Abnormal QRS Morphology
Brugada Pattern and Syndrome
Pathogenesis
Brugada syndrome is an autosomally dominant inherited disease (with incomplete penetrance) known for causing sudden death in patients with structurally normal hearts. More than half of cases, however, are sporadic. The disease has been linked to the SCN5A gene that encodes for a subunit of the sodium channel within epicardial cells of the right ventricle.
Clinical Course
Patients present with syncope or sudden cardiac death from polymorphic ventricular tachycardia. Mean age of sudden cardiac death is 41 years. The disease is markedly more predominant in males and, while present worldwide, is endemic in Southeast Asia.1
ECG Features
RBBB Pattern
The classic ECG findings are right bundle branch block pattern and ST elevation in V1-V3. These findings in a patient with Brugada syndrome may be absent at any given time. Fever, drugs (sodium channel-blocking medications), electrolyte disturbances, and high vagal tone are known to exaggerate or unmask ST elevation in the right precordial leads. The Brugada pattern in patients with Brugada syndrome is variable over time and may be absent on some ECGs.
ST Elevation in V1-V3
There are three different Brugada patterns. Only type I Brugada pattern is diagnostic of Brugada syndrome. Further provocative testing with intravenous administration of class I antiarrhythmic drugs is required for types II and III.2
 FIGURE 6.1 Different Brugada patterns. |
Differential Diagnosis
The differential for ST elevation in right precordial leads should be considered. More common causes of ST elevation such as early repolarization, septal MI, bundle branch block, left ventricular hypertrophy, right ventricular strain, and right ventricular infarction should be ruled out. Toxicity from cocaine and tricyclic antidepressants can mimic the Brugada pattern (through sodium channel-blocking mechanisms). One of the more difficult syndromes to differentiate from Brugada on ECG is arrhythmogenic right ventricular dysplasia. Brugada pattern can also appear for several hours after cardioversion or defibrillation and lead to an incorrect diagnosis of Brugada syndrome.
Treatment
ICD implantation is recommended for patients with Brugada syndrome who have survived cardiac arrest (class I), have documented ventricular arrhythmia (class I), or have a history of syncope judged to be likely caused by ventricular arrhythmias (class IIa).2
Arrhythmogenic Right Ventricular Dysplasia
Pathogenesis
Arrhythmogenic right ventricular dysplasia (ARVD) is an inherited cardiomyopathy characterized by progressive fatty or fibrofatty infiltration of the right ventricular myocardium. Despite the name given to this disease, involvement of the left ventricle can occur in as many as 60% of patients. Replacement of cardiac myocytes by fibrofatty tissue leads to wall thinning, aneurysm formation of the right ventricle, and conduction abnormalities.3 Inheritance is most commonly autosomal dominant with variable penetrance although recessive forms have been identified. The most common genetic abnormality identified to date is a mutation in desmosomal proteins that provide mechanical binding between myocytes.
Clinical Course
Initially, patients with ARVD may have only localized infiltration and involvement of part of the right ventricle. They are asymptomatic but still at risk for sudden cardiac death.3 As infiltration becomes more diffuse in the right ventricle and starts to involve the left ventricle, patients may develop symptomatic arrhythmias (ventricular tachycardia). In late stages of the disease, patients suffer from biventricular heart failure that becomes difficult to distinguish from dilated cardiomyopathy.
ECG Features
Conduction abnormalities in ARVD are located in the right ventricle. The ECG abnormalities in ARVD can be appreciated in the right precordial leads:
T-Wave Inversion
Inverted T waves in the right precordial leads (V1-V3) is the most common repolarization abnormality seen.4
Epsilon Wave
An epsilon wave is a low-amplitude deflection occurring at the end of the QRS segment or early ST segment. It represents delayed conduction in the right ventricle. This is a subtle finding but is very specific to ARVD.
Prolonged QRS
Delayed activation of the right ventricle results in prolongation of the QRS. This can take the form of an incomplete or complete right bundle branch block pattern. The QRS in the right precordial leads may be longer than that in the left precordial leads.3
 FIGURE 6.2 Incomplete RBBB morphology and epsilon wave. |
Arrhythmias
PVCs
Frequent PVCs in younger adults may indicate myocardial disease.
Ventricular Tachycardia
Nonsustained or sustained VT with left bundle branch morphology results from an ectopic focus in the right ventricle. ARVD should be considered as an underlying cause of an ECG demonstrating right ventricular outflow tract (RVOT) tachycardia.
Ventricular Fibrillation
This is the mechanism of sudden cardiac death in athletes and young adults.
Diagnosis
The diagnosis of ARVD is based on ECG findings, histopathology from endomyocardial biopsy, family history, and imaging findings. Cardiac MRI, 2D echocardiography, and RV angiography are all used to identify RV dysfunction.
Management
Young adults are advised to refrain from vigorous exercise as ventricular arrhythmias are more likely to occur in the setting of an adrenergic surge. Some patients are advised to undergo placement of an ICD.
Wolff-Parkinson-White Pattern
Pathogenesis
In Wolff-Parkinson-White (WPW) syndrome, atrial conduction is able to reach the ventricles through an accessory pathway that bypasses the normal AV conduction system. Because this pathway bypasses AV nodal tissue where conduction is delayed, a portion of the ventricle becomes prematurely excited.
ECG Features
Wide QRS
The QRS wave can be thought of as a fusion complex. It reflects ventricle that is pre-excited by the bypass tract and depolarized by the normal conduction system.
Short PR Interval
Time from atrial to ventricular conduction, represented by the PR interval, is shorter because the accessory pathway bypasses the AV node.
Delta Wave
Represents early depolarization (pre-excitation) of the ventricle by the bypass tract. Though ventricular activation is premature, conduction through this tissue is slower, making the slope of the inscribed delta wave less steep than the rest of the QRS complex.
ST/TW Changes
Secondary to the abnormal depolarization pattern of the ventricles. These changes are discordant with the delta wave (e.g., T waves following a positive delta wave are inverted).
Location
The bypass tract can be left- or right-sided. If the tract is left-sided, conduction will travel from the left ventricle to right ventricle and result in a positive delta wave or tall R wave in V1. Left-sided free wall bypass tracts are most common.5
When the tract is right-sided, conduction spreads from the right to left ventricle, and a negative delta wave with a deep S is present in V1. Patients with Ebstein anomaly commonly have right-sided bypass tracts.
 FIGURE 6.3 Appearance of (A) left- and (B) right-sided bypass tracts in V1. |
 FIGURE 6.4 A. Anterograde conduction down a left-sided bypass tract and normal conduction system. B. Resultant ECG morphology (from the view of a right-sided lead). |
Direction of Conduction
The bypass tract can be limited to conducting in a single direction or have the ability to conduct in both directions. Bypass tracts that conduct anterograde from the atria to the ventricle are recognizable by their ventricular pre-excitation patterns on ECG. Bypass tracts capable of only retrograde conduction are considered to be concealed pathways with no characteristic ECG findings.
Arrhythmias
A patient who has symptoms of tachycardia related to one of these bypass tracts has WPW syndrome.
Atrial Fibrillation
Most feared arrhythmia associated with WPW because it can rarely degenerate into ventricular fibrillation. QRS complex morphology varies dramatically from beat to beat.
AV Reciprocating Tachycardia (AVRT)
Most common arrhythmia associated with WPW. QRS complexes may be wide or narrow depending on the direction of conduction in the bypass tract.
Hypothermia
The conduction system is more sensitive to cold temperatures than the myocardium. With progressive hypothermia, the action potential duration becomes prolonged. PR prolongation results from delay at the AV node. QRS widening and QTc prolongation then develop. P waves may disappear or be difficult to identify when obscured by tremor artifact. The most common rhythm in hypothermic patients is atrial fibrillation with slow ventricular response.5 The ECG findings of hyperkalemia may be masked by hypothermia. However, hypokalemia resulting in QTc prolongation is more commonly observed in accidental hypothermia and during induction of therapeutic hypothermia.6,7
Degrees of Hypothermia
Associated ECG Findings
 ECG 6.1 Tremor artifact. |
 ECG 6.2 J waves. |
 ECG 6.3 AFib with slow ventricular response. |
Thermogenesis from shivering is maximal at this temperature.
The J wave is a deflection in the same direction of the R wave in the terminal portion of the QRS complex. They are most commonly seen in leads II, III, aVF, V5, and V6; they may be seen more diffusely in all leads when hypothermia becomes more severe.
In general, the lower the temperature, the larger the amplitude of the J wave.8
AFib with slow ventricular response is common at temperatures ≤ 32°C and present in more than 50% of patients with moderate hypothermia.8,9
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