Posts Tagged ‘Inhalation’

Curator: Aviva Lev-Ari, PhD, RN

iNO – Clinical Trials and Meta Analysis Studies: Recent Findings

Clinical perspectives with long-term pulsed inhaled nitric oxide for the treatment of pulmonary arterial hypertension

1Department of Pediatrics and Medicine, Columbia University, New York, New York, US
2Department of Pediatrics and Medicine, Massachusetts General Hospital, Boston, Massachusetts, US
3Department of Pediatrics, University of Colorado School of Medicine, Children’s Hospital Colorado, Aurora, Colorado, US
4Ikaria, Inc., Hampton, New Jersey, USA
Address correspondence to: Dr. Robyn J. Barst, 31 Murray Hill Road, Scarsdale, NY 10583, USA ; Email: robyn.barst@gmail.com
This article has been corrected. See Pulm Circ. 2012; 2(3): iv.


Pulmonary arterial hypertension (PAH) is a chronic, progressive disease of the pulmonary vasculature with a high morbidity and mortality. Its pathobiology involves at least three interacting pathways –
  • prostacyclin (PGI2),
  • endothelin, and
  • nitric oxide (NO).
Current treatments target these three pathways utilizing PGI2 and its analogs, endothelin receptor antagonists, and phosphodiesterase type-5 (PDE-5) inhibitors.
Inhaled nitric oxide (iNO) is approved for the treatment of hypoxic respiratory failure associated with pulmonary hypertension in term/near-term neonates. As a selective pulmonary vasodilator, iNO can acutely decrease pulmonary artery pressure and pulmonary vascular resistance without affecting cardiac index or systemic vascular resistance. In addition to delivery via the endotracheal tube, iNO can also be administered as continuous inhalation via a facemask or a pulsed nasal delivery. Consistent with a deficiency in endogenously produced NO, long-term pulsed iNO dosing appears to favorably affect hemodynamics in PAH patients, observations that appear to correlate with benefit in uncontrolled settings. Clinical studies and case reports involving patients receiving long-term continuous pulsed iNO have shown minimal risk in terms of adverse events, changes in methemoglobin levels, and detectable exhaled or ambient NO or NO2. Advances in gas delivery technology and strategies to optimize iNO dosing may enable broad-scale application to long-term treatment of chronic diseases such as PAH.
Keywords: drug, hypertension, inhalation administration, nitric oxide, pulmonary arterial hypertension, pulmonary circulation, pulmonary hypertension, pulmonary/physiopathology, pulse therapy, vasodilator agents


In summary, uncontrolled observational studies of long-term use (>1 month) of continuous pulsed iNO (as monotherapy or as part of combination therapy) in a total of 14 patients with PAH across five studies [Ref 46-48, 54,55]

have reported no significant adverse events, no elevated metHb levels, and no detectable exhaled or ambient NO or NO2. In one study, a patient experienced three episodes of severe epistaxis over two years while on a combination of pulsed iNO and epoprostenol.[46]

In a case report of a patient awaiting heart-lung transplantation, the patient experienced hypotensive bradycardia upon an attempt to wean from iNO therapy. In addition, a recurrence in hypotensive bradycardia resulted in the increase of iNO dose (40–106 ppm), followed by a decrease to 70 ppm (along with administration of bicarbonate and reintroduction of prostacyclin) after increasing metabolic acidosis.[55]

There is evidence that pulsed delivery may allow utilization of lower NO concentrations compared with continuous face mask administration, potentially minimizing the risk of associated adverse events as well as resulting in a more practical delivery system.[49]

The consensus on treatment for PAH encompasses numerous goals, the most important being to improve overall quality of life by decreasing symptoms while minimizing treatment-related side effects.[2]

Additional goals include enhancing functional capacity, i.e., exercise capacity, improving hemodynamic derangements (lowering PVR and PAP, and normalizing RAP and CO), and preventing, if not reversing, disease progression. Finally, improving survival, although certainly desirable, is rarely an end point in trials examining PAH treatment.[2]

The availability of novel treatments and the improvement in survival rates have allowed the goals of PAH therapy to expand from improving survival and preventing disease progression to also improving HRQOL.[71]

Potential advances in long-term PAH treatment, such as ambulatory iNO administration, may allow for greater improvements in HRQOL. Pérez–Peñate et al. observed that ambulatory pulsed iNO treatment did not diminish quality of life beyond the consequences of the disease itself.[47]

Eight of eleven patients who led a nonsedentary life were able to leave their home daily, with four returning to work while on long-term iNO therapy.

An ideal drug-device for long-term PAH treatment should emphasize portability and safety features for outpatient use. Advances in iNO gas delivery technology and strategies to optimize dosing should allow for randomized controlled trials of iNO and, hopefully, may lead to broad-scale application of iNO in the treatment of chronic diseases such as PAH.[45]



Anesth Analg. 2011 Jun;112(6):1411-21. doi: 10.1213/ANE.0b013e31820bd185.
Epub 2011 Mar 3.

Inhaled nitric oxide for acute respiratory distress syndrome and acute lung injury in adults and children: a systematic review with meta-analysis and trial sequential analysis.

Afshari ABrok JMøller AMWetterslev J.


Department of Anesthesiology, Rigshospitalet, University of Copenhagen, Anestheisa, Juliane Marie Centre, Copenhagen, 2100, Denmark.



Acute hypoxemic respiratory failure, defined as acute lung injury and acute respiratory distress syndrome, are critical conditions associated with frequent mortality and morbidity in all ages. Inhaled nitric oxide (iNO) has been used to improve oxygenation, but its role remains controversial. We performed a systematic review with meta-analysis and trial sequential analysis of randomized clinical trials (RCTs). We searched CENTRAL, Medline, Embase, International Web of Science, LILACS, the Chinese Biomedical Literature Database, and CINHAL (up to January 31, 2010). Additionally, we hand-searched reference lists, contacted authors and experts, and searched registers of ongoing trials. Two reviewers independently selected all parallel group RCTs comparing iNO with placebo or no intervention and extracted data related to study methods, interventions, outcomes, bias risk, and adverse events. All trials, irrespective of blinding or language status were included. Retrieved trials were evaluated with Cochrane methodology. Disagreements were resolved by discussion. Our primary outcome measure was all-cause mortality. We performed subgroup and sensitivity analyses to assess the effect of iNO in adults and children and on various clinical and physiological outcomes. We assessed the risk of bias through assessment of trial methodological components. We assessed the risk of random error by applying trial sequential analysis.


We included 14 RCTs with a total of 1303 participants; 10 of these trials had a high risk of bias. iNO showed no statistically significant effect on overall mortality (40.2%versus 38.6%) (relative risks [RR] 1.06, 95% confidence interval [CI] 0.93 to 1.22; I² = 0) and in several subgroup and sensitivity analyses, indicating robust results. Limited data demonstrated a statistically insignificant effect of iNO on duration of ventilation, ventilator-free days, and length of stay in the intensive care unit and hospital. We found a statistically significant but transient improvement in oxygenation in the first 24 hours, expressed as the ratio of Po₂ to fraction of inspired oxygen (mean difference [MD] 15.91, 95% CI 8.25 to 23.56; I² = 25%). However, iNO appears to increase the risk of renal impairment among adults (RR 1.59, 95% CI 1.17 to 2.16; I² = 0) but not the risk of bleeding or methemoglobin or nitrogen dioxide formation.


iNO cannot be recommended for patients with acute hypoxemic respiratory failure. iNO results in a transient improvement in oxygenation but does not reduce mortality and may be harmful.


Clinical Policy Bulletin:

Nitric Oxide, Inhalational (INO) Number: 0518

Aetna Policy

      Aetna considers inhaled nitric oxide (INO) therapy medically necessary as a component of the treatment of hypoxic respiratory

      failure in term and near-term (born at 34 or more weeks of gestation) neonates when both of the following criteria are met:

  •                         Neonates do not have congenital diaphragmatic hernia; and
  •                         When conventional therapies such as administration of high concentrations of oxygen, hyperventilation, high-frequency
  •                         ventilation, the induction of alkalosis, neuromuscular blockade, and sedation have failed or are expected to fail.

      Note: Use of INO therapy for more than 4 days is subject to medical necessity review.

      Aetna considers the diagnostic use of INO medically necessary as a method of assessing pulmonary vaso-reactivity in persons

      with pulmonary hypertension.

      Aetna considers INO therapy experimental and investigational for all other indications because of insufficient evidence in the

      peer-reviewed literature, including any of the following:

                        Acute bronchiolitis; or

                        Acute hypoxemic respiratory failure in children (other than those who meet the medical necessity criteria above) and in adults; or

Adult respiratory distress syndrome or acute lung injury; or

Post-operative management of pulmonary hypertension in infants and children with congenital heart disease; or

Premature neonates (less than 34 weeks of gestation); or

Prevention of ischemia-reperfusion injury/acute rejection following lung transplantation; or

Treatment of persons with congenital diaphragmatic hernia; or

Treatment of vaso-occlusive crises or acute chest syndrome in persons with sickle cell disease (sickle cell vasculopathy).




NO is naturally produced in the body by the enzyme NO synthase, which converts L-arginine to L-citrulline and NO in the presence of oxygen and certain cofactors. Both constitutive and inducible forms of NO synthase are present in endothelium and various other tissues.39–,41 NO has several important physiological roles, including involvement in smooth muscle relaxation, neurotransmission, host defense responses, and platelet function. NO produced by the vascular endothelium causes local vasodilatation, thereby regulating vasomotor tone. Circulating NO is present in only picomolar amounts and is rapidly inactivated by reaction with hemoglobin. Because of this short circulating half-life (3–5 seconds), inhalation of subtoxic levels of NO causes vasodilatation of the pulmonary vasculature with little or no systemic vasodilatation. Therapeutic administration of NO by inhalation thus provides a means of selectively lowering pulmonary arterial blood pressure, potentially improving hemodynamic status and gas exchange.11–13,15,17,18,23

Inhaled NO has been widely studied in adults with pulmonary hypertension and acute lung injury, and it is currently approved by the Food and Drug Administration for treatment of hypoxic respiratory failure in neonates with pulmonary hypertension. Three potential hazards associated with inhaled NO therapy are recognized:

(1) direct pulmonary toxic effects of NO,

(2) pulmonary toxic effects due to NO2 produced by oxidation of NO, and

(3) development of methemoglobinemia.

Studies of exposure to toxic levels of NO and NO2 in various species indicated that high concentrations of these gases can be lethal. Pulmonary edema, hypoxemia, acidosis, and hypotension developed in dogs exposed to 0.5% to 2% NO or NO2, and most animals died within 7 to 50 minutes of exposure.42 In rats, inhaled NO2 concentrations of 127 ppm were lethal within 30 minutes in 50% of animals (LC50).43 The LC50 in primates exposed to NO2 for 30 to 60 minutes is 100 to 200 ppm.43 Methemoglobinemia is detectable by measurement of blood levels of methemoglobin and is manifested clinically as cyanosis and hypoxia. Methemoglobinemia developed in animals exposed to high concentrations of NO or NO2, although not uniformly. In one instance, a methemoglobin level of 1.00 developed in a dog exposed to 2% NO for 50 minutes.42

In humans, NO at 10 to 20 ppm can cause irritation of the eyes and nose, 25 ppm can be irritating to the respiratory tract and cause chest pain, 50 ppm can cause pulmonary edema, and 100 ppm can be fatal.1,4

Legally permissible exposure limits for NO and NO2 have been issued by the Occupational Safety and Health Administration. For NO, this threshold is 25 ppm (30 mg/m3), averaged over an 8-hour work shift.10 This value corresponds to the threshold limit value promulgated by the American Conference of Governmental Industrial Hygienists.2 Adherence to this limit is thought to provide adequate protection against methemoglobinemia and other toxic effects. Concentrations of 100 ppm and higher (30-minute mean) are deemed to be an immediate threat to life and health by the National Institute for Occupational Safety and Health.44 The Occupational Safety and Health Administration ceiling limit for NO2 is 1 ppm (1.8 mg/m3), and this limit is not to be exceeded at any time during the work shift.10 The threshold limit for TWA concentration of NO2 issued by the American Conference of Governmental Industrial Hygienists is 3 ppm,2 and the National Institute for Occupational Safety and Health requires that NO2exposures not exceed 1 ppm.10,44

These threshold values are thought to represent maximum concentrations to which nearly all workers can be exposed on a regular basis without adverse effects. Nevertheless, evidence suggests that lower levels of exposure can have deleterious effects. For example, irreversible emphysematous changes to the lungs occurred in beagles exposed to 0.6 ppm NO2 for 16 h/d for 68 months and then to clean air for 32 to 36 months.45 In a study of exposure of humans to NO at 1.0 ppm, small but significant increases in airway resistance occurred in half the subjects.46 Similarly, inhalation of NO2 at 0.7 to 2 ppm for 10 minutes increased airflow resistance in healthy subjects.1 Exposure to NO2 at 2.3 ppm for 5 hours reportedly altered alveolar permeability in humans.47 Brief exposure to NO2 levels as low as 0.4 ppm may augment the response to challenge with specific allergens, and exposure to 0.1 to 0.5 ppm may affect pulmonary function in patients with asthma or chronic obstructive lung disease.1,5,7,48,49

Limited information is available on occupational exposure to NO in the healthcare setting. Using stationary chemiluminescence monitoring, Mourgeon et al50 determined ambient concentrations of NO and NO2 in the main corridor of an ICU. They found that mean ambient NO concentrations within the ICU were 0.237 ppm (SD 0.147 ppm) during the therapeutic use of inhaled NO at 5 ppm or less in 1 or more patients and 0.289 ppm (SD 0.147 ppm) during times when inhaled NO therapy was not used. The institution where this study50 was performed is located on a main street in Paris, and Mourgeon et al concluded that the ICU corridor values were entirely dependent on prevailing outdoor concentrations. Markhorst et al51 examined ambient levels of NO and NO2 in well-ventilated and poorly ventilated pediatric ICU rooms in which administration of inhaled NO at 20 ppm was simulated. As in the study by Mourgeon et al, sampling was done from a stationary position (in the study by Markhorst et al, 65 cm from the high-frequency oscillator used) at a height of 150 cm. During the simulation, maximum NO and NO2levels were 0.462 and 0.064 ppm, respectively. Phillips et al52 used occupational hygiene techniques similar to those we used to examine exposure levels in medical personnel during administration of inhaled NO to 6 patients in a pediatric ICU. In all instances, TWA concentrations were less than the limits of detection for the assay used. The patients’ sizes and minute volumes were not specified, although 3 of the patients were classified as neonatal.

▪ Nitric oxide therapy does not appear to expose nurses to excessive levels of nitric oxide or nitrogen dioxide during routine patient care in the ICU.

We examined the occupational exposure of ICU nurses to NO during NO therapy at delivery levels of 5 and 20 ppm in adult patients with acute respiratory distress syndrome. The maximum TWA exposures in our study were 0.45 ppm for NO and 0.28 ppm for NO2, well below the legally permissible exposure limits mandated by the Occupational Safety and Health Administration, and the involved nurses reported no respiratory or other signs or symptoms. The maximum outdoor background concentrations of NO and NO2 in our county during the periods of study ranged from 0.006 to 0.030 ppm for NO and 0.018 to 0.090 ppm for NO2. For comparison, the primary national ambient air quality standard issued by the Environmental Protection Agency is 0.053 ppm (100 μg/m3), calculated as an annual arithmetic mean.53 We did not assess methemoglobin levels in the nurses; however, methemoglobinemia did not develop in the treated patients. Marked methemoglobinemia is uncommon in patients treated with inhaled NO at concentrations similar to those used in our study.11,12,15,16,18,23

In the simulation study of Markhorst et al,51 ambient NO concentrations were measured at distances of 15 to 200 cm from a high-frequency oscillator, yielding levels ranging from 1.2 to 0.4 ppm. Our measurements yielded similar results (see Figure); however, in our study, NO levels at the ventilator exhaust port were nearly 10 times higher (9.2 ppm) than those 15 cm away (1.0 ppm). NO concentrations decreased rapidly; the mean was about 0.030 ppm in the area between 0.6 m from the ventilator and 0.6 m outside the patient’s room. For comparison, in homes with gas cooking stoves, ambient NOx levels of 0.025 to 0.075 ppm are typical.9

A number of factors determine the concentrations of NO and NO2 to which personnel are exposed during the therapeutic use of inhaled NO. These include the concentration of NO delivered to the patient, the patient’s minute volume, room size, room ventilation, and whether special ventilator exhaust routing or chemical scavenging devices are used. Baseline ambient levels of NO and NO2 depend on outdoor environmental factors such as proximity to motor vehicle traffic or heavy industry, climate, wind, and sky clarity.50Depending on the mode of administration, the actual concentration of NO delivered to a patient can fluctuate from the intended level. Continuous delivery during the entire respiratory cycle can produce more atmospheric contamination than does sequential administration limited to the inspiratory phase.54 The amount of NO2 formed during NO therapy varies according to the concentrations of oxygen and NO delivered, the time the 2 gases remain in contact, total gas flow, and minute volume.55 Thus, higher fractions of inspired oxygen will lead to increased formation of NO2 during inhaled NO therapy.

Because of differences in minute volume, therapeutic administration of inhaled NO to adult patients will result in substantially greater release of NO than will administration to infants or children. For example, to achieve a delivered NO concentration of 20 ppm, the required flow from a 1000-ppm NO source varies from 20 mL/min for a minute volume of 1 L/min to more than 200 mL/min for a minute volume of 11 L/min19 (our patients’ minute volumes exceeded 11 L/min). Simultaneous treatment of multiple patients in the same room or unit might increase exposure levels. The time spent by healthcare providers in the patient’s room and their average exposure distance from the ventilator exhaust port are also important factors. Room ventilation is clearly a factor. Ventilation in our negative-pressure isolation rooms exceeded that mandated by the Centers for Disease Control and Prevention (ie, ≥6 air changes per hour for existing rooms and ≥12 air changes per hour where possible and in new hospital construction).56 Our study design did not allow analysis of the effects of any of these factors; however, the methods we used provide data for real-world examples of ICU nurses caring for typical adult patients receiving inhaled NO. These techniques also constitute the standard method for evaluations of occupational exposure to toxic gases. Studies in which these methods are used, but involving larger samples of nurses and patients in various settings, would allow better definition of variance and the effects that factors such as room ventilation have on exposure to ambient NO and NO2.

In summary, we found that inhaled NO therapy at doses up to 20 ppm does not appear to pose a risk of excessive occupational exposure to NO or NO2 to healthcare workers during the routine delivery of critical care nursing in typical adult ICU settings. These findings lend support to the occupational safety of this therapeutic modality.


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Exposure of Intensive Care Unit Nurses to Nitric Oxide and Nitrogen Dioxide During Therapeutic Use of Inhaled Nitric Oxide in Adults With Acute Respiratory Distress Syndrome

1.  Mohammed A. Qureshi, MD,

2. Nipurn J. Shah, MD,

3. Carol W. Hemmen, RN, BSN

4. Mary C. Thill, RN, MSN and

5. James A. Kruse, MD

Am J Crit Care March 2003 vol. 12 no. 2 147-153


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Lung Cancer (NSCLC), drug administration and nanotechnology

Author: Tilda Barliya PhD

Dr. Saxena has greatly introduced us to lung cancer , the associated drug treatments and their market share in the post titled ” NSCLC and where the future lie?”. Since lung cancer is the most leading cause of death in both man and women, and have gained lots of attention I am interested in elaborating on NSCLC and explore the potential use of nanotechnology in this matter.

As previously mentioned, there are 3 common types of lung cancer:

  • Adenocarcinomas are often found in an outer area of the lung. (Most common)
  • Squamous cell carcinomas are usually found in the center of the lung next to an air tube (bronchus).
  • Large cell carcinomas can occur in any part of the lung. They tend to grow and spread faster than the other two types. (Least common).

Figure 1. The Signs and symptoms of lung cancer anatomy.


Since each type develops in different areas/part of the lung, it is hypothesized that they might need different routs of administration. The possible routes of administration are:

  • IV (systemic)————->through the blood
  • Inhaled aerosols (more localized)———–>through the airways

In order to understand what does “different routs of administration” refers to, we need to dig into the anatomy of the lung, i.e, airways and blood circulation as well as understand the lung-blood barriers components that may affect drug absorption.

The Blood Circulation

Two different circulatory systems, the bronchial and the pulmonary, supply the lungs with blood (Staub, 1991). The bronchial circulation is a part of the systemic circulation and is under high pressure. It receives about 1% of the cardiac output and supplies the airways (from the trachea to the terminal bronchioles), pulmonary blood vessels and lymph nodes with oxygenated blood and nutrients and conditions the inspired air (Staub, 1991). In addition, it may be important to the distribution of systemically administered drugs to the airways and to the absorption of inhaled drugs from the airways (Chediak et al., 1990). The pulmonary circulation comprise an extensive low pressure vascular bed, which receives the entire cardiac output. It perfuses the alveolar capillaries to secure efficient gas exchange and supplies nutrients to the alveolar walls. Anastomoses between bronchial and pulmonary arterial circulations have been found in the walls of medium-sized bronchi and bronchioles (Chediak et al., 1990; Kröll et al.,1987)



  • Fast: 15–30 seconds to 1-2 hours
  • suitable for drugs not absorbed by the digestive system
  • IV can deliver continuous medication


  • Patients are not typically able to self-administer
  • It is the most dangerous route of administration because it bypasses most of the body’s natural defenses, exposing the user to health problems, known as chemo side affects.
  • Finally dose at the organ site is much lower than the administrated dose

Most of the conventional chemotherapy are mainly administrated IV (Docetaxel, Paxlitaxel, Gemcitiabine, Avastin etc).

The Airways

The human respiratory system can be divided in two functional regions: the conducting airways and the respiratory region. The conducting airways, which are composed of the nasal cavity and associated sinuses, the pharynx, larynx, trachea, bronchi, and bronchioles, filter and condition the inspired air. From trachea to the periphery of the airway tree, the airways repeatedly branch dichotomously into two daughter branches with smaller diameters and shorter length than the parent branch (Weibel, 1991). For each new generation of airways, the number of branches is doubled and the crosssectional area is exponentially increased. The conducting region of the airways generally constitutes generation 0 (trachea) to 16 (terminal bronchioles). The respiratory region, where gas exchange takes place, generally constitutes generation 17-23 and is composed of respiratory bronchioles, the alveolar ducts, and the alveolar sacs.

The air-blood barrier of the gas exchange area is composed of the alveolar epithelial cells (surface area 140 m2) on one side and the capillary bed (surface area 130 m2) on the other side of a thin basement membrane (Simionescu, 1991; Stone et al., 1992). The extensive surface area of the air-blood barrier in combination with its extreme thinness (0.1-0.5 μm) permit rapid gas exchange by passive diffusion (Plopper, 1996).


The lung is a very attractive target for drug delivery. It provides direct access to disease in the treatment of respiratory diseases, while providing an enormous surface area and a relatively low enzymatic, controlled environment for systemic absorption of medications. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1884307/)


  • Can be self medicated
  • Easy to use
  • Reduced side effects associated with systemic delivery


  • Slower route of action
  • Potential problem of deposition to the deeper alveolar (higher generations, like G 8-10)
  • Immuno-defense system
  • Difficulty in measuring the exact dose inside the lung
  • inhaled aerosol is entrapped in the mucus in the conducting airways

Need to be reminded that in addition, a drug’s efficacy may be affected by where in the respiratory tract it is deposited, its delivered dose and the disease it may be trying to treat.

Major components of the lung – barriers to drug absorption
As one of the primary interfaces between the organism and the environment, the respiratory system is constantly exposed to airborne particles, potential pathogens, and toxic gases in the inspired air (Plopper, 1996). As a result a sophisticated respiratory host defense system, present from the nostrils to the alveoli, has evolved to clear offending agents (Twigg, 1998).

The system comprises of:

  • mechanical (i.e. air filtration,cough, sneezing, and mucociliary clearance),
  • chemical (antioxidants, antiproteases and surfactant lipids),
  • immunological defense mechanisms and is tightly regulated to minimize inflammatory reactions that could impair the vital gas-exchange

**Intratracheal inhalation is another  administration option but will be left out of the discussion for now

From a drug delivery perspective, the components of the host defense system comprise barriers that must be overcome to ensure efficient drug deposition and absorption from the respiratory tract.

Generally, lung physiological investigations show that the airway and alveolar epithelia, not the interstitium and the endothelium, constitute the main barrier that restricts the movement of drugs and solutes from the airway lumen into the cells or the blood circulation.

Aerosols are defined as An aerosol is a suspensions of fine solid particles or liquid droplets in a gas.The major aspect affect the efficacy of aerosols as a drug delivery system is Drug Deposition.

Aerosol Drug deposition is affected by:

  • particle properties (e.g. size, shape, density, and charge),
  • respiratory tract morphology,
  • the breathing pattern (e.g. airflow rate and tidal volume)

These parameters determine not only the quantity of particles that are deposited but also in what region of the respiratory tract the particles are deposited.

Particle properties

As the cross-sectional area of the airways increases, the airflow rate rapidly decreases, and consequently the residence time of the particles in the lung increases from the large conducting airways towards the lung periphery. The most important mechanisms of particle deposition in the respiratory tract are (1) inertial impaction, (2) sedimentation, and (3) diffusion.

  • Inertial impaction – Inertial impaction occurs predominantly in the extrathoracic airways and in the tracheobronchial tree, where the airflow velocity is high and rapid changes in airflow direction occurs. Generally, particles with a diameter larger than 10 μm are most likely deposited in the extrathoracic region, whereas 2- to 10-μm particles are deposited in the tracheobronchial tree by inertial impaction. A long residence time of the inspired air favors particle deposition by sedimentation and diffusion.
  • Sedimentation – Sedimentation is of greatest importance in the small airways and alveoli and is most pronounced for particles with a diameter of 0.5-2 μm, Ultrafine particles (<0.5 μm in diameter) are deposited mainly by diffusional transport in the small airways and lung parenchyma where there is a maximal residence time of the inspired air.

Most therapeutic aerosols are almost always heterodisperse, consisting of a wide range of particle sizes and described by the log-normal distribution with the log of the particle diameters plotted against particle number, surface area or volume (mass) on a linear or probability scale and expressed as absolute values or cumulative percentage (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1884307/)

Optimal drug delivery to the lungs depends on an interaction between;

  • the inhaler device,
  • the drug formulation properties,
  • the inhalation maneuver

The devices currently available for pulmonary drug administration of pharmaceutical aerosols in clinical therapy include nebulizers, pressurized metered dose inhalers (pMDIs), and dry powder inhalers (DPIs).

However, much effort is put into the development of new inhaler devices and formulations to optimize the pulmonary delivery system for local or systemic drug targeting.

One of the major problems in aerosol delivery is

One disadvantage of the aerosol inhalation is, however, that a substantial portion of the aerosolized drug is not delivered to the lungs (i.e. delivered to the nose, mouth, skin, exhaled). only 10–15% of the emitted dose in the lungs.

In general the aerosol exposure techniques have a low dosing effectiveness, which often requires longer exposure times to administer the target dose and renders investigations of rapid kinetic events difficult. In addition, aerosol exposure requires an advanced equipment for exposure and ml-quantities of test formulation to fill up the device.

Airway geometry and humidity

Progressive branching and narrowing of the airways encourage impaction of particles. The larger the particle size, the greater the velocity of incoming air, the greater the bend angle of bifurcations and the smaller the airway radius, the greater the probability of deposition by impaction. The lung has a relative humidity of approximately 99.5%. The addition and removal of water can significantly affect the particle size of a hygroscopic aerosol and thus deposition. Drug particles are known to be hygroscopic and grow or shrink in size in high humidity, such as in the lung. A hygroscopic aerosol that is delivered at relatively low temperature and humidity into one of high humidity and temperature would be expected to increase in size when inhaled into the lung. The rate of growth is a function of the initial diameter of the particle, with the potential for the diameter of fine particles <1 µm to increase five-fold compared with two-to-three-fold for particles >2 µm. he increase in particle size above the initial size should affect the amount of drug deposited and particularly, the distribution of the aerosolized drug within the lung,

Lung Clearance Mechanism

Once deposited in the lungs, inhaled drugs are either cleared from the lungs, absorbed into the systemic circulation or degraded via drug metabolism. Drug particles deposited in the conducting airways are primarily removed through mucociliary clearance and, to a lesser extent, are absorbed through the airway . epithelium into the blood or lymphatic system. a low-viscosity periciliary or sol layer covered by a high-viscosity gel layer. Insoluble particles are trapped in the gel layer and are moved toward the pharynx (and ultimately to the gastrointestinal tract) by the upward movement of mucus generated by the metachronous beating of cilia. In the normal lung, the rate of mucus movement varies with the airway region and is determined by the number of ciliated cells and their beat frequency. Movement is faster in the trachea than in the small airways and is affected by factors influencing ciliary functioning and the quantity and quality of mucus.

Drugs deposited in the alveolar region may be phagocytosed and cleared by alveolar macrophages or absorbed into the pulmonary circulation. Alveolar macrophages are the predominant phagocytic cell for the lung defence against inhaled microorganisms, particles and other toxic agents. There are approximately five to seven alveolar macrophages per alveolus in the lungs of healthy nonsmokers. Macrophages phagocytose insoluble particles that are deposited in the alveolar region and are either cleared by the lymphatic system or moved into the ciliated airways along currents in alveolar fluid and then cleared via the mucociliary escalator.

Very little is known about how the drug-metabolizing activities of the lung affect the concentration and therapeutic efficacy of inhaled drugs. All metabolizing enzymes found in the liver are found to a lesser extent in the lung. Therefore assuming, drug deposition could have been calculated it would be hard to impossible to evaluate it’s metabolism.

In summary:

As the end organ for the treatment of local diseases or as the route of administration for systemic therapies, the lung is a very attractive target for drug delivery. It provides direct access the site of disease for the treatment of respiratory diseases without the inefficiencies and unwanted effects of systemic drug delivery. It provides an enormous surface area and a relatively low enzymatic, controlled environment for systemic absorption of medications. But it is not without barriers. Airway geometry, humidity, clearance mechanisms and presence of lung disease influence the deposition of aerosols and therefore influence the therapeutic effectiveness of inhaled medications. A drug’s efficacy may be affected by the site of deposition in the respiratory tract and the delivered dose to that site. To provide an efficient and effective inhalant therapy, these factors must be considered. Aerosol particle size characteristics can play an important role in avoiding the physiological barriers of the lung, as well as targeting the drug to the appropriate lung region.

Drug formulations and chemo drug delivery will be further discussed in a another post.


1. N R Labiris and M B Dolovich. “Pulmonary drug delivery. Part I: Physiological factors affecting therapeutic effectiveness of aerosolized medications”. Br J Clin Pharmacol. 2003 December; 56(6): 588–599. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1884307/.

2. Tronde A. “Pulmonary drug absorption”. Acta Universities Upsalninesis Uppsala 2002. uu.diva-portal.org/smash/get/diva2:161887/FULLTEXT01

3. Naushad Khan Ghilzai. Pulmonary drug delivery. http://www.drugdel.com/Pulm_review.pdf.

Read Full Post »


Author: Larry Bernstein, MD


Creagh-BrownBC, Griffiths MJD, Evans TW. “Bench-to-bedside review: Inhaled nitric oxide therapy in adults”. Crit Care.  2009;  13(3): 221. Published online 2009 May 29. doi:  10.1186/cc7734. PMCID: PMC2717403.

This article is modified from a review series on Gaseous mediators, edited by Peter Radermacher.  Other articles in the series can be found online athttp://ccforum.com/series/gaseous_mediators


Part I.   Basic and downstream effects of inhaled NO

Inhaled nitric oxide (NO), a mediator of vascular tone produces pulmonary vasodilatation with low pulmonary vascular resistance. The route of administration delivers NO selectively improving oxygenation. Developments in our understanding of the cellular and molecular actions of NO may help to explain the results of randomised controlled trials of inhaled NO.


Nitric oxide (NO), a determinant of local blood flow is formed by the action of NO synthase (NOS) on L-arginine in the presence of molecular oxygen. Inhaled NO results in preferential pulmonary vasodilatation it lowers pulmonary vascular resistance (PVR), correcting hypoxic pulmonary vasoconstriction (HPV). However, in the therapeutic use of gaseous NO to patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), and related conditions, evidence of a benefit is disappointing.

Administration of inhaled nitric oxide to adults

The licensed indication of inhaled NO is restricted to persistent pulmonary hypertension in neonates. Pharma-ceutical NO is costly, and raises concerns over potential adverse effects of NO. Therefore, an advisory board under the auspices of the European Society of Intensive Care Medicine and the European Association of Cardiothoracic Anaesthesiologists published recommendations in 2005 [1]. The sponsor had no authorship or editorial control over the content of the meetings or any subsequent publication.

The NO is administered as a NO/nitrogen mixture to the tubing of ventilated patients, and the NO and NO2 concen-trations are monitored, with methemoglobin levels measured regularly. Even though rapid withdrawal induces rebound pulmonary hypertension, it is avoided by gradual withdrawal [2]. There is variation in vasodilatory response to administered NO between patients [2] and in the same patient, and there is a leftward shift in the dose-response curve with use. Toxicity and loss of the therapeutic effect is a risk of excessive NO administration [3]. A survey of 54 intensive care units in the UK as well as results of a European survey revealed that the most common usage was in treating ARDS, followed by pulmonary hypertension [4], [5]. The only use of therapeutic inhaled NO usage in US adult patients reported from a single medical site (2000 to 2003) reveals that the most common application was in the treatment of RVF in patients after cardiac surgery and then, in surgical and medical patients for refractory hypoxemia[6].

Inhaled nitric oxide in acute lung injury and acute respiratory distress syndrome

ALI and ARDS are characterised by hypoxemia despite high inspired oxygen (PaO2/FiO[arterial partial pressure of oxygen/fraction of inspired oxygen] ratios of less than 300 mm Hg [40 kPa] and less than 200 mm Hg [27 kPa], respectively) in the context of evidence of pulmonary edema, and the absence of left atrial hypertension suggestive of a cardiogenic mechanism [7]. Pathologically, there is alveolar inflammation and injury leading to increased pulmonary capillary permeability and a serous alveolar fluid with inflammatory infiltrate. This is manifest clinically as hypoxemia, inadequate alveolar perfusion, venous-arterial shunting, atelectasis, and reduced compliance.

Since 1993, when the first investigation on the effects of NO on adult patients with ARDS was published [8], there have been several randomised controlled trials (RCTs) examining the effect in ALI/ARDS  ​(Table 1). The first systematic review and meta-analysis [9] found no beneficial effect on mortality or ventilator-free days. A more recent meta-analysis that considered 12 RCTs with a total of 1,237 patients [10] concluded: [1] no mortality benefit, [2] improved oxygenation at 24 hours (13% improvement in PaO2/FiOratio) at the cost of increased risk of renal dysfunction (relative risk 1.50, 95% confidence interval 1.11 to 2.02). Based on a trend to increased mortality in patients receiving NO, the authors suggested that it not be used in ALI/ARDS.  Why the NO fails to improve patient outcomes requires clarifying the effects of inhaled NO that occur outside the pulmonary vasculature.


Published online 2009 May 29. doi: 10.1186/cc7734

Table 1

Studies of inhaled nitric oxide in adult patients with acute lung injury/acute respiratory distress syndrome

The biological action of inhaled nitric oxide

NO was first identified as an endothelium-derived growth factor (EDGF) and an important determinant of local blood flow [11]. NO reacts very rapidly with free radicals, certain amino acids, and transition metal ions. The action of NOS on the semi-essential amino acid L-arginine in the presence of molecular oxygen and its identity with EDGF was the basis for the Nobel discovery of Furthgott and others [12]. Three isoforms of NO are: neuronal NOS, inducible NOS (iNOS or NOS2), and endothelial NOS (eNOS or NOS3). Calcium-independent iNOS generates higher concentrations of NO [13] than the other isoforms and its role has been implicated in the pathogenesis septic shock.

Exogenous NO is administered by controlled inhalation or through intravenous administration of NO donors. It was thought to have no remote or non-pulmonary effects. The effect NO has on circulating targets is shown. (Figure 1).


Published online 2009 May 29. doi: 10.1186/cc7734

Figure 1

New paradigm of inhaled nitric oxide (NO) action. Figure 1 illustrates the interactions between inhaled NO and the contents of the pulmonary capillaries. Although NO was considered to be inactivated by hemoglobin (Hb), proteins including Hb and albumin contain reduced sulphur (thiol) groups that react reversibly with NO causing it to lose its vasodilating properties. A stable derivate, in the presence of oxyhemoglobin, is formed by a reaction resulting in nitrosylation of a cysteine residue of the β subunit of Hb.  The binding of NO to the heme iron predominates in the deoxygenated state [14]. If circulating erythrocytes store and release NO peripherally in areas of low oxygen tension, this augments peripheral blood flow and oxygen delivery via decreased systemic vascular resistance [15]. Thus, NO can act as an autocrine or paracrine mediator but when stabilised may exert endocrine influences [16]. In addition to de novo synthesis, supposedly inert anions nitrate (NO3) and nitrite (NO2) can be recycled to form NO, and nitrite might mediate extra-pulmonary effects of inhaled NO [17]. In the hypoxic state, NOS cannot produce NO and deoxy-hemoglobin catalyses NO release from nitrite, potentially providing a hypoxia-specific vasodilatory effect. Given that effects of inhaled NO are mediated in part by S-nitrolysation of circulating proteins, therapies aiming at directly increasing S-nitrosothiols have been developed.

Introduce another effect. When inhaled with high concentrations of oxygen, gaseous NO slowly forms the toxic product NO2, but other potential reactions include nitration (addition of NO2+), nitrosation (addition of NO+), or nitrosylation (addition of NO), and reaction with reactive oxygen species such as superoxide to form reactive nitrogen species (RNS) such as peroxynitrite (ONOO). These reactions of NO, potentially cytotoxic NO2 , and covalent nitration of tyrosine in proteins by RNS lead to measures of oxidative stress.

In a small observational study, inhaled ethyl nitrite safely reduced PVR without systemic side effects in persistent pulmonary hypertension of the newborn [18]. In animal models, pulmonary vasodilatation was maximal in hypoxia and had prolonged duration of action after cessation of administration [19].


  1. Germann P, Braschi A, Della Rocca G, Dinh-Xuan AT, et al. Inhaled nitric oxide therapy in adults: European expert recommendations.  Intensive Care Med. 2005;31:1029–1041. [PubMed]
  2. Griffiths MJ, Evans TW. Inhaled nitric oxide therapy in adults. N Engl J Med. 2005;353:2683–2695. [PubMed]
  3. Gerlach H, Keh D, Semmerow A, Busch T, et al. Dose-response characteristics during long-term inhalation of nitric oxide in patients with severe acute respiratory distress syndrome: a prospective, randomized, controlled study. Am J Respir Crit Care Med. 2003;167:1008–1015. [PubMed]
  4. Cuthbertson BH, Stott S, Webster NR. Use of inhaled nitric oxide in British intensive therapy units. Br J Anaesth. 1997;78:696–700.[PubMed]
  5. Beloucif S. A European survey of the use of inhaled nitric oxide in the ICU. Working Group on Inhaled NO in the ICU of the European Society of Intensive Care Medicine. Intensive Care Med. 1998;24:864–877.[PubMed]
  6. George I, Xydas S, Topkara VK, Ferdinando C, et al. Clinical indication for use and outcomes after inhaled nitric oxide therapy. Ann Thorac Surg. 2006;82:2161–2169. [PubMed]
  7. Bernard GR, Artigas A, Brigham KL, Carlet J,et al. The American-European Consensus Conference on ARDS. Definitions, mechanisms, relevant outcomes, and clinical trial coordination. Am J Respir Crit Care Med. 1994;149:818–824. [PubMed]
  8. Rossaint R, Falke KJ, López F, Slama K, Pison U, Zapol WM. Inhaled nitric oxide for the adult respiratory distress syndrome. N Engl J Med.1993;328:399–405. [PubMed]
  9. Sokol J, Jacobs SE, Bohn D. Inhaled nitric oxide for acute hypoxic respiratory failure in children and adults: a meta-analysis. Anesth Analg. 2003;97:989–998. [PubMed]
  10. Adhikari NK, Burns KE, Friedrich JO, Granton JT, Cook DJ, Meade MO. Effect of nitric oxide on oxygenation and mortality in acute lung injury: systematic review and meta-analysis.  BMJ. 2007;334:779.[PMC free article] [PubMed]
  11. Palmer RM, Ferrige AG, Moncada S. Nitric oxide release accounts for the biological activity of endothelium-derived relaxing factor.  Nature. 1987;327:524–526. [PubMed]
  12. Nitric Oxide: The Nobel Prize in Physiology or Medicine 1998 Robert F. Furchgott, Louis J. Ignarro, Ferid Murad. Leaders in Pharmacutical Intelligence.  A blog specializing in Pharmaceutical Intelligence and Analytics
  13. McCarthy HO, Coulter JA, Robson T, Hirst DG. Gene therapy via inducible nitric oxide synthase: a tool for the treatment of a diverse range of pathological conditions. J Pharm Pharmacol. 2008;60:999–1017. [PubMed]
  14. Coggins MP, Bloch KD. Nitric oxide in the pulmonary vasculature.   Arterioscler Thromb Vasc Biol. 2007;27:1877–1885. [PubMed]
  15. McMahon TJ, Doctor A. Extrapulmonary effects of inhaled nitric oxide: role of reversible S-nitrosylation of erythrocytic hemoglobin. Proc Am Thorac Soc. 2006;3:153–160. [PMC free article] [PubMed]
  16. Cokic VP, Schechter AN. Effects of nitric oxide on red blood cell development and phenotype. Curr Top Dev Biol. 2008;82:169–215. [PubMed]
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  18. Moya MP, Gow AJ, Califf RM, Goldberg RN, Stamler JS. Inhaled ethyl nitrite gas for persistent pulmonary hypertension of the newborn. Lancet  2002; 360:141–143. [PubMed]

Creagh-BrownBC, Griffiths MJD, Evans TW. “Bench-to-bedside review: Inhaled nitric oxide therapy in adults”. Crit Care.  2009;  13(3): 221. Published online 2009 May 29. doi:  10.1186/cc7734. PMCID: PMC2717403.

This article is modified from a review series on Gaseous mediators, edited by Peter Radermacher.

Other articles in the series can be found online athttp://ccforum.com/series/gaseous_mediators

Part II. Application of inhaled NO and circulatory effects

Cardiovascular effects

NO activates soluble guanylyl cyclase by binding to its heme group to form cyclic guanosine 3’5′-monophosphate (cGMP)   activating a protein kinase. Consequently, myosin sensitivity to calcium-induced contraction is reduced lowering the intracellular calcium concentration as a result of activating calcium-sensitive potassium channels and inhibiting release of calcium. The smooth muscle cell (SMC) relaxation with decrease in pulmonary vascular resistance (PVR) and decreased RV after load could improve cardiac output. However, left ventricular impairment associated with decrease in PVR allows increased RV output to a greater extent than the left ventricle can accommodate and the increase in left atrial pressure reinforces pulmonary edema.

Inhaled NO augments the normal physiological mechanism of hypoxic pulmonary ventilation (HPV) and improves systemic oxygenation ​(Figure 2). The effects of inhaled NO on systemic oxygenation are limited. Experiments show that intravenously administered vasodilators counteract HPV [3]. However, the non-pulmonary effects of inhaled NO include increased renal and hepatic blood flow and oxygenation [14].


Published online 2009 May 29. doi: 10.1186/cc7734

Figure 2

Hypoxic pulmonary vasoconstriction (HPV).       (a) Normal ventilation-perfusion (VQ) matching. (b) HPV results in VQ matching despite variations in ventilation and gas exchange between lung units. (c) Inhaled nitric oxide (NO) augmenting VQ matching by vasodilating.

Non-cardiovascular effects relevant to lung injury

Neutrophils are important cellular mediators of ALI. Limiting neutrophil production of oxidative species and proteolysis reduces lung injury. In neonates, prolonged administration of NO diminished neutrophil-mediated oxidative stress [19]. Neutrophil deformability and CD18 expression were reduced in animal models [20] accomp-anied by decreases in adhesion and migration [21]. These changes limit damage to the alveolar-capillary membrane and the accumulation of protein-rich fluid within the alveoli. Platelet activation and aggregation, intra-alveolar thrombi, contribute to ALI. Inhaled NO attenuates the procoagulant activity in animal models of ALI [22] and a similar effect is seen in patients with ALI [23], but also in healthy volunteers [23,24]. In patients with ALI, decreased surfactant activity in the alveoli and noncompliance, as we recall is hyaline membrane disease accompanied by impaired pulmonary function [25].  The deleterious effects of the NO damages the alveolar wall with loss of surfactant by reactions with RNS [26]. Finally, prolonged exposure to NO in experimental models impairs cellular respiration [27].

The failure of inhaled NO to improve outcome in ALI/ARDS is therefore potentially due to several factors. First, patients with ALI/ARDS die of multi-organ failure, as the actions of NO are not expected to improve the outcome of multi-organ failure, which is a cytokine driven process leading to circulatory collapse. Indeed, the expected beneficial effect of inhaled NO is abrogated by detrimental downstream systemic effects discussed. Second, ALI/ARDS is a heterogeneous condition with diverse causes. Finally, using inhaled NO without frequent dose titration risks unwanted systemic effects without the expected benefits.

Other clinical uses of inhaled nitric oxide

Pulmonary hypertension and acute right ventricular failure

RVF may develop when there is abnormally elevated PVR and/or impaired RV perfusion.  ​Table 2 lists common causes of acute RVF. The RV responds poorly to inotropic agents but is exquisitely sensitive to after load reduction.


Published online 2009 May 29. doi: 10.1186/cc7734

Table 2

Reducing PVR will have beneficial effects on cardiac output and therefore oxygen delivery. In the context of high RV afterload with low systemic pressures or when there is a limitation of flow within the right coronary artery [28], RV failure triggers a backward failure of venous return, as diagrammatically represented in  ​Figure 3.


Published online 2009 May 29. doi: 10.1186/cc7734

Figure 3

Pathophysiology of right ventricular failure. CO, cardiac output; LV, left ventricle; PAP, pulmonary artery pressure; PVR, pulmonary vascular resistance; RV, right ventricle.

Inhaled NO is used when RV failure complicates cardiac surgery, as cardiopulmonary bypass per se causes diminished endogenous NO production [29]. There is marked variation in response to inhaled NO between patients [30] and in the same patient over time. After prolonged use, there is a leftward shift in the dose-response curve.  The risk of excessive NO administration is associated with toxicity and loss of the therapeutic effect without regular titration against a therapeutic goal [31].  Further, cardiac transplantation may be complicated by pulmonary hypertension and RVF that are improved with NO [32]. Early ischemia-reperfusion injury after lung transplantation manifests clinically as pulmonary edema and is a cause of significant morbidity and mortality [33,34]. Although NO has been administered in this circumstance [35], it hasn’t prevented ischemia-reperfusion injury in clinical lung transplantation [36]. Inhaled NO has been used successfully in patients with cardiogenic shock and RVF associated with acute myocardial infarction [37,38,46], and in patients with acute RVF following acute pulmonary venous thrombo-emboli [39, 47].  An explanation is needed in view of the downstream effects of systemic vasoconstriction and MOF previously identified. No systematic evaluation of inhaled NO and its effect on clinical outcome has been conducted in these conditions.

Acute chest crises of sickle cell disease

Acute chest crises are the second most common cause of hospital admission in patients with sickle cell disease (SCD) and are responsible for 25% of all related deaths [40]. Acute chest crises are manifest by fever, respiratory symptoms or chest pain, and new pulmonary infiltrate on chest  x-ray. The major contributory factors are related to vaso-occlusion. Hemolysis of sickled erythrocytes releasing Hb into the circulation generates reactive oxygen species and reacts with NO [41]. In SCD, the free Hb depletes NO. In addition arginase 1 is released, depleting the arginine needed for NO production, [42]. While secondary PVH is common in adults with SCD the physiological rationale for the use of inhaled NO needs to be considered, except for the complication just referred to. Thus far, iNO has failed to demonstrate either persistent improvements in physiology or beneficial effects on any accepted measure of outcome in clinical trials (other than its licensed indication in neonates). Therefore, inhaled NO is usually reserved for refractory hypoxemia.

Potential problems in designing and conducting RCTs in the efficacy of inhaled NO are numerous. Blinded trials will be difficult to conduct as the effects of inhaled NO are immediately apparent. Recruitment is limited as there is little time for consent/assent or randomization. Finally, industry funding might cast doubt on the independence of the trial results.

Inhaled NO is an unproved tool in the intensivist’s armamentarium of rescue therapies for refractory hypoxemia even though it has an established role in managing complications of cardiac surgery and in heart/lung transplantation. The current place for inhaled NO in the management of ALI/ARDS, acute sickle chest crisis, acute RV failure, and acute pulmonary embolism is a rescue therapy.


ALI: acute lung injury; ARDS: acute respiratory distress syndrome; Hb: haemoglobin; HPV: hypoxic pulmonary vasoconstriction; iNO: inhaled nitric oxide; iNOS: inducible nitric oxide synthase; NO: nitric oxide; NO2: nitrogen dioxide; NOS: nitric oxide synthase; PaO2/FiO2: arterial partial pressure of oxygen/fraction of inspired oxygen; PVR: pulmonary vascular resistance; RCT: randomised controlled trial; RNS: reactive nitrogen species; RV: right ventricle; RVF: right ventricular failure; SCD: sickle cell disease; SMC: smooth muscle cell.

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