key: cord-0786130-c7bxzwpf
authors: Gianni, Stefano; Morais, Caio C.A.; Larson, Grant; Pinciroli, Riccardo; Carroll, Ryan; Yu, Binglan; Zapol, Warren M.; Berra, Lorenzo
title: Ideation and assessment of a nitric oxide delivery system for spontaneously breathing subjects
date: 2020-08-21
journal: Nitric Oxide
DOI: 10.1016/j.niox.2020.08.004
sha: 518751d731c544ae90e1f8f39d361ced67daa8ee
doc_id: 786130
cord_uid: c7bxzwpf

BACKGROUND: There is an increasing interest in safely delivering high dose of inhaled nitric oxide (NO) as an antimicrobial and antiviral therapeutics for spontaneously breathing patients. A novel NO delivery system is described. METHODS: We developed a gas delivery system that utilizes standard respiratory circuit connectors, a reservoir bag, and a scavenging chamber containing calcium hydroxide. The performance of the system was tested using a mechanical lung, assessing the NO concentration delivered at varying inspiratory flows. Safety was assessed in vitro and in vivo by measuring nitrogen dioxide (NO(2)) levels in the delivered NO gas. Lastly, we measured the inspired and expired NO and NO(2) of this system in 5 healthy subjects during a 15-minute administration of high dose NO (160 parts-per-million, ppm) using our delivery system. RESULTS: The system demonstrated stable delivery of prescribed NO levels at various inspiratory flow rates (0-50 L/min). The reservoir bag and a high flow of entering air minimized the oscillation of NO concentrations during inspiration on average 4.6 ppm for each 10 L/min increment in lung inspiratory flow. The calcium hydroxide scavenger reduced the inhaled NO(2) concentration on average 0.9 ppm (95% CI -1.58, -0.22; p=0.01). We performed 49 NO administrations of 160 ppm in 5 subjects. The average concentration of inspired NO was 164.8 [Formula: see text] 10.74 ppm, with inspired NO(2) levels of 0.7 [Formula: see text] 0.13 ppm. The subjects did not experience any adverse events; transcutaneous methemoglobin concentrations increased from 1.05 [Formula: see text] 0.58 to 2.26 [Formula: see text] 0.47%. CONCLUSIONS: The system we developed to administer high-dose NO for inhalation is easy to build, reliable, was well tolerated in healthy subjects.

Administration (FDA) in 1999 for the treatment of "term and near-term (>34 weeks) neonates with hypoxic respiratory failure associated with clinical or echocardiographic evidence of pulmonary hypertension where it improves oxygenation and reduces the need for extracorporeal membrane oxygenation". 1 In addition to its pulmonary vasodilator effect, NO produces broad antimicrobial activity on bacteria 2 and viruses such as SARS CoV 3,4 , the virus responsible for the SARS epidemic in 2003. Based on the established anti-viral effects, several clinical trials are now testing the efficacy of NO inhalation on patients infected by SARS CoV-2, in the midst of the ongoing pandemic. [5] [6] [7] In spontaneously breathing patients, NO gas is traditionally delivered through a mechanical ventilator or through a high flow nasal cannula (HFNC) system. 8 Nitric oxide is blended with medical air and oxygen in the ventilator and may be delivered to the patient through a snug fitting facemask. 8 Despite this approach's ability to administer a high concentration of NO gas (>100 parts per million [ppm]), widespread adoption is challenged by the lack of a safe delivery system. In addition, in the phase of the SARS-CoV-2 pandemic, delivering NO via HFNC or ventilator-driven respiratory systems potentially aerosolizes droplets with virus, which raises further concerns for safety.

Two major patient safety aspects of administering high-dose NO are the generation of nitrogen dioxide (NO 2 ) and methemoglobin (MetHb). Nitrogen dioxide is formed by the reaction between NO and oxygen, and when combined with water in the airways, NO 2 forms nitric acid, leading to a caustic burn of the airways. When inhaling NO, methemoglobin is generated by oxidation of the iron contained in circulating hemoglobin. Methemoglobin cannot bind oxygen, so levels must be closely monitored in all subjects receiving NO (particularly high doses). Methemoglobin levels of 10%, or less, are well tolerated in a healthy subject. 9 After cessation of NO treatment, intracellular methemoglobin reductase rapidly reduces RBC MetHb levels.

An in-hospital system that is simple, inexpensive, and capable of delivering a constant and predictable concentration of NO over time, while minimizing NO 2 , without generating aerosolized particles is needed to allow use of high-dose NO outside the intensive care unit (ICU).

In this study, we designed and developed a breathing system capable of delivery high concentrations of NO. We evaluated performance and safety of the device both in vitro and in healthy adults by accurately sampling and measuring NO and NO 2 concentrations in the inhaled and exhaled breath.

J o u r n a l P r e -p r o o f

The system incorporates standard respiratory circuit connectors (Figure 1 ). The distal portion of the inspiratory limb begins with a one-way valve (Hudson RCI, Wayne, PA, USA) and two gas inlet connectors (Hudson RCI, Wayne, PA, USA), which inject medical air and NO gas, respectively. A T-connector joins a 3L bag to the inspiratory limb. The bag serves as an NO reservoir to stabilize the NO concentration throughout the inspiratory phase. A scavenger (internal diameter = 60 mm, internal length = 53 mm, volume = 150 mL) containing 100g of calcium hydroxide (Spherasorb™, Intersurgical Ltd, Berkshire, UK) absorbs the NO 2 generated in the gas mixture 10 . A flexible connector was inserted to accommodate patient movement and positioning. Two gas inlet connectors act as oxygen inlet and NO/NO 2 sampling line, respectively. A second inspiratory one-way valve was placed after the set reservoir/scavenger to avoid additional gas mixing due to expired backflow. This system was created for the treatment of subjects with coronavirus disease 2019 (COVID-19), and a high-efficiency particulate air (HEPA) filter was connected between the Y-piece and the patient interface (full face mask or mouthpiece) to remove any aerosolized virus.

Active humidification was not added, and relative humidity ratio was not tested, as the device, here described, has been built for delivering intermittent, short periods of high dose nitric oxide. 11

The NO delivery system performance was tested using a bench testing lung (Dual Adult Test Lung, Michigan Instruments, Michigan, USA) ( Figure 2 ) and a mechanical ventilator to simulate an inspiratory effort (Hamilton G5, Hamilton Medical AG, Bonaduz, Switzerland). The ventilator was connected to the right lung, which acts as the "diaphragm" to lift the left lung by a coupling clip. An inspiratory sinusoid flow waveform was produced by the ventilator during a volume-controlled ventilation mode. NO was delivered at 50 ppm or 250 ppm, respectively. One should note, however, that the delivery system we described here is independent from the tank of NO employed.

By introducing a standard gas connector, an operator could use our delivery system with any desirable NO source.

To reduce the fluctuations of delivered NO, we examined the efficacy of adding a reservoir bag to stabilize the concentration of the inspired NO. The experimental setup involved a respiratory rate (RR) of 15 breaths/min; tidal volume (V T ) of 0.25, 0.5, 0.75 and 1L; an inspiratory time of 1 second, and a sinusoidal flow wave. During this test the calcium hydroxide scavenger was incorporated into the system. The average inspiratory flow required to archive the set V T was used as an independent variable in the analysis.

Nitric oxide concentration was measured over 2 minutes during the inspiratory phase of each set of tidal volume, with and without the reservoir bag. We used three NO concentrations: 50, 150 and 250 ppm. FiO 2 was set at 0.21.

Similarly, we evaluated the inspired concentration of NO 2 , using the same experimental settings, with or without the 3L reservoir bag.

We examined the effect of various levels of air flow on NO concentration during ventilation. We measured the inspiratory NO concentrations at 5, 10, and 15 L/min of air flow. At every level of air flow, before starting ventilation, we set the NO gas flow to achieve a static concentration of 180 ppm NO in the inspiratory limb of the circuit.

Ventilator settings included a respiratory rate of 20 bpm, a tidal volume of 0.5 L, an inspiratory time of 1 sec and sinusoidal flow wave.

We tested the performance of the system to find the NO and oxygen flow required to obtain the desired concentration of inspired NO. Mechanical ventilation was set with a tidal volume 0.5 L, a respiratory rate of 20 bpm, an inspiratory time of 1 sec and a sinusoidal flow wave. We tested 3 different target NO concentrations: 50, 150, and 250 ppm at different FiO 2 : 0.21, 0.30, and 0.40. NO 2 levels were also measured.

To assess the efficacy of the calcium hydroxide scavenger in reducing the inspiratory levels of NO 2 , we used the same mechanical ventilator settings as above, a range of target NO concentrations (50, 150, and 250 ppm), two different levels of FiO2 (0.21 and 0.40) and measured NO 2 levels in the inspiratory limb with and without the scavenger.

We administered high-dose NO with our system to healthy adult subjects as part of a randomized controlled trial 7 conducted in our center (NCT04312243). Each administration lasted for 15 minutes. FiO 2 , NO and NO 2 concentrations were monitored in the inspiratory limb of the circuit. Peripheral oxygen saturation (SpO 2 ) and J o u r n a l P r e -p r o o f methemoglobin (MetHb) were continuously and non-invasively monitored with a pulse co-oximeter (Masimo rainbow SET, Irvine, CA 92618) 12,13 .

Additionally, we evaluated the exhaled concentration of NO 2 in one healthy subject. We administered 150 ppm NO using the previously described system. To monitor the expiratory NO 2 concentration we placed the sampling line between the mouthpiece and the HEPA filter (figure 1). Since the continuous gas flow from the inspiratory limb of the circuit can interfere with the measure, a 3-way stopcock (2100 series, Hans Rudolph INC. Shawnee, KS, USA) was positioned before the Y: its closure at the beginning of exhalation stopped the washout effect of fresh gas coming from the inhalation arm of the circuit, allowing a sampling of exhaled gas only.

We used linear mixed models to analyze the effect of the reservoir and the scavenger on the system's performance. Statistical significance was assumed at a two-tailed P 

The total inspiratory resistance, from the inspiratory one-way valve to the HEPA filter, was on average 8.6 cmH 2 O/L/s. The total expiratory resistance, measured from the HEPA filter to expiratory one-way valve, was on average 7.2 cmH 2 O/L/s. 

The scavenger positioned in the inspiratory limb of the circuit reduced the inhaled NO 2 concentration to an average of 0.9 ppm (95% CI -1.58, -0.22; p =0.01) (Figure 7) . At 150 ppm of inhaled NO, the NO 2 concentration was maintained below 1.2 ppm with FiO 2 from 0.21 to 0.40. Our data suggest that the scavenger can efficiently reduce NO 2 in the circuit for NO delivery. Similar NO 2 values were found when 857 ppm NO/N 2 tanks were used (see Table 1 ).

These data provide a reference for adjusting desired NO delivery in our designed system.

We administered NO to 5 adult health care subjects: 2 males and 3 females. Median age was 32. The subjects had no history of cardiovascular or lung disease. The total number of NO administrations was 48. The average concentration of inspired NO was 164.8 ± 10.74 ppm with NO 2 levels of 0.7 ± 0.13 ppm; these levels remained stable throughout the administration (Figure 8 ). We administer oxygen to keep FiO2 0.21 (see table 1 ). During 15 minutes of administration of gaseous NO, methemoglobin levels increased from a baseline value of 1.05 ± 0.58 % to 2.26 ± 0.47%. The subjects did not experience any discomfort during the procedure. No adverse events were reported.

Despite the small number of administrations, these results indicate that breathing high concentration of NO for short period of time using our newly developed NO breathing system is feasible and well tolerated without adverse events.

The average inspired NO and NO 2 concentration were 153 ppm and 0.51 ppm, respectively, using an FiO 2 of 0.205. At the end of exhalation NO 2 concentration decreased to 0.03 ppm (see Figure 9 ). Exhaled FiO 2 was 0.195. 16 Additionally, inhaled NO therapy has also shown to be a potent anti-inflammatory agent, reducing lung thrombosis after lung transplant. 17 In the setting of the current COVID-19 pandemic, we are examining whether the administration of high-dose NO in spontaneously breathing COVID-19 patients leads to a reduced rate of hospital admission (NCT04338828) and respiratory failure requiring intubation and mechanical ventilation (NCT04305457). Furthermore, we recently published a case series of 6 COVID 19 positive pregnant patients that received high dose (160-200 ppm) nitric oxide using our delivery system. 18 To avoid overwhelming COVID-19 systemic inflammation, we designed a way to deliver NO treatment early. Therapy with NO inhalation starts in the emergency department or upon hospital admission to the general care wards. One could envision treatments with NO gas with a similar prototype of NO delivery system for home use. 19 J o u r n a l P r e -p r o o f

The system we designed and evaluated in this study allows for the administration of NO outside the ICU setting without a mechanical ventilator. If evidence continues to mount that high-dose inhaled NO is an effective anti-inflammatory, anti-thrombotic and antimicrobial agent, the system described can be used to treat or prophylactically treat many patients without the need for a dedicated mechanical ventilator. It has potential applications on the front lines, in an emergency room or rural clinic setting, or in lowresource settings, in the face of a pandemic due to a susceptible respiratory pathogen.

The system is reproducible, inexpensive, reliable, and easy to build and maintain.

There are two important limitations of this system. First, this system does not continuously measure NO and NO 2 during administration without the use of external gas analyzers. However, once calibrated, using consistent gas flows and concentrations, and fresh calcium hydroxide, the NO and NO 2 levels showed to remain consistent in repeated laboratory tests. Second, to set medical air, NO, and oxygen flows, we used a high precision digital flowmeter. These flowmeters are not in widespread clinical use. The flowmeters commonly used in the clinical setting cannot reach this level of resolution, which may introduce some unexpected variability in the predicted NO and NO 2 levels.

In conclusion, we built an NO delivery system that provides an alternative to a ventilator-based system 8 to give high dose NO to spontaneously breathing patients.

Despite some limitations, this system can efficient to allow administering high concentrations of NO via a comfortable fitting mask. • In a healthy subject breathing 153 ppm of NO, the exhaled NO 2 was 0.03 ppm

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