BackgroundPeak ratios of pulmonary gas-exchange to ventilation during exercise ((V) over dotO(2)/VE and (V) over dotCO(2)/VE, termed circulatory equivalents) are sensitive to heart failure (HF) severity, likely reflecting low and/or poorly distributed pulmonary perfusion. We tested whether peak (V) over dotO(2)/VE and (V) over dotCO(2)/VE would: (1) distinguish HF patients from controls; (2) be independent of incremental exercise protocol; and (3) correlate with lactate threshold (LT) and ventilatory compensation point (VCP), respectively. Methods and ResultsTwenty-four HF patients (6111years) with reduced ejection fraction (31 +/- 8%) and 11 controls (63 +/- 7years) performed ramp-incremental cycle ergometry. Eighteen HF patients also performed slow (5 +/- 1W/min), medium (9 +/- 4W/min), and fast (19 +/- 6W/min) ramps. Peak (V) over dotO(2)/VE and (V) over dotCO(2)/VE from X-Y plot, and LT and VCP from 9-panel plot, were determined by 2 independent, blinded, assessors. Peak (V) over dotO(2)/VE (31.2 +/- 4.4 versus 41.8 +/- 4.8mL/L; P<0.0001) and (V) over dotCO(2)/VE (29.3 +/- 3.0 versus 36.9 +/- 4.0mL/L; P<0.0001) were lower in HF than controls. Within individuals, there was no difference across 3 ramp rates in peak (V) over dotO(2)/VE (P=0.62) or (V) over dotCO(2)/VE (P=0.97). Coefficient of variation (CV) in peak (V) over dotO(2)/VE was lower than for LT (5.1 +/- 2.1% versus 8.2 +/- 3.7%; P=0.014), and coefficient of variation in peak (V) over dotCO(2)/VE was lower than for VCP (3.3 +/- 1.8% versus 8.7 +/- 4.2%; P<0.001). In all participants, peak (V) over dotO(2)/VE was correlated with, but occurred earlier than, LT (r(2)=0.94; mean bias, -0.11L/min), and peak (V) over dotCO(2)/VE was correlated with, but occurred earlier than, VCP (r(2)=0.98; mean bias -0.08L/min). ConclusionsPeak circulatory equivalents during exercise are strongly associated with (but not identical to) LT and VCP. Peak circulatory equivalents are reliable, objective, effort-independent indices of gas-exchange abnormality in HF.