Blood lactate as an endpoint for submaximal testing in patients with heart failure: Clinical application of a constant work rate exercise protocol

Maximal exercise tests, according to the modified Naughton protocol, and 6 minute submaximal exercise tests, at a constant work rate which corresponded to 80% of peak oxygen uptake, were serially performed by 20 patients with stable congestive heart failure (CHF) enrolled in a 4-week placebo-controlled trial of an investigations cardiotonic agent, pimobendan. Respiratory gas exchange was continuously monitored during all exercise tests. Deep femoral vein oxygen content and lactate concentration and forearm vein Lactate concentration were determined at rest and after 2, 4 and 6 minutes of submaximal exercise. Peak oxygen uptake significantly increased from a baseline value of 11.1 ± 0.8 to 13.0 ± 0.6 mL/kg/min (p < 0.05) in patients receiving pimobendan, but did not change in the placebo group. Femoral vein oxygen content was significantly greater at the end of 6 minutes of submaximal exercise after 4 weeks administration of pimobendan (4.6 ± 0.6 vs 3.4 ± 0.4 mL/100 mL, p < 0.05), while both deep femoral vein and forearm vein lactate concentrations were significantly lower (1.5 ± 0.1 vs 2.4 ± 0.3 mmol/L and 1.2 ± 0.1 vs 1.6 ± 0.2 mmol/L, respectively, both p < 0.05). Blood oxygen content and lactate concentration did not significantly change from baseline after 4 weeks of placebo. Thus, improved exercise capacity in patients with CHF is associated with a decrease in venous blood lactate concentration during submaximal exercise at a constant work rate. Blood lactate concentration determined from either the deep femoral or forearm venous blood appears to be a clinically useful endpoint during submaximal exercise testing at a constant work rate.