Monitoring Drug-Induced Prolonged QT Interval: Why 12 Leads Are Best

GE Healthcare

Many drugs are known to cause prolonged QT interval—including some antidepressants, antipsychotics, antibiotics, and other medications, such as the antimalarial drug hydroxychloroquine, which had its COVID-19 emergency use authorization removed by the FDA in June 2020, partly due to this risk.

As Healio notes, because patients with prolonged QT who take these medications are at risk of fatality from torsades de pointes, many labels indicate that QT intervals should be measured before the therapeutic regimen begins. That way, providers can assess the patient's safety and establish baselines for monitoring future changes.

Still, this precaution may not always take place at the point of care, which can put patients at risk for drug-induced prolonged QT. The careful placement of 10 electrodes requires additional time and contact, increasing COVID-19 infection risk, so practitioners may be wondering whether single-lead ECG devices can be a suitable alternative. While they do present a viable—albeit imperfect—option for assessing QT intervals in some clinical situations, they should not be used exclusively.

The Pros and Cons of Single-Lead ECGs

The appeal of using single-lead ECGs is clear: with just one lead to read and none to superimpose, they are simple to interpret, comparatively inexpensive, and ubiquitous in both the clinic and the home, thanks to the advent of digital app technology. Importantly, they also require less of the technician's time. Single readings, for example, include sensors only on the limbs—and not on the chest, as required by 12-lead ECGs.

However, single-lead ECGs do have limitations. For one, limb leads have been known to have flattened T waves, compared to chest leads on a standard 12-lead ECG. And though normal U waves should not be included in QT, disparities in U-wave abnormalities between single-lead and 12-lead machines can also complicate diagnoses; chest leads inform more discernible U waves, which can be more subtle and thus harder to interpret on single-lead devices. Because large and irregular U waves commonly precede torsades de pointes, these gaps in diagnostic knowledge can dramatically impede informed care planning.

12-Lead ECG Is the Gold Standard

Due to the limitations of single-lead machines, in contrast to the comprehensive insights from 12-lead ECG, the latter is considered the gold standard for initial and ongoing QT assessment. As such, guidelines adopted by the American Heart Association now include two critical evidence-based best practices from the Common Standards for Quantitative Electrocardiography:

  • ECGs should illustrate beat alignment that best depicts a selective complex average.
  • Intervals from multiple lead sources should be taken to help identify the earliest and latest points of onset and offset for ECG waveforms.

However, these recommendations don't preclude single-lead ECGs or render them useless. They still have utility for QT surveillance, if they are used correctly. For instance, single-lead ECG can be helpful when there's a need to track changes in QT interval after a baseline 12-lead has already been conducted and therapy has been administered. If the single-lead ECG presents abnormalities or changes develop over time, the 12-lead can step in to confirm.

 


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Strategies for Detecting Long QT with 12-Lead ECGs

Once you have determined that the 12-lead ECG would be the best choice for your current case, recognizing drug-induced prolonged QT will require a multistep process. The steps are as follows:

  • Measure the QT (QRS complex, ST segment, T wave, and only U wave if it's abnormal).
  • Measure the RR (peak of QRS complex, ST segment, T wave, U wave, peak of subsequent QRS).
  • Correct for heart rate using one of the many available formulas, such as Bazett: QTc = QT multiplied by (HR/60)1/2. Since all heart correction formulas have limitations, when possible, acquire the ECG during a fixed heart rate between 60 and 100 beats per minute.
  • Assess for obvious long QT if QT is greater than half of RR.
  • Compare QT with normal sex-based averages (under 470 msec for men and under 480 msec for women).

Seek additional troubleshooting methods if you encounter the following difficulties:

  • A biphasic T wave in lead two: Define the baseline by QRS onset, not the end of the T wave. Use all the leads to assist.
  • A varying heart rate: Determine an overall QT interval and RR by employing one of the many available methods. For instance, you could use the QT of the median beat and the average RR.
  • Excessive artifact tracings: Repeat ECG until a delineated waveform is established, or use median complexes from superimposed beats for a cleaner depiction.

Above all, it is critical that providers have fast and accurate tools available for waveform analysis, both before drug administration and during therapy. The use of 12-lead ECG, in combination with single-lead devices where appropriate, can play a vital role in spotting signs before they progress into life-threatening complications.