree heart block, the signal is just slowed down a little as it travels along the defective part of the conduction system so that it arrives late traveling from the atrium to the ventricle.
In second-degree heart block, not every impulse reaches its destination. The block may affect every other beat, every second or third beat, or be very rare. If the blockage is frequent, it results in an overall slowing of the heart called bradycardia.
Third-degree block, also called complete heart block, is the most serious. When no signals can travel through the AV node, the heart uses its backup impulse generator in the lower portion of the heart. Though this impulse usually keeps the heart from stopping entirely, it is too slow to be an effective pump.
Causes and symptoms
First-degree heart block is fairly common. It is seen in teenagers, young adults and in well-trained athletes. The condition may be caused by rheumatic fever, some types of heart disease and by some drugs. First-degree heart block produces no symptoms.
Some cases of second-degree heart block may benefit from an artificial pace-maker. Second-degree block can occasionally progress to third-degree.
Third-degree heart block is a serious condition that affects the heart's ability to pump blood effectively. Symptoms include fainting, dizziness and sudden heart failure. If the ventricles beat more than 40 times per minute, symptoms are not as severe, but include tiredness, low blood pressure on standing, and shortness of breath.
Young children who have received a forceful blunt chest injury, can experience first-, or second-degree heart block.
Diagnosis of first-, and second-degree heart block is made by observing it on an electrocardiograph (ECG).
Third-degree heart block usually results in symptoms such as fainting, dizziness and sudden heart failure, which require immediate medical care. A physical exam and ECG confirm the presence of heart block.
Some second- and almost all third-degree heart blocks require an artificial pacemaker. In an emergency, a temporary pacemaker can be used until an implanted device is advisable. Most people need the pacemaker for the rest of their lives.
Most people with first- and second-degree heart block don't even know they have it. For people with third-degree block, once the heart has been restored to its normal, dependable rhythm, most people live full and comfortable lives.
American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-6300. 〈http://www.americanheart.org〉.
Atrioventricular node (AV node)— Highly specialized area of the heart muscle which transmits electrical impulses.
Bradycardia— A slow heart rate, usually under 60 beats per minute.
Bundle Branch Block
Bundle Branch Block
Bundle branch block (BBB) is a disruption in the normal flow of electrical pulses that drive the heart beat.
Bundle branch block belongs to a group of heart problems called intraventricular conduction defects (IVCD). There are two bundle branches, right and left. The right bundle carries nerve impulses that cause contraction of the right ventricle (the lower chamber of the heart) and the left bundle carries nerve impulses that cause contraction of the left ventricle. The two bundles initially are together at a junction called the bundle of His. Nerve impulses come through the sinus node of the heart to the bundle of His and then move into the right and left bundle branches. Bundle branch block is a slowing or interruption of nerve impulses. A problem may exist in any of the three bundles.
Patients with BBB are generally without symptoms unless the disease is severe enough to cause a complete infranodal A-V block and very slow heart rate. In patients with right bundle branch block (RBBB), the nerve impulse is conducted slowly or not at all. The right ventricle finally receives the impulse through muscle-to-muscle spread, outside the regular nerve pathway. This mechanism of impulse transmission is slow and results in a delayed contraction of the right ventricle. There are several types of left bundle branch block (LBBB), each producing its own characteristic mechanism of failure. In each case, the nerve impulse is blocked or delayed. Patients with LBBB may have left ventricular disease or cardiomyopathy.
Causes and symptoms
Left bundle branch block usually happens as a consequence of other diseases such as arteriosclerosis, rheumatic fever, congenital heart disease, myocarditis, myocardial infarction, metastatic heart tumors, or other invasions of the heart tissue. Right bundle branch block happens less often from underlying heart disease.
Detection of BBB usually takes place during a normal physical examination. The block shows up as a widening of the second heart sound. Confirmation of BBB is obtained by electrocardiogram (ECG). The pattern seen in the electrocardiogram indicates pulses in a heart beat and their duration. A QRS duration of greater than 110 milliseconds is a diagnostic indication of BBB. There is a unique ECG pattern for blocks in each of the three bundles.
There is no specific therapy for BBB. Patients are usually treated for associated heart diseases.
The prognosis of blockage in any of the three bundle branches depends on the prognosis of the associated heart disease. The associated diseases determine the outcome of the patient's health. Occasionally, disruptions in bundle branches lead to complete infranodal A-V block, a more serious blockage of nerve impulses. Approximately 2% of patients with BBB develop infranodal A-V blockage and these patients often require artificial pacemakers.
Alexander, R. W., R. C. Schlant, and V. Fuster, editors. The Heart. 9th ed. New York: McGraw-Hill, 1998.
Electrocardiogram— The pattern of the heart's electrical impulses that indicate the order and condition of the heart's components.
QRS— A pattern seen in an electrocardiogram that indicates the pulses in a heart beat and their duration. Variations from a normal QRS pattern indicate heart disease.
Gerbezius, an early eighteenth-century German physician, described the pulse of a man he attended as being ‘so very slow, that before the subsequent pulsations followed that which went before, three pulsations would certainly have pass'd in another healthy person’. But it was two Dublin clinicians more than a century later who gave heart block and its effects the eponym Stokes–Adams syndrome. Adams and Stokes both reported patients with persistent pulse rates around 30. The exceptionally slow heart rates in these patients (and in others recognized subsequently) became from time to time even slower, causing them to pass out in a manner which could be confused with epilepsy or ‘apoplexy’. Such a black-out became known as a ‘Stokes– Adams attack’, occurring because the brain was deprived of blood flow when a ‘subsequent pulsation’ did not follow for far too long.
The mechanism whereby, normally, the ‘subsequent pulsation’ follows after a proper interval of one second or less, depends on transmission of electrical signals from the heart's pacemaker, the sino-atrial (SA) node, through the atria to the only, and restricted, pathway through which it can reach the ventricles — via the atrio-ventricular (AV) node and thence along the rapidly conducting modified muscle cells known as the Bundle of His and its branching fibres, to spread through the whole ventricular muscle mass. Should the signal fail to get through, it cannot stimulate the ventricles to beat. But there are cells beyond the AV node which can take over as pacemakers, generating regular signals at a slower pace than that which is normally imposed. The SA node keeps firing off at its own faster rate, but to no avail as far as ventricular contraction is concerned. The electrocardiogram (ECG) shows P waves and QRS complexes which are completely dissociated from one another. The QRS waves, and therefore the heart beats, are 40 or fewer per minute. This is the condition of complete, or third degree, heart block.
The importance of the normal sequence of activation lies in the fact that a wave of contraction is initiated first in the atria then, after a brief delay, in the ventricles. The relative timing of this sequence, as well as the sequence itself, are important determinants of the effectiveness with which the heart pumps the blood. Since heart block is a major disturbance to the smooth progress of the triggering wave, its consequences can be serious.
There are lesser degrees of block, where some activations get through and others do not (shown by the presence of any isolated P waves) and in its mildest form there is simply a slowing down of transmission from atria to ventricles (shown by a prolonged P–R interval). It is also possible for just one of the main branches of the Bundle of His to be blocked.
The cause of heart block is damage to the conducting fibres, by deprivation of blood supply picking out that part of the heart when there is coronary artery disease, or as a result of infection; it may also be congenital.
Nowadays, heart block can be treated by implanting an artificial pacemaker to drive the ventricles.
David J. Miller, and Niall G. MacFarlane
See also electrocardiogram; heart; pacemaker.