Oxygen Supply in Hospitals: Requisites in the Current Pandemic.

Medical oxygen is an essential medicine as is aptly stated by the World Health Organization. With the advent of coronavirus disease 2019 (COVID-19) pandemic and with increased lung involvement, oxygen has become a precious life-saving drug. With more than 200 million cases worldwide, this pandemic has put tremendous pressure on scarce healthcare resources. In a step toward our readiness for further COVID-19 waves, we would like to bring forth the information regarding oxygen supply in the hospitals. We searched various published literature in journals and books, as well as scientific databases, including PubMed Central, Google Scholar, National Medical Library, and Medline, using search terms as "oxygen sources," "oxygen supply," and "hospitals." The relevant articles published during the period of 1990-2021 and in the English language were selected. This article is an attempt to enrich the readers in further strengthening the oxygen supplies in hospitals during such pandemics and other natural disasters. Copyright:

INTRODUCTION

Coronavirus disease 2019 (COVID-19) pandemic accounts for more than 200 million confirmed cases worldwide, with India alone reporting more than 31 million cases.[1] The primary cause of death from COVID-19 is a respiratory failure as a result of acute respiratory distress syndrome.[2] The Lancet Commission on Global Surgery 2015 revealed in their report that approximately one-quarter of hospitals surveyed in resource-limited countries lack sufficient oxygen supply.[3] The above facts highlight the burden this pandemic has put on already scarce health resources, including oxygen.

Medical oxygen is a regulated commodity that must be at least 82% pure, free from any contamination, and generated by an oil-free compressor. Oxygen is a colorless, odorless, tasteless gas. It changes from gas to liquid at a temperature of −182.96°C. The liquid oxygen can be solidified or frozen at a temperature of −218°C.[4]

Oxygen supply in hospitals can come from oxygen plants consisting of high-pressure oxygen cylinders, liquid oxygen storage tanks, or large oxygen concentrators.

OXYGEN CYLINDERS

Oxygen cylinders have oxygen compressed to the pressure of 1900–2200 pounds per square inch (psi). They come in various sizes ranging from 13 to 51 inch in length and in different volumes and weights. Conventionally, cylinders were manufactured from molybdenum steel. Lighter-weight cylinders made of aluminum with nonmagnetic valve are becoming more common. Steel cylinders are more economical but heavier. Aluminum cylinders are moderately priced, light weight, and can be manufactured to be safe in magnetic resonance imaging environment.[5678] Recently, light-weight carbon fiber-wrapped cylinders with built-in regulator have become available, which eliminates the need for costly purchase and maintenance of a separate population of regulators.[5] The cylinder content present in this state (compressed gas) does not change into a liquid at room temperature, regardless of the pressure applied, since the room temperature is above the critical temperature of gas.[69]

Calculation of duration the cylinder content will last can be done using Boyle's law. Atlas equation is used to calculate the duration of e-cylinders, based on conversion factor. Conversion factors are different for different types of cylinder. Nomograms have also been devised to calculate this duration.[101112]

Oxygen cylinders can be used as standalone in hospitals, as well as in manifold [Figure 1]. When solely used in hospitals, strong supply chain is required. Only qualified personnel should refill the cylinders. Full and empty cylinders are kept separate. They are stored in cool, dry, well-ventilated areas, away from exposure to weather. Full cylinders are usually placed with tamper-evident seal. Cylinders must be stored upright, straight, and secured. If a cylinder cannot be stored upright, it is safer to store it on its side. Always transport cylinder using cart or carrier. They should never be dragged, rolled, or slid, even for a short distance. Gases in their compressed state can cause disastrous effects in the absence of proper handling, storage, and transport. An oxygen cylinder blast case due to improper handling of cylinders in an oxygen filling factory has been reported, which claimed three lives.[513]

Figure 1

Gas cylinder manifold at authors’ institute

Safety checks are done by manufacturers every 5 years, and details are printed on the neck of the cylinder. The valve should always be opened slowly before use; a sit prevents adiabatic changes from occurring. An adiabatic process is defined as one that occurs without the exchange of heat energy from the surroundings.[6]

An alternate to manifold is to use a pressure regulator and gauze directly fitted to the cylinder. Flexible hosing with a Schrader probe or appropriate screw connection is directly attached to the regulator and then supplies the distribution system. This is less expensive than the cylinder manifold but requires manual changeover between cylinders; supply may therefore be interrupted.[6]

LIQUID MEDICAL OXYGEN

It is the best means of storing large volumes of oxygen with small footprints.[14] One liter liquid oxygen provides approximately 860 liters of gaseous oxygen, making this the most efficient system of transportation and storage.[814] Hospitals typically rely on large liquid medical oxygen (LMO) supplies as their primary source.

LMO is commonly produced by the process of fractional distillation in air separation units. In this method, gases from air are separated into various components after cooling them into a liquid state, and then, liquid oxygen is extracted from it. Atmospheric air is cooled to −181°C. Oxygen liquefies at this point. Since the boiling point of nitrogen is −196°C, it remains in a gaseous state. However, argon has a boiling point similar to that of oxygen (−186°C), and hence, significant amount of argon liquefies along with oxygen. The resultant mixture of oxygen and argon is drained, decompressed, and passed through a second low-pressure distillation vessel for further purification. The final purified liquid oxygen is transported using cryogenic liquid containers. These tanks are highly insulated containers, specifically designed for safe and economical transportation and storage of liquefied gases at cryogenic temperatures.[15]

LMO is stored in a vacuum-insulated evaporator, a double-walled large insulating flask – inner stainless steel shell is separated from carbon steel casing by a layer of perlite (insulating material) with a high-performance vacuum [Figure 2]. As liquid oxygen evaporates, its mass decreases, reducing the pressure at the bottom. If lesser demand, the pressure inside the vessel rises, and to prevent this, a safety relief valve opens. The inner vessel is maintained at pressure near 130 psi. LMO is withdrawn from the vessel and routed through the vaporizer that converts it into a gas and then through a series of regulators that reduce the pressure down to hospital working pressure of approximately 50 psi.[716] Copper alloy pipework is used to supply the individual medical gases to the required location, ending at a self-sealing terminal outlet. Copper alloy is used as it does not interact with medical gases and is also bacteriostatic.[6]

Figure 2

Liquid oxygen tank

Single patient liquid oxygen system does not offer any distinct advantage unless they are already in wide use in a hospital due to the following reasons. They slowly dissipate their contents into atmosphere, when not in use, as liquid slowly evaporates to maintain a stable pressure inside the vessel. Second, a portable system requires periodic refilling from satellite station; this process requires a modest amount of training. Third, there are logistic issues too. The large-capacity mobile systems are available in different configurations. This type of system provides federal, state, and local agencies, as well as hospitals, with an option for their disaster plan.[71417]

OXYGEN CONCENTRATORS

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, and rest other gases (argon, carbon dioxide, neon, helium, and hydrogen). Hence, air comprises two gases, mainly nitrogen and oxygen (together 99%). Oxygen concentrator concentrates oxygen from a gas supply (typically ambient air) by selectively removing nitrogen to supply oxygen-enriched air.[18] Conventionally, they are used to support low-flow oxygen therapy for home care patients via a portable concentrator. They have advantage over cylinder gas because they generate their own supply of oxygen and do not need to be refilled, avoiding many logistic issues.[719]

The large oxygen generating system used in hospitals range in a variety of sizes from portable trailer-based designs to fixed systems catering to the entire hospital [Figure 3]. Such large oxygen generators include air compressors and gas dryers.[7] Oxygen is produced using selective adsorption method, known as pressure swing adsorption. This method leverages the property that under high pressure, gases tend to be attracted to solid surfaces. The higher the pressure, the more the adsorption of gas.[15]

Figure 3

Large oxygen concentrators at authors’ institute

Synthetic zeolite (adsorbent) used for the production of oxygen consists of a rigid framework of silica and aluminum with extra caution to make up the positive charge deficit in the structure. Compressed air is applied to zeolite granule bed that absorbs nitrogen. By switching to a second, separate zeolite bed, the extracted oxygen is diverted to a holding chamber. The process of switching between two sieves is termed pressure swing adsorption and is continuously repeated to provide a continuous flow of nearly pure oxygen.[720] Adsorption efficiency can be enhanced by a modest increase in operating pressure. Regeneration of each zeolite column is accomplished by relieving the pressure in the system to wash-out adsorbed nitrogen from the zeolite bed at the end of each cycle and then briefly flushing with a small quantity of the just-generated oxygen. Or alternatively, by the application of a brief negative pressure at the end of each cycle to suck-out the absorbed nitrogen. The product gas produced contains not less than 90% and not more than 96%, by volume, of oxygen, the remainder consisting mostly of argon and nitrogen.[21] The amount of oxygen delivered varies depending upon several factors, including the conserving ratio of the conserver used in the oxygen concentrators, their delivery method (for example, pulse delivery), and patient factors.[22] Thus, monitoring of oxygen concentration at end-use terminals is important.[23]

There are certain drawbacks of oxygen generating system. Like LMO, they are large, requiring much space. They must be protected from environment extremes. They are not “off-the-shelf” systems, and one must be designed and built according to the individual specifications. The cost could be higher than LMO. The cost of required electricity could be substantial as the system is completely dependent on electric power. Time required from ordering to installation could be substantial.[7142324]

In a variety of remote settings and at high altitude, oxygen concentrators have proved effective and reliable. The study by Dobson showed a significant cost saving in favor of concentrators, ranging from around 25% for the smallest hospital to 75% for large district hospital. A concentrator-based system has additional advantage of superior logistics in remote locations since experience has shown that in such circumstances, the reliability of oxygen deliveries is poor and supplies often fail; further savings may result if provision of reliable oxygen supply removes the need to transport patients to central hospitals for surgery.[23]

CONCLUSION

The detailed insight into the existing healthcare resources is the foremost requirement for planning for any unanticipated devastating situation. This article discusses the important sources of oxygen supply, which not only caters to the big cities but also highlights their feasibility to reach low-resource areas where erratic electricity supply and poor transport facilities make access difficult. It equips us to deal with oxygen crisis in a more fathomable way so as to have the maximum timely advantage of scarce resources with minimum expenditure.

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Conflicts of interest

There are no conflicts of interest.