The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

UNDERESTIMATION OF DELIVERED NO AND NOx IN INHALATIONAL NITRIC OXIDE THERAPY WHEN USING CHEMILUMINESCENCE MONITORING

Edward Snyder AE, CBET, Michael Antunes MD, William Holt RRT, RPFT, Sean Weekerly AE, CBET, Dicter Kita, and Alan Spitzer MD Thomas Jefferson University Hospital, Philadelphia, PA and Thermo environmental Instruments Inc., Franklin MA

BACKGROUND: Inhalational nitric oxide (NO) therapy has been shown to improve oxygenation and decrease morbidity in some infants with severe respiratory failure. Assessment of inspired oxides of nitrogen is critically important for response and monitoring toxicity. Chemiluminescence is a techniquc for monitoring NO and NOx concentration in which NO reacts with ozone to form electrically excited NO2, which produces a characteristic luminescence that can be spectroscopically measured. In the presence of significant O2, CO2, or water vapor concentrations, an altemative reaction pathway can limit the luminescent energy produced. This process is commonly referred to as "quenching." and can cause and underestimation of actual NO and NOx. In clinical applications, the chemiluminescent analyzers are typically calibrated with known concentrations of NO in N2, but actual measurements are made in a mixture far different than the calibration gas (usually FiO2> 0.90). thus introducing error in measured nitrogen oxides due to quenching. This error can be averted through calibration of the analyzer under sampling conditions (high FiO2).

METHOD: To investigate possible alterations in measurement with this technique at high FiO2, we used a chemilumincscence analyzer (Model 42H, Thermo Environmental Instruments, Franklin MA), calibrated with known NO at 0.90 FiO2 (Oxygen Cal) and in N2 (Nitrogen Cal) to detect differences in delivered NO and NOx fractions. NO was delivered through our clinical circuit, gas flow at 12 lpm consisted of 0.90 02, NO and th remainder gas N2, Sampling of non-humidified gas was performed at the same site with the analyzer calibrated in O2, then in N2. NO concentrations were varied from 24 to 100 ppm. Results:

OXYGEN CAL NITROGEN CAL % CHANGE

NO NOx NO NOx \deltaNO \deltaNOx

23.6 23.6 20.6 19.9 -14.46 -18.59

39.2 39.3 38.3 36.9 -2.35 -6.5

60.0 60.9 50.9 49.8 -17.88 -22.29

80.5 81.5 71.8 70.4 -12.12 -15.77

100.1 102.0 91.7 91.5 -9.16 -11.48

CONCLUSION: This data shows that measurement of NO and NOx at high FiO2 using an analyzer calibrated with NO in N2 can lead to an understimation of actual NO and NOx. The magnitude of the reduction varied from 2.35 to 17.88 for NO and 6.5 to 22.29 for NOx. and was present at all concentrations studied. This suggests that the chemiluminescent technique may need to be modified when used at high FiO2 to account for the effects of quenching.

Reference: OF-96-018

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