Relationship between total cell-free plasma Hb and the physiologic effects of hemolysis and inhaled NO. Upper panels: The difference in response from 0 to 6 hours between baseline and intervention studies for each of the 4 treatment groups is shown for MAP and SVRI. In animals receiving D5W (nonhemolyzing control groups), inhaled NO had no net effect on MAP and SVRI. Compared with these nonhemolyzing controls, free water–induced intravascular hemolysis caused significant increases in MAP and SVRI, which were attenuated by the concurrent inhalation of NO gas (P = 0.0003 for interaction of NO and hemolysis for both variables). Lower panels: Relationship between change in MAP and SVRI and total plasma Hb levels (concentration in terms of heme groups) during the intervention studies in the hemolyzing groups (free water and free water plus NO groups). Despite similar total plasma Hb levels in these 2 groups, the relationships between change in MAP and SVRI and total plasma Hb levels were significantly different (P = 0.003 and P = 0.001, respectively). As total plasma Hb levels increased, MAP and SVRI increased more in the free water group than in the free water plus NO group.