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 Table of Contents  
REVIEW ARTICLE
Year : 2021  |  Volume : 10  |  Issue : 2  |  Page : 171-181

Oxygen Sources and delivery devices: Essentials during COVID-19


1 Department of Respiratory Therapy, Manipal College of Health Professions, Manipal, Karnataka, India
2 Department of Critical Care Medicine, Kasturba Medical College, Manipal Academy of Higher Education, Manipal, Karnataka, India
3 Department of Anaesthesia, Kasturba Medical College, Manipal Academy of Higher Education, Manipal, Karnataka, India
4 Former Professor and Head, Department of Anaesthesia, Kasturba Medical College, Manipal Academy of Higher Education, Manipal, Manipal, Karnataka, India

Date of Submission22-May-2021
Date of Decision25-May-2021
Date of Acceptance28-May-2021
Date of Web Publication14-Jun-2021

Correspondence Address:
Dr. Souvik Chaudhuri
Associate Professor, Department of Critical Care Medicine, Kasturba Medical College, Manipal Academy of Higher Education, Manipal, Karnataka State
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijrc.ijrc_63_21

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  Abstract 


The coronavirus disease of 2019 (COVID-19) is an ongoing pandemic which is known to predominantly affect the respiratory system. Oxygen (O2) therapy has a profound role in the treatment of COVID-19 patients. The pandemic has drawn special attention to ensure uninterrupted O2 supply to all hospitals, especially those catering to COVID-19 patients. During the pandemic, a rational use of O2 therapy is essential. This includes optimal supplemental O2 therapy, careful monitoring of patients, and escalation as well as de-escalation of O2 therapy when indicated. We summarize the various sources of O2 to health-care establishments and various O2 delivery devices which are of paramount importance to ensure seamless O2 supply in the pandemic scenario.

Keywords: Hypoxemia, oxygen devices, oxygen sources, oxygen therapy, supplemental oxygen therapy


How to cite this article:
Todur PM, Chaudhuri S, Eeshwar M V, Teckchandani D, Venkateswaran R. Oxygen Sources and delivery devices: Essentials during COVID-19. Indian J Respir Care 2021;10:171-81

How to cite this URL:
Todur PM, Chaudhuri S, Eeshwar M V, Teckchandani D, Venkateswaran R. Oxygen Sources and delivery devices: Essentials during COVID-19. Indian J Respir Care [serial online] 2021 [cited 2021 Sep 16];10:171-81. Available from: http://www.ijrc.in/text.asp?2021/10/2/171/318330




  Introduction Top


Oxygen (O2), discovered by Scheele and Priestley in the 17th century, is vital for sustaining life. Within a decade of its discovery, supplemental O2 started gaining importance in the medical system. The first reported publication of treating a tuberculosis patient with O2 was in 1783.[1] In the current world, the use of O2 therapy has been extended from inpatient to outpatient settings and has gained tremendous significance during the coronavirus disease of 2019 (COVID-19) pandemic.

Severe acute respiratory syndrome coronavirus-2 virus invokes an extensive inflammatory reaction, resulting in the damage of the Type I and Type II alveolar cells. This triggers an acute respiratory distress syndrome like response, resulting in hypoxemia which could vary in severity. Hypoxemia is defined as a lack of O2 in the blood. O2 helps in reducing hypoxemia which is a major cause of mortality in COVID-19. The COVID-19 pandemic exposed deficiencies in health-care establishments across many nations, specifically in the aspect of medical O2 production capabilities and distribution systems. The disparity between the huge demand and meager supply of O2 led to higher fatalities. There was a false sense of security among those who procured O2 cylinders at home but could not get them refilled when empty. This scarcity of O2 ravaged not just India but also many other countries such as Brazil as well.[2] There was a huge demand for O2 concentrators early on during the second wave of COVID-19 in India. People have been scrambling for O2 sources and devices, some even hoarding them without really knowing what is actually beneficial for them.

O2 is a simple medical intervention that can save many lives around the world. An appropriate knowledge of various sources of O2 and the delivery devices is the need of the hour. A wide array of O2 delivery devices is available and choosing an appropriate device for a particular patient or health facility is important. O2 must be prescribed and administered in the right manner in terms of appropriate device selection and mode of delivery. While choosing a delivery device, we must consider 3 P's: Purpose, patient, and performance.[3] This narrative review focuses on the source of medical O2, its storage, and the clinical use of individual O2 delivery systems.


  Oxygen Sources and Storage Top


Atmospheric air contains 21% O2 and 79% nitrogen, some carbon dioxide, traces of rare gases, and a varying amount of water vapor. The general principle involved in the manufacture of O2 is fractional distillation of air. When atmospheric air is cooled, O2 and nitrogen get separated by fractional distillation because of their differing boiling points. O2 is then compressed and stored as liquid O2. It can be maintained in a liquid state only at very low temperatures and in a compressed form. That is the reason why cryogenic containers are required for its transport. O2 can also be prepared by O2 concentrators. The process involves drawing in of ambient atmospheric air, adsorbing nitrogen onto an adsorbent called zeolite (aluminum silicate), concentrate and dry the gas before supplying it as 95% pure O2.

The predominant source of O2 supply at hospitals is from cryogenic liquid O2, pressure swing adsorption (PSA) O2 generating plants or from the O2 concentrator.[4],[5] O2 from O2 plants is stored in cylinders which are ubiquitous across all medical setups.

Medical grade O2 consists of 99.5% pure O2, should be devoid of any contaminants, and is usually generated using an oil-free compressor.[6],[7] Only high quality, medical-grade O2 should be administered to patients. O2 sources at health-care setups are usually one of the following types – cryogenic liquid O2 plants, PSA O2 plant, vacuum PSA O2 plant, and O2 concentrators.[6]

Liquid oxygen tank

Liquid O2 is a cryogenic liquid. Cryogenic liquids are liquefied gases that have a normal boiling point below − 130°F (−90°C). Liquid O2 has a boiling point of − 297°F (−183°C). Cryogenically produced liquid O2 is always produced away from hospitals and never at a medical facility. Medical facilities often have large bulk liquid O2 tanks that are refilled periodically from special trucks by the supplier.[6] The liquid O2 tank is the source of supply to a central piped O2 system within the entire health-care setup. Conversion to gaseous O2 is by self-vaporization, a process that does not need power supply. Although it is a financially more feasible source, its concerns are that it needs a constant, uninterrupted external supply and extreme caution during transport and storage, as it involves dealing with O2 at high pressures.[6] Liquid O2 for medical use is expressed in m3 of liquid. Once the total flows are known in L/min of gas, total volume of liquid can be calculated over a specified period of time, using the following formula: 1 L of liquid O2 = 861 L O2 gas (21°C at sea level) and 1 m3 O2 gas = 1000 L of O2 gas.[6]

Liquid O2 is stored in units called vacuum-insulated evaporators.[8] Liquid O2 at − 160°C is present at the bottom of the container at a pressure of 10 atmospheres, and O2 gas is present on top at a pressure of 10.5 bar or about 152 pounds per square inch gauge (psig) or 7875.64 mmHg.[8] As liquid O2 evaporates during use, its mass decreases and therefore the pressure at the bottom reduces. Gaseous O2 at higher pressure passes through a series of pressure regulators and is eventually supplied at 58 pounds per square inch gauge (psig) through the central pipeline system.[8]

Pressure swing adsorption oxygen plant

PSA O2 plant is utilized as a large, central source of O2 production using PSA technology (which is similar to that used in O2 concentrators). PSA O2 plants may be constructed on site at medical facilities.[6] An O2 generator separates this O2 from compressed air through a process called PSA [Figure 1]. This process of production of enriched O2 gas from environmental air incorporates the ability of a synthetic zeolite molecular sieve to adsorb mainly nitrogen. While nitrogen concentrates and is adsorbed in the pore system of the zeolite, O2 gas is produced as a product. O2 from a PSA plant can be supplied directly to the bedside within patient care areas. Using a booster compressor, it could also be utilized to refill O2 cylinders.[6] O2 plants require a constant uninterrupted power supply (UPS). Therefore, it is recommended to have cylinders as a backup supply at any health-care setup.
Figure 1: Pressure swing adsorption oxygen plant

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Vacuum pressure swing adsorption oxygen plant

Here, the process of O2 generation is similar to that of a conventional PSA plant. However, these plants have two adsorber units where nitrogen is adsorbed. When the first adsorber is in the adsorption phase adsorbing nitrogen from pressurized air, the second adsorber is in regeneration or desorption phase by being connected to a vacuum pump. The vacuum pump withdraws nitrogen, carbon dioxide, and other residual gases from the second adsorber and vents it. Thus, the vacuum pump enables the second adsorber to get desorbed.

O2 plants are being set up in health-care facilities during the pandemic in order to meet the ever increasing requirements of O2. We must know the requirement and production capacity of each O2 plant, in order to efficiently cater to the needs of the facility.

Illustration

Production capacity of O2 of such plants is expressed in tons.

1 ton = 1000 kg

1 kg = 1000 g, therefore 1000 kg = 1000 × 1000 g = 1,000,000 g

1 gram mole of O2 = 32 g.

As per Avogadro's hypothesis, 1 g mole of any particular gas in liquid form will yield 22.4 L of that gas at standard temperature and pressure (STP).

Therefore, 1 g mole of liquid O2 or 32 g liquid O2 will yield 22.4 L gaseous O2 at STP.

Hence, 1 ton liquid O2 = 1,000,000/32 g moles of O2 = 31,250 g moles of liquid O2

Thus, 31,250 g moles of liquid O2 will yield 31,250 × 22.4 L of gaseous O2 = 700,000 L gaseous O2.

If we need to give 5 L/min O2 to any patient, that patient will require 5 × 60 × 24 L gaseous O2 per day = 7200 L gaseous O2 per day.

1 ton liquid O2 yields 700,000 L gaseous O2.

Therefore, 1 ton liquid O2 in the above case will cater to 700,000/7200 = 97 patients per day.

Hence, any COVID-19 hospital dealing with 100 ward patients per day will need a PSA plant with at least 1 ton capacity (700,000 L gaseous O2) capacity. However, if O2 requirement increases in several patients at once, the system can get overwhelmed, if it depends only on the PSA system. For that reason, a backup manifold of bulk cylinders is required.

Oxygen concentrators

An O2 concentrator is an electrically driven device designed to concentrate O2 from ambient air. An O2 concentrator utilizes PSA technology to draw in air from the environment and remove the nitrogen in order to ensure a constant supply of more than 90% O2 [Figure 2].[6] O2 concentrator should not be used if the O2 concentration drops to <82%.[6] O2 concentrators are portable and can be mobilized into different parts of the hospital setup. They could even be placed stationary in certain patient areas.
Figure 2: Parts of an oxygen concentrator

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Concentrators which serve for portable medical O2 support are available in models that can generate different flow rates of between 0.5 L/min and 10 L/min.[6] Accordingly, they are classified as low flow O2 concentrators and high flow O2 concentrators.[7] Low flow O2 concentrators can deliver O2 flows between 0.5 L/min and 5 L/min, whereas the high flow O2 concentrators can deliver flows up to 10 L/min.[7] O2 concentrators can deliver an FIO2 between 0.85 and 1.0. However, the final O2 concentration that a patient breathes depends on the particular mode of delivery, that is, nasal prongs or facemask.[9] Using a portable O2 concentrator to provide O2 through the Bain circuit, Burn et al. achieved an FIO2 of 0.95 at a flow rate of 0.5 L/min and 0.96 at flow rate of 1–3 L/min.[10]

Nonportable O2 concentrators used in health-care setups usually weigh up to 25 kg and require a constant power source. An UPS should be used when power fluctuations and interruptions are frequent. They supply O2 with an FIO2 ranging from 0.90 to 0.96.[11] When used with a flow meter stand for splitting flow, concentrators can provide a continuous supply of O2 to multiple patients at the same time.

Concentrators ensure a safe and economical source of O2, but they do require a source of continuous and reliable power and regular preventive maintenance to ensure proper functioning.[6] During regular functioning of O2 concentrators, the users should be well versed with various alarms which indicate malfunction.[9] Some manufacturers indicate the drop in FIO2 below 0.82 by lights that illuminate on the panel of the O2 concentrator. A user manual is always provided with every O2 concentrator and should be referred to for troubleshooting. It is best practice to also have cylinders as a backup supply.

Oxygen cylinder

O2 cylinders are commonly used during intra- and inter-hospital transport, at remote locations and during resuscitation, especially in out-of-hospital settings as well as at home. Cylinders for storage of medical grade O2 are made of lightweight chrome molybdenum steel, aluminum, or a composite (such as aluminum wrapped in carbon fiber). Aluminum cylinders are compatible in magnetic resonance imaging suites.[12] Cylinders have four important components – body, shoulder, neck, and valve.[12] Medical O2 cylinders in India have a black body with a white shoulder and a pin index configuration of 2–5.[12]

In India, the three sizes of O2 cylinders commonly used have a capacity of 660 L (“E” type cylinders used on anesthesia workstations), 1360 L (“F” type cylinders used in wards for O2 therapy), and 6900 L (“H” type cylinders used on cylinder manifolds supplying hospital pipeline systems) [Table 1]. While transporting or resuscitating a patient using an O2 cylinder with a capacity of 660 L, it is vital to know the time remaining before the cylinder gets empty.
Table 1: Oxygen cylinder sizes with their capacity, pressure and the type of valves used

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The duration for which a cylinder would last would depend on its size (determining the initial volume) and the rate of use of O2 (flow rate). We can calculate the time an O2 cylinder will last using the following equation (derived from Boyle's law):



Where,

Tr = Time remaining in hours

Pr = Remaining pressure

Vi = Initial volume of O2 available in the full cylinder

Pi = Initial pressure in the full cylinder (approximately 2000 psi or 140 × 100 kPa)

Q = Flow rate of O2 required.

Illustration

If 1200 psi is the remaining pressure in an E cylinder and the O2 flow rate being administered to the patient is 6 L/min, then the “remaining time” of that cylinder is calculated as follows:

For an “E” cylinder, the initial volume (Vi) in a full cylinder is ≅ 600 L. Thus,



The initial volume of O2 available from a full F cylinder would be 1400 L, whereas a H cylinder has 7000 L [Table 1]. Thus, the time remaining for any cylinder can be easily calculated with the same formula as given above. One just needs to know the initial volume of oxygen available from that cylinder, pressure remaining, and the flow rate of O2. For a more detailed description, refer [Table 2].
Table 2: Calculation of oxygen available from an oxygen cylinder

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At a time when O2 use has to be used optimally, its storage should also be done with utmost precaution. The cylinders should be stored in a cool, dry, well-ventilated area which has been constructed with fire resistant material.[12] Empty and full cylinders should be stored separately and never in direct sunlight.[12]

Cylinder manifold

An average cylinder manifold consists of a group of bulk cylinders which is used to supply O2 in a hospital setup. The configuration consists of two banks containing an equal number of bulk cylinders with an output pressure of 58.8 psig.[13] The cylinders in a manifold are divided into two groups: primary cylinders (on-duty bank) and secondary (standby bank) [Figure 3]. The two groups of cylinders are alternately used to ensure uninterrupted O2 supply through the pipelines. The number of cylinders used at a particular time depends on the demand. All the cylinders in a particular group are connected to the manifold using a flexible pig-tail copper pipe with a gas-specific connection and seal.[13] The storage capacity of the manifold is calculated on the basis of required O2 supply for 1 week. There should be a minimum of 2-day supply in each bank and a supply of at least 3 days of spare cylinders kept reserve in the manifold room. In each bank, all the cylinder valves are kept in an open position. This ensures that the cylinders empty simultaneously. The O2 supply can be set to automatically change to the standby bank when the on-duty bank is almost empty. In hospitals, devices such as O2 concentrators may not work in the event of power failure. In such a scenario, the cylinder manifold comes to the rescue. Care should be taken to ensure that both the banks in the manifold can provide O2 supply in the case of such an eventuality.[8] The banks in the manifold should be able to provide adequate O2 supply till power supply is restored as equipment such as O2 concentrators can function only after restoration of power supply.[8]
Figure 3: Cylinder manifold

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  Oxygen Therapy in Coronavirus Disease of 2019 Top


Apart from reducing symptoms due to distress and cardiopulmonary workload, the main purpose for administering O2 is to correct arterial hypoxemia. Hypoxemia can cause headache, shortness of breath, restlessness, dizziness, confusion, and rapid breathing. If untreated, hypoxemia can jeopardize the functioning of the brain and heart, eventually leading to death. Hypoxemia can damage organ function, the extent of which will depend on the degree and duration of hypoxemia. Therefore, while treating a hypoxemic COVID-19 patient, an O2 system is necessary which involves equipment to detect hypoxemia and also to provide O2. Usually, O2 saturation of blood is measured using a pulse oximeter which detects hypoxia. To provide O2 therapy, we need an O2 source; appropriate O2 delivery devices selected on the basis of design and performance; and technical equipment such as flow meters and O2 tubing. In addition, a trained health-care worker (HCW) and round-the-clock biomedical maintenance are vital.

O2 therapy has a profound role in the treatment of moderate-to-severe COVID-19 patients. Supplemental O2 therapy is the first line of treatment in COVID-19 patients to enhance oxygenation and avoid, if possible, the need for intensive care unit admission and intubation. However, this therapy needs to be rationalized. It comprises initial O2 therapy, followed by proper monitoring and escalation without delaying intubation, de-escalation of O2 therapy as indicated, and also the consideration of protection of HCW and minimizing the risk of transmission.[14]

In COVID-19 units, apart from the 3Ps – purpose, patient, and performance, a fourth 'P, prevention of aerosolization should be kept in mind while choosing an O2 therapy device. The quantum of aerosols generated with the use of these devices poses a threat of nosocomial infection to HCWs. In COVID-19 patients, one has to weigh the risk of aerosol-generating potential of a device before selecting it for a patient. An in-depth understanding of O2 therapy device is essential for the selection of appropriate gadget for O2 delivery.[15]

Based on performance and patient dependency, O2 delivery systems are classified into:

  1. Low flow and low-performance devices (low dependency)
  2. High-flow devices and reservoirs (medium dependency)
  3. Mechanical ventilation-invasive/noninvasive (high dependency).


Performance refers to the fraction of inspired O2 (FIO2) delivered to the patient and can be fixed or variable depending on certain factors. These include:

  1. Equipment factors: O2 flow rate, mask volume, quality of mask fit, and areas of potential leak between the mask and face
  2. Patient factors: Respiratory rate, tidal volume, and peak inspiratory flow rate (PIFR)
  3. Additional factors (presence of other gases or vapors).


The flow rate is determined by the design of the device. As shown in [Figure 4], a low-flow device provides O2 at a flow rate that is lower than the patient's PIFR. The additional volume is drawn from the surrounding room air. This results in dilution, and FIO2 becomes lower and variable. High-flow devices deliver O2 flow at higher rates than the PIFR of the patient, thereby achieving a fixed FIO2. In a reservoir, the device stores the reserve volume within the device that equals or is greater than the patient's tidal volume.
Figure 4: Flows provided by oxygen therapy devices in relation to peak inspiratory flow rate of patient

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Low-flow delivery devices

Low flow devices deliver O2 at a flow rate that is less than the normal PIFR (approximately 30 L/min and increased in dyspnea). To meet the PIFR of the patient, low flow devices draw in room air from the surrounding, resulting in lowering of FIO2. The delivered FIO2 also varies with PIFR and the set input O2 flow.

Nasal cannula

Nasal cannula is the most common O2 delivery device used in COVID-19 patients. It is a low flow, variable performance device made of soft plastic tube with two short prongs, straight or curved, that are inserted into the external nares. The cannula is held in place by two small pieces of tubing that fit over the ears and can be tightened with a tie that fits under chin [Figure 5]. The range of flow is 0.25–6 L/min, and it delivers an FIO2 range of 0.22–0.4. The nasal cannula can be used in stable patients with spontaneous breathing. It has the advantages of being well tolerated, easy to use, less expensive, and comfortable while feeding and talking. The disadvantages at higher flows include dryness, mucosal damage, and bleeding.
Figure 5: Nasal cannula

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Reservoir system

Reservoir cannula (nasal and pendent) and reservoir masks (simple mask, partial rebreathing masks, and nonrebreathing masks [NRBM]) are the currently available reservoir O2 devices. During patient's expiratory phase and pause, the reservoir system allows accumulation of O2 from the continuous source flow. When the patient's inspiratory flow exceeds the O2 flow into the device, the gas from the reservoir system flows into the patient, resulting in increase in inspired FIO2.

Simple mask, partial rebreathing mask, and NRBM are the most common reservoir masks used in COVID-19 patients. The flow rate, range of FIO2 and performance of each reservoir device are explained in [Table 3].
Table 3: Common oxygen delivery devices, their category, delivered flow rate, and fraction of inspired oxygen

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Simple mask

Simple mask is a single plastic unit that fits over the mouth and nasal bridge and has an elastic strap attached to the peripheral edges of the mask that helps to secure the device in place. The mask has holes on either side which serve as exhalation ports to vent out the exhaled gas. A long, small bore tubing connects the body of the mask to the O2 source [Figure 6]. The mask adds an additional reservoir volume of around 100–200 mL. The delivered FIO2 is variable as air dilution occurs during inspiration through the ports and sides around the body of the mask. The delivered FIO2 depends on the O2 input flow, air leakage around mask, and breathing pattern. A simple mask can deliver O2 flows of 5–10 L/min and an FIO2 range from 0.35 to 0.5. The delivered FIO2 is variable based on the patient's breathing pattern. Advantages of simple mask are that they are easy to use, low cost, and are disposable. Simple masks cannot be used in patient's needing a precise FIO2. Besides, they can cause CO2 rebreathing if the set input flow is <5 L/min or in case of disconnection. They are also uncomfortable and need to be removed while eating and talking.
Figure 6: Simple mask

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Partial rebreathing mask

A partial rebreathing mask consists of a mask that resembles a simple mask, the base of which is attached to a reservoir bag [Figure 7]. An O2 tubing connects the O2 source and the device at the junction of the mask and reservoir bag. As the patient inhales, O2 is drawn from the bag, the source gas flowing into the mask and from room air through exhalation ports. On exhalation, as no valves separate the mask and bag, exhaled gas that occupied the dead space enters the reservoir bag, and the remaining moves out through exhalation port. The gas that is inhaled during the next inhalation contains negligible amounts of CO2 from the dead space along with gas enriched with O2. The O2 flow rate in a partial rebreathing mask is >10 L/min (must avoid bag collapse during inspiration) and the delivered FIO2 ranges from 0.4 to 0.7. The advantages of partial rebreathing masks include that of simple mask with higher FIO2 and disadvantages is that any disconnection from input source can be life threatening.
Figure 7: Partial rebreathing mask

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Nonrebreathing mask

A NRBM is similar to a partial rebreathing mask except for the presence of three one-way valves [Figure 8]. Two of the one-way valves cover the exhalation port during inhalation, preventing room air from entering the mask during inspiration. During exhalation, these valves allow the exit of expired gas from the mask. Another valve is located between the mask and the reservoir bag [Figure 9]. This valve prevents expired gas from entering the reservoir bag during expiration. During inhalation, this valve opens allowing fresh gas to flow from the bag to mask. The design minimizes dilution from room air and can deliver higher FIO2. [Table 3] describes the category, flow rate, range of FIO2 delivered, and the performance of individual O2 therapy devices.
Figure 8: Nonrebreathing mask

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Figure 9: Three valves of nonrebreathing mask

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High-flow delivery system

High-flow devices have higher flow rates and the delivered FIO2 is stable as the set flow meets and often exceeds the patient's PIFR. Air entrainment mask (AEM), blending system, and high-flow nasal cannula system are high flow delivery devices that are used in COVID-19 patients. [Table 2] summarizes the high flow delivery devices, their category, delivered flow rate, and FIO2.

Air entrainment mask

AEM is a high flow O2 delivery device with precise FIO2 at flows greater than patient's PIFR. They operate on the principle of jet mixing (Venturi) principle. The O2 tubing is connected to a small nozzle or jet surrounded by air entrainment ports. As O2 passes through the nozzle, it accelerates. The fast-moving O2 molecules collide with the stationary molecules in the room air and entrain them through the entrainment ports. The mixture of O2 and room air reaches the patient through the mask. The amount of air entrainment through the ports is directly proportional to the size of the entrainment ports and the velocity of O2 flow. The AEM provides 6 FIO2 settings ranging from 0.24 to 0.50 (namely, 0.24, 0.28, 0.32, 0.35, 0.40, and 0.50). The flow rate of gas is provided as per the manufacturer's recommendation (usually ranging from 4 L/min to 15 L/min).

Advantages of air entrainment devices are that they can deliver a precise FIO2. Disadvantages include patient discomfort, noise, and potential suffocation hazard and need to remove while feeding.

Oxygen blenders

O2 blenders are compact, lightweight devices that allow precise mixing of medical grade O2 and air within the blending unit [Figure 10]. When air entrainment devices cannot deliver higher FIO2, a blender can be used. Blenders can deliver flows >60 L/min with fixed FIO2. Gas from the blender is delivered through an aerosol mask or T-tube. This kind of O2 delivery is used in patients needing high FIO2 and higher inspiratory demands. It can deliver wide range of a precise FIO2 (0.24–1).
Figure 10: Oxygen blender

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High-flow nasal oxygen

High-flow nasal oxygen (HFNO) is a therapeutic device that provides patients with relatively constant FIO2, heated and humidified, at flow rates up to 60 L/min [Figure 11]. The device includes air – O2 mixing device, a heated humidification system, connecting circuits, and an interface (nasal cannula). The range of FIO2 delivered is 0.21–1 at temperatures ranging between 31°C and 37°C and flows of 20–60 L/min. HFNO can be used in patients with high and variable minute ventilation, requiring supplemental O2 with higher flows and humidification. Advantages are a wide range of FIO2 and provision of humidified gases. Disadvantages include risk of infection, need for reliable input source, expensive, and increased complexity.
Figure 11: High-flow nasal oxygen system

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Supplemental oxygen therapy in coronavirus disease of 2019 patients

O2 supplementation is initiated when the pulse oximeter (SpO2) reading drops below 90%. The SpO2 target for pregnant women is 92%. The normal range of partial pressure of O2 in arterial blood (PaO2) is 80–100 mm Hg. This corresponds to O2 saturation of hemoglobin (SaO2) of 93% to 97% in the oxyhemoglobin dissociation curve (OHDC). The OHDC is a sigmoid or S-shaped curve that represents the relationship between PaO2 and SaO2 in the arterial blood. The flat portion of the curve suggests that a significant change in PaO2 (80–100 mmHg) has small change in SaO2 (93%–97%) indicating that the patient's oxygenation status is better protected at this flat portion. At the steep lower part of the curve where the PaO2 is between 40 and 60 mm Hg, the change in SaO2 is drastic.[16] Clinically, this indicates that, when SpO2 drops below 90%, the patient can potentially suffer hypoxic damage. Therefore, when the SpO2 reading goes down to 93%–94%, it is important to contact a physician, as a drop below 90% corresponds to steeper drop in PaO2 between 40 and 60 mm Hg because of the shape of the OHDC. When patients present to the hospital with 60% or 70% O2 saturation, some tissue hypoxia and acidosis would have already set in. This must be avoided by seeking help earlier. When the patient receives supplemental O2 therapy as guided by the SpO2, the PaO2 increases. At the same time, if saturation remains steady at 94%–95% even after a 6-min walk test, they could wait and adopt other measures to improve oxygenation such as awake proning and deep breathing exercises.

Nasal cannula is most preferred O2 therapy device in mild COVID-19 cases because of its simplicity and ease to use. Nasal cannula is considered to have minimal aerosol generation and thereby has lower potential to transmit infection.[17] A maximum FIO2 of 0.4 can be delivered through the nasal cannula with a flow rate of 6 L/min.

If the patient is still hypoxemic and requires higher O2 flows to achieve a higher FIO2, masks can be a preferred choice. Simple masks deliver slightly higher FIO2 than nasal cannula, achieving an FIO2 of 0.35–0.5 with an O2 flow rate of 5–10 L/min. Simple masks deliver variable FIO2 depending on the patient's breathing efforts. On the other hand, AEMs have an advantage of delivering fixed FIO2.[14] Masks with reservoir bags, especially the NRBM, can provide higher flow with fixed FIO2. NRBMs can provide O2 flow of 15 L/min and FIO2 up to 0.8. NRBM is used as initial therapy in patients with severe COVID-19.

HFNO, used in severe cases COVID-19, can deliver heated and humidified gases at higher flows (60 L/min) and FIO2 up to 1.[18],[19] HFNO can be delivered either through a mechanical ventilator or through a stand-alone system such as Airvo/Optiflow®. HFNO can create positive pressure within the pharynx and trachea. It improves oxygenation, reduces dead space, improves mucociliary clearance, and improves work of breathing due to the high-flow rate produced by the device.[18],[20] HFNO decreases mortality rate and the need for intubation in patients with acute respiratory failure.[20],[21] When HFNO was used as a first-line therapy in COVID-19 patients, it was found to have clinical improvement within first 2 h and reduce intubation rate by 15%.[22] The World Health Organization, the Society of Critical Care Medicine, the Australian and New Zealand Intensive Care Society, and the Chinese Medical Association recommend the use of HFNO in COVID-19 patients with acute respiratory failure.[14],[23],[24],[25] However, with O2 shortage affecting treatment of COVID-19 patients, the use of HFNO has now become restricted in many centers across India.

A few patients whose main problem is decreased lung compliance may not benefit with just increased FIO2. Positive end-expiratory pressure (PEEP) helps in splinting the collapsed alveoli and improves oxygenation. PEEP can be generated with noninvasive ventilators (NIV) that can provide continuous positive airway pressure or bi-level positive airway pressure. The use of NIV in COVID-19 patients before intubation reported higher mortality rates.[26],[27] It is reported that NIV may delay intubation in severe COVID-19 cases and is hence not recommended.[28],[29] NIV can be used for only short duration in COVID-19 patients with close monitoring.[19] A helmet interface is the preferred interface for NIV in COVID-19 as it is more comfortable for the patient, reduces air leakage, and minimizes aerosol dispersion.

Mechanical ventilators

Invasive mechanical ventilation is used in patients when all other O2 therapy modalities fail. About 10%–17% of COVID-19-infected patients eventually require endotracheal intubation or tracheostomy and need for invasive mechanical ventilation.[30] Mechanical ventilators deliver O2-enriched breaths as set by the clinician.

For portable ventilators connected to a source such as a cylinder (most commonly), the amount of O2 consumed and the time for which a cylinder might last can be calculated as below.[31]

Gas consumption = (minute ventilation + bias flow) × ([FIO2-0.2]/0.8) + cycling requirement.

Oxygen therapy and aerosol generation and risk of transmission in COVID-19 infection

COVID-19 patients should be treated in a negative pressure room. O2 therapy devices generate different amounts of aerosols and have the potential to transmit infection to the HCW.[32] Wearing of surgical masks by patients on O2 therapy can help reduce the spread of droplets.[33] While using ventilators, appropriate bacterioviral filters (high efficiency particulate air filter or HEPA-filter is most preferred) must be used before the expiratory port. This will prevent the aerosol dispersion from the patient. The interface of the patient must be closely fitted, and ventilator circuit disconnection must be minimized to reduce aerosol dispersion. Coughing and sneezing can increase the dispersion and production of aerosol. All HCWs dealing with COVID-19-infected patients receiving O2 therapy must wear personnel protective equipment comprising N95 mask, face shield, gloves, and gown.


  Conclusion Top


O2 therapy is critical for the successful treatment of hypoxemic COVID-19 patients. It improves oxygenation and reduces mortality in COVID-19 patients. This review summarizes various sources of O2, its storage, and calculations required in COVID-19 hospitals in order to ensure seamless O2 supply to all patients. Selection of the appropriate O2 therapy device is most crucial for the success of O2 therapy. This review article also focuses on various O2 therapy devices used, its performance and characteristics, uses, advantages, and disadvantages. O2 therapy is associated with aerosol generation and dispersion which is a potential risk of disease transmission to HCW. In the current pandemic scenario, with O2 scarcity being apparent irrespective of the size of the health setup, utmost sagacity is required for its optimal utilization so that a maximum number of patients can benefit from the judicious use of O2.

Acknowledgment

We thank Dr. Deepika Teckchandani and Ms. Susmitha Surendran for their help with illustrations and figures.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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