Background
Brugada syndrome is a disorder characterized by sudden death associated with one of several electrocardiographic (ECG) patterns characterized by incomplete right bundle-branch block and ST elevations in the anterior precordial leads. See the image below.
Three types of ST-segment elevation in Brugada syndrome, as shown in the precordial leads on ECG in the same patient at different times. Left panel shows a type 1 ECG pattern with pronounced elevation of the J point (arrow), a coved-type ST segment, and an inverted T wave in V1 and V2. The middle panel illustrates a type 2 pattern with a saddleback ST-segment elevated by >1 mm. The right panel shows a type 3 pattern in which the ST segment is elevated < 1 mm. According to a consensus report (Antzelevitch, 2005), the type 1 ECG pattern is diagnostic of Brugada syndrome. Modified from Wilde, 2002. Image courtesy of Richard Nunez, MD, and EMedHome.com (http://www.emedhome.com/).
In the initial description of Brugada syndrome, the heart was reported to be structurally normal, but this concept has been challenged. Subtle structural abnormalities in the right ventricular outflow tract have been reported.
Brugada syndrome is genetically determined and has an autosomal dominant pattern of transmission in about 50% of familial cases (see Etiology). The typical patient with Brugada syndrome is young, male, and otherwise healthy, with normal general medical and cardiovascular physical examinations.
Patients with Brugada syndrome are prone to develop ventricular tachyarrhythmias that may lead to syncope, cardiac arrest, or sudden cardiac death. Infrahisian conduction delay and atrial fibrillation may also be manifestations of the syndrome.
About 5% of survivors of cardiac arrest have no clinically identified cardiac abnormality. About half of these cases are thought to be due to Brugada syndrome.
At present, implantation of an automatic implantable cardiac defibrillator (ICD) is the only treatment proven effective in treating ventricular tachycardia and fibrillation and preventing sudden death in patients with Brugada syndrome
Pathophysiology
Brugada syndrome is an example of a channelopathy, a disease caused by an alteration in the transmembrane ion currents that together constitute the cardiac action potential. Specifically, in 10-30% of cases, mutations in the SCN5A gene, which encodes the cardiac voltage-gated sodium channel Nav 1.5, have been found. These loss-of-function mutations reduce the sodium current (INa) available during the phases 0 (upstroke) and 1 (early repolarization) of the cardiac action potential.
This decrease in INa is thought to affect the right ventricular endocardium differently from the epicardium. Thus, it underlies both the Brugada ECG pattern and the clinical manifestations of the Brugada syndrome.
The exact mechanisms underlying the ECG alterations and arrhythmogenesis in Brugada syndrome are disputed. The repolarization-defect theory is based on the fact that right ventricular epicardial cells display a more prominent notch in the action potential than endocardial cells. This is thought to be due to an increased contribution of the transient outward current (Ito) to the action potential waveform in that tissue.
A decrease in INa accentuates this difference, causing a voltage gradient during repolarization and the characteristic ST elevations on ECG. Research has provided human evidence for a repolarization gradient in patients with Brugada syndrome using simultaneous endocardial and epicardial unipolar recordings. See the image below.
Schematics show the 3 types of action potentials in the right ventricle: endocardial (End), mid myocardial (M), and epicardial (Epi). A, Normal situation on V2 ECG generated by transmural voltage gradients during the depolarization and repolarization phases of the action potential. B-E, Different alterations of the epicardial action potential that produce the ECG changes observed in patients with Brugada syndrome. Adapted from Antzelevitch, 2005.
When the usual relative durations of repolarization are not altered, the T wave remains upright, causing a saddleback ECG pattern (type 2 or 3). When the alteration in repolarization is sufficient to cause a reversal of the normal gradient of repolarization, the T wave inverts, and the coved (type 1) ECG pattern is seen. In a similar way, a heterogeneous alteration in cardiac repolarization may predispose to the development of reentrant arrhythmias, termed phase 2 reentry, that can clinically cause ventricular tachycardia and ventricular fibrillation
An alternative hypothesis, the depolarization/conduction disorder model, proposes that the typical Brugada ECG findings can be explained by slow conduction and activation delays in the right ventricle (in particular in the right ventricular outflow tract).
One study used ajmaline provocation to elicit a type 1 Brugada ECG pattern in 91 patients, and found that the repolarization abnormalities were concordant with the depolarization abnormalities and appeared to be secondary to the depolarization changes. Using vectorcardiograms and body surface potential maps, investigators were able to show that depolarization abnormalities and conduction delay mapped to the right ventricle.
Etiology
The prototypical case of Brugada syndrome has been associated with alterations in the SCN5A gene, of which nearly 300 mutations have been described.Mutations in other genes have been proposed to cause a variant of Brugada syndrome, including the genes coding for alpha1- and beta2b-subunits of the L-type calcium channel (CACNA1C and CACNB2), which are thought to cause a syndrome of precordial ST elevation, sudden death, and short QT interval.
Mutations in the genes GPD1-Land SCN1Bhave been identified in a few familial cases. Cases in which a mutation in the SCN5A gene cannot be demonstrated may be due to mutations of these genes, due to other unidentified genes, or located in regions of the coding sequence or promoter region of SCN5Athat are not routinely sequenced in lab tests.
Many clinical situations have been reported to unmask or exacerbate the ECG pattern of Brugada syndrome. Examples are a febrile state, hyperkalemia, hypokalemia, hypercalcemia, alcohol or cocaine intoxication, and the use of certain medications, including sodium channel blockers, vagotonic agents, alpha-adrenergic agonists, beta-adrenergic blockers, heterocyclic antidepressants, and a combination of glucose and insulin.
Source emedecine.com
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