Understanding the use of oxygen delivery devices
Intended for healthcare professionals
Evidence and practice    

Understanding the use of oxygen delivery devices

Andrew Parry Senior lecturer (adult nursing), School of Healthcare Sciences, College of Biomedical and Life Sciences, Cardiff University, Cardiff, Wales

Why you should read this article:
  • To refresh your knowledge of the physiological principles of gas exchange

  • To enhance your understanding of the use of different types of oxygen delivery devices

  • To recognise the need to support patients using oxygen delivery devices

Many nurses will care for a patient who requires oxygen therapy, either to treat an acute respiratory condition, such as a lung infection, or a chronic condition, such as chronic obstructive pulmonary disease. This article discusses the physiology of gaseous exchange and provides an overview of some of the main respiratory conditions that may result in the need for oxygen therapy. The author also describes the oxygen delivery devices commonly used in clinical practice, such as simple face masks and nasal cannulas, to assist nurses in selecting the most appropriate device.

Nursing Standard. doi: 10.7748/ns.2023.e12175

Peer review

This article has been subject to external double-blind peer review and checked for plagiarism using automated software



Conflict of interest

None declared

Parry A (2023) Understanding the use of oxygen delivery devices. Nursing Standard. doi: 10.7748/ns.2023.e12175

Published online: 20 November 2023

At some point in their career, many nurses will care for a patient with respiratory failure. Oxygen therapy forms an integral part of the initial treatment of respiratory failure, with the nurse providing the patient with sufficient oxygen until the cause of the respiratory failure – for example, any type of infection can increase the body’s oxygen demand – is identified through assessment and then treated. In addition, there is a significant cohort of patients with chronic lung conditions, such as chronic obstructive pulmonary disease (COPD), who require long-term oxygen therapy (Lacasse et al 2022).

With the increasing acuity of the patient population, due to people living longer and the related increase in comorbidities such as respiratory and cardiac conditions, the use of oxygen therapy is increasing. Oxygen delivery devices are used for patients with chronic conditions, such as COPD and chronic heart failure, and in acute situations, for example where patients present with pneumonia or severe haemorrhage (Resuscitation Council UK 2021).

This article explores the physiological principles of gas exchange and provides a brief overview of common respiratory conditions that require supplementary oxygen therapy. The range of variable and fixed performance oxygen delivery devices are also discussed, with an emphasis on how they function to benefit patients. The author explores essential nursing considerations involved in oxygen therapy throughout the article, as it is essential that nurses are aware of the challenges experienced by patients wearing oxygen delivery devices and that support and reassurance is fundamental to providing best practice.

Physiology of gas exchange

To understand which oxygen delivery device may be most effective for a particular patient, it is important that nurses understand the physiology of gas exchange between the alveoli (microscopic air sacs situated at the end of the bronchioles in the lungs) and the circulation. This process starts with ventilation of the alveoli in the lungs through contraction of the intercostal muscles and the diaphragm, causing an increase in the volume of the thoracic cavity, which in turn results in a drop in pressure within the alveoli below the atmospheric pressure (Tortora and Derrickson 2017). This difference in pressure means that atmospheric air, which comprises 21% oxygen, flows into the airways and subsequently into the alveoli.

Once the oxygen has flown into the alveoli, it diffuses across the thin alveolar wall into the pulmonary circulation (the system of blood vessels that forms a closed ‘circuit’ between the lungs and the heart). This is because the partial pressure of oxygen in the alveoli is higher than the partial pressure of oxygen in the blood of the pulmonary capillaries; therefore, the oxygen diffuses from an area of high pressure in the alveoli to an area of lower pressure in the pulmonary circulation (Tortora and Derrickson 2017, Thompson and Hardy 2021). The constant flow of pulmonary circulation maintains this pressure difference and, in turn, maintains the process of diffusion.

Following diffusion, oxygen attaches to haemoglobin within red blood cells and is carried away from the pulmonary circulation to the left side of the heart and out into the systemic arterial system to the body’s tissues, while deoxygenated blood flows into the pulmonary circulation from the right side of the heart. This deoxygenated blood will offload carbon dioxide into the alveoli to be exhaled (Lumb and Thomas 2020).

For this process to be effective it is important that the following three factors are in place (Hall and Hall 2020):

  • There is sufficient alveolar membrane surface area for diffusion.

  • The pressure difference between the alveoli and the pulmonary circulation must be maintained, as described above.

  • The alveolar membrane must be thin to minimise the diffusion distance, which enables efficient diffusion. Therefore, the alveolar membrane, which divides the alveoli and the pulmonary circulation, is one cell thick.

These three factors are best summarised by Fick’s law, which states: ‘The rate of diffusion is proportional to both the surface area and concentration difference and is inversely proportional to the thickness of the membrane’ (Lumb and Thomas 2020). Therefore, when the nurse is administering supplementary oxygen therapy, the concentration of oxygen in the alveoli will be greater than normal because a portion of the inspired air will be 100% oxygen; this creates a greater concentration difference and thereby enhances diffusion.

In healthy individuals, the factors in Fick’s law are maintained, but these factors are disrupted in ill-health or disease and can be broken down into diffusion issues (such as chronic inflammation caused by some respiratory conditions) or carriage issues (such as a lack of haemoglobin caused by blood loss). Both diffusion and carriage issues can bring about either type 1 respiratory failure (hypoxia [lack of oxygen in the tissues]) or type 2 respiratory failure (hypoxia and hypercapnia [excess carbon dioxide in the blood]) (Norris and Lalchandani 2018). Some of the main conditions that result in diffusion and carriage issues are summarised in Table 1.

Table 1.

Some of the main conditions that result in diffusion and carriage issues

ConditionDiffusion or carriage issueProcess
Emphysema (categorised under chronic obstructive pulmonary disorder [COPD])Diffusion
  • Exposure to smoking or other toxic substances leads to chronic inflammation

  • This chronic inflammation breaks down the alveolar membrane, reducing its surface area over time

Chronic bronchitis
(categorised under COPD)
(SA and DD)
  • Over-production of mucous by goblet cells in the respiratory system due to goblet cell hyperplasia (increased cell production)

  • This process occurs in response to long-term exposure to noxious substances or irritants, for example smoking

(SA and DD)
  • Significant infection and inflammation cause the breakdown of the alveolar membrane and cause fluid from the circulation to flood the alveoli

  • This process further breaks down the alveolar membrane and increases the diffusion distance through the presence of fluid and pus

  • The presence of systemic inflammation causes blood vessel walls to leak fluid into the body’s tissues

  • When this occurs in the lungs it leads to increased volumes of water in the lung tissue, thereby increasing the diffusion distance

Coronavirus disease 2019Diffusion
(SA and DD)
  • Viral infiltration of the alveolar cells that comprise the alveolar membrane causes significant levels of inflammation

  • This process leads to breakdown of the alveolar surface area and flooding of the alveoli with fluid from the circulation

  • Increasing anaerobic respiration (cellular respiration that develops in the absence of oxygen) in the tissues leads to increasing lactic acid levels, which depresses cardiac contractions

  • This process leads to compromised pulmonary circulation, which adversely affects pressure differences and therefore the carriage of oxygen

  • Loss of blood and therefore haemoglobin leads to reduced oxygen-carrying capacity in the circulation

  • Reduced production of haemoglobin leads to reduced oxygen-carrying capacity

Surface area

diffusion distance

As well as understanding the respiratory system and the issues that can arise as a result of various conditions, the nurse should be familiar with the devices used to provide supplementary oxygen to patients experiencing hypoxia in clinical practice.

Oxygen flow meters

To ensure the oxygen delivery device is providing the predicted concentration, it is important that nurses understand how to use oxygen flow meters accurately. These devices (Figure 1) control the flow of oxygen from the hospital’s or unit’s main system or from a portable cylinder. The desired oxygen flow is set by adjusting the flow valve to the volume of oxygen prescribed, to a maximum of 15L per minute (L/min). Nurses must be careful to ensure that the centre of the flow meter ‘ball’ rests on the desired flow graduation, which is guided by clinical need (Al-Shaikh and Stacey 2018).

Figure 1.

Oxygen flow meter


The nurse must exercise caution when delivering oxygen at 15L/min through a flow meter, because they may inadvertently open the valve further to the flush setting (which is used to flush out the valve when it is not connected to a patient). The flush setting can deliver flows of up to 65-75L/min (Arora et al 2021) which is wasteful and potentially harmful to the patient, as high oxygen concentrations can be toxic and damage the body’s tissues (Lumb and Thomas 2020).

Key points

  • Understanding the physiology of gas exchange between the alveoli and the circulation supports selection of the most effective oxygen delivery device for the patient

  • Accurate use of oxygen flow meters is essential to ensure the oxygen delivery device provides the predicted concentration

  • Oxygen delivery devices can be broadly categorised as variable or fixed performance devices

  • Nurses should recognise the challenges experienced by patients wearing oxygen delivery devices

  • Support and reassurance are fundamental to best practice in the care of patients wearing oxygen delivery devices

Variable and fixed performance devices

Oxygen delivery devices can be placed broadly into two categories – variable or fixed performance devices. Variable performance devices, such as Hudson or simple face masks, deliver a fixed flow of oxygen, which is set on the oxygen flow meter but blended with atmospheric air, meaning that the fraction of inspired oxygen (FiO2) (an estimation of the oxygen content a person inhales) will vary depending on the patient’s inspiratory flow rate. Conversely, fixed performance devices such as Venturi valves and masks can deliver a fixed concentration of oxygen via a system of flow settings and valves (Al-Shaikh and Stacey 2018).

Variable performance devices

Nasal cannulas

The nasal cannula is ideal for addressing mild hypoxia by delivering a limited concentration of supplementary oxygen, for example to patients whose oxygen demand is only slightly elevated by conditions such as chronic respiratory diseases or chest infections of low severity (Lee et al 2016). A nasal cannula is a flexible tube with two nasal ‘prongs’ that are applied to the nostrils and which deliver the oxygen into the nasal cavity. These devices can deliver an oxygen flow of 1-6L/min, although oxygen flows of around 4L/min or more can become uncomfortable due to drying of the nasal mucosa (Hardavella et al 2019).

Table 2 details the levels of FiO2 that can be delivered via a nasal cannula in relation to oxygen flow rates. However, when using a nasal cannula, the patient may blend atmospheric air with the delivered oxygen as they may ‘mouth breathe’, meaning some atmospheric air will enter their airway. Therefore, the actual FiO2 delivered can be highly variable between patients even at the same flow rates (Hardinge et al 2015). For this reason, Table 2 should be viewed as a guide.

Table 2.

Possible levels of fraction of inspired oxygen delivered via a nasal cannula in relation to oxygen flow rates

Oxygen flow (litre per minute)Fraction of inspired oxygen (FiO2) (%)
10.24 (24)
20.28 (28)
30.32 (32)
40.36 (36)
50.40 (40)
60.44 (44)

Using a nasal cannula means that the patient can continue to communicate, eat and drink because the cannula is less restrictive than a full-face mask. However, nasal cannula can be easily dislodged when the patient moves – particularly if they are agitated – so it is advisable that nurses check regularly that the cannula are still in the correct place. The nurse must also take care to fit a nasal cannula correctly, ensuring that it is not too tight or too loose by using the ‘toggle’ on the device.

Based on the author’s clinical experience, the cannula should be fitted with the nasal prongs first placed in each nostril, then the tubing passed over and around the back of the ear and finally adjusted with the toggle, which should rest under the patient’s chin (see steps in Table 3). The nurse must also be mindful of potential pressure ulcers in areas such as cheekbones, the tops of the ears and inside the nasal cavity, therefore regular inspection of these areas is essential.

Table 3.

Steps involved in fitting a nasal cannula

  • 1. Place the nasal prongs in each nostril

  • 2. Pass the tubing over and around the back of the ears

  • 3. Adjust the ‘toggle’ under the patient’s chin to ensure even tension on each side of the tubing


Images supplied by Andrew Parry

Hudson or simple face masks

Hudson or simple face masks, which are among the most commonly used devices in hospitals, are used to deliver supplementary oxygen in situations such as the post-operative phase or in patients with pneumonia. These masks can deliver an FiO2 of 35-55% depending on the oxygen flow rate. Table 4 indicates the FiO2 level supplied by simple face masks based on various oxygen flow rates.

Table 4.

Possible levels of fraction of inspired oxygen delivered via a simple face mask in relation to oxygen flow rates

Oxygen flow (litre per minute)Fraction of inspired oxygen (FiO2) (%)
20.24 (24)
40.35 (35)
60.50 (50)
80.55 (55)
100.60 (60)
120.65 (65)
150.70 (70)

(Adapted from Lister et al 2020)

Although 100% oxygen is delivered into simple face masks, the gaps in the mask’s seal on the patient’s face, and the ports on the side of the mask, can draw in atmospheric air which reduces the total FiO2 delivered (Hardavella et al 2019). Therefore, it is important that the nurse fits the mask comfortably on the patient’s face using the elastic strap. These gaps and ports also enable venting of exhaled carbon dioxide.

Simple face masks can only deliver oxygen at the rate at which the oxygen flow meter is set, and the reservoir of available oxygen is contained within the mask itself, which re-fills during the short pause between expiration and inspiration. Therefore, the FiO2 quoted in Table 4 would be lowered if the patient’s respiratory flow rate was to increase, as the available oxygen in the mask would be inspired more quickly than it could be replenished by the oxygen flow (Lee et al 2016). Increases in oxygen demand must therefore be met with increases in oxygen flow; increases in oxygen demand are indicated by changes in the patient’s oxygen saturation of haemoglobin (SaO2) levels, which the nurse should check regularly using pulse oximetry (Siemieniuk et al 2018).

Wearing a simple face mask can make it challenging for a patient to communicate, eat and drink, so the nurse must be mindful of this (Lister et al 2020). Patients can often feel claustrophobic wearing a mask and may pull it away from their face. This response is driven by mechanoreceptors in the tissues of the face. These receptors detect pressure and increase the patient’s sense of dyspnoea (shortness of breath) and elicit a protective response, where the patient will seek to free their airway (Fukushi et al 2021). Therefore, it is important that the nurse uses their communication skills to allay the patient’s anxiety as far as possible to ensure that they keep the mask on to receive the required oxygen therapy.

The functions and principles of using a simple face mask also apply to tracheostomy masks, which can be applied over a tracheostomy tube (Figure 2). A tracheostomy tube bypasses the upper airway, therefore much of the required humidification (the process of adding heat and moisture to a gas) of inspired gases cannot occur. It is important, therefore, that the oxygen circuit on a tracheostomy mask is humidified to prevent the patient’s airways from drying out (Hardinge et al 2015).

Figure 2.

Tracheostomy mask


Non-rebreathe masks

Non-rebreathe masks are used to deliver elevated rates of FiO2 in patients experiencing severe hypoxia, such as in haemorrhage or sepsis, and where a significant amount of oxygen is required at once but where the patient is still ventilating well (Hardavella et al 2019). These masks are designed with a 1L reservoir bag, which has a one-way valve connecting it to a face mask (Figure 3). Oxygen flows from the oxygen source into the reservoir bag and a one-way valve connects the reservoir bag to the mask. When the patient inhales, oxygen moves from the reservoir into the mask.

Figure 3.

Non-rebreathe mask


Before placing the face mask on the patient, the nurse must connect the tubing to the oxygen supply, set it at 15L/min and place a finger over the one-way valve until the reservoir bag is full of oxygen.

The oxygen supply fills the reservoir bag during expiration, meaning the bag is full when the patient inspires, leading to a consistently high delivery of FiO2 on every inspiration. The one-way valve in the reservoir closes during expiration and the patient vents carbon dioxide through ports at the side of the mask and via the gaps in the seal around the face. Although the reservoir is the main source of gas inspired by the patient, the mask enables some atmospheric air to be entrained, therefore it will not deliver an FiO2 of 100%, but can be titrated to ranges of 80-95% FiO2 at an oxygen flow of 10-15L/min respectively (Auerbach et al 2019).

When the patient’s condition improves and their oxygen requirements fall below 10L/min, the non-rebreathe mask should be changed to a simple face mask (Herren et al 2017). As with simple face masks, non-rebreathe masks must be well-fitted to the patient’s face using the elastic strap.

High-flow nasal cannulas

High-flow nasal cannulas provide a high flow of warmed, humidified oxygen via a single limb circuit (comprising a single limb of tubing), which can generate up to 60L/min of flow and deliver an FiO2 range of 21-100% (Vargas et al 2015, Hardavella et al 2019). The high flow of oxygen into the patient’s airways from the cannula reduces the entrainment of atmospheric air and any subsequent reduced oxygen dilution (Al-Shaikh and Stacey 2018).

The high flow of oxygen also means that the delivery of the FiO2 is consistent and generates positive pressure within the airways and alveoli causing them to expand, thus further enhancing the delivery of oxygen (Sharma et al 2023).

High-flow nasal cannulas have been shown to have positive effects in patients with acute respiratory failure and low SaO2 despite having received maximal supplemental oxygen via a simple face mask. Such situations may include patients experiencing acute hypoxic respiratory failure associated with pneumonia, coronavirus disease 2019 and post-extubation (Roca et al 2010, Mauri et al 2017). High-flow nasal cannulas have also been found to be effective in enabling healthcare professionals to avoid intubating patients in respiratory failure where intubation would yield a high mortality rate, for example in patients who are immunocompromised (Frat et al 2017).

It has been suggested that another benefit of high-flow nasal cannulas is that they decrease the effort of breathing for patients (Roca et al 2010), which is important in those with fatigue-prone respiratory muscles. Other benefits include increased carbon dioxide elimination, which limits respiratory acidosis, and greater secretion clearance due to humidification of the delivered oxygen (Mauri et al 2017). There is also evidence that high-flow nasal cannulas are better tolerated by patients and are more comfortable than face masks (Roca et al 2010).

Fixed performance devices

Venturi valve and mask

The Venturi valve and mask is named after the attached Venturi valve (Figure 4), which enables the delivery of high-flow oxygen at a prescribed FiO2. This is important in patients with hypercapnic respiratory failure (for example COPD) where the FiO2 must be carefully controlled to reduce the risk of bradypnea (abnormally slow breathing) or apnoea (temporary cessation of breathing), which would cause carbon dioxide retention (Hardinge et al 2015).

Figure 4.

Venturi valve


The Venturi valve works on the Bernoulli principle, which dictates that as a gas’s speed increases its pressure reduces (Thompson and Hardy 2021). The structure of the Venturi valve means that as oxygen passes through the narrow inlet, the oxygen velocity increases and its pressure decreases – this reduced pressure draws in atmospheric air at a rate dictated by the size of the holes in the side of the valve. The smaller the holes in the valve, the less atmospheric air is entrained, meaning that the FiO2 rate will be raised; similarly, larger holes in the valve mean that more atmospheric air will be entrained achieving a lowered FiO2. Therefore, the valve enables blending of pure oxygen from the oxygen supply with atmospheric air to achieve a prescribed FiO2 (Al-Shaikh and Stacey 2018).

It is important to note that each valve requires a distinct flow setting on the oxygen flow meter. These settings are moulded or printed on the side of the valve and the nurse must ensure they make this adjustment if the patient’s oxygen requirements change.

As with other face masks, the Venturi valve and mask requires appropriate fitting, and the nurse should be mindful that the valve itself can be heavy and pull the mask downwards; some Venturi masks have a short length of corrugated tubing to counter this issue. There are also tracheostomy versions of the Venturi device, which have a T-piece arrangement to enable attachment to the tracheostomy tube.


When using oxygen delivery devices, it is important that the nurse has a high level of knowledge of the physiology of the respiratory system and the pathophysiology that can adversely affect gas exchange. By using this knowledge, the nurse can make informed decisions about which oxygen delivery device is the most appropriate in any given clinical situation, as well as the degree of support a patient will require.


  1. Al-Shaikh B, Stacey SG (2018) Essentials of Equipment in Anaesthesia: Critical Care and Perioperative Medicine. 5. Elsevier, Edinburgh.
  2. Arora N, Dennis A, Willson J et al (2021) Delivery of oxygen by standard oxygen flowmeters. Anaesthesia. 76, 11, 1546-1547. doi: 10.1111/anae.15548
  3. Auerbach PS, Constance BB, Freer L (2019) Field Guide to Wilderness Medicine. 5. Elsevier, Philadelphia PA.
  4. Frat JP, Coudroy R, Marjanovic N et al (2017) High-flow nasal oxygen therapy and noninvasive ventilation in the management of acute hypoxemic respiratory failure. Annals of Translational Medicine. 5, 14, 297. doi: 10.21037/atm.2017.06.52
  5. Fuentes S, Chowdhury YS (2023) Fraction of Inspired Oxygen. StatPearls Publishing, Treasure Island FL.
  6. Fukushi I, Nakamura M, Kuwana S-I (2021) Effects of wearing facemasks on the sensation of exertional dyspnea and exercise capacity in healthy subjects. PLoS One. 16, 9, e0258104. doi: 10.1371/journal.pone.0258104
  7. Hall JE, Hall ME (2020) Guyton and Hall Textbook of Medical Physiology. 14. Elsevier, Philadelphia PA.
  8. Hardavella G, Karampinis I, Frille A et al (2019) Oxygen devices and delivery systems. Breathe. 15, 3, e108-e116. doi: 10.1183/20734735.0204-2019
  9. Hardinge M, Annadale J, Bourne S et al (2015) British Thoracic Society guidelines for home oxygen use in adults: accredited by NICE. Thorax. 70, i1-143. doi: 10.1136/thoraxjnl-2015-206865
  10. Herren T, Achermann E, Hegi T et al (2017) Carbon dioxide narcosis due to inappropriate oxygen delivery: a case report. Journal of Medical Case Reports. 11, 204. doi: 10.1186/s13256-017-1363-7
  11. Lacasse Y, Casaburi R, Sliwinski P et al (2022) Home oxygen for moderate hypoxaemia in chronic obstructive pulmonary disease: a systematic review and meta-analysis. The Lancet. 10, 11, 1029-1037. doi: 10.1016/S2213-2600(22)00179-5
  12. Lee CC, Mankodi D, Shaharyar S et al (2016) High flow nasal cannula versus conventional oxygen therapy and non-invasive ventilation in adults with acute hypoxemic respiratory failure: a systematic review. Respiratory Medicine. 121, 100-108. doi: 10.1016/j.rmed.2016.11.004
  13. Lister S, Hofland J, Grafton H (Eds) (2020) The Royal Marsden Manual of Clinical Nursing Procedures. 10. Wiley-Blackwell, Chichester.
  14. Lumb AB, Thomas CR (2020) Nunn and Lumb’s Applied Respiratory Physiology. 9. Elsevier, Edinburgh.
  15. Mauri T, Alban L, Turrini C et al (2017) Optimum support by high‑flow nasal cannula in acute hypoxemic respiratory failure: effects of increasing flow rates. Intensive Care Medicine. 43, 1453-1463. doi: 10.1007/s00134-017-4890-1
  16. Norris TL, Lalchandani R (2018) Porth’s Pathophysiology: Concepts of Altered Health States. 10. Lippincott Williams and Wilkins, Singapore.
  17. Resuscitation Council UK (2021) Advanced Life Support. 8. Resuscitation Council, London.
  18. Roca O, Riera J, Torres F et al (2010) High-flow oxygen therapy in acute respiratory failure. Respiratory Care. 55, 4, 408-413.
  19. Sharma S, Danckers M, Sanghavi DK et al (2023) High Flow Nasal Cannula. StatPearls Publishing, Treasure Island FL.
  20. Siemieniuk RA, Chu DK, Kim LH-Y et al (2018) Oxygen therapy for acutely ill medical patients: a clinical practice guideline. BMJ. 363, k4169. doi: 10.1136/bmj.k4169
  21. Thompson N, Hardy L (2021) Physics of gases. Anaesthesia and Intensive Care Medicine. 22, 1, 33-36. doi: 10.1016/j.mpaic.2020.11.002
  22. Tortora GJ, Derrickson BH (2017) Tortora’s Principles of Anatomy and Physiology. 15. John Wiley and Sons, Hoboken NJ.
  23. Vargas F, Saint-Leger M, Boyer A et al (2015) Physiologic effects of high-flow nasal cannula oxygen in critical care subjects. Respiratory Care. 60, 10, 1369-1376. doi: 10.4187/respcare.03814
  24. West JB, Lucks AM (2021) West’s Pulmonary Pathophysiology: The Essentials. 10. Wolters Kluwer, Philadelphia PA.

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