WATER-VAPOR AND CARBON-DIOXIDE DECREASE NITRIC-OXIDE READINGS

Measurement of nitric oxide levels in exhaled ah-is commonly performed using a chemiluminescence detector. However, water vapour and carbon dioxide affect the chemiluminescence process, The influence of these g ases at the concentrations present in exhaled air has not vet been stu died. For this in vitro study, mixtures of 50, 100 and 200 parts per b illion (ppb) NO in air mere prepared and fed into the NO analyser eith er directly or bubbled through water, Mixtures with CO2 were prepared by adding 0-10% CO2 to the nitric oxide diluent air. We found a signif icant decrease in NO readings in the water-saturated samples compared to the dry gas (p<0.001), strongly dependent on the partial pressure o f water, NO levels in exhaled air (mean 10+/-2 ppb) showed a decrease of 17+/-3% when waer vapour was not absorbed, From the experiments wit h CO2 we found a decrease in NO reading of 1.04+/-0.07% per volume CO2 (%). Presence of water vapour, thus, leads to a systematic underestim ation of NO levels, Insertion of a water absorber might, therefore, be advantageous. The influence of CO2 concentrations in the normal respi ratory range is negligible, With high expiratory CO2 levels as applied in permissive hypercapnia, the effects may be substantial.