Echocardiographic determinants of peak aerobic capacity and breathing efficiency in patients with undifferentiated dyspnea

Diastolic function and E/e′ correlate with peak aerobic capacity (VO₂) in patients with heart failure, but the echocardiographic correlates of abnormal gas exchange in patients without heart failure are not well defined. We sought to determine the echocardiographic correlates of peak VO₂ and breathing efficiency (estimated using the ratio of minute ventilation to carbon dioxide production, or VE/VCO₂ nadir) in patients with unexplained dyspnea. We identified 232 patients with unexplained dyspnea who underwent echocardiography at rest followed by stress echocardiography with simultaneous measurement of peak VO₂ and VE/VCO₂ nadir. At baseline, 17 patients (5%) had an E/e′ of ≥15 while 31 patients (17%) had a right ventricular systolic pressure (RVSP) of >35 mm Hg. E/e′ ≥15 and RVSP >35 mm Hg were associated with lower peak VO₂ (14.1 ± 4.4 vs 21.0 ± 6.9 and 15.2 ± 3.6 vs 21.8 ± 6.8 ml/kg/min, respectively, p <0.0001). E/e′ ≥15 (sensitivity 0.13, specificity 0.99, area under the curve 0.64) and RVSP >35 mm Hg (sensitivity 0.38, specificity 0.93, area under the curve 0.76) were highly specific for predicting limited peak VO₂. Age and RVSP at rest were independent correlates with VE/VCO₂, but diastolic function was not. However, the risk of having abnormal VE/VCO ₂ nadir was only elevated in subjects with elevated RVSP in the setting of abnormal diastolic function (hazard ratio 2.4, 95% confidence interval 1.3 to 4.6, p = 0.02). In conclusion, both E/e′ ≥15 and RVSP >35 mm Hg are highly specific markers of exercise limitation in patients without heart failure, but RVSP at rest may offer better overall diagnostic power than E/e′ to predict low peak VO₂ in this group.