Relationship between cell-free plasma Hb levels (levels reported in terms of heme) and (A) blood flow responses to intra-arterial infusions of sodium nitroprusside, showing that patients with higher plasma cell-free Hb have blunted vasodilation responses to NO donors (reproduced with permission from Reiter et al⁴ ), (B) flow-mediated vasodilation, a measure of endothelial NO production from NO synthase during shear-stress, which is blunted in patients with higher plasma cell-free Hb levels (see Figure 4 in the article by Detterich et al that begins on page 703), and (C) the TR jet velocity, a noninvasive estimate of pulmonary artery systolic pressure that is higher in patients with higher levels of plasma cell-free Hb (see Figure 3 in the article by Detterich et al that begins on page 703).