Inhalation of NO and oxygen improved CTO in PH. In multivariate regression analysis CTO at maximal exercise predicted the work load achieved when controlled for age , pulmonary vascular resistance and mixed venous oxygen saturation ( R ( 2 ) = .419 , p < .000 ) ; in addition , the 6MWD was predicted by CTO ( adjusted R ( 2 ) = .511 , p < .000 ) .