Validation of Nomogram to Predict Exercise Capacity from Treadmill Speed and Grade in Cardiac Patients

Abstract

Using treadmill speed and grade to estimate peak VO2 (VO2pk) in cardiac patients results in overestimation when compared to VO2pk directly measured with spirometry. Recently, Ades et al.(Circulation 2006; 113:2706-2712) developed a nomogram for converting VO2pk estimated from treadmill speed and grade to a more accurate VO2pk in patients with cardiac disease. PURPOSE: To investigate the accuracy of this nomogram in an independent sample of cardiac patients. METHODS: Data were analyzed from males who underwent a cardiopulmonary exercise test at entry into a cardiac rehabilitation program, or were 3 months post-coronary artery bypass (CABG) surgery. A symptom-limited exercise test was performed on a treadmill and VO2pk was measured with spirometry. Patients selected a brisk walking pace that was kept constant while grade was increased each minute until volitional fatigue. The ACSM metabolic equation for walking was used to estimate VO2pk from treadmill speed and grade. The Ades nomogram was used to correct ACSM-derived VO2pk to a new estimated VO2pk specific for cardiac patients. Data from ACSM VO2pk and Ades VO2pk were compared to directly measured VO2pk. RESULTS: Ninety men (80% post-CABG, 10% post-PCI; 10% stable angina) were included in this analysis. Demographic data (mean + SD) were: age =60.8+8.3 (yr); BMI=28.2+4.5; LVEF=48.3+11 (%). Patients achieved a measured VO2pk=20.3+4.8 ml/kg/min, with an ACSM predicted VO2pk=24.9+9.8 ml/kg/min and Ades nomogram corrected VO2pk= 19.4+2.9 ml/kg/min. The average absolute value of error with ACSM equation = 6.6+5.3 ml/kg/min versus 2.9+2.2 ml/kg/min with Ades correction. The mean absolute value of error in VO2pk was significantly lower with the Ades correction compared to ACSM prediction equation alone p< 0.001. CONCLUSIONS: The ACSM walking metabolic equation overestimated VO2pk in male cardiac patients, with an average difference approximating 1.9 METs. The Ades nomogram successfully corrected the ACSM metabolic equation to within ∼0.8 MET of measured VO2pk on average in these patients. This study validates the utility of the Ades nomogram for predicting VO2pk from treadmill speed and grade in male cardiac patients.