A A commonly used formula that overestimates the risk of an adverse cardiac event can lead to incorrect chemotherapy decisions according to a
With many cancer drugs, there is a risk of prolongation of the QT interval, which promotes fatal cardiac arrhythmias in patients.Because of this risk, cancer patients often undergo ECG monitoring, paying special attention to the QT interval.
The aim is to detect extensions early so that the drug dosage can be changed to avoid arrhythmias.
For the calculation of heart rate corrected QT intervals (QTc), three formulas are common, which differ mathematically from each other.
The first study, which examined how these formulas affect treatment decisions for cancer patients, has now raised concerns.
After evaluating nearly 20,000 ECGs from cancer patients, the researchers say it’s time for oncologists to decide on and maintain a formula to calculate corrected QT intervals.
They came to this conclusion after finding that a commonly used formula, the Bazett formula, likely overestimated the frequency of prolonged QT intervals and could lead to inappropriate changes in chemotherapy.
The researchers found that the Bazett formula was associated with a threefold higher rate of grade 3 QT extensions compared to Fridericia’s and Framingham’s formulas, which are also commonly used to monitor ECGs.
Since there is no “clear guidance on how to adequately calculate QTc, clinical decisions based on prolonged QTc levels can vary greatly depending on the formula used.”
This can be “detrimental to patient outcomes,” the researchers warn.They suspect that undue withholding of chemotherapy as a result of the use of the Bazett formula “is likely to occur in routine oncology practice.”
The first important step in solving the problem is to standardize QTc monitoring in oncology,” the researchers said.
The researchers calculated and compared QTc intervals using the Bazett, Fridericia, and Framingham formulas using 19,955 ECGs from 6,881 adult patients treated between at the University of North Carolina Cancer Clinic.
The medium QTc values according to Bazett were 26.4 milliseconds higher than with the Fridericia formula and 27.8 milliseconds higher than with the Framingham formula.
The differences were “substantially higher than the average differences reported by non-oncology populations,” the team notes.
9% of patients had QTc prolongation grade 3 defined as a QTc value greater than 500 milliseconds compared to only 1.8% according to Framingham and 2.8% according to Fridericia.
Of 1,786 ECGs classified as Grade 3 by Bazett, 81% were classified as Grade 2 or less by either Fridericia or Framingham.
Of 2,340 ECGs from 421 patients who received chemotherapy known to prolong the QT interval, 12.5% had QTc grade 3 according to Bazett, compared to only 2.7% according to Framingham and 4.5% according to Fridericia.
“While we know that there is no standard criterion to check whether the bazett formula “correctly” adjusts QT time to heart rate, our results show that there is a significantly increased risk of QTc overestimation when using the Bazett formula for cancer patients,” the researchers said.
The team also examined 496 ECGs from 142 patients to evaluate the impact of formula selection on chemotherapy decisions.
The detection of QT prolongation resulted in chemotherapy and/or concomitant QT-prolonging drugs being withheld, reduced or discontinued in 28 cases.5 changes (17.9%) were inappropriate according to either the Fridericia or Framingham formula.
“We suspect that in this study, the actual number of clinical changes, including inappropriate changes, was underestimated, as some of them were probably not documented in the clinical records,” the investigators said.
In addition, only 6% of cases documented which QTc formula was used for decision-making.
Three commonly used formulas
Although it is already known that the Bazett formula overestimates the QTc value compared to the Fridericia and Framingham formulas, it is often used as the standard formula in ECG software to calculate the QTc value, which many oncologists probably do not know.
In addition, in oncology, in the labelling of medicinal products and in the notification unwanted Events often required QTc monitoring, but not generally recommended one formula over another.
“In the absence of such a standard, the best formula is the one … that minimizes both adverse events and inappropriate interventions,” according to the authors of an accompanying editorial, Benjamin Starobin, MD, and Kevin Kwaku, MD, PhD.
The study was funded by the National Institutes of Health.
A version of this article first appeared on Medscape.com, part of the Medscape Professional Network.
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Source – https://www.univadis.de/viewarticle/ubliche-berechnung-des-qtc-intervalls-kann-zu-falschen-chemotherapieentscheidungen-fuhren