HBG2 and pulmonary arterial hypertension: Methemoglobin results from the oxidation of the hemoglobin iron to a ferric (Fe+++) state, a reaction that occurs when oxyhemoglobin reacts with nitric oxide (NO) [11]; therefore levels of metHb may track with levels of NO, a potent vasodilator implicated in the pathogenesis of PAH [12].