Hypoxic pulmonary vasoconstriction (HPV). (a) Normal ventilation-perfusion (VQ) matching. (b) HPV results in VQ matching despite variations in ventilation and gas exchange between lung units. (c) Inhaled nitric oxide (NO) augmenting VQ matching by vasodilating vessels close to ventilated alveoli. (d) Intravenous vasodilation counteracting HPV leads to worse oxygenation. (e) In disease states that are associated with dysregulated pulmonary vascular tone, such as sepsis and acute lung injury, failure of HPV leads to worse oxygenation. (f) Accumulation of NO adducts leads to loss of HPV-augmenting effect. Reprinted with permission from the Massachusetts Medical Society . Copyright© 2005 Massachusetts Medical Society. All rights reserved.