![]() ![]() In literature, there are many papers describing QT interval variation and mortality in patients with cardiac or non-cardiac pathologies. QT interval variations and mortality rate However, less is known regarding shortened QT intervals, especially at the population level. Therefore, arrhythmias are even more likely to be malignant in short, compared with long, QT syndrome ( 13). This creates a substrate for functional re-entry, similar to that of long QT syndrome but with hastened recovery and reduced refractoriness in the ventricle ( 13). Similar to QT prolongation, shortening of the QT interval is not uniform in time and space in the ventricle ( 7), producing an exaggerated heterogeneity of repolarization. The progressive association of QT-interval duration with mortality reflects the increased likelihood of ventricular arrhythmias associated with increasing heterogeneity in the ventricular action potential duration ( 7). This allows for the development of functional reentry, in which still-activated regions of the ventricular myocardium re-enter and reactivate regions with shorter action potentials, producing polymorphic ventricular tachycardias (such as torsade de pointes) ( 7). Longer QT intervals reflect longer ventricular action potentials and a reduction in the repolarizing reserve that is associated with exaggerated spatial and temporal heterogeneity of electrical recovery of the ventricle ( 12). This can progress to ventricular fibrillation and sudden cardiac death. Early afterdepolarizations of sufficient amplitude can generate premature action potentials that lead to cardiac arrhythmias. Prolongation of the QT interval is associated with early afterdepolarizations, in which an abnormal depolarization occurs during phase 2 or 3 of the action potential before repolarization has been completed. Mechanism of ventricular arrhythmias in QT interval variations 1), both in daily clinical practice and for scientific purposes. Therefore, automated measurements should be manually confirmed ( Fig. Manual measurements of the QTc interval are better than digital (with the 12SL algorithm) because the latter may lack a prolonged QTc interval diagnostic statement ( 11). ![]() Heart rate correction of the QT interval is fraught with problems. The report proposed a reference range for linear function-adjusted QT-interval durations from 390 to 450 ms for men and 390 to 460 ms for women. In addition, heart rate, sex, and age should be incorporated into QT adjustments because women and elderly individuals tend to have longer QT intervals. The 2009 recommendations for the standardization and interpretation of the QT interval from the American Heart Association Electrocardiography and Arrhythmias Committee Council on Clinical Cardiology, American College of Cardiology Foundation, and Heart Rhythm Society were to use linear regression functions (the R-R interval-adjusted QT-interval duration) for QT correction instead of Bazett’s formula ( 10). However, Bazett’s correction has a strong residual correlation with heart rate. Finally, within-person variability and measurement error are additional sources of variability in the QT interval length.īazett’s correction is still recommended for the diagnosis of congenital long and short QT syndrome ( 8, 9). Other factors associated with QT interval length variability include age, sex, hypertension, body mass index, medication usage, low-calorie diets, serum potassium levels, and common genetic variants ( 7). Marked prolongations in the QT interval may be caused by genetic disorders (e.g., long QT syndrome), pharmacological agents (e.g., antiarrhythmics, antipsychotics, and antibiotics), electrolyte abnormalities (e.g., hypokalemia and hypomagnese-mia), and their interactions ( 7). Normal values for the QTc range from 350 to 450 ms for adult men and from 360 to 460 ms for adult women however, 10%-20% of otherwise healthy persons may have QTc values outside this range. The electrocardiographic QTc is approximately normally distributed in the general population. Bazett’s formula is the most commonly used method to calculate QTc and adjusts for the heart rate, although it tends to underestimate the duration of repolarization when the heart rate is particularly slow (or overestimate when the heart rate is fast). Frequently, reference ranges for the QT-interval in the general population are expressed in terms of QTc, a corrected form of the QT interval ( 3- 6). ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |