| Left bundle branch block | |
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| ECG characteristics of a typical LBBB showing wide QRS complexes with abnormal morphology in leads V1 and V6. | |
| Specialty | Cardiology, Emergency Medicine |
Left bundle branch block (LBBB) is a conduction abnormality in the heart that can be seen on an electrocardiogram (ECG). [1] In this condition, activation of the left ventricle of the heart is delayed, which causes the left ventricle to contract later than the right ventricle.
Among the causes of LBBB are: [2]
Slow or absent conduction through the left bundle branch means that it takes longer than normal for the left ventricle to fully depolarise. [3] This can be due to a damaged bundle branch that is completely unable to conduct, but may represent intact conduction that is slower than normal. LBBB may be fixed, present at all times, but may be intermittent for example occurring only during rapid heart rates. This may be due to the bundle having a longer refractory period than usual. [3]
LBBB is diagnosed on a 12-lead ECG. In adults, it is seen as wide QRS complexes lasting ≥120ms with characteristic QRS shapes in the precordial leads, although narrower complexes are seen in children. [4] In lead V1, the QRS complex is often entirely negative (QS morphology), although a small initial R wave may be seen (rS morphology). In the lateral leads (I, aVL, V5-V6) the QRS complexes are usually predominantly positive with a slow upstroke last >60ms to the R-wave peak. [4] Notching may be seen in these leads but this is not universal. The small Q-waves that are usually seen in the lateral leads are absent in LBBB. [4] T-waves usually point in the opposite direction to the terminal portion of the preceding QRS - positive QRS complexes have negative T-waves while negative QRS complexes have positive T-waves. The ST segments typically slur into the T-wave and often appear elevated in leads with negative QRS complexes. [3] The axis may be normal but may be deviated to the left or right. [4]
There are also partial blocks of the left bundle branch: "left anterior fascicular block" (LAFB) [5] and a "left posterior fascicular block" (LPFB). [5] This refers to the block after the bifurcation of the left bundle branch.
The presence of LBBB results in that electrocardiography (ECG) cannot be used to diagnose left ventricular hypertrophy or Q wave infarction, because LBBB in itself results in a widened QRS complex and changes in the ST segment consistent with ischemia or injury. [6]
Given the difficulty diagnosing an acute myocardial infarction (AMI) in patients with pre-existing LBBB, a multicenter retrospective cohort study was done to help improve diagnosis of AMI in this patient population. Sgarbossa criteria and its Modified Criteria have historically been used to determine AMI in patients with LBB, but the clinical utility has a high specificity (90%) with a low sensitivity (36%).
Two new approaches to the ECG were elaborated to improve the diagnostic sensitivity of ECG in patients with LBBB and suspected AMI. First, since any ST deviation concordant with the QRS should be regarded as abnormal, it was hypothesized that not only concordant ST elevation but also concordant ST depression might be a sign of AMI. The Sgarbossa rule of concordant ST depression in leads V1 to V3 was then extended to include any other lead to, in theory, cover the electrocardiographic projection of acute ischemia in different myocardial regions. Second, the presence of an appreciable (≥1 mm or 0.1 mV) discordant ST deviation in low‐voltage QRS complexes was considered as a positive criterion for AMI. This criterion was included because in the absence of ischemia, these complexes usually show isoelectric ST‐segment potentials.
The results of the study resulted in the development of the BARCELONA algorithm. Under these criteria, an ECG is positive for an AMI in the presence of LBBB if any of the following criteria are present:
The BARCELONA algorithm attained the highest sensitivity (95%), significantly higher (P<0.01) than Sgarbossa and Modified Sgarbossa rules, as well as the highest negative predictive value (97%), while maintaining 89% specificity. The global performance of the BARCELONA algorithm was significantly better than previous algorithms: It achieved the highest efficiency (91%) and the highest area under the ROC curve (0.92), which was significantly higher (P<0.01) than the ones obtained by the Sgarbossa and Modified Sgarbossa rules. The BARCELONA algorithm also allowed a significant improvement in the ability to predict the occurrence of an AMI, as shown by Integrated Discrimination Improvement and Net Reclassification Improvement indexes (both indexes showed P<0.01 comparing BARCELONA algorithm with Sgarbossa and Modified Sgarbossa rules). [7]
Electrocardiography is the process of producing an electrocardiogram, a recording of the heart's electrical activity through repeated cardiac cycles. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances, inadequate coronary artery blood flow, and electrolyte disturbances.
Third-degree atrioventricular block is a medical condition in which the electrical impulse generated in the sinoatrial node in the atrium of the heart can not propagate to the ventricles.
The cardiac conduction system transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through the body's circulatory system. The pacemaking signal travels through the right atrium to the atrioventricular node, along the bundle of His, and through the bundle branches to Purkinje fibers in the walls of the ventricles. The Purkinje fibers transmit the signals more rapidly to stimulate contraction of the ventricles.
First-degree atrioventricular block is a disease of the electrical conduction system of the heart in which electrical impulses conduct from the cardiac atria to the ventricles through the atrioventricular node more slowly than normal. First degree AV block does not generally cause any symptoms, but may progress to more severe forms of heart block such as second- and third-degree atrioventricular block. It is diagnosed using an electrocardiogram, and is defined as a PR interval greater than 200 milliseconds. First degree AV block affects 0.65-1.1% of the population with 0.13 new cases per 1000 persons each year.
Ventricular tachycardia is a cardiovascular disorder in which fast heart rate occurs in the ventricles of the heart. Although a few seconds of VT may not result in permanent problems, longer periods are dangerous; and multiple episodes over a short period of time are referred to as an electrical storm. Short periods may occur without symptoms, or present with lightheadedness, palpitations, or chest pain. Ventricular tachycardia may result in ventricular fibrillation (VF) and turn into cardiac arrest. This conversion of the VT into VF is called the degeneration of the VT. It is found initially in about 7% of people in cardiac arrest.
In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period or vulnerable period. The T wave contains more information than the QT interval. The T wave can be described by its symmetry, skewness, slope of ascending and descending limbs, amplitude and subintervals like the Tpeak–Tend interval.
A bundle branch block is a defect in one of the bundle branches in the electrical conduction system of the heart.
The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram. It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the heart and contraction of the large ventricular muscles.
A right bundle branch block (RBBB) is a heart block in the right bundle branch of the electrical conduction system.
Atrioventricular block is a type of heart block that occurs when the electrical signal traveling from the atria, or the upper chambers of the heart, to ventricles, or the lower chambers of the heart, is impaired. Normally, the sinoatrial node produces an electrical signal to control the heart rate. The signal travels from the SA node to the ventricles through the atrioventricular node. In an AV block, this electrical signal is either delayed or completely blocked. When the signal is completely blocked, the ventricles produce their own electrical signal to control the heart rate. The heart rate produced by the ventricles is much slower than that produced by the SA node.
Bifascicular block is characterized by right bundle branch block with left anterior fascicular block, left bundle branch block, or right bundle branch block with left posterior fascicular block on electrocardiography. Complete heart block could be the cause of syncope that is otherwise unexplained if bifascicular block is seen on electrocardiography. It is estimated that less than 50% of patients with bifascicular block have high-degree atrioventricular block, although the exact incidence is unknown.
The electrical axis of the heart is the net direction in which the wave of depolarization travels. It is measured using an electrocardiogram (ECG). Normally, this begins at the sinoatrial node ; from here the wave of depolarisation travels down to the apex of the heart. The hexaxial reference system can be used to visualise the directions in which the depolarisation wave may travel.
Left anterior fascicular block (LAFB) is an abnormal condition of the left ventricle of the heart, related to, but distinguished from, left bundle branch block (LBBB).
A left posterior fascicular block (LPFB), also known as left posterior hemiblock (LPH), is a condition where the left posterior fascicle, which travels to the inferior and posterior portion of the left ventricle, does not conduct the electrical impulses from the atrioventricular node. The wave-front instead moves more quickly through the left anterior fascicle and right bundle branch, leading to a right axis deviation seen on the ECG.
An intraventricular block is a heart conduction disorder — heart block of the ventricles of the heart. An example is a right bundle branch block, right fascicular block, bifascicular block, trifascicular block.
ST depression refers to a finding on an electrocardiogram, wherein the trace in the ST segment is abnormally low below the baseline.
In electrocardiography, left axis deviation (LAD) is a condition wherein the mean electrical axis of ventricular contraction of the heart lies in a frontal plane direction between −30° and −90°. This is reflected by a QRS complex positive in lead I and negative in leads aVF and II.
Electrocardiography in suspected myocardial infarction has the main purpose of detecting ischemia or acute coronary injury in emergency department populations coming for symptoms of myocardial infarction (MI). Also, it can distinguish clinically different types of myocardial infarction.
Sgarbossa's criteria are a set of electrocardiographic findings generally used to identify myocardial infarction in the presence of a left bundle branch block (LBBB) or a ventricular paced rhythm.
A tachycardia-dependent bundle branch block is a defect in the conduction system of the heart, and is distinct from typical bundle branch blocks due to its reliable, reproducible onset related to an increase in the rate of cardiac contraction. Tachycardia-dependent bundle branch block can prevent both ventricles from contracting efficiently and can limit the cardiac output of the heart.