TECHNICAL CONSIDERATIONS FOR INHALED NITRIC-OXIDE THERAPY - TIME RESPONSE TO NITRIC-OXIDE DOSING CHANGES AND FORMATION OF NITROGEN-DIOXIDE

The aim of the present study was to analyse the time response to nitri c oxide (NO) dosing changes as well as the formation of nitrogen dioxi de (NO2) with different ventilation systems, respirator settings and a pplication sites during NO inhalation. The inspired NO and NO2 concent rations were continuously measured using chemiluminiscence within a du mmy ventilatory system equipped with two different respirator systems (Siemens Servo 900c and Bear BP 2001). NO was either introduced into t he afferent limb of the ventilatory circuit close to the endotracheal tube (site A) or into the so-called low pressure port of the Servo 900 c respirator, far away from the endotracheal tube (site B). In additio n, the decay of the inspired NO concentration after cessation of the N O gas flow was studied. This decay was considerably prolonged when NO was introduced at site B (time constants: tau = 7.19 min versus tau = 0.29 min). Within the concentration range studied (0-25 ppm NO) a line ar correlation between the NO and NO2 concentration was found. At site A and an inspired oxygen concentration of > 0.95 NO2 formation amount s to 1.14% +/- 0.11% of the NO concentration. Using this value one can calculate the NO2 formation for a given NO dose. For example, when 40 ppm NO are applied, a concentration of 0.45 ppm NO2 can be expected, which is well below the relevant toxic concentrations. However, when N O was introduced at site B, NO2 formation was significantly increased to 1.61% +/- 0.16%. Passage of the ventilated gas through soda lime le d only to a slight and insignificant reduction in NO2 concentration. T he continuous flow respirator BP 2001 showed a significantly lower NO2 concentration when compared to the non-continuous flow respirator Ser vo 900c (0.64 +/- 0.11% vs. 1.14 +/- 0.11%).Conclusion The application of NO close to the endotracheal tube is associated with a much faster response of the actual inspired NO concentration to: dosing changes a nd shows the lowest NO2 formation, In order to avoid toxic NO2 concent rations, an upper limit of 40 ppm NO is recommended for continuous NO inhalation.