![]() ![]() However, the Fridericia method is unreliable at higher heart rates. In a study of 108 patients, the Bazett formula was found to have a sensitivity for detecting manually measured QT prolongation of 54% compared to 100% sensitivity of the Fridericia formula (11). The Fridericia formula (QTc = QT / RR 1/3) was also developed in 1920 (10). As patients are at increased risk of TdP at slower heart rate, the under-correction by the Bazett method at slower heart rates has prompted investigation into alternative formulae. However, the Bazett formula under-corrects the QTc at slower heart rates while overcorrecting at faster heart rates (7,8,9). Bazett published the first version of this formula in 1920 using ECGs from 39 young subjects and was subsequently updated by Taran and Szilagyi in 1947 (5,6). The most frequently used method for calculating the QTc is the Bazett formula, QTc = QT / √RR. In all the following formulae as written below the RR, QT, and QTc are expressed in seconds. Note that in the following formulae, RR is the interval between two consecutive R waves. The maximum slope intercept method can be particularly useful in patients with tachycardia when the subsequent P wave can obscure the terminal portion of the T wave. The point at which this tangent line crosses the isoelectric line is the end of the T wave. The end of the T wave is determined using the maximum slope intercept method, in which a tangent line is drawn through the maximum down slope of the T wave. Smaller U waves and those separate from the T wave should be excluded from the measurement (4). U waves greater than 50% of the T wave amplitude and fused to the T wave should be included in the QT interval measurement. The QT interval should be measured and averaged over 3 or more beats (3). The QT interval should be measured in leads clearly demonstrating all portions to be measured, most frequently lead II or V5-6. The QT interval is measured from the beginning of the QRS complex to the end of the T wave. A QTc of greater than 500ms is associated with an increased risk of TdP (1). The QTc is considered prolonged if greater than 440ms in males or 460ms in females. By correcting the QT interval for the heart rate, the physician is able to compare QT intervals at different heart rates over time and assess for increased risk of dysrhythmias. These calculated measurements are referred to as the corrected QT interval (QTc) and estimate the QT interval at a standard heart rate of 60 beats per minute. Several formulae have been developed to include both the QT interval and the heart rate to determine the risk of ventricular dysrhythmia. ![]() The QT interval alone is not enough to determine the likelihood of developing dysrhythmias as the QT interval is inversely proportional to the heart rate, with the QT interval shortening at faster heart rates and lengthening at slower heart rates. A prolonged QTc interval increases the repolarization period of cardiac myocytes, and a premature ventricular contraction (PVC) that occurs during this repolarization period can cause TdP in the R on T phenomenon, which itself can degenerate into ventricular fibrillation (2). It often occurs in hospitalized patients receiving QT prolonging interventions in the setting of underlying cardiac disease or electrolyte disturbances (1). TdP is a rare version of polymorphic ventricular tachycardia characterized by a pattern of alternating amplitudes, or “twisting points,” on ECG. QT prolongation puts patients at risk of dysrhythmias such as torsades de pointes (TdP). Other QT-prolonging conditions include hypothermia, myocardial ischemia, increased intracranial pressure, and congenital long QT syndrome. Several electrolyte disturbances can also prolong the QT interval including hypokalemia, hypomagnesemia, and hypocalcemia. There are many drugs associated with QT prolongation including but not limited to methadone several classes of antimicrobials antifungals antiretrovirals antiemetics antipsychotics and class IA, IC, and III antidysrhythmics. The QT interval corresponds to the time period from ventricular depolarization and contraction to ventricular repolarization and relaxation. One effect many drugs, both pharmaceutical and recreational, have on the heart is to prolong the QT interval. When caring for the poisoned patient, it is essential to rapidly detect dysrhythmias or to determine the risk of the patient developing a dysrhythmia. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |