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In the previous post on oxygen and pregnancy, we discussed how oxygen enters the body through the lungs, and left off with the idea that there are different ways to deliver oxygen, as well as air, when the lungs are not working adequately to get enough oxygen into the blood. Such a situation can result from a variety of conditions. One such condition is pneumonia, which can develop from a range of causes, including COVID-19, but another lung complication that is even more of a game changer in COVID-19 patients is called acute respiratory distress syndrome (ARDS). When you hear about people with COVID-19 getting admitted to an intensive care unit (ICU), a major question that the ICU team must answer is whether the patients are developing ARDS and require mechanical ventilation. To give a famous example, on April 5, it was reported that UK Prime Minister and COVID-19 patient Boris Johnson had been transferred from a regular hospital care setting to the ICU. Then, within two days, reports about Johnson varied from “likely to need a ventilator” to “not on a ventilator”.
I’m writing this story on April 7th, and it’s possible that Johnson will be on a ventilator by the time that you read it, but you may now be wondering what happens between needing just a little extra oxygen and needing mechanical ventilation. As with the range of severities that COVID-19 can take, from no symptoms to failure of organs throughout the body, there also is a spectrum of methods to deliver air and oxygen to the body. For some of these methods, such as an oxygen tube into the nose you can be in a regular hospital ward. At the other end of the spectrum is mechanical ventilation, which means that a machine –a ventilator– is breathing for the patient. It also means, generally, that the air is going in and out of the patient through what doctors call an endotracheal (ET) tube, because it passed through the throat, into the trachea, the airway that leads to the lungs.
I should mention that there is also a way to provide oxygen to a patient without even using the patient’s lungs. It’s called ECMO and it puts oxygen (O2) directly into the blood while removing carbon dioxide (CO2). Researchers in some Asian countries are checking whether ECMO can help COVID-19 patients, but there aren’t enough ECMO machines in the United States for this tactic to become a major force in the battle against the current pandemic. Thus, we are talking here about helping with air and oxygen delivery to the patient’s lungs. An oxygen tube into the nose is the simplest way to do this and mechanical ventilation is the most complicated, plus, along the way, there are some intermediate methods.
While a tube going into the nose has the advantage of allowing you to talk and eat, this method is limited in the amount of oxygen that it can supply. In the previous post, we mentioned that room air (which is the same as air from outside, the air in Earth’s atmosphere) is about 21 percent oxygen and that most of the rest is nitrogen. Depending on how the flow of oxygen is set, a tube going into the nose can bring oxygen up, at best, to about 40 percent of the air entering the person’s lungs. This is okay, if the level of oxygen in your blood is just a little low, due to some difficulty breathing while you are pregnant, or if you are admitted to the hospital with a case of COVID-19 whose symptoms are a little bit to difficult to manage at home.
Stepping up from a tube in the nose, you can be given a facemask that fits over the nose and mouth. The simplest masks do not seal all the way around, plus you rebreathe some of what you exhale. Even so, with the right amount of oxygen flowing into the mask, the air reaching your lungs can be up to about 70 percent oxygen. Both nose tubes and simple masks can be used on a regular hospital ward. In fact, you have probably seen plenty of people getting oxygen with these methods outside of the hospital. A disadvantage, however, is you don’t really know if you’re getting 40 percent oxygen using a nose tube or 70 percent using a mask. Those numbers are simply the estimated maximum amounts. On the other hand, there are some fancier types of oxygen masks that allow doctors to control the percentage of oxygen. These include what’s called a non-rebreathing mask, which is able to provide air that is more than 90 percent oxygen.
While nose tubes and masks can use oxygen from a tank to increase the percentage of oxygen in your air, your lungs and breathing muscles are functioning the same way that they do in everyday life. Although enhanced with extra oxygen, air is moving in and out of your lungs, because your breathing muscles and the stretchiness of your lungs and chest change the pressure constantly in your chest and lungs. However, there also are machines that provide different mixtures of air and oxygen while also changing the pressure of air in your respiratory system. These devices help get air deep into your lungs by blowing air, meaning increasing the air pressure, during the inhalation part of your breathing cycle. Importantly, they also can be set so that, when it’s time to exhale, they do not allow the pressure inside the lungs to go all the way down. Instead, they stop the exhalation at a certain point, like when you have an inflated balloon and you start letting the air out but then close your fingers again around the opening while the balloon is still a little bit inflated. This is extremely important in people with COVID-19 when they have ARDS, or are developing it. This is because in ARDS, the air sacs of the lungs are collapsing, but keeping the lungs a little bit inflated at the end of exhalation keeps air sacs working. Keeping the lungs a little bit inflated actually recruits more air sacs that otherwise would be collapsed and not helping to get oxygen into the blood and CO2 out.
With some of the more sophisticated machines and oxygen systems, the patient needs to be in the ICU, but COVID-19 introduces various problems that can make mechanical ventilation look like the best option. One major concern, for instance, is that ventilating a patient through the nose and mouth (rather than through an ET tube that goes deep into the airway) may spray the 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 another problem is that lungs that are being exposed to air under pressure can be injured easily, if the patient resists the machine, consciously or unconsciously.
For these reasons and a range of others, the severe form of COVID-19 often reaches the point when mechanical ventilation is necessary. In such cases, an ET tube is inserted through the patient’s throat, into the trachea. The patient is also given medications that keep her in a deep sleep and keep her muscles paralyzed. This includes the muscles that control breathing. Essentially, the patient is in a kind of controlled coma. This prevents her from fighting the ventilator and allows doctors to decide and continuously adjust a range of things that the ventilator does. These include the amount of air that goes in an out of the lungs with each pump, the rate of breathing, and the percentage of oxygen in the air. While doing this, the team in the ICU must also account for numerous other factors, such as how the ventilator settings affect the return of blood from the body circulation to the heart, and how they affect the removal CO2 from the body and the acidity of the blood.
So what does this all have to do with pregnancy? The answer is that pregnancy tends to complicate things. With COVID-19 patients who develop ARDS, as with the machines that use special masks, mechanical ventilation is set in a way that it keeps the lungs a little bit inflated even during exhalation, which helps to recruit more air sacs. However, an additional way to get the most out of the lungs is to put the patient on the bed with her tummy down (the prone position). But that’s kind of hard to do during the late phases of pregnancy. Furthermore, the process of intubation –inserting the ET down the throat into the trachea– can be challenging during pregnancy, and can take longer than usual, and such delays increase the danger. Fortunately, early study suggests that the severe form of the COVID-19 –the kind that leads to ARDS and other complications requiring ventilation– is not so common in younger women, including those who are pregnant.