Right bundle branch block with left anterior hemiblock

Classification and Epidemiology

Classification

Bundle branch and fascicular blocks are classified on the basis of where the disruption occurs within the His-Purkinje system.

• Bundle branch blocks:
  • Right bundle branch block (RBBB)
  • Left bundle branch block (LBBB)
• Fascicular blocks:
  • Left anterior fascicular block or hemiblock (LAFB)
  • Left posterior fascicular block or hemiblock (LPFB)

Bundle branch and fascicular blocks arise because of obstruction of electrical current Electrical current The flow of charged particles from one point to another (in physiology, usually across a cell membrane) Cardiac Physiology through the His-Purkinje system and are named on the basis of the location of that disruption.

Epidemiology

• RBBB:
  • Prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency:
    • In the general population: < 1%
    • Increases with age
  • Can occur in young, healthy people
• LBBB:
  • Prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency:
    • In the general population: < 1%
    • Increases with age
  • Occurs infrequently in young, healthy people
• LAFB:
  • Prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency in the general population: 1%–2.5%
  • Prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency increases with age
• LPFB:
  • An isolated LPFB is rare.
  • Prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency: 0.1%–0.6%
  • Often occurs in association with RBBB

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Etiology

RBBB

• Structural disease:
  • Includes conditions resulting in right ventricular pathology:
    • Hypertrophy Hypertrophy General increase in bulk of a part or organ due to cell enlargement and accumulation of fluids and secretions, not due to tumor formation, nor to an increase in the number of cells (hyperplasia). Cellular Adaptation
    • ↑ Pressure
    • Dilation
    • Injury
    • Inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation/infiltration
  • Examples:
    • Pulmonary hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension and cor pulmonale Cor Pulmonale Cor pulmonale is right ventricular (RV) dysfunction caused by lung disease that results in pulmonary artery hypertension. The most common cause of cor pulmonale is chronic obstructive pulmonary disease. Dyspnea is the usual presenting symptom. Cor Pulmonale
    • Pulmonary embolism Pulmonary Embolism Pulmonary embolism (PE) is a potentially fatal condition that occurs as a result of intraluminal obstruction of the main pulmonary artery or its branches. The causative factors include thrombi, air, amniotic fluid, and fat. In PE, gas exchange is impaired due to the decreased return of deoxygenated blood to the lungs. Pulmonary Embolism
    • Myocardial ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage or infarction ( MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction)
    • Myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis
    • Cardiomyopathy Cardiomyopathy Cardiomyopathy refers to a group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function in the absence of other heart disorders (coronary artery disease, hypertension, valvular disease, and congenital heart disease). Cardiomyopathy: Overview and Types
    • Valvular disease
    • Atrial septal defect Atrial Septal Defect Atrial septal defects (ASDs) are benign acyanotic congenital heart defects characterized by an opening in the interatrial septum that causes blood to flow from the left atrium (LA) to the right atrium (RA) (left-to-right shunt). Atrial Septal Defect (ASD)
    • Chagas disease Chagas disease Infection with the protozoan parasite trypanosoma cruzi, a form of trypanosomiasis endemic in central and south america. It is named after the brazilian physician carlos chagas, who discovered the parasite. Infection by the parasite (positive serologic result only) is distinguished from the clinical manifestations that develop years later, such as destruction of parasympathetic ganglia; chagas cardiomyopathy; and dysfunction of the esophagus or colon. Trypanosoma cruzi/Chagas disease
• Idiopathic Idiopathic Dermatomyositis cardiac conduction disease (Lenegre or Lev disease)
• Iatrogenic Iatrogenic Any adverse condition in a patient occurring as the result of treatment by a physician, surgeon, or other health professional, especially infections acquired by a patient during the course of treatment. Anterior Cord Syndrome trauma to the conduction system:
  • Right-heart catheter insertion
  • Septal reduction therapy with ethanol Ethanol A clear, colorless liquid rapidly absorbed from the gastrointestinal tract and distributed throughout the body. It has bactericidal activity and is used often as a topical disinfectant. It is widely used as a solvent and preservative in pharmaceutical preparations as well as serving as the primary ingredient in alcoholic beverages. Ethanol Metabolism ablation
  • Cardiac surgery Cardiac surgery Cardiac surgery is the surgical management of cardiac abnormalities and of the great vessels of the thorax. In general terms, surgical intervention of the heart is performed to directly restore adequate pump function, correct inherent structural issues, and reestablish proper blood supply via the coronary circulation. Cardiac Surgery
• Can also occur in those with no heart disease

LBBB

• Structural disease:
  • Includes conditions resulting in left ventricular pathology:
    • Hypertrophy Hypertrophy General increase in bulk of a part or organ due to cell enlargement and accumulation of fluids and secretions, not due to tumor formation, nor to an increase in the number of cells (hyperplasia). Cellular Adaptation
    • ↑ Pressure
    • Dilation
    • Injury
    • Inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation/infiltration
  • Examples:
    • MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction/ ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage (most common)
    • Cardiomyopathy Cardiomyopathy Cardiomyopathy refers to a group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function in the absence of other heart disorders (coronary artery disease, hypertension, valvular disease, and congenital heart disease). Cardiomyopathy: Overview and Types
    • Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension
    • Endocarditis Endocarditis Endocarditis is an inflammatory disease involving the inner lining (endometrium) of the heart, most commonly affecting the cardiac valves. Both infectious and noninfectious etiologies lead to vegetations on the valve leaflets. Patients may present with nonspecific symptoms such as fever and fatigue. Endocarditis (particularly with abscess Abscess Accumulation of purulent material in tissues, organs, or circumscribed spaces, usually associated with signs of infection. Chronic Granulomatous Disease)
    • Myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis
    • Valvular disease
    • Congenital Congenital Chorioretinitis defects
• Idiopathic Idiopathic Dermatomyositis cardiac conduction disease
• Iatrogenic Iatrogenic Any adverse condition in a patient occurring as the result of treatment by a physician, surgeon, or other health professional, especially infections acquired by a patient during the course of treatment. Anterior Cord Syndrome:
  • Septal myectomy Septal myectomy Hypertrophic Cardiomyopathy
  • Transcatheter aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy implantation Implantation Endometrial implantation of embryo, mammalian at the blastocyst stage. Fertilization and First Week

LAFB and LPFB

These fascicular blocks can occur because of many of the same causes of RBBB or LBBB, most notably:

• Aortic valvular disease (LAFB)
• Myocardial ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage or MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction
• Cardiomyopathy Cardiomyopathy Cardiomyopathy refers to a group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function in the absence of other heart disorders (coronary artery disease, hypertension, valvular disease, and congenital heart disease). Cardiomyopathy: Overview and Types
• Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension
• Chagas disease Chagas disease Infection with the protozoan parasite trypanosoma cruzi, a form of trypanosomiasis endemic in central and south america. It is named after the brazilian physician carlos chagas, who discovered the parasite. Infection by the parasite (positive serologic result only) is distinguished from the clinical manifestations that develop years later, such as destruction of parasympathetic ganglia; chagas cardiomyopathy; and dysfunction of the esophagus or colon. Trypanosoma cruzi/Chagas disease
• Infiltrative and inflammatory diseases
• Congenital Congenital Chorioretinitis defects
• Idiopathic Idiopathic Dermatomyositis cardiac conduction disease

Pathophysiology

Normal physiology

• The cardiac impulse is generated by pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells in the sinoatrial (SA) node and moves through the atria → depolarization Depolarization Membrane Potential → atrial contraction
• Transmitted to the atrioventricular (AV) node → His-Purkinje system → depolarization Depolarization Membrane Potential → ventricular contraction
• His-Purkinje system:
  • Provides rapid electrical conduction for the ventricles → synchronized ventricular depolarization Depolarization Membrane Potential and contraction
  • Contains:
    • Bundle of His Bundle of His Small band of specialized cardiac muscle fibers that originates in the atrioventricular node and extends into the membranous part of the interventricular septum. The bundle of his, consisting of the left and the right bundle branches, conducts the electrical impulses to the heart ventricles in generation of myocardial contraction. Heart: Anatomy
    • The bundle splits in 2 → right and left bundles branches
    • Left bundle branch splits → left anterior fascicle and left posterior fascicle
    • Purkinje fibers Purkinje fibers Modified cardiac muscle fibers composing the terminal portion of the heart conduction system. Heart: Anatomy

Normal electrophysiology of the heart:
The conduction impulse starts at the sinoatrial (SA) node and travels through the atrium to the atrioventricular (AV) node. From there, it moves through the bundle of His Bundle of His Small band of specialized cardiac muscle fibers that originates in the atrioventricular node and extends into the membranous part of the interventricular septum. The bundle of his, consisting of the left and the right bundle branches, conducts the electrical impulses to the heart ventricles in generation of myocardial contraction. Heart: Anatomy down through both bundle branches (and fascicles) to the Purkinje fibers Purkinje fibers Modified cardiac muscle fibers composing the terminal portion of the heart conduction system. Heart: Anatomy. The movement of this electrical impulse can be recorded on an ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG).
P wave P wave Electrocardiogram (ECG) (blue): depolarization Depolarization Membrane Potential of atrial myocardium Myocardium The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Heart: Anatomy
QRS complex QRS complex Electrocardiogram (ECG) (orange): depolarization Depolarization Membrane Potential of ventricular myocardium Myocardium The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Heart: Anatomy
T wave T wave Electrocardiogram (ECG) (yellow): repolarization Repolarization Membrane Potential of ventricular myocardium Myocardium The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Heart: Anatomy

Diagram outlining the electrical pathway through the heart

Bundle branch blocks

• Impairment in a bundle causes disruption of downward cardiac impulse transmission.
• This subsequently causes the impulse to be conducted through the opposite branch.
• The contralateral ventricle will depolarize first.
• The ipsilateral ventricle will depolarize later (electrical impulse travels slowly through the muscle, reaching the conduction system below the block).
• This will appear as prolongation of the QRS interval on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG).

Diagram demonstrating how electrical conduction moves through the ventricles in a right bundle branch block:
The electrical impulse runs through the left bundle branch, through the septum and left ventricle, and then through the right ventricle. These phases result in the electrical conduction vectors shown above (drawn in relation to a cross section of the thorax with the precordial leads Precordial leads Recording obtained from the corresponding chest electrode and the central terminal. Electrocardiogram (ECG) attached), which correlate with the corresponding ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) waveforms.

Diagram demonstrating how electrical conduction moves through the ventricles in a left bundle branch block:
The electrical impulse runs through the right bundle branch, through the septum and right ventricle, and then through the left ventricle. These phases result in the electrical conduction vectors shown above (drawn in relation to a cross section of the thorax with the precordial leads Precordial leads Recording obtained from the corresponding chest electrode and the central terminal. Electrocardiogram (ECG) attached), which correlate with the corresponding ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) waveforms.

Fascicular blocks

• The pathophysiology is similar to that of bundle branch blocks, though the bundle branch is not completely affected → depolarization Depolarization Membrane Potential of the left ventricle is dependent on the opposite fascicle
• This causes axis Axis The second cervical vertebra. Vertebral Column: Anatomy deviation, but has less effect on QRS duration.

Diagram of left anterior and posterior fascicular blocks:
In left anterior fascicular block or hemiblock (LAFB), the resultant electrical vector results in significant left axis Axis The second cervical vertebra. Vertebral Column: Anatomy deviation. In left posterior fascicular block or hemiblock (LPFB), the electrical vector is deviated a bit rightward but is not significantly displaced from the normal QRS axis Axis The second cervical vertebra. Vertebral Column: Anatomy range.

Clinical Presentation and Diagnosis

Clinical presentation

• Usually asymptomatic
• Most people are unaware.
• Rarely, may cause syncope Syncope Syncope is a short-term loss of consciousness and loss of postural stability followed by spontaneous return of consciousness to the previous neurologic baseline without the need for resuscitation. The condition is caused by transient interruption of cerebral blood flow that may be benign or related to a underlying life-threatening condition. Syncope or presyncope Presyncope Syncope (may have associated AV block AV block Atrioventricular (AV) block is a bradyarrhythmia caused by delay, or interruption, in the electrical conduction between the atria and the ventricles. Atrioventricular block occurs due to either anatomic or functional impairment, and is classified into 3 types. Atrioventricular block (AV block))
• A split S1 S1 Heart Sounds is common in RBBB.
• A split S2 S2 Heart Sounds can also be noted: 
  • Persistent in RBBB → delayed pulmonic valve closure (due to delayed activation of the right ventricle)
  • Paradoxical in LBBB → delayed aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy closure (due to delayed activation of the left ventricle)

Audio:

This audio clip is an example of a split S2 S2 Heart Sounds in the setting of an RBBB. The 2 sounds occurring during S2 S2 Heart Sounds result from delayed closure Delayed Closure Gastroschisis of the pulmonic valve in relation to the aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy.

A diagram of a paradoxical split in closure of the aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy which is delayed:
The name “paradoxical” is due to the fact that the split narrows with inspiration Inspiration Ventilation: Mechanics of Breathing (right). This can be heard in some individuals with a left bundle branch block.

Right bundle branch block on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)

• QRS duration ≥ 120 msec
• Leads V1 and V2:
  • Rsr′, rsR’ or rSR’ (many variations)
  • The R’ or r’ deflection is usually wider than the initial R wave.
  • Appears as “rabbit ears”
• Leads I and V6 will have an S wave that is:
  • Deep
  • Of longer duration
  • Slurred 
• T waves tend to be discordant to the terminal QRS vector.

QRS morphology QRS morphology Electrocardiogram (ECG) seen in right bundle branch block:
The RsR’ (and variations of this) give the appearance of “rabbit ears.” The S wave in leads I and V6 will appear broad, deep, and slurred.

ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) demonstrating a right bundle branch block:
The QRS duration is prolonged, at 140 msec. Note the rSR’ and RSr’ in leads V1 and 2, along with the deep and broad S waves in I and V6.

LBBB on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)

• QRS duration > 120 msec in adults
• Broad notched (R,R’) or slurred R wave in leads I, aVL, V5, and V6
• Absent Q waves in lateral leads
• Large S wave in V1 and V2
• ST segments and T waves are usually discordant to QRS complex QRS complex Electrocardiogram (ECG).

ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) patterns in left bundle branch block:
A large S wave will be seen in V1, while a large, notched R wave occurs in V5 and V6. Note that the ST-segment and T-wave directions are discordant to the QRS.

ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) demonstrating a left bundle branch block:
Note the widened QRS; the large, notched R waves in V5 and V6; and the large, broad S waves in V1 and 2. The ST segments and T waves are also generally discordant to the QRS complex QRS complex Electrocardiogram (ECG).

Incomplete RBBB and LBBB

A bundle branch block may be considered incomplete if the usual RBBB or LBBB pattern is seen but the QRS duration is 110–119 msec.

Left anterior fascicular block on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)

• QRS duration < 120 msec
• Left-axis deviation (approximately 45–90 degrees)
• R-peak time ≥ 45 msec in lead aVL (measured from the start of the Q wave to the peak of the R wave)
• qR complexes in leads I and aVL
• rS complexes in leads II, III, and aVF

ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) demonstrating left anterior fascicular block:
Here, the axis Axis The second cervical vertebra. Vertebral Column: Anatomy is deviated to –60 degrees and a small Q wave is noted in aVL. The QRS is slightly prolonged, but still < 120 msec.

Left posterior fascicular block on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)

• QRS duration < 120 msec
• Right-axis deviation (90–180 degrees)
• qR complexes in leads II, III, and aVF
• rS complexes in leads I and aVL

ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) demonstrating a left posterior fascicular block:
There is right axis Axis The second cervical vertebra. Vertebral Column: Anatomy deviation (+ 100 degrees), small Q waves in II, III, and aVF, rS complexes in I and aVL. The QRS complex QRS complex Electrocardiogram (ECG) duration is also < 120 msec.

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