Introduction: Inhaled nitric oxide (iNO) is used to treat premature babies with severe hypoxemia. Hemoglobin and iNO combine to form methemoglobin (metHb) which is converted back to hemoglobin by metHb reductase. Activity of this enzyme system is unknown in premature babies. Objective: To establish a normal range of metHb levels in well and sick preterm babies in a NICU. Methods: Prospective cohort comprising consecutive admissions over a 9 month period. MetHb was assayed by cooximeter (Ciba Corning Model 270 Australian Diagnostics Corporation, Victoria) on samples drawn for routine blood gas analysis on days 1 - 7 inclusive. Data on birthweight (bwt), gestational age (GA), illness severity(arterial:alveolar (a:A) oxygen ratio, severe ≤ 0.22), blood transfusions, inotropes and parenteral nutrition use were collected. Results: 402 assays from 118 babies (mean bwt 1200g, range 375-2490, GA 28 weeks, 23-35 weeks) performed. Median metHb level = 0.4%, 99% reference range 0.1-1.8%. No level was > 2.0%. No variable was found to have a significant affect on metHb levels (see Fig for bwt plot). Conclusion: Based on these data metHb levels are tightly controlled in premature babies. A level greater than 1.8% should be considered abnormal.

figure 1