Oxygen As A Medicine

Provided in the form of supplemental oxygen, oxygen is administered to a person who requires more oxygen than he or she is able to get from breathing in regular, or normal, air.

Regular, or normal, air found within the Earth’s atmosphere consists of a mixture of a number of different gases – including oxygen. The percent of oxygen found in the air we breathe is commonly referred to as the fraction of inspired oxygen, or FiO2.

The FiO2 in air is actually the percentage of pressure that the oxygen contributes to the total pressure of the air. Since the pressure of each particular gas, such as oxygen, makes up only a portion of the total pressure of the air, the actual pressure of that gas is called the partial pressure of that gas. While this may sound complex, its important information to be aware of, primarily because the partial pressure of oxygen is what doctors are really adjusting when they adjust the percentage of oxygen in the air.

Understanding Oxygen At Different Altitudes

In cases of diseases affecting the lungs, such as COVID-19, one of the most important things to be aware of is that nitrogen dilutes oxygen (which normally comprises 21 percent of the air).

What this means is that oxygen has a partial pressure 160 mmHg at sea level (making up 21 percent of air), while nitrogen has a partial pressure of 593 mmHg, making up 78 percent of air (small amounts of other gases comprise the remaining 1 percent of the air).

As you move to higher altitudes, and although oxygen continues to comprise 21 percent of the air, the atmosphere becomes thinner. This thinning of the atmosphere causes the pressure from nitrogen, oxygen, and all of the gases to decrease, meaning that less oxygen is available for a patient to extract and use from the normal or regular air that we breathe.

As an example, let’s use Salt Lake City, which is located at a higher elevation - about 1,300 meters above sea level. Because there is an overall lower air pressure in areas of higher elevation – like Salt Lake City – when compared to sea level (650 mmHg or 0.86 atm in Salt Lake City compared to ___ mmHg or 1.0 atm at sea level), there is less overall oxygen available in the air there.

For healthy people who are not about to run a marathon, the difference in the amount of oxygen available in areas of higher elevation doesn’t typically matter. Generally, this is because healthy people do not need the full 160 mmHg of oxygen pressure that Earth provides at sea level. Using this same reasoning, it’s why most passengers on airplanes – including women who are pregnant - do not need to wear oxygen masks, even though the cabin is pressurized at an elevation even higher than Salt Lake City.

Oxygen, Altitude, and Health Issues

Unfortunately, the same is not true when someone has a health issue affecting the ability of the lungs to allow oxygen from the air sacs into the blood, the ability of air to get to the parts of the lungs where blood in need of oxygen is perfusing, or the ability of the heart to get blood through the lungs.

These health issues, including cases of COVID-19, can reach a point that a person cannot efficiently breath on their own, even with the 160 mmHg of oxygen that the atmosphere provides at sea level. In these cases, supplemental oxygen, or oxygen comprising more than 21 percent of the breathing gas is required. But just as with any other medicine, not everybody requires the same amount of oxygen.

Some people might need just 25 or 30 percent oxygen while others might require anywhere from 40-50 percent of the air that they breathe to be oxygen. There are even some very critically ill people who need close to 100 percent oxygen, and in rare cases, patients require more than 100 percent O2, (which requires a special chamber called a hyperbaric chamber). But, for the purpose of this article, we will cover supplemental oxygen reaching no higher than 100 percent.

How Much Supplemental Oxygen Do You Need?

The amount of supplemental oxygen that you need depends on the O2 saturation (SpO2), often called “O2 sat” for short. SpO2 is measured with a device that clips onto your finger, toe, or earlobe called a pulse oximeter.

Each molecule of hemoglobin in your red blood cells can carry up to four molecules of O2. An O2 sat of 100 percent means that the red blood cells in your arterial blood are fully loaded with as much oxygen as their supply of hemoglobin is able to carry. This means that just about every hemoglobin molecule in your systemic arteries (the arteries that deliver blood to body tissues) is loaded up with four O2 molecules.

As long as you have enough hemoglobin in your blood —meaning, as long as you are not anemic— an O2 sat anywhere from 95-100 percent indicates that the lungs are supplying enough oxygen into the blood to support tissue throughout the body.

In people with no health problems, pulse oximetry readings usually show an O2 sat of 98 or 99 percent, but anything 95 percent and higher is considered normal.

Severe Respiratory Infection of COVID-19 In Pregnant Women

In cases of severe respiratory infection of COVID-19 in pregnant women, the World Health Organization (WHO) recommends keeping SpO2 in the range of 92 to 95 percent. Other organizations, such as the International Federation of Gynecology and Obstetrics and the Royal College of Obstetricians and Gynaecologists have also suggested 95 percent as a cutoff SpO2 for giving supplemental oxygen.

Increasing SpO2 percentage into the high 90s has not been shown to add any additional benefit for the fetus. This is primarily due to the fact that the fetus already has special hemoglobin which attracts oxygen more strongly than adult hemoglobin does.

SpO2 Ranges and Lactation

These specific SpO2 cutoffs and ranges also apply to lactating women. In addition, it’s important to point out that if you are lactating and have COVID-19 symptoms, even mild symptoms, your doctor will recommend that you use a pulse oximeter to monitor your oxygen levels from home.

Should the pulse oximeter read 94 percent or below, your doctor will most likely ask you to come into the office for further evaluation. It’s important to point out that your doctor will also likely ask you to come in if your O2 sat is 95 or 96 percent while you are sitting or at rest but drops to 94 percent or lower when you exert yourself mildly (mild exertion includes walking across the room, walking up a flight of stairs, of similar activities).

In these cases, your doctor is proactively trying to avoid sending you to the emergency department when there is a good chance that you do not need hospital admission. That’s because hospitals are typically very busy with patients with COVID-19 of severity that requires more attention.

Nevertheless, if your O2 sat at home is below 92 percent, your doctor will either ask you to come to the office to have that verified or will send you directly to the hospital emergency department.

As a point of reference, it’s helpful to note that in any non-pregnant person, an SpO2 below 90 percent means that he or she will definitely need supplemental oxygen, while an SpO2 in the 90-91 percent range means that he or she will most likely need oxygen.

Pregnancy and SpO2 Ranges

Pregnancy puts you into the category of definitely requiring oxygen, along with hospitalization - even if your saturation level is in that borderline range of 90-91 percent. Furthermore, if your saturation level is in the 92-94 percent range, but you have concerning symptoms or concerning test results, this too can trigger the need for admission to the hospital.

In the cases of mild case COVID-19 progressing toward moderate COVID-19 in a pregnant woman who is not in labor, you can most likely expect admission to a general medical unit to be monitored, to receive supplemental oxygen, and to receive other therapies, such as dexamethasone.

Supplemental oxygen is given to get as a way to get you into the range of 92-96 percent saturation. Then, the supplemental oxygen is typically titrated in order to keep you in that range. If the O2 sat rises above 95 or 96 percent, your supplemental oxygen is decreased, or turned off, as long as the monitoring of your fetus shows that the fetus is doing well. The supplemental oxygen is turned back up again if your saturation drops to 92 percent.

Different Types of Oxygen-Delivery Devices

There are several different oxygen delivery devices that can be used to help maintain your O2 sat at the optimal levels; they are also often used as a way to account for various medical situations.

Low-Flow Nasal Cannula

The simplest oxygen-delivery device is the low-flow nasal cannula. This is a tube that that delivers oxygen from a tank (which, in hospitals, usually comes through an attachment fixture on the wall) through two prongs, one going into each nostril. The dose of oxygen is defined as a particular volume of oxygen (how many liters) delivered over a particular amount of time (each minute). Named low flow nasal cannula for a reason, the device can only deliver up to a maximum of 6 liters per minute. The device is designed to prevent higher flow rates, specifically because high flows have been observed to dry the upper airways, cause nosebleeds and lead to other problems.

Non-Rebreather Mask

Another type of mask, called a non-rebreather mask, is also designed to accommodate patients breathing rapidly, but can supply even higher FiO2 than a Venturi mask. With the Venturi mask, the oxygen entering the mask is diluted by the air that the patient exhales. In contrast, a non-rebreather mask has a bag that hangs from the mask. Pure oxygen from a tube enters the bag until the bag is full, this isolates the oxygen from the air that the patient exhales into the mask. Then, when the patient inhales, all of the gas that enters his or her lungs is comes directly from the bag. Like a Venturi mask, the flow through a non-rebreather mask is generated by the patient – allowing it to be able to keep up with rapid breathing.

High-Flow Nasal Cannula

Even more effective than a non-rebreather mask is a device called a high-flow nasal cannula. Typically, by the time a patient is switched to a high-flow nasal cannula, the patient is in the intensive care unit (ICU).

Unlike the low-flow nasal cannula, this device receives oxygen that is heated and humidified; this allows oxygen to flow into the patient’s nose in much greater volume per minute compared the rate allowed with a low flow nasal cannula.

A high-flow nasal cannula can also supply very high FiO2 and is generally more comfortable for patients than a face mask. The high-flow nasal cannula also can provide pressure as a patient finishes exhaling; doctors call this pressure PEEP. PEEP is very important when people with COVID-19 start developing a lung complication called acute respiratory distress syndrome (ARDS).

One of the many things that happens in ARDS is that the alveoli , which are the balloon-like sacs where gases move between the air and the blood, can collapse. But having a good PEEP —or a good pressure at the end of exhalation - can help to prevent the collapse.

CPAP/BiPAP Devices

Another way to deliver oxygen, in addition to PEEP, is a category of devices known as CPAP and BiPAP. You may have heard of CPAP/BiPAP machines that are used at night to prevent sleep apnea. These devices are useful in sleep apnea, because they keep the airway open by supplying positive air pressure throughout the breathing cycle.

CPAP and BiPAP are also used in COVID-19 patients, because the positive air pressure is PEEP when it is present at the end of exhalation. Since oxygen can be added to CPAP/BiPAP to increase the FiO2, this system is often used in an effort to give COVID-19 patients adequate ventilation without going to the next step, which is invasive ventilation. Invasive ventilation means mechanical ventilation through a tube that is inserted through the mouth and into the airway.

Unfortunately, studies indicate that putting people on CPAP/BiPAP in an effort to delay or avoid mechanical ventilation can have an overall negative effect on the person’s recovery.

Additionally, there are concerns that ventilating a patient through the nose and mouth (rather than through an endotracheal [ET] tube that goes deep into the airway) may spray the COVID-19 virus out into the room air.

To confront this problem and hold off on mechanical ventilation through an ET tube, some medical centers have tried using helmets that enclose the head of patients who need help with breathing. But information on the benefits of the various helmet systems in COVID-19 is limited (as of January 2022).

Mechanical Ventilation

The advantage of mechanical ventilation seem to increase as a patient becomes increasingly sick. This is because mechanical ventilation can do everything that other berating devices can do, and more.

Among the benefits, mechanical ventilation can supply numerous different types of air flows with any patterns and FiO2 that doctors decide is required. This enables the delivery of oxygen in situations where the other non-invasive ventilation devices are unable to get the person’s oxygen saturation high enough.

Other reasons for putting a person on mechanical ventilation include that his or her lungs are not removing carbon dioxide well enough or that the cardiovascular system is becoming unstable.

Another reason is to avoid airway injury. Lungs exposed to air under pressure can be injured, especially if the patient consciously or unconsciously resists the machine.

In mechanical ventilation of a COVID-19 patient, however, the patient is put into a kind of reversible coma where they are sedated with a medication, such as propofol (a drug that is used frequently in general anesthesia for surgery) and given other medication to paralyze his or her muscles - including the muscles that control breathing. Taking these steps prevents the patient her from fighting the ventilator and allows doctors to better monitor and adjust a range of things that the ventilator does.

Extracorporeal Membrane Oxygenation (ECMO)

Finally, there is another technology called extracorporeal membrane oxygenation, or ECMO. In ECMO, the supplemental oxygen is delivered directly into the blood. Basically, the machine becomes a substitute for the patient’s lungs, but the amount of ECMO can be tapered down as the patient’s lungs begin to recover.

Pregnancy tends to complicate things when a women’s oxygen therapy needs increase, especially as a result of a worsening case of COVID-19. Along with the various devices discussed above, an additional way to get the most out of the lungs is to put the patient on the bed face down. This is a position called proning, and while it’s proven to be effective, it becomes increasing difficult for a pregnant woman to lay this way during the late phases of pregnancy.

Additionally, the process of intubation - inserting the ET down the throat into the trachea – can also be challenging during pregnancy; this process typically take longer than usual with these delays increase the danger associated with the procedure.

References

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Graves CR. Pneumonia in pregnancy. Clin Obstet Gynecol. 2010 Jun;53(2):329-36. doi: 10.1097/GRF.0b013e3181de8a6f. PMID: 20436308.

Sobrevilla LA, Cassinelli MT, Carcelen A, Malaga JM. Human fetal and maternal oxygen tension and acid-base status during delivery at high altitude. Am J Obstet Gynecol. 1971 Dec 15;111(8):1111-8. doi: 10.1016/0002-9378(71)90113-x. PMID: 5129566.

World Health Organization. Clinical management of severe acute respiratory infection when novel coronavirus (nCoV) is suspected. Jan 2020. Accessed January 24, 2022.