Inhaled nitric oxide (NO) may be useful in the treatment of adult respiratory distress syndrome and other diseases characterized by pulmonary hypertension and hypoxemia. NO is rapidly converted to nitrogen dioxide (NO2) in oxygen (O2) environments. We hypothesized that in patients whose lungs are mechanically ventilated and in those with a long residence time for NO in the lungs, a clinically important [NO2] may be present. We therefore determined the rate constants for NO conversion in adult mechanical ventilators and in a test lung simulating prolonged intrapulmonary residence of NO.
NO (800 ppm) was blended with nitrogen (N2), delivered to the high-pressure air inlet of a Puritan-Bennett 7200ae or Siemens Servo 900C ventilator, and used to ventilate a test lung. The ventilator settings were varied: minute ventilation (VE) from 5 to 25 l/min, inspired O2 fraction (FIO2) from 0.24 to 0.87, and [NO] from 10 to 80 ppm. The experiment was then repeated with air instead of N2 as the dilution gas. The effect of pulmonary residence time on NO2 production was examined at test lung volumes of 0.5-4.0 l, VE of 5-25 l/min, FIO2 of 0.24-0.87, and [NO] of 10-80 ppm. The inspiratory gas mixture was sampled 20 cm from the Y-piece and from within the test lung. NO and NO2 were measured by chemiluminescence. The rate constant (k) for the conversion of NO to NO2 was determined from the relation 1/[NO]t-1/[NO]o = k x [O2] x t, where t = residence time.
No NO2 was detected during any trial with VE 20 or 25 l/min. With N2 dilution and the Puritan-Bennett 7200ae, NO2 (< or = 1 ppm) was detected only at a VE of 5 l/min with an FIO2 of 0.87 and [NO] > or = 70 ppm. In contrast, [NO2] values were greater with the Servo 900C ventilator than with the Puritan-Bennett 7200ae at similar settings. When NO was diluted with air, clinically important [NO2] values were measured with both ventilators at high [NO] and FIO2. Rate constants were 1.46 x 10(-9) ppm-2.min-1 when NO was mixed with N2, 1.17 x 10(-8) ppm-2.min-1 when NO was blended with air, and 1.44 x 10(-9) ppm-2.min-1 in the test lung.
[NO2] increased with increased FIO2 and [NO], decreased VE, blending with air, and increased lung volumes. Higher [NO2] was produced with the Servo 900C ventilator than the Puritan-Bennett 7200ae because of the greater residence time. With long intrapulmonary residence times for NO, there is a potential for NO2 production within the lungs. The rate constants determined can be used to estimate [NO2] in adult mechanical ventilation systems.
Key words: Gases: nitric oxide; nitrogen dioxide. Ventilation: mechanical.
INHALED nitric oxide (NO), a selective pulmonary vasodilator, may be useful in the treatment of adult respiratory distress syndrome (ARDS) and other lung diseases characterized by pulmonary hypertension and hypoxemia. [1–4] The potential toxic effects of inhaled NO are not completely known but include methemoglobinemia and nitrogen dioxide (NO2) production. NO2is produced spontaneously from NO and oxygen (Oxygen2). [5–11] Although the Occupational Safety and Health Administration has set safety limits for NO2at 5 ppm,  airway reactivity  and parenchymal lung injury  have been reported with inhalation of as little as 2 ppm.
The conversion rate of NO to NO2is determined by the square of [NO], [Oxygen2], and the residence time of NO with Oxygen sub 2.  The second-order kinetics are described by the relation Equation 1. Because [Oxygen2] is typically much greater than [NO], it is assumed that [Oxygen2] remains constant. It also is assumed that all of the conversion of NO is to NO2. Integration of Equation 1yields the following equation:Equation 2where t = residence time;[NO]1=[NO] after t;[NO]0= initial [NO]; and k = the rate constant for the conversion of NO to NO2. The difference between [NO]1and [NO]0is [NO2]. Glasson and Tuesday in 1963 reported the value of the rate constant as (1.57 plus/minus 0.09) x 109ppm sup -2 *symbol* min sup -1 at 23 degrees Celsius and 1 atm.  This rate constant was determined in static dry conditions, however, which may differ from those of dynamic systems such as adult mechanical ventilation.
We were interested in developing a system for NO administration that minimized NO2production during adult mechanical ventilation. We were concerned that ventilator systems with large internal volumes may produce high [NO2]. We therefore determined the rate constant for NO conversion using two ventilators with different internal volumes, using nitrogen (Nitrogen2) or air to dilute the NO proximal to the ventilator inlet, and using a variety of combinations of minute ventilation (V with dotE) and inspired Oxygen2fraction (FIO2). Because NO may be converted to NO2in lungs with long residence times for NO, we also used a lung model at various lung volumes and V with dotEto determine the rate constant for intrapulmonary conversion of NO to NO2.
Materials and Methods
(Figure 1) shows the experimental apparatus. NO (800 ppm in Nitrogen2, Airco, Riverton, NJ) was mixed with Nitrogen2using two blenders (Bird Products, Palm Springs, CA) in series. Stock NO cylinders were certified by the supplier to contain < 8 ppm NO2, and thus the [NO2] delivered to the ventilators in this study was < 0.8 ppm. NO was connected to the Oxygen2inlet of the first blender, and Nitrogen2(Airco) was connected to the air inlets of the first and second blenders. The outlet of the first blender led to the Oxygen2inlet of the second blender. Two blenders allowed more precise gas mixing at lower [NO], consistent with our clinical practice. NO and Nitrogen2pressures were set at 50 lb/in2. The outlet of the second blender was delivered to the high-pressure air inlet of a Puritan-Bennett 7200ae (Puritan-Bennett, Carlsbad, CA) or Siemens Servo 900C (Siemens-Elema, Sola, Sweden) ventilator. Including the ventilator blender, [NO] was thus reduced in three stages.
The ventilator was connected to a test lung (Dual Adult TTL (Training Test Lung) model 160, Michigan Instruments, Grand Rapids, MI) with a disposable ventilator circuit (Aerosol Hose 5022, Seamless, Ocala, FL). Each ventilator was set to deliver V with dotEof 5, 10, 15, 20, and 25 l/min in the controlled mechanical ventilation mode. FIO2of the ventilators was set at 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9, but actual FIO2(7820 Oxygen2monitor, Puritan- Bennett) was 0.24, 0.37, 0.50, 0.62, 0.75, and 0.87, respectively, because a nitric oxide/nitrogen mixture was delivered to each ventilator instead of air.
The inspiratory gas was aspirated 20 cm from the Y-piece and analyzed for NO and NO2using a calibrated chemiluminescence analyzer (Chemiluminescent NO-NO2-NOxanalyzer model 10, Thermo Environmental Instruments, Franklin, MA). The analyzer was calibrated with 0 and 80 ppm NO. Chemiluminescent analyzers have been shown to be linear over the range of concentrations that we evaluated.  The blenders were set to deliver NOx ([NO + NO2]) of 10, 20, 30, 40, 50, 60, 70, and 80 ppm. NO and NO2measurements were recorded after equilibration. A servo-controlled humidifier (Conchatherm III, Temecula, CA) was placed in the inspiratory limb and the temperature at the Y-piece of the circuit was kept at 30–32 degrees Celsius. Gas from the ventilator and the analyzer was scavenged by wall vacuum.
Dilution of Nitric Oxide with Air
NO gas was diluted with air instead of Nitrogen2, and the experiment described above was repeated, NO (800 ppm in Nitrogen2) was blended with air from a wall pressure source, and one blender was used to control [NO] leading to the air inlet of the ventilator. We did not use two blenders for this experiment because pilot studies showed that very high [NO2](> 5 ppm) resulted when NO was diluted with air in two blenders. The length of the air hose (conductive K/l) from the blender to the ventilator was 3.6 m. When [NO2] > 5 ppm was detected in the inspiratory limb, measurements at higher [NO2] were not performed at that FIO2and V with dotE. To allow comparisons with Nitrogen2dilution, each ventilator was set at an FIO2of 0.24, 0.37, 0.50, 0.62, 0.75, and 0.87.
Effect of Pulmonary Residence Time
We examined the effect of pulmonary residence time on NO2production in a mechanical lung model. NO (800 ppm) was mixed with Nitrogen2using two blenders in series and connected to the high- pressure air inlet of a Puritan-Bennett 7200ae ventilator. The lung model was set at a compliance of 0.1 l/cmH2O. The ventilator settings were those listed in Table 1, except that positive end- expiratory pressure was applied to create end-expiratory volumes of 0.5, 1.0, 2.0, 3.0, and 4.0 l in the lung model. End-expiratory flow as 0 l/min for all settings. Gas was sampled from the inspiratory limb and from within the lung bellows, and [NO] and [NO2] were measured by chemiluminescence at all V with dotEand test lung volumes.
Determination of Rate Constant
The rate constant was determined from the relation shown in Equation 2. Rate constants were determined for NO blended with Nitrogen2, NO blended with air, and simulated intrapulmonary residence of NO.
Because virtually no NO2(less or equal to 1 ppm) was produced with the Puritan-Bennett 7200ae ventilator when NO was mixed with Nitrogen2, the rate constant for NO mixed with Nitrogen2was determined using only the Servo ventilator data. To determine the rate constant for NO mixed with air, data was pooled for the Puritan- Bennett 7200ae and Servo 900C ventilators. The [Oxygen2] was converted to parts per million to determine the rate constants. Equation 2was solved for the rate constant by using the nonlinear regression module of SPSS (Chicago, IL).
To determine the rate constant for intrapulmonary residence of NO, the residence time was estimated by dividing the residence time was estimated by dividing the test lung volume by V with dotE. To determine the rate constant for NO conversion to NO2in the ventilator systems, the residence time was calculated by dividing the volume of the system by the V with dotE. The ventilatory circuit in our study was 213 cm in length and had a volume of 0.7 l. The Servo 900C has an internal volume of 1.0 l because of the reservoir inside the ventilator (0.9 l) and the additional internal circuity (0.1 l). Because the Puritan-Bennett 7200ae does not have an internal bellows, its internal volume is only 0.25 l.* Thus, the total volume of the Servo 900C ventilator system was 1.7 l, and the total volume of the Puritan- Bennett 7200ae system was 0.95 l.
Dilution of Nitric Oxide with Nitrogen
No NO2was detected during any trial with a V with dot sub E of 20 or 25 l/min. With the Puritan-Bennett 7200ae, NO2(less or equal to 1 ppm) was detected only at a V with dotEof 5.0 l/min when FIO2was 0.87 and [NO] was 70 or 80 ppm. In contrast, with the Servo 900C (Table 2), at a V with dotEof 5.0 l/min, 0.5–3.5 ppm NO2was present when FIO2was greater or equal to 0.37 and [NO] greater or equal to 50 ppm; at a V with dotEof 10 l/min, 0.5–1.5 ppm NO2was present when FIO2was greater or equal to 0.62 and [NO] greater or equal to 60 ppm; at a V with dotEof 15 l/min, 0.5 ppm NO2was present when FIOsub 2 was 0.87 and [NO] 80 ppm.
Dilution of Nitric Oxide with Air
(Figure 2and Figure 3) show NO2formation with the Servo 900C and the Puritan-Bennett 7200ae ventilators, respectively. Compared with Nitrogen2, dilution with air resulted in substantially increased [NO2] at all settings with both ventilators. At the same settings, [NO2] was greater with the Servo than with the Puritan-Bennett 7200ae. At an FIO2of 0.87 and [NO] of 60 ppm with the Puritan-Bennett 7200ae, or an FIO2of 0.87 and [NO] of 40 ppm with the Servo 900C, NO2was > 5 ppm even at a V with dotEof 25 l/min. At all settings, the greater the FI sub O2and [NO] and the less the V with dotE, the greater the [NO2].
Effect of Pulmonary Residence Time
At a V with dotEof 20 or 25 l/min, less or equal to 1 ppm NO2was measured in the test lung bellows (simulated intrapulmonary residence of NO). At a V with dotEof 15 l/min, [NO] greater or equal to 60 ppm, test lung volume greater or equal to 1 l, and FIO2greater or equal to 0.75, 1–3 ppm NO2was detected in the bellows. Table 3shows NO2production at a VEof 5 or 10 l/min. With greater FIO2, greater [NO], greater test lung volume, and lesser V with dotE, more NO2was produced.
Rate Constants for Conversion of Nitric Oxide to Nitrogen Dioxide
When NO was blended with NO2before entering the ventilator, the rate constant was (1.46 plus/minus 0.07) x 10 sup -9 ppm sup -2 *symbol* min sup -1 (r2= 0.999). When NO was blended with air, the rate constant increased eightfold to (1.17 plus/minus 0.04) x 10 sup -8 ppm sup -2 *symbol* min sup -1 (r2= 0.997). For the simulated intrapulmonary residence of NO, the rate constant was (1.44 plus/minus 0.02) x 10 sup -9 ppm sup -2 *symbol* min sup -1 (r2= 0.9997).
This study demontrates that the [NO2] values measured in the inspiratory limb of a ventilator circuit are greater when the Servo 900C rather than the Puritan-Bennett 7200ae ventilator is used and when NO is diluted with air rather than with Nitrogen2*symbol*[NO2] values also were greater with increased [NO], greater FIO2, or lesser V with dotE. There also is a potential for intrapulmonary NO2production in lung units with long residence times, as simulated in our lung model.
Rate constants for NO conversion to NO2have been published by others (Table 4). [8–11] The rate constants calculated in the current study for NO blended with Nitrogen2and for the simulated intrapulmonary residence of NO are similar to those published by Glasson and Tuesday.  With the exception of our data for NO blended with air, rate constants of 0.79 x 10 sup -9 - 2.26 x 10 sup -9 have been reported. These differences are likely caused by differences in experimental methods, including static versus dynamic conditions, temperature, and humidity differences. Our current study verifies these values in clinically relevant conditions with modern adult critical care ventilators. Figure 4graphs the residence times required to generate 2 ppm NO2using the rate constant of the current study and those of three other studies. [9–11] These differences may be clinically important and emphasize the need to measure [NO2] during NO administration. Regardless of the rate constant used, however, a residence time > 2.75 min is needed to generate more than 2 ppm NO2at [NO] less or equal to 20 ppm and FIO2less or equal to 0.9.
When NO was blended with air, the rate constant was about eight times greater than with Nitrogen2blending. With this method, blending of NO and Oxygen2begins inside the high-pressure hose proximal to the ventilator and thus increases the residence time. Further, NO and Oxygen2react under high pressure (50 lb/in2). Clinically, however, blending NO with compressed air decreases the effect of NO delivery on FIO2and is more convenient and economical during prolonged administration.
Low [NO](less or equal to 20 ppm) has been reported to improve oxygenation for patients with ARDS. [3,4] Our data indicate that these concentrations can be administered with the Puritan-Bennett 7200ae ventilator without significant NO2production, even when the NO is blended with air. With the Servo 900C ventilator, [NO] less or equal to 20 ppm can be administered without significant NO2production at an FIO2less or equal to 0.75 and an V with dot sub E of at least 10 l/min when air blending is used. When NO was blended with Nitrogen2, NO2was not produced in either mechanical ventilation system at an [NO] of less than 50 ppm.
We chose the Puritan-Bennett 7200ae and the Servo 900C as examples of adult mechanical ventilators with different internal volumes. Our results may apply to other ventilator systems with similar internal volumes. Some ventilators for adults other than the Servo 900C have large internal volumes, and gas is compressed in some of these reservoirs,  a condition that may further affect the rate constant for NO conversion to NO2.
There is a potential for NO conversion to NO2in lungs with long residence times. From our experience, exhaled [NO] that is 50- 75% of inhaled concentrations is common when inhaled NO is administered to patients with ARDS. Our lung model data, however, indicate that a relatively long residence time is required to produce 2 ppm NO2at the [NO] typically used in the treatment of ARDS (i.e., less or equal to 20 ppm). At a rate constant of 1.44 x 10 sup -9 ppm sup -2 *symbol* min sup -1, the time required to produce 2 ppm NO2is 4.6 min during breathing of 90% Oxygen2and 6.8 min during breathing of 60% Oxygen sub 2. This prolongation of residence time is unlikely during ARDS but may occur in patients with severe chronic airflow obstruction. Because ours was a study in a lung model, further work is needed to determine whether conversion of NO to NO2does occur in the lungs during therapeutic NO administration.
Because NO2is an atmospheric pollutant, its toxic pulmonary effects have been investigated in many studies.  Inhaled NO2potentially affects the parenchyma and the airways of the lungs. Animal studies evaluating the parenchymal effects of high concentrations of inhaled NO2(> 10 ppm) have reported pulmonary edema, hemorrhage, changes in the surface tension properties of surfactant, reduction in the number of alveoli, and death. [18–21] Other animal studies have shown that inhaled NO2at concentrations as low as 2 ppm produced alveolar cell hyperplasia, alteration in surfactant hysteresis, changes in the epithelium of the terminal bronchiole, and loss of cilia. [14,22] In humans, 2.3 ppm NO2has been shown to affect alveolar permeability.  Several studies evaluating airway responses to low concentrations of inhaled NO2have reported conflicting results.  Some, however, have reported increased airway responsiveness at inhaled [NO2] less than or equal to 2 ppm. [13,24,25] Although the Occupational Safety and Health Administration has set safety limits at 5 ppm,  our clinical goal is to maintain inhaled [NO2] as low as possible. Inhaled NO2may remain in the lungs for prolonged periods because it reacts with H sub 2 O to produce HNO3and undergoes irreversible reactive absorption by the pulmonary epithelial lining fluid.  Exhaled [NO sub 2] therefore may not be a sensitive indicator of toxic pulmonary concentrations.
A soda lime (94% Ca(OH)2, 5% NaOH, and 1% KOH) absorber in the inspiratory gas circuit can be used to remove NO2.  Stenqvist et al.  evaluated the effectiveness of soda lime to absorb NO2and reported that 3 ppm NO2decreased to less than 1 ppm after passing through the absorber. However, soda lime also absorbs NO2; its effectiveness decreases with time; and some forms of soda lime may be ineffective in removal of NO2.  The use of a soda lime canister introduces other potential problems during mechanical ventilation such as increased resistance to breathing, increased risk of system leaks, more difficult triggering of the ventilator, and modification of inspiratory flow waveforms.
It has been our practice to dilute NO with Nitrogen2or air and then introduce this gas mixture to the air inlet port of the ventilator. We have found this system reliable in the laboratory and in clinical use, and it maintains a constant [NO] and FIO2with changes in V with dotEor inspiratory flow pattern. A method very similar to ours was recently described by Channick et al.  Similar to our findings, they reported very low [NO2] with the Puritan-Bennett 7200ae when blending NO with Nitrogen2before delivery to the air inlet of the ventilator. Wessel et al.  recently described a method to administer NO with the Servo 900C. They added NO and Oxygen2to the low-pressure inlet at a rate equal to the V with dotEand in a proportion to produce the desired [NO] and FIO2. If the nitric oxide/oxygen flow matches the V with dot sub E1 the Servo 900C bellows should remain relatively empty. They found that this method usually but not always resulted in an [NO2] of less than 3 ppm. We have not used this method but believe that it may be difficult to use in patients with changing V with dotE.
Delivery of NO distal to the ventilator outlet has been described as a method to avoid NO2production within the ventilator circuit. Benzing et al.  introduced an adjustable amount of NO gas (600 ppm NO) into the inspiratory limb with a special injection nozzle. Others have used a nebulizer module that injects NO into the circuit during inspiration. [2,4] NO has also been injected at the Y- piece of the ventilator circuit.  These techniques may not deliver a constant [NO] during inspiration, and the additional flow may affect tidal volume and FIO2. Some methods also complicate analysis of [NO] and [NO2](e.g., injection at the Y-piece). With any method of NO inhalation, monitoring of [NO] and [NO2] is essential.
*Sanborn W: Personal communication, 1994.