Inhaled nitric oxide fraction is influenced by both the site and the mode of administration

Inhaled nitric oxide (NO) can be delivered continuously or sequentially (= during inspiration) at different locations of the ventilation circuit. We have tested the influence of locations, modes of NO administration and the ratio of the inspiratory time over the respiratory cycle time (I/I + E ratio) on the accuracy of NO fractions, delivered by 2 devices: Opti-NO and Flowmeter.

We used a simplified lung model consisting of a ventilation circuit with a Y piece, a tracheal tube, a 150 ml dead-space volume and a 5 liter balloon. Three fractions (3, 6, 9 ppm) were administered continuously or sequentially, in controlled volume, in 4 different sites on the inspiratory branch above the Y piece: i) just after the water trap, ii) just before the Y piece; below the Y piece: iii) just after the Y piece, iv) into the endotracheal tube. In addition, different I/I + E ratios (25, 33, 50, 80%) were studied. The delivered NO fractions were measured in the balloon by chemiluminescence (CLD 700, Ecophysics). A linear regression analysis was used to test the relationship between administered and measured NO fractions for the 3 fractions (3, 6 and 9 ppm) in sequential and continuous modes. Intercept values were compared to zero and slopes to the identity line.

When NO was administered in the continuous mode upstream the Y piece, NO fractions measured in the balloon corresponded to the administered fractions. In contrast, below the Y piece, the measured NO fractions were significantly lower than the administered NO fractions. In the sequential mode, above and below the Y piece, the delivered NO fractions were within the manufacturer's range.

For the continuous NO delivery, locations above the Y piece are mandatory. However, locations below the Y piece imposes a sequential system, which can also be used for the sites located above the Y piece.