Administration of inhaled NQ

Inhaled NO is generally administered to mechanically ventilated patients and can be added before the ventilator at a fresh gas inlet. The source is a tank containing a known fraction of NO in nitrogen which ranges from 25 to 2000 ppm (1 ppm = 40 nM). A gas-mixing system dilutes NO in additional pure nitrogen from another tank. The NO-N2 mixture is connected to the gas inlet of the ventilator. A home-made version is provided on the Siemens Servo 900C ventilator. Another prototype NO mixing method was recently described with the new Siemens Servo 300 ventilator.

When NO is delivered at the fresh gas inlet, a soda lime canister may be added to the inspiratory limb downstream of the ventilator to minimize the fraction of nitrogen dioxide (NO2). NO2 is produced by oxidation of NO in the presence of a high oxygen fraction, particularly if there is prolonged contact between these two gases. The final acceptable NO2 fraction delivered to the ventilator must be less than 1 ppm.

Another delivery system has been described where NO in N 2 is directly administered to the inspiratory limb either continuously or during the inspiratory phase. NO is delivered via a nitrogen flowmeter which delivers flows between 10 and 1500 ml/min.

Because NO reacts rapidly with oxygen, we have chosen a delivery system that limits the time of contact between the two gases ( Payen.efa./ 1993). At the majority of centers in France, NO is administered continuously via a fenestrated silicone catheter of diameter 1.3 mm (Vygon, Paris, France) positioned within the tracheal tube so that the distal tip of the catheter is 2 cm above the distal end of the tube. Thus the tracheal tube protects the tracheal wall from direct contact with NO which could induce local inflammation and damage. The tube and catheter positions are easily checked by chest radiography. NO is driven into the tracheal tube by continuous flow from a cylinder containing 225 ppm NO in nitrogen (CFPO, Meudon La Fort, France). The flow rate is adapted to provide the therapeutic fraction (5-15 ppm) according to the following formula:

NO flaw rate {l/min} = x minute vçmilaïkin (l/min).

For example, if the minute ventilation is 9 l/min, the NO flow rate must be 0.6 l/min to ensure delivery of 15 ppm NO. Tidal volume is reduced to compensate for the added volume of NO and nitrogen, and to keep the minute ventilation constant. An additional advantage of this delivery system is that, if the ventilator fails, hand ventilation does not interrupt NO administration. However, ventilator breakdown will result in the provision of the tank fraction of NO (225 ppm), which might be toxic.

NO can be administered to non-intubated patients through a face-mask connected to a non-rebreathing circuit. Such a system consists of a 5-liter reservoir bag and a one-way valve to separate inspired and expired gas. The fraction of NO-N 2 mixture delivered to the mixing reservoir ranges from 5 to 40 ppm.

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