Inhaled Nitric Oxide and Methemoglobinemia in Term Infants with Persistent Pulmonary Hypertension of the Newborn

Abstract 1306 Poster Session I, Saturday, 5/1 (poster 17)

Objective: Methemoglobinemia (Met) is a recognized adverse effect of inhaled nitric oxide (iNO). Several papers have suggested that significant Met can be avoided by limiting iNO exposure to no more than 40 ppm, but we are unaware of any published data defining the relationship between iNO exposure and the development of Met. We therefore reviewed our own experience to better assess this relationship and to ask whether iNO concentration greater than 40 ppm was highly predictive of subsequent Met.

Design and Methods: Medical charts of term infants admitted to our Intensive Care Nursery between 1/92 and 10/97 were selected based on: (1) a discharge diagnosis of persistent pulmonary hypertension; (2) iNO therapy provided; and (3) methemoglobin (metHb) measurements recorded.

Results: Twenty-eight charts were selected. The average duration of iNO therapy was 77.8 (±60.1) hours. The maximum iNO dosage ranged from 10 to 80 ppm. The average number of metHb measurements per infant was 20 (± 16). In general, a max iNO ≥ 40 ppm was a poor predicator of subsequent Met. Of eleven infants who received ≥ 40 ppm of iNO, only five developed Met > 4% and five infants had max Met < 2%. However, we found that cumulative iNO (ΣiNO) exposure; ppm × hours) was a better predictor of Met. Of nineteen infants who received a ΣiNO ≤ 2000 (e.g. 20 ppm for 4 days), only one developed Met > 4%. Of nine infants receiving (Σ iNO) > 2000, seven developed Met > 4% and all nine had met > 2% (see (Graph).

Fig 1

Graph 1. Cumulative iNO exposure vs. MetHb

Conclusion: The cumulative iNO exposure is a better predictor of subsequent Met than absolute iNO concentration, within the range of iNO concentrations typically used in term infants with persistent pulmonary hypertension.