Medical Illustrations by Patrick Lynch, generated for multimedia teaching projects by the Yale University School of Medicine, Center for Advanced Instructional Media, 1987-2000.

Dr. Wright joins us to discuss acute respiratory distress syndrome (ARDS). There has been a lot said about ARDS recently given some of it’s similarities to the COVID-19 disease process. That being said ARDS has been around for a long time and is very much its own disease process. We discuss how to define it, how to recognize it, and how we treat it.


Let’s start with the anatomy-

When we think of the airway, we typically think of the more macro picture- the nose, mouth, trachea, bronchi. But let’s remember that the airway physiology is quite complex and incredibly elaborate.  If we work our way through the path that oxygen takes, it starts in the larynx, travels through the trachea, and then into the bronchi. The bronchi then further divide into the bronchioles, which divides then become the alveoli. The very thin, single-cell layer alveoli interface with the capillaries to participate in gas exchange – so remember oxygen is crossing through the cell layer form the alveoli to the capillaries, to then join the pulmonary circulation and be transported to the body.

Similarly, the capillaries are offloading their CO2 into the alveoli, which then works its way “backwards,” through the bronchioles, the bronchi, the trachea and the larynx. Take a second to just envision from you’re a/P days this path that oxygen takes, it is going to help when we start talking about how things go wrong.

So we just covered what should happen, the “normal.” Now let’s talk about the abnormal.

Remember, there are different things that can cause respiratory failure- this can by a problem with OXYGENATION or a problem with VENTILATION.

  • Problems with OXYGENATION occur when there is difficulty delivering oxygen to the organs throughout the body because of problems with the distal airways- issues getting oxygen into the capillaries and successfully delivering them throughout
  • Problems with VENTILATION refer to issues in mechanically moving oxygen into the distal areas, and mechanically removing CO2
  • When we are talking about problems with oxygen delivery to the organs, there are a number of different ways this can go wrong- specifically, how much oxygen is getting into the lungs, how efficiently the gas exchange is occurring at the level of alveoli, and how well the blood is moving through the lungs to receive the oxygen. There are a number of very complex math equations that describe these different ways that oxygen delivery to the tissues can be insufficient, but for the sake of a COVID focus and ARDS, we are going to review something called the P:F ratio.

The P:F ratio is the PaO2:FiO2 ratio. This is the ratio of arterial oxygen partial pressure (i.e. if you take a sample of arterial blood, how much oxygen is in there), compared to the FiO2, or the fractional inspired oxygen, meaning the fraction of O2 that is in the lungs, or how much oxygen you are delivering to the alveoli.

Now for some definitions:

Treatment

Patient lying supine in the ICU
Proned Patient

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