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Figure 3 (A, B) Correlation of World Health Organization (WHO) functional class with peak oxygen uptake (peak VO2) and 6-min walk (6 MW) in primary pulmonary hypertension. (A) Correlation of WHO functional class with peak VO2; r = –0.73; p < 0.001; (B) correlation of WHO functional class with 6 MW; r = –0.86; p < 0.001.