Measuring the QT/QTc Interval in the Presence of Wide QRS Complexes

Abstract

Scenario: The electrocardiographic (ECG) rhythm strip below (leads V1 and II) is from a 68-year-old woman with pneumonia who was admitted to the step-down unit from a long-term care facility. Vital signs upon arrival included blood pressure, 107/61 mm Hg; body temperature, 98.9 °F (37.2 °C); heart rate, 68/min; respiratory rate, 21/min; and oxygen saturation (SpO2), 96% on 4 L of oxygen. The attending physician ordered intravenous levofloxacin based on culture of a blood sample collected in the emergency department. Per unit policy, because intravenous levofloxacin is known to prolong the QT/QTc, the nurse must measure and document the QT/QTc before starting the medication in order to determine whether QT/QTc prolongation occurs after administration. How should the QT/QTc be measured in this patient?Sinus rhythm, right bundle branch block (RBBB), possible prior anteroseptal myocardial infarction (MI), and old inferior wall MI.When measuring the QT/QTc in a patient with BBB (RBBB, in this case), because ventricular depolarization is delayed (ie, left to right ventricle), the QT/QTc is prolonged (compare “normal QRS” above). Therefore, the QT/QTc measured in patients with BBB (right or left) has to be adjusted to avoid overestimation. The Bogossian method can be used in patients with BBB, where the modified QT (QTM) is calculated using QT – (QRS/2).1 The QTM is then used to calculate the QTc to account for heart rate using the Bazett formula, or QTM/RR. In this example, QT/QTc would be calculated as follows: QTM = 440 – (160/2) = 360 milliseconds, and RR is 800 milliseconds or 0.8 seconds, so QTc=0.36/0.8=0.402 seconds or 402 milliseconds, which is within the normal range. In comparison, whereas the QT interval is within normal range without using the Bogossian formula, when corrected for heart rate the QTc was prolonged (ie, QTc without Bogossian correction = 0.44/0.8 = 0.492 seconds = 492 milliseconds); hence, adjusting QTc to account for the RBBB in this patient was important.In patients with RBBB, while conduction down the right bundle branch is “blocked,” an initial r wave should be present in lead V1, resulting in the classic rsR’ pattern associated with RBBB (see normal QRS above). However, in patients with a prior septal wall MI, the initial r wave will not be present because depolarization is disrupted in this myocardial region, as is seen in this example. Acute ischemia may be present when examining lead V1 because one would expect the ST segment to be sloping downward and not to be isoelectric with the PR interval. A Q wave is also seen in lead II, which is ~40 milliseconds, suggesting that this patient may also have a prior inferior wall MI, although this feature may be associated with the RBBB.Based on the QT/QTc calculated using the Bogossian correction and Bazett formulas, the antibiotic can be administered to this patient safely. However, QT/QTc should be measured every 8 to 12 hours as it can become prolonged after antibiotic administration. Because of the ECG waveform changes suggestive of prior septal and inferior wall MI and possible acute ischemia, a 12-lead ECG should be obtained to rule out these conditions. In this patient, prior inferior wall (abnormal Q wave in leads III and aVF) and septal wall (missing r wave in lead V2) MI was confirmed. The 12-lead ECG also showed possible acute myocardial ischemia in the other precordial leads; hence, a cardiac workup (ie, cardiac troponin blood test, serial ECGs, and echocardiogram) was ordered. The QT/QTc measured in lead V1 of the 12-lead ECG did not differ from the above measurement. Careful continuous ECG monitoring for ventricular arrhythmias (ie, torsades de pointes, ventricular tachycardia/fibrillation) is paramount in this patient because of the possibility of QT/QTc prolongation and ECG changes suggestive of acute ischemia. Ultimately, this patient was treated for both pneumonia and myocardial ischemia. She did not have QT/QTc prolongation or ventricular arrhythmias and was discharged back to the long-term care facility.