Six-minute walking performance in patients with moderate-to-severe heart failure; is it a useful indicator in clinical practice? - Opasich,C.; Pinna,G.D.; Mazza,A.; Febo,O.; Riccardi,R.; Riccardi,P.G.; Capomolla,S.; Forni,G.; Cobelli,F.; Tavazzi,L. (Eur Heart J, 2001)