A safe clinical system for nitric oxide (NO) inhalation therapy was developed. The system consists of three parts: a NO controller, a NO monitor, and a patient circuit. NO gas flow and carrier gas flow are controlled by a special rust-proof thermal mass flowmeter. Standard gas quality NO gas (10,000 ppm, balance nitrogen) is used. The outlet of the NO gas tank is connected to the distal end of a heated humidifer that is very close (12 mL) to the patient, to decrease acidic water precipitation and decrease contact time between NO and oxygen (02).
Fail-safe mechanisms to prevent the delivery of a hypoxic mixture or excessive NO concentration are incorporated. lnspiratory NO concentration is continuously monitored by a modified electrochemical NO meter. The patient circuit consists of a breathing circuit and a ventilator with a scavenging unit. A modified Mapleson D type circuit is used. Fresh gas, humidified and mixed with NO, is introduced to the patient connection port. A mechanical ventilator, either of conventional or of high-frequency oscillation type, is connected to the expiratory limb of the Mapleson D circuit. A coaxial scavenging unit including activated charcoal is placed in between the expiratory limb and the ventilator.
The adjustment of inspiratory NO concentration (y) was accurate over a wide range (1-80 ppm) of concentrations (x) (y = 0.36 + 0.96x, R2 = 0,999, n = 45) and showed good agreement with the chemilurninescence method. lnspiratory nitrous oxide (NO,) concentration was less than 0.3 ppm, and acidic water accumulation as measured by NO, and NO, was less than 5 ppm, even at an extremely high NO concentration of 80 ppm with an Fi02 of 1.0 and 10 Umin of fresh gas flow. Environmental NO and NOp concentrations in the ICU remained below 0.005 and 0.05 ppm, respectively. This system was used clinically on 214 pediatric patients and proved to be accurate, safe, and useful.