Functional characteristics of QT prediction formulas. The concepts of QTmax and QT rate sensitivity.

A community-based population sample of 14,379 North American children and adults aged from birth to 75 years with normal ECG was used to investigate functional properties of 13 QT prediction formulas over a wide range of heart rates (HR) in normal sinus rhythm. QT measurements were made by a computer program from simultaneously sampled standard 12-lead or orthogonal XYZ leads and all QT measurements were visually verified using a high-resolution display terminal. New concepts of QT rate sensitivity (dQT/dHR) and maximum predicted QT after a long pause (QTmax) are introduced. Drastically diverging functional behavior is documented for different QT prediction formulas regarding QT rate sensitivity and QTmax. The simplest formula for predicted QT (QTp) meeting reasonable physiological constraints regarding QTmax and functional behavior of QT rate sensitivity while producing the best QT prediction accuracy was a function with only one independently adjusted parameter (QTmax) of the form: QTp = QTmax/(1 + 0.01HR), with QTmax = 656 ms. This formula also suggests that QT rate sensitivity is inversely proportional to the square of HR and that individual values of QTmax can be predicted from ambulatory or exercise ECGs using the formula QTmax = 2 x QT100, where QT100 is QT at HR = 100/min.