NO measurements

The most widely employed methods for monitoring NO and NO2 in respiratory gas are chemiluminescence and electrochemical detection. Clinicians usually attempt to measure a very small fraction of NO2 in an oxygen-rich gas mixture with a substantial level of NO. In addition, this mixture undergoes cyclic pressure changes and may have a high humidity.

Chemiluminescence monitors in use at present measure NO and NOx, and the NO2 fraction is given by the difference between these two values. Fractions of NO and NOx are measured continuously using a fast-response-time chemiluminescence apparatus (Eco Physic, Massy, France). Intratracheal gas is continuously aspirated through the endotracheal tube. At an aspiration flow rate of 150 ml/min, the response time is approximately 30 s and only mean fractions of NO and NOx can be measured accurately. When an aspiration flow rate of 1000 ml/min is selected, the response time is approximately 200 ms and inspiratory and expiratory NO fractions can be measured.

Electrochemical techniques for the measurement of NO and NO2 levels are rapidly gaining popularity as they are not as costly as the chemiluminescence technique. Although measurements of NO levels have an acceptable accuracy, there have been difficulties with measurement of NO 2 levels.

The frequency of measurement is a subject of much debate. Some authors measure NO and NO2 levels continuously for days and weeks, so that each patient requires an NO monitor. In our institution, the NO fraction is regularly checked (two to six times daily) on the ventilator expiratory limb and in the tracheal tree by a chemiluminsecence method. This allows us to monitor the NO and NOx fractions of several patients.

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