ECG Assessment
The 12-lead ECG is the most frequently used technique for obtaining the surface electrocardiographic signal for evaluation of ventricular repolarization. Manual ECG readings are performed using visual determinations ("eyeball"/caliper techniques), digitizing methods, and/or on-screen computerized methods. The accuracy of the automatic measurements of the corrected QT (QTc) interval is questionable in many cases and should be supplemented by manual reading.
Inconsistency between manufacturers in terms of the algorithms used for calculation of the intervals is another problem in the interpretation of computerized readings. Some digitizing methods employ a digitizing pad, magnifying lamp, and pointing device to identify the beginning and end of the QT interval, with an accuracy level of 5 ms. A more technologically advanced option is to display digitally recorded ECGs on a computer screen, where they can be measured using computer-driven, on-screen calipers. This latter approach provides high-quality ECG data and is recommended at core laboratories performing centralized analyses of a large ECG database. Scanned paper-recorded ECGs can also be subjected to on-screen measurements.
The accuracy levels of manual determination with a caliper is 20–40 ms. A standard 12-lead ECG tracing at 25 mm/s paper speed at 10 mm/mV amplitude is generally adequate for accurate measurement of QT-interval duration. Higher speeds (e.g., 50 mm/sec) may lead to distortion of low-amplitude waves such as U waves. The QT interval should be determined as a mean value derived from at least 3–5 cardiac cycles (heartbeats), and is measured from the beginning of the earliest onset of the QRS complex to the end of the T wave. The QT measurement should be made in leads II and V5 or V6, with the longest value being used. The main difficulty lies in identifying correctly the point where the descending limb of the T wave intersects the isoelectric line, particularly when there are T and U waves that are close together. We identify the end of the T wave when its descending limb returns to the TP baseline when it is not followed by a U wave or if it is distinct from the following U wave. When T-wave deflections of equal or near-equal amplitude result in a biphasic T wave, the QT interval is measured to the time of final return to baseline. If a second low-amplitude repolarization wave interrupts the terminal portion of the T wave, it is difficult to determine whether the second deflection is a biphasic T wave or an early-occurring U wave.
In such cases, it is best to record both the QT interval (T-wave offset measured as the nadir between the T and U wave) and the QTU interval (repolarization offset measured at the end of the second wave). In general, biphasic T waves are frequently present in multiple leads, whereas discrete and separate low-amplitude U waves are best seen in the lateral precordial leads. The end of the U wave is defined as the intersection point of the descending limb of the U wave and the isoelectric baseline. This method reflects accurately the real duration of ventricular repolarization, but it introduces a large degree of subjectivity, particularly when biphasic T waves are present or when large U waves interrupt the return of the T wave to the baseline. The method can be effectively applied for manual measurements, but is less suitable for computer analysis because it requires the definition of a given threshold for the amplitude below which T or U wave potentials return to baseline.
The QRS interval can be modified by several factors (such as bundle branch block, Class 1c antiarrhythmic drugs, or preexcitation); these changes in depolarization can alter repolarization in unexpected ways, and thus the QT interval may not be an accurate reflection of repolarization duration. In these patients, the measure of the JT from the S-wave offset to T-wave end may be used, but normal standards for the JT interval are not well established.
|
