Second a study published on JAMA Oncology, a commonly used formula, which overestimates the risk of a cardiac adverse event, can lead to inappropriate chemotherapy decisions.
Many cancer drugs carry a risk of QT prolongation, which predisposes patients to fatal cardiac arrhythmias.Because of this risk, cancer patients are often monitored with ECGs, with a focus on the QT interval.
The idea is to detect any prolongations early so that medication dosages can be modified to prevent arrhythmias.
3 different mathematical formulas are commonly used to calculate heart rate-corrected QT intervals.
Now, the first study to examine how these formulas affect treatment decisions for cancer patients has raised concerns.
After examining almost 20,000 ECG of cancer patients, investigators say it’s time for oncologists to choose 1 formula to calculate the correct QT intervals and not change it.
They came to their conclusion after discovering that the commonly used 1 formula, Bazett’s formula, probably overestimates the incidence of QT prolongation and could result in inappropriate chemotherapy changes.
The researchers determined that Bazett’s formula is associated with a grade 3 tripling of QT prolongations compared to Fridericia and Framingham formulas, which are also commonly used to monitor ECGs.
Without «clear guidelines on how to correctly calculate QTc, clinical decisions based on QTc prolongation values can vary widely» depending on the formula used.
This could «negatively affect patient outcomes,» the researchers warn.They suspect that «routine oncology practice is likely to occur» inappropriate temporary suspensions of chemotherapy following the use of Bazett’s formula.
The «essential first step in solving» the problem «is to standardize practice for QTc monitoring in oncology,» they suggest.
The researchers calculated and then compared QTc intervals using Bazett, Fridericia and Framingham’s formulas of 19,955 ECGs obtained from 6,881 adult patients treated at the University of North Carolina Oncology Hospital from .
The median QTc values calculated with Bazett’s formula were 26.4 milliseconds higher than Fridericia’s formula and 27.8 milliseconds higher than Framingham’s formula.
The differences are «considerably greater than the average differences reported in non-cancer populations,» the team notes.
9% of patients had grade 3 QTc prolongation, defined as QTc greater than 500 milliseconds, with the Bazett formula, compared with only 1.8% with the Framingham formula and 2.8% with the Fridericia formula.
On 1.786 ECGs classified as grade 3 with Bazett’s formula, 81% were grade 2 or lower with Fridericia or Framingham’s formula.
Among 2,340 ECG of 421 patients who received chemotherapy known to prolong the QT interval, 12.5% had grade 3 QTc according to Bazett’s formula, compared with only 2.7% with the Framingham formula and 4.5% with the Fridericia formula.
«While acknowledging that there is no standard criterion for testing whether Bazett’s formula is correcting QT for heart rate, our results indicate that there is a very high risk of overestimating QTc using Bazett’s formula in cancer patients,» the investigators state.
The team also looked at 496 ECG of 142 patients to assess the impact of the selected formula on chemotherapy decisions.
The result of QT prolongation led to temporary discontinuation, reduction or permanent discontinuation of chemotherapy and/or concomitant QT-prolonging drugs in 28 cases; 5 changes (17.9%) were found to be inappropriate according to Fridericia or Framingham formulas.
«We suspect that this study underestimated the actual number of clinical changes, including inappropriate changes, because some were probably not documented in medical records,» the investigators say.
Moreover, only in 6% of cases was the formula used to calculate the QTc in the decisions indicated.
Three commonly used formulas
Although it is already known that Bazett’s formula overestimates QTc compared to Fridericia and Framingham’s formulas, it is often the default formula used by ECG software to calculate QTc, which, investigators suspect, many oncologists do not know.
Also, although drug labels and adverse event reporting schemes often require QTc monitoring in oncology, they generally do not recommend 1 formula over another.
«In the absence of such a standard, the best formula is the one that…minimizes adverse events and inappropriate interventions,» say the authors of an accompanying editorial editorial accompanying , Dr. Benjamin Starobin and Dr. Kevin Kwaku, PhD.
The study was funded by the National Institutes of Health.
A version of this article first appeared on Medscape.com, part of Medscape Professional Network.
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Source — https://www.univadis.it/viewarticle/un-metodo-comune-per-il-calcolo-degli-intervalli-qtc-puo-portare-a-decisioni-sulla-chemioterapia-non-accurate