Relationship between double product break point, lactate threshold, and ventilatory threshold in cardiac patients.
A double product break point (DPBP) occurs simultaneously with both ventilatory threshold (VT) and lactate threshold (LT) in normal subjects. We sought to determine whether a DPBP also occurs in cardiac patients and to investigate correlations between DPBP, VT and LT (study 1). We also evaluated a non-invasive DPBP measurement system that determined blood pressure automatically by the cuff method (study 2). Study 1 comprised 15 patients [8 men and 7 women, mean (SD) age 47.7 (11.1) years] who performed cardiopulmonary exercise tests on a cycle ergometer. The double product was defined as the product of heart rate and direct systolic blood pressure. Arterial blood samples were obtained for measurement of lactate, pyruvate, pH, and norepinephrine levels. VT was determined by gas analysis, and LT was determined as the lactate/pyruvate ratio. DPBPs were detected in all 15 patients. Double product slopes above the DPBP were significantly greater than those below the DPBP (286.2 vs 98.5/W, P<0.001). The lactate/pyruvate ratio increased, arterial pH decreased, and plasma norepinephrine concentration increased above the DPBP. DPBP had strong positive correlations with VT ( r=0.93) and LT ( r=0.95). Study 2 comprised 65 cardiac patients. The DPBP was detected in 89.2% of patients and correlated closely with VT. We conclude that DPBP occurs near VT and LT in cardiac patients during incremental exercise, that the noninvasive DPBP measurement method is comparable to the invasive method, and that DPBP may be as useful an index of exercise intensity in patients with cardiac disease as VT or LT.