How is QTc calculated using Bazett's formula?

How is QTc calculated using Bazett's formula?

• A. QTc = QT / sqrt(RR) with QT and RR in seconds
• B. QTc = QT * sqrt(RR) with QT and RR in seconds
• C. QTc = QT + RR
• D. QTc = QT / RR

Answer: (A)

Bazett's formula corrects the QT interval for heart rate by normalizing it to a standard 60 bpm. The key idea is that the QT interval changes with how fast the heart is beating, so we adjust it using the square root of the RR interval. The correct expression uses QT divided by the square root of RR, with both QT and RR in seconds. This reflects what the QT would be if the heart rate were 60 bpm. For example, if QT is 0.40 s and RR is 0.80 s, QTc ≈ 0.40 / sqrt(0.80) ≈ 0.40 / 0.894 ≈ 0.447 s (447 ms). The other forms don’t properly account for the rate effect: multiplying by sqrt(RR) would overestimate correction at slower rates, adding QT and RR has no basis for rate normalization, and dividing by RR doesn’t mirror the established relationship Bazett describes.

Bazett's formula corrects the QT interval for heart rate by normalizing it to a standard 60 bpm. The key idea is that the QT interval changes with how fast the heart is beating, so we adjust it using the square root of the RR interval. The correct expression uses QT divided by the square root of RR, with both QT and RR in seconds. This reflects what the QT would be if the heart rate were 60 bpm. For example, if QT is 0.40 s and RR is 0.80 s, QTc ≈ 0.40 / sqrt(0.80) ≈ 0.40 / 0.894 ≈ 0.447 s (447 ms). The other forms don’t properly account for the rate effect: multiplying by sqrt(RR) would overestimate correction at slower rates, adding QT and RR has no basis for rate normalization, and dividing by RR doesn’t mirror the established relationship Bazett describes.