Electrocardiographic Diagnosis of Myocardial Infarction during Left Bundle Branch Block

Left bundle branch block and the evolving role of QRS morphology in selection of patients for cardiac resynchronization

BACKGROUND

The clinical significance of left bundle branch block (LBBB) has recently expanded with the discovery of a strong association with better outcomes in patients receiving cardiac resynchronization therapy.

METHODS

Several milestones have contributed to the current understanding on the role of LBBB in clinical practice.

RESULT

Sunao Tawara described the arrangement of components of what he called the cardiac conduction system from the atrioventricular node to the terminal Purkinje fibers that connect to the working myocardium, and his hypotheses on how it functions remain current. Mauricio Rosenbaum and colleagues developed the bifascicular model of the left-sided conduction system that explains the characteristic electrocardiographic changes associated with propagation disturbances in its components. Andrés Ricardo Pérez-Riera and others have disputed the bifascicular model as oversimplified and have emphasized the role of the left septal fascicle. Marcelo Elizari and colleagues have explained the importance of masquerading bundle branch block. Elena Sgarbossa and colleagues developed a scheme to recognize ST elevation myocardial infarction in patients with left bundle branch block which remains current after more than 20 years. Enrique Cabrera and others identified electrocardiographic signs of remote myocardial infarction.

CONCLUSION

Substantial progress has been made in the understanding of LBBB, yet its role in clinical practice continues to evolve and important gaps remain to which research should be directed.

Diagnosis of myocardial infarction and ischemia in the setting of bundle branch block and cardiac pacing

The diagnosis of myocardial infarction (MI) in the presence of left bundle branch block (LBBB) or during ventricular pacing (VP) is challenging because of inherent changes in the sequence of ventricular depolarization and repolarization associated with both conditions. Although LBBB and right ventricular (RV) pacing may both produce abnormalities in the ECG, it is often possible to diagnose an acute MI (AMI) or an old MI based on selected morphologic changes. Primary ST-segment changes scoring 3 points or greater according to the Sgarbossa criteria are highly predictive of an AMI in patients with LBBB or RV pacing. The modified Sgarbossa criteria are useful for the diagnosis of AMI in patients with LBBB; however, these criteria have not yet been studied in the setting of RV pacing. Although changes of the QRS complex are not particularly sensitive for the diagnosis of an old MI in the setting of LBBB or RV pacing, the qR complex and Cabrera sign are highly specific for the presence of an old infarct. Diagnosing AMI in the setting of biventricular (BiV) pacing is challenging. To date there is minimal evidence suggesting that the traditional electrocardiographic criteria for diagnosis of AMI in bundle branch block may be applicable to patients with BiV pacing and positive QRS complexes on their ECG in lead V1. This report is a careful review of the electrocardiographic criteria facilitating the diagnosis of acute and remote MI in patients with LBBB and/or VP.

Translating airway biomarker information into practice: from theoretical science to applied medicine

Biomarkers ranging from simple to sophisticated have been used by man for many years of his existence. The main use for biomarkers over that time has been to assess relative states health and well-being, including the presence of functional limitations that presage debilitation and even death. In recent years, there has been intense interest in the development of non-invasive biomarkers to accurately predict disease state and progression, as well as potential drug therapy to assist in early mitigation of morbidity and possibly, forestall premature mortality. The development of biomarkers of airway status has followed a similar pattern, and in recent years, several biomarkers have followed the progression from basic and pre-clinical development, to clinical/translational application, and finally to potential clinical therapeutic application. Inherent in this progression is the refinement of technology that has allowed measurement of these biomarkers in a fast, convenient, and reliable fashion, such that they can be obtainable within a clinical practice setting, to allow the physician to make treatment decisions for diseases such as asthma and COPD. While the clinical therapeutic application of airway biomarkers such as exhaled nitric oxide and β(2)-adrenoreceptor Arg-16 polymorphism are still in their infancy, they have followed this common pathway of development, and now will require some years of application to demonstrate their true utility as predictive biomarkers of airway status and treatment response.