New Formula to Predict Heart Rate at Anaerobic Threshold That Considers the Effects of β-Blockers in Patients With Myocardial Infarction

Purpose: It is recommended that patients with myocardial infarction (MI) be prescribed exercise by target heart rate (HR) at the anaerobic threshold (AT) via cardiopulmonary exercise testing (CPX). Although percent HR reserve using predicted HR max (%HRR pred ) is used to prescribe exercise if CPX or an exercise test cannot be performed, %HRR pred is especially difficult to use when patients take β-blockers. We devised a new formula to predict HR at AT (HR AT ) that considers β-blocker effects in MI patients and validated its accuracy. Methods: The new formula was created using the data of 196 MI patients in our hospital (derivation sample), and its accuracy was assessed using the data of 71 MI patients in other hospitals (validation sample). All patients underwent CPX 1 mo after MI onset, and resting HR, resting systolic blood pressure (SBP), and HR AT were measured during CPX. Results: The results of multiple regression analysis in the derivation sample gave the following formula ( R 2 = 0.605, P < .001): predicted HR AT = 2.035 × (≥65 yr:−1, <65 yr:1) + 3.648 × (body mass index <18.5 kg/m 2 :−1, body mass index ≥18.5 kg/m 2 :1) + 4.284 × ( β 1-blocker(+):−1, β 1-blocker(−):1) + 0.734 × (HR rest ) + 0.078 × (SBP rest ) + 36.812. This formula consists entirely of predictors that can be obtained at rest. HR AT and predicted HR AT with the new formula were not significantly different in the validation sample (mean absolute error: 5.5 ± 4.1 bpm). Conclusions: The accuracy of the new formula appeared to be favorable. This new formula may be a practical method for exercise prescription in MI patients, regardless of their β-blocker treatment status, if CPX is unavailable.