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A Novel Inhalation Mask System to Deliver High Concentrations of Nitric Oxide Gas in Spontaneously Breathing Subjects

doi: 10.3791/61769 Published: May 4, 2021
Riccardo Pinciroli1, Lisa Traeger1, Anna Fischbach1, Stefano Gianni1, Caio Cesar Araujo Morais1, Bijan Safaee Fakhr1, Raffaele Di Fenza1, Dianna Robinson3, Ryan Carroll2, Warren M. Zapol1, Lorenzo Berra1


Nitric Oxide (NO) is administered as gas for inhalation to induce selective pulmonary vasodilation. It is a safe therapy, with few potential risks even if administered at high concentration. Inhaled NO gas is routinely used to increase systemic oxygenation in different disease conditions. The administration of high concentrations of NO also exerts a virucidal effect in vitro. Owing to its favorable pharmacodynamic and safety profiles, the familiarity in its use by critical care providers, and the potential for a direct virucidal effect, NO is clinically used in patients with coronavirus disease-2019 (COVID-19). Nevertheless, no device is currently available to easily administer inhaled NO at concentrations higher than 80 parts per million (ppm) at various inspired oxygen fractions, without the need for dedicated, heavy, and costly equipment. The development of a reliable, safe, inexpensive, lightweight, and ventilator-free solution is crucial, particularly for the early treatment of non-intubated patients outside of the intensive care unit (ICU) and in a limited-resource scenario. To overcome such a barrier, a simple system for the non-invasive NO gas administration up to 250 ppm was developed using standard consumables and a scavenging chamber. The method has been proven safe and reliable in delivering a specified NO concentration while limiting nitrogen dioxide levels. This paper aims to provide clinicians and researchers with the necessary information on how to assemble or adapt such a system for research purposes or clinical use in COVID-19 or other diseases in which NO administration might be beneficial.


NO inhalation therapy is regularly used as a life-saving treatment in several clinical settings1,2,3. In addition to its well-known pulmonary vasodilator effect4, NO displays a broad antimicrobial effect against bacteria5, viruses6, and fungi7, particularly if administered at high concentrations (>100 ppm).8 During the 2003 Severe Acute Respiratory Syndrome (SARS) outbreak, NO showed potent antiviral activity in vitro and demonstrated therapeutic efficacy in patients infected with the SARS-Coronavirus (SARS-CoV)9,10. The 2003 strain is structurally similar to SARS-Cov-2, the pathogen responsible for the current Coronavirus Disease-2019 (COVID-19) pandemic11. Three randomized controlled clinical trials are ongoing in patients with COVID-19 to determine the potential benefits of breathing high-concentration NO gas to improve outcomes12,13,14. In a fourth ongoing study, the prophylactic inhalation of high concentrations of NO is being investigated as a preventive measure against the development of COVID-19 in healthcare providers exposed to SARS-CoV-2-positive patients15.

The development of an effective and safe treatment for COVID-19 is a priority for the healthcare and scientific communities. To investigate the administration of NO gas at doses > 80 ppm in non-intubated patients and volunteering healthcare workers, the need to develop a safe and reliable non-invasive system became apparent. This technique aims to administer high NO concentrations at different fractions of inspired oxygen (FiO2) to spontaneously breathing subjects. The methodology described here is currently in use for research purposes in spontaneously breathing COVID-19 patients at the Massachusetts General Hospital (MGH)16,17. Following the guidelines of MGH's human research ethics committee, the proposed system is currently in use to conduct a series of randomized controlled trials to study the following effects of high concentrations of NO gas. First, the effect of 160 ppm NO gas is being studied in non-intubated subjects with mild-moderate COVID-19, admitted either in the Emergency Department (IRB Protocol #2020P001036)14 or as inpatients (IRB Protocol #2020P000786)18. Second, the role of high-dose NO is being examined to prevent SARS-CoV-2 infection and the development of COVID-19 symptoms in healthcare providers routinely exposed to SARS-CoV-2-positive patients (IRB Protocol # 2020P000831)19.

This simple device can be assembled with standard consumables routinely used for respiratory therapy. The proposed apparatus is designed to non-invasively deliver a mixture of NO gas, medical air, and oxygen (O2). Nitrogen dioxide (NO2) inhalation is minimized to reduce the risk of airway toxicity. The current NO2 safety threshold set by the American Conference of Governmental Industrial Hygienists is 3 ppm over an 8-h time-weighted average, and 5 ppm is the short-term exposure limit. Conversely, the National Institute for Occupational Safety and Health recommends 1 ppm as a short-term limit of exposure20. Given the increasing interest in high-dose NO gas therapy, the present report provides the necessary description of this novel device. It explains how to assemble its components to deliver a high concentration of NO for research purposes.

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NOTE: See the Table of Materials for the materials needed to assemble the delivery system. Sources of medical air, O2, and NO gases should also be available on site. The device has been developed for investigation use in research protocols that underwent rigorous review by the local Institutional Review Board (IRB). Under no circumstances should providers operate solely based on the indications included in this manuscript, assembling and using this device without seeking prior appropriate institutional regulatory approval. Starting from the proximal end of the device, assemble the pieces in the following order (Figure 1).

1. Building the patient interface

  1. Take a snug-fitting, standard, non-invasive ventilation face mask of the appropriate size for the subject.
  2. Connect the mask's built-in elbow port to a high-efficiency particulate air (highly hydrophobic bacterial/viral filter, HEPA class 13) filter through the 22 mm outer diameter (O.D.)/15 mm inner diameter (I.D.) connector.
  3. (Optional) To facilitate the subject's movement and reduce the risk of disconnection, add a 15 mm O.D. x 22 mm O.D./15 mm I.D. (length 5 cm-6.5 cm) flexible patient connector for an endotracheal or tracheostomy tube between the mask interface and the HEPA filter.
    ​NOTE: Make every effort to avoid leakage of the mask interface. The "patient end" of the device could also consist of a mouthpiece. A nose clip must be added in such a configuration.

2. Building the Y-piece and preparation of the O2 supply

  1. Take a 22 mm to 22 mm and 15 F Y-piece connector with 7.6 mm ports. Create the circuit's expiratory and inspiratory limbs on the two distal ends of the Y-piece through two opposite-sense, low-resistance, 22 mm male/female, one-way valves.
    1. Expiratory limb: On one end of the Y-piece, place the one-way valve connector allowing a proximal-to-distal flow only (arrow pointing downward).
    2. Inspiratory limb: On the other end of the Y-piece, connect a one-way valve allowing a distal-to-proximal flow only (arrow pointing upward).
  2. Connect the proximal end of the Y to the HEPA filter.
  3. Using standard, kink-resistant, vinyl gas tubing with universal adaptors at both ends, connect the O2 source to the Y-piece's inspiratory limb. Choose tubing of appropriate length considering the distance between the patient and the source of the gas.
    ​NOTE: The Y-piece connector must have a sampling port on the inspiratory limb. If not, an additional straight connector with a sampling port must be used to supply O2.

3. Building and attaching the scavenging chamber

  1. Connect a 22 mm x 22 mm silicon rubber, flexible connector adapter to the proximal end of a scavenger chamber (internal diameter = 60 mm, internal length = 53 mm, volume = 150mL) containing 100 g of calcium hydroxide (Ca(OH)2).
  2. Attach a 15 mm O.D. x 22 mm O.D./15 mm I.D., 5 cm-6.5 cm, flexible, corrugated tube to the silicon rubber adapter.
  3. Connect another 22 mm x 22 mm silicon rubber, flexible connector adapter to the distal end of the scavenger.
  4. Add the scavenging chamber and tubing assembly to the Y piece's inspiratory limb using a 15 mm-22 mm two-step adapter.

4. Building and attaching the NO reservoir system

  1. Assemble a 3-L latex-free breathing reservoir bag and a 90° ventilator elbow connector without ports (22 mm ID x 22 mm).
  2. Connect the other end of the elbow to the central opening of the aerosol T-piece (horizontal ports 22 mm O.D., vertical port 11 mm I.D./22 mm O.D.).
  3. Attach the T-piece to the scavenging chamber's distal end by advancing it until it fits the silicon rubber connector tightly.

5. Building the NO and medical air supply system

  1. Build the NO/air gas supply system by attaching two consecutive 15 mm O.D. x 15 mm I.D./22 mm O.D. connectors with 7.6 mm sampling ports and flip-top caps.
    NOTE: Once the caps are removed, the sampling accesses will function as gas inlet ports.
  2. At the distal end of the NO/air supply system, attach another one-way inspiratory valve (arrow pointing upwards).
  3. At the proximal end of the NO/air supply system, connect a 15/22 mm two-step adapter.
  4. Connect the proximal two-step adapter to the remaining free inlet of the green T-piece from the NO reservoir system.

6. Attach the air and NO gas flow lines by using standard, kink-resistant, star-lumen vinyl oxygen gas tubing for the following steps.

  1. Connect medical air to the most distal gas inlet port.
  2. Connect NO gas from an 800 ppm medical-grade NO tank (size AQ aluminum cylinders containing 2239 L of 800 ppm of NO gas at standard temperature and pressure, balanced with nitrogen; delivered volume 2197 L) to the next port downstream.
    ​NOTE: Tubing must be of appropriate length to reach the gases' sources comfortably. Different tanks or generators of NO can be used as sources of gas.

7. Use in spontaneously breathing subjects

  1. Set the air, O2, and NO gas flow according to the desired FiO2 and NO concentration.
    NOTE: The recommended flow rates for administering NO at 80, 160, or 250 ppm are listed in Table 1 (applicable to 800 ppm cylinders only).
  2. Position the tight-fitting mask on the patient's face, similar to a non-invasive ventilation interface setup.
  3. Start the inhalation session for the desired duration.

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Representative Results

A 33-year-old respiratory therapist working at the ICU at MGH during the surge of ICU admission for COVID-19 volunteered to receive NO as part of the trial involving healthcare workers15,19. The trial tested the efficacy of 160 ppm of NO as a virucidal agent, thereby preventing disease occurrence in lungs at risk for viral contamination. The first session of the inhalation prophylaxis was administered before starting a shift through the described device for 15 min. For research purposes, concentrations of inhaled NO, NO2, and O2 were continuously measured. NO gas was administered at 3.5 L/min from an 800 ppm gas tank and mixed with air at a flow rate of 15 L/min and an O2 flow rate of 1 L/min to maintain a FiO2 at 21%.

The resulting NO concentration was 160 ppm at a total gas flow rate of 19.5 L/min, measured by three standard 15 L/min flowmeters. Oxygen saturation (SpO2), methemoglobin (MetHb), and heart rate were continuously monitored. SpO2 remained stable at around 97%. MetHb peaked at 2.3% during NO administration before rapidly returning to the baseline value upon suspension of the gas. The subject did not experience any side effects during or after the session. The NO concentration remained stable throughout the whole period of inhalation. NO2 peaked at 0.77 ppm and was therefore safely below the recommended toxicity threshold. A representative portion of the recorded tracings of NO and NO2 signals is depicted in Figure 2.

Figure 1
Figure 1: Graphic representation of the delivery device. The single components are indicated in the figure, as named in the text and the Table of Materials. The system comprises four major parts: the patient interface; Y-piece and oxygen supply; scavenging chamber; and the NO reservoir system and NO and medical air supply system. Abbreviations: HEPA = high-efficiency particular air; NO = nitric oxide. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Representative tracing of NO and NO2 concentrations during the 160 ppm NO inhalation in a healthy healthcare worker. Abbreviations: NO = nitric oxide; NO2 = nitrogen dioxide; ppm = parts per million. Please click here to view a larger version of this figure.

Target NO (ppm) FiO2 (%) Flow setup (L/min) Measured NO2 (ppm)
NO O2 Air
80 21 1.67 1.28 15 0.32
30 1.89 3.28 15 0.32
40 2.21 7.24 15 0.37
160 21 3.87 1.78 15 0.81
30 4.38 4.31 15 1.05
40 5.38 9.59 15 1.2
250 21 6.99 2.1 15 1.57
30 9.1 7.3 15 2.35
40 11.91 17.4 15 2.61

Table 1: Setup of NO, O2, and air gas flows. Gas flows to deliver target NO concentrations at varying FiO2, as measured with a lung simulator in a bench experiment. NO, and O2 flow (in L/min) were set to obtain target NO inspiratory concentration (80, 160, and 250 ppm) at the desired FiO2 (21%, 30%, 40%). A constant medical air flow rate (15 L/min) was used in every setting. A commonly available 800 ppm NO cylinder balanced with nitrogen was used. Abbreviations: L/min: L per min; NO: Nitric Oxide; NO2 = nitrogen dioxide; FiO2: Fraction of Inspired Oxygen, O2: Oxygen; ppm: parts per million.

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Given the increasing interest in NO gas therapy for non-intubated patients, including those with COVID-198, the present report describes a novel custom device and how to assemble its components to deliver NO at concentrations as high as 250 ppm. The proposed system is built out of inexpensive consumables and safely delivers a reproducible concentration of NO gas in spontaneously breathing patients. The ease of assembly and use, together with the safety data published elsewhere16,17, makes this system the ideal embodiment to administer a high NO gas concentration at varying FiO2 in non-intubated patients. The methodology described herein is currently in use at MGH to investigate the effect of high concentrations of NO to treat, or prevent, COVID-1914,18,19. The method can be adjusted based on the local availability of specific consumables, which may differ in brand and size from those described here. Nevertheless, a few critical steps of the protocol must be followed.

The sequence of each gas supply line, the reservoir bag, and the unidirectional valves must not be altered for any reason. A HEPA filter must also be present, particularly in case of any risk of infected bio-aerosol dispersion to the environment. Air leaks might impact the delivery of appropriate NO concentrations. Care should be exercised to use appropriately positioned and sized face masks and to avoid disconnection at any point of the system. The availability of a scavenger chamber with at least the reported amount (100 g) of Ca(OH)2 is also essential to prevent the accumulation of NO2 and avoid nitric acid formation upon reaction with water in the lungs. The Ca(OH)2 scavenger is designed to undergo a chemical dye reaction upon consumption, functioning as an indicator of its residual absorbent properties. To ensure the efficiency of the scavenger in reducing NO2 levels, the component should be changed when two-thirds of the canister have changed color. Bench tests showed that NO2 remained below 1 ppm for the first 60 min and never exceeded 1.3 ppm even after 5 h of exposure to 160 ppm NO17. Sessions longer than five hours will likely require the scavenger to be changed.

In case a cylinder is used as a source of NO gas, attention must be paid to the native NO concentration in the tank, as reported by the manufacturer. The NO, air, and O2 flow settings for a standard NO high-pressure cylinder are reported (Table 1). The use of cylinders with different gas concentrations, or alternative NO generating devices21,22,23, would impact the flow settings necessary to deliver gas mixtures with the desired NO and O2 concentrations. NO is diluted in nitrogen as a balance gas in most high-pressure cylinders. The higher the NO concentration, the lower is the net FiO2 administered to the patient if no supplemental O2 is added to the mixture. This interplay between NO concentration and FiO2 must be considered, especially when NO is administered to an already hypoxic patient, or while assessing the efficacy of NO in terms of oxygenation improvement. The resulting SpO2 increase might be blunted if FiO2 is not maintained constant during NO administration. Importantly, if no supplemental O2 is administered, a hypoxic mixture can potentially be generated by mixing high-dose NO and air.

NO has a very favorable safety profile. The molecule's very short half-life further limits the few potential adverse effects. Methemoglobinemia is the most important threat, particularly in the setting of prolonged high-dose exposure because of which MetHb levels should always be monitored closely. MetHb is formed in the blood upon breathing NO by the oxidation of iron present in circulating hemoglobin. Measurements can be obtained through rapid blood testing or non-invasively through SpMet % monitoring. Levels up to 10% are usually well-tolerated in healthy subjects24. Hemodynamic deterioration can rarely occur following NO inhalation. Rebound pulmonary hypertension is another possible risk if the prolonged administration of NO is abruptly interrupted25. The device can be modified to sample gas concentrations if needed. A NO/NO2 sampling access (15 mm straight connector with port) can be placed at the inspiratory limb, before the Y-piece. In that case, to safely add O2 to the admixture, an additional 15 mm straight connector with a port must be placed upstream and used as an oxygen inlet. However, monitoring the inspired gas concentrations of NO and NO2 is most likely not clinically feasible because of technical difficulties and the need for dedicated equipment to measure ppm levels of these gases at the bedside. Despite using the same tank, slight variations in the administered concentration might occur, compared to those reported in Table 1, based on the patient's minute ventilation. Additionally, standard gas rotameters (0-15 L/min with a stainless steel ball float) do not allow increments smaller than 0.5 L. The availability of high-precision digital flowmeters, similar to those for the setup shown in Table 1, would increase the precision of the dose being administered.

The limitations of the described methodology mainly include the scarce data currently available on the proposed device's human use. Although convincingly performing in bench experiments and testing on volunteers and patients17, to date, data are based on experience limited to a single center16. Operators should engage in the use of this novel system and the administration of high-dose NO only if already experienced in the use of NO gas therapy to treat critically ill patients. Depending on the local institutional policy and agreements in force, tanks or other NO gas sources might be challenging to obtain and use as freely adjustable gas sources, outside of the limitations imposed by the delivery devices currently available on the market. NO is an endogenously produced vasodilator26. Its administration as a gas therapy is currently approved by the U.S. Food and Drug Administration "for the treatment of term and near-term neonates with hypoxic respiratory failure associated with clinical or echocardiographic evidence of pulmonary hypertension"27. However, NO is also routinely used in adults for pulmonary vasoreactivity testing28 and as rescue therapy in hypoxemic critically ill patients with or without pulmonary hypertension2,29,30,31. The safety and tolerance of a high concentration (160 ppm) of NO have been consistently reported in studies addressing the drug's virucidal, bactericidal, and fungicidal effects5,6,7,27. To administer high-dose NO for research purposes, IRB approval was sought and obtained14,18,19,32.

To date, the administration of inhaled NO mainly relies on gas tanks and associated bulky machinery. Tank-based delivery devices are commonly designed to administer NO gas concentrations up to 80 ppm. Commercially available systems offer software-based capabilities to deliver an adjustable amount of NO based on the total gas flow being provided to the patient and the desired NO concentration. NO inhalation can be continuous or synchronized with the patient's inspiration. Measuring NO, NO2 and O2 concentrations through an electrochemical sensor cell is always possible. Such expensive devices may offer technical and safety advantages compared to the proposed construction. However, they are expensive and rarely present in more than a few units, being generally used within selected ICUs in intubated patients. As a result, the availability of NO therapy for patients outside the ICU is very limited, even at large institutions. Furthermore, the majority of currently marketed devices do not allow the off-label administration of concentrations higher than 80 ppm. Not surprisingly, by means of currently available devices, it is virtually impossible to administer NO at high concentrations on a large scale in a limited-resource setting, such as that mandated by a surge of patients and a shortage of medical supplies. Under such circumstances, the need for a simple and inexpensive, yet safe and open-source, device for the administration of this potentially beneficial therapy is critical.

This system might be implemented in the future by more investigators and clinicians to safely and reliably administer NO in a reproducible way in COVID-19 and other disease states for which NO properties might be beneficial. In the described methodology, the source of NO is usually a standard gas tank. Other NO sources can be adapted to be used with this delivery system, including tankless devices and generators.

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L.B. receives salary support from K23 HL128882/NHLBI NIH as a principal investigator for his work on hemolysis and nitric oxide. L.B. receives technologies and devices from iNO Therapeutics LLC, Praxair Inc., Masimo Corp. L.B. receives a grant from iNO Therapeutics LLC. A.F. and L.T. reported funds from the German Research Foundation (DFG) F.I. 2429/1-1; TR1642/1-1. WMZ receives a grant from NHLBI B-BIC/NCAI (#U54HL119145), and he is on the scientific advisory board of Third Pole Inc., which has licensed patents on electric NO generation from MGH. All other authors have nothing to declare.


This study was supported by the Reginald Jenney Endowment Chair at Harvard Medical School to L.B., by L.B. Sundry Funds at MGH, and by laboratory funds of the Anesthesia Center for Critical Care Research of the Department of Anesthesia, Critical Care and Pain Medicine at MGH.


Name Company Catalog Number Comments
90° ventilator elbow connector without ports 22 mm ID x 22 mm OD Teleflex, Wayne, PA, USA 1641
Aerosol tee connector: horizontal ports 22 mm OD, vertical port 11 mm ID/22 mm OD Teleflex, Wayne, PA, USA 1077
Flexible patient connector for endotracheal or tracheostomy tube (15 mm OD x 22 mm OD/15 mm ID, length 5 cm to 6.5 cm) Vyaire Medical Inc., Mettawa, IL, USA 3215
High-efficiency particulate air (highly hydrophobic bacterial/viral filter,  HEPA class 13) filter (22 mm ID/15 mm OD x 22 mm OD/15 mm ID connector) Teleflex, Wayne, PA, USA 28012
Latex-free 3-L breathing reservoir bag CareFusion, Yorba Linda, CA, USA 5063NL
Nitric Oxide tank 800 ppm medical-grade (size AQ aluminum cylinders containing 2239 L at STP of 800 ppm NO gas balanced with nitrogen, volume 2197 L) Praxair, Bethlehem PA, USA MM NO800NI-AQ
One-way valve 22 mm male/female (arrow pointing towards female end) Teleflex, Wayne, PA, USA 1664 N=2 inspiratory limb (upward arrow)
One-way valve 22 mm male/female (arrow pointing towards male end) Teleflex, Wayne, PA, USA 1665 N=1 expiratory limb (downward arrow)
Rad-57 Handheld Pulse Oximeter with Rainbow SET Technology Masimo Corporation, Irvine, CA, USA 3736 Including SpMet Option
Scavenger (ID = 60 mm, internal length = 53 mm, volume = 150 mL) containing 100 g of calcium hydroxide Spherasorb, Intersurgical Ltd, Berkshire, UK
Silicon rubber flexible connectors 22 mm F x 22 mm F Tri-anim Health Services, Dublin, OH, USA 301-9000
Snug-fit standard face mask of appropriate size
Star Lumen standard medical grade vynil oxygen tubing with universal connectors Teleflex, Morrisville, NC, USA 1115 Variable length according to distance from source of gas. 2.1 m length used in protocol
Straight connector with a 7.6 mm sampling port (15 mm OD x 15 mm ID/22 mm OD) Mallinckrodt, Bedminster, NJ, USA 502041
Two-step adapter (15 mm to 22 mm) Airlife Auburndale, FL, USA 1824
Y-piece connector with 7.6 mm ports (22 mm to 22 mm and 15 F) Vyaire Medical Inc., Mettawa, IL, USA 1831



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Cite this Article

Pinciroli, R., Traeger, L., Fischbach, A., Gianni, S., Morais, C. C. A., Fakhr, B. S., Di Fenza, R., Robinson, D., Carroll, R., Zapol, W. M., Berra, L. A Novel Inhalation Mask System to Deliver High Concentrations of Nitric Oxide Gas in Spontaneously Breathing Subjects. J. Vis. Exp. (171), e61769, doi:10.3791/61769 (2021).More

Pinciroli, R., Traeger, L., Fischbach, A., Gianni, S., Morais, C. C. A., Fakhr, B. S., Di Fenza, R., Robinson, D., Carroll, R., Zapol, W. M., Berra, L. A Novel Inhalation Mask System to Deliver High Concentrations of Nitric Oxide Gas in Spontaneously Breathing Subjects. J. Vis. Exp. (171), e61769, doi:10.3791/61769 (2021).

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