Physiological dead space during exercise in patients with heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is associated with cardiopulmonary abnormalities that may increase physiological dead space to tidal volume (VD/VT) during exercise. However, studies have not corrected VD/VT for apparatus mechanical dead space (VDM), which may confound the accurate calculation of VD/VT. We evaluated whether calculating physiological dead space with (VD/VT_VDM) and without (VD/VT) correcting for VDM impacts the interpretation of gas exchange efficiency during exercise in HFpEF. Fifteen HFpEF (age: 69 ± 6 yr; V^_ O_2peak: 1.34 ± 0.45 L/min) and 12 controls (70 ± 3 yr; V^_ O_2peak: 1.70 ± 0.51 L/ min) were studied. Pulmonary gas exchange and arterial blood gases were analyzed at rest, submaximal (20 W for HFpEF and 40 W for controls), and peak exercise. VD/VT was calculated as Pa_{CO 2} – PE_CO2 /Pa_{CO 2} . VD/VT_VDM was calculated as Pa_{CO 2} – PE_CO2 /Pa_{CO 2} –VDM/VT. VD/VT decreased from rest (HFpEF: 0.54 ± 0.07; controls: 0.32 ± 0.07) to submaximal exercise (HFpEF: 0.46 ± 0.07; controls: 0.25 ± 0.06) in both groups (P < 0.05), but remained stable (P > 0.05) thereafter to peak exercise (HFpEF: 0.46 ± 0.09; controls: 0.22 ± 0.05). In HFpEF, VD/VT_VDM did not change (P = 0.58) from rest (0.29 ± 0.07) to submaximal exercise (0.29 ± 0.06), but increased (P = 0.02) thereafter to peak exercise (0.33 ± 0.06). In controls, VD/VT_VDM remained stable such that no change was observed (P > 0.05) from rest (0.17 ± 0.06) to submaximal exercise (0.14 ± 0.06), or thereafter to peak exercise (0.14 ± 0.05). Calculating physiological dead space with and without a VDM correction yields quantitively and qualitatively different results, which could have impact on the interpretation of gas exchange efficiency in HFpEF. Further investigation is required to uncover the clinical consequences and the mechanism(s) explaining the increase in VD/VT_VDM during exercise in HFpEF. NEW & NOTEWORTHY Calculating VD/VT with and without correcting for VDM yields quantitively and qualitatively different results, which could have an important impact on the interpretation of V/Q mismatch in HFpEF. The finding that V/Q mismatch and gas exchange efficiency worsened, as reflected by an increase in VD/VT_VDM during exercise, has not been previously demonstrated in HFpEF. Thus, further studies are needed to investigate the mechanisms explaining the increase in VD/VT_VDM during exercise in patients with HFpEF.