Estimating exercise PaCO2 in patients with heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) patients exhibit cardiopulmonary abnormalities that could affect the predictability of exercise PaCO2 from the Jones (PJCO2) equation (PJCO2=5.5+0.9xPETCO2-2.1xVT). Since the dead space to tidal volume (VD/VT) calculation also includes PaCO2 measurements, estimates of VD/VT from PJCO2 may also be affected. Because using noninvasive estimates of PaCO2 and VD/VT could save patient discomfort, time, and cost, we examined whether PETCO2 and PJCO2 can be used to estimate PaCO2 and VD/VT in 13 HFpEF patients. PETCO2 was measured from expired gases measured simultaneously with radial arterial blood gases at rest, constant-load (20W), and peak exercise. VD/VT[art] was calculated using the Enghoff modification of the Bohr equation, and estimates of VD/VT were calculated using PETCO2 (VD/VT[ET]) and PJCO2 (VD/VT[J]) in place of PaCO2. PETCO2 was similar to PaCO2 at rest (-1.46±2.63, P=0.112) and peak exercise (0.66±2.56, P=0.392), but overestimated PaCO2 at 20W (-2.09±2.55, P=0.020). PJCO2 was similar to PaCO2 at rest (-1.29±2.57, P=0.119) and 20W (-1.06±2.29, P=0.154); but, underestimated PaCO2 at peak exercise (1.90±2.13, P=0.009). VD/VT[ET] was similar to VD/VT[art] at rest (-0.01±0.03, P=0.127) and peak exercise (0.01±0.04, P=0.210), but overestimated VD/VT[art] at 20W (-0.02±0.03, P=0.025). Although VD/VT[J] was similar to VD/VT[art] at rest (-0.01±0.03, P=0.156) and 20W (-0.01±0.03, P=0.133), VD/VT[J] underestimated VD/VT[art] at peak exercise (0.03±0.04, P=0.013). Exercise PETCO2 and VD/VT[ET] provide better estimates of PaCO2 and VD/VT[art] than PJCO2 and VD/VT[J] does at peak exercise. Thus, estimates of PaCO2 and VD/VT should only be used if sampling arterial blood during CPET is not feasible.