Some COVID-19 patients have very low oxygen, but don’t seem to know it. They aren’t breathing hard, and while they’re sick, they can talk and sit up and generally seem to be functioning better than someone whose body is starving for oxygen.
When doctors check their lungs on CT scans, they seem mostly clear, a marked contrast from the images seen in pneumonia patients whose lungs are cloudy and filled with fluid.
These patients are presenting a challenge to doctors who are scrambling to understand how they could have such low oxygen but also lungs that appear to be open.
In a clinical update posted in JAMA, John J. Marini, MD, professor of Medicine at the University of Minnesota and Intensivist at Regions Hospital in Minneapolis/St. Paul and Luciano Gattinoni, MD, at the Medical University of Gottingen, in Germany, say the problem for these patients doesn’t only lie in the tiny air sacs of the lungs, but also in the legions of blood vessels that ferry blood through them.
Normally, when a part of the lung becomes damaged, tiny blood vessels constrict to redirect blood to areas of the lung that are still working. This mechanism protects the body from a sudden drop in oxygen.
Emerging evidence suggests that the virus that causes COVID-19 can infect the lining of blood vessels. That infection plus an outsized immune response by the body, prevents the vessels from constricting the way they normally would. This causes blood to flow through damaged parts of the lungs where it can’t pick up oxygen. Oxygen levels fall.
WebMD spoke with Marini about why this happens and how doctors might need to adapt their treatment plans to improve outcomes for patients.
WebMD: What are you hearing from you colleagues who are treating COVID-19 patients?
Marini: Doctors that I know personally are very concerned because they do not understand what’s going on. These patients are not responding in the expected ways to their usual interventions.
WebMD: I know this is mostly a debate that’s happening between doctors, but can you try to explain what’s going on in a way that patients might understand?
Marini: I think I can. The viral infection is of the endothelium, the lining of the blood vessels, throughout the body. The lung gets the most blood flow of any organ of the body, so the potential for trouble to develop in the lung due to the vascular insult is very high.
Once that happens, it destroys normal mechanisms that match oxygen need to oxygen availability in the air, and people become blue.
However, unlike most cases of severe pneumonia, the lung stays flexible. Doctors call this respiratory system compliance. In the early phase of the disease in many COVID-19 patients, lungs are highly compliant, which is not usually the case in acute respiratory distress syndrome, or ARDS.
Many people don’t even feel very short of breath. They know something is wrong, but they don’t feel very short of breath, despite the low oxygen going to their organs.
Later in the infection, when the lung becomes very damaged from the virus, and the patient starts to really make strenuous efforts to breathe, the lung begins to pick up fluid. The lung shrinks, forming what we call a baby lung, and you get the more conventional ARDS.
WebMD: There are guidelines that doctors use to treat ARDS. They tell doctors how to use a ventilator.
Marini: Yes. They try to make sure that enough air sacs of the lung stay open, but not too stretched, since too much pressure from the ventilator can damage the lung.
The guidelines that are out there are not bad guidelines for established ARDS, and they do apply to the later phase of COVID-19 ARDS pneumonia.
WebMD: You’ve proposed calling respiratory distress in COVID patients “CARDS.”
Marini: Yes, for COVID-19 Acute Respiratory Distress Syndrome, to distinguish it and call attention to the highly unusual way in which lung damage and low blood oxygen levels first develop — from the vascular side.
WebMD: You’re not saying don’t use ventilators in these patients.
Marini: Oh no. If you can avoid putting them on a ventilator because they’re comfortable and things seem to be working with non-invasive ventilation, like high-flow nasal oxygen or a mask, if they’re not struggling and they’re comfortable and they’re getting good gas exchange, follow the trend.
If they begin to deteriorate, get agitated, short of breath, that is the time to intervene and intubate [with a ventilator].
WebMD: Are you suggesting changes to the ventilator guidelines?
Marini: I strongly recommend lung-protective ventilation. Early on, the patient may be able to take deeper breaths without excessive lung stretch and may feel more comfortable in doing so. In that case, it is preferable to use a ventilator setting with a larger tidal volume [the size of the breath being delivered by the ventilator] but lower positive end expiratory pressure, or PEEP. Current guidelines call for PEEPS in the 8-12 range depending on oxygen needs, but you may be able to lower the PEEP to 5, for example.
Currently, some doctors are restricting the tidal volume early on for fear of overstretching the lung and causing damage. If a patient is not forcefully breathing, I would not advise enforcing a low breath size that is not appropriate for the high level of ventilation. This may make the patient feel as though they’re not getting enough air and may increase their agitation. The increased agitation can cause them to try to breathe too forcefully. That forceful effort to breathe can also damage the lungs.
Minimizing oxygen and ventilation demands — reducing agitation with medication, for example, avoids the need for aggressively meeting them and reduces the vascular stresses, as well.
If you ignore what’s going on from the vascular side, you’re likely to make wrong decisions early in the course or in mild cases that will precipitate more problems.
In the beginning, if you try to open lung units that don’t need to be opened, with pressure from the ventilator, you direct the blood flow the wrong way.
WebMD: What should be done differently, in your opinion, than is happening now?
Marini: Pay attention to the patient’s breathing comfort. That’s the first point. Secondly, understand the physiology enough so that you don’t put excessive demands for breathing on the patient, for example by not sedating them when you should, by not alleviating any pain that they might have.
You want to reduce the stretch on the lung and the stretch on the blood vessels and the amount of blood flowing through the damaged lung. That, in the early phase will help you a lot.
