Covid-19 is a respiratory virus. The majority of patient may experience no or minimal symptoms. But small subset of those infected will unfortunately progress to have significant pulmonary dysfunction. Some will even require mechanical ventilation. Oxygenation in COVID 19 patients with severe symptoms is altered. This is the due to the changes caused by the virus.

Normal Physiology

Oxygen (O2) is exchanged with Carbon Dioxide(CO2) in the lungs. The CO2 is exhaled and the O2 is taken up by the blood. This high O2 continuing blood is then pumped to every single organ. With complete distribution network of capillary vessels, every cell then gets access to the O2 rich blood. Hemoglobin is the carrier that transports the O2.

The O2 is removed and dissociated from the Hgb  depends on a number of variables.  Each red cell Hgb has four binding site for the oxygen. The  affinity and strength of each one of those four units for oxygen changes based on a number variables. These are CO2, Acidity (PH), DPG and temperature.

Oxygen Dissociation Curve

The oxygen dissociation curve has a long “S” shape. On the low end of oxygen in the blood most of the Hgb site are occupied. As the oxygent level increase there is little change to the saturation.

Normally the relationship of the blood in the lungs (horizontal axis) and the amount of the O2 in the red cell (vertical axis) is following the red line. When the amout of oxygen insired is 25mmhg  the blood saturation is at 50% (A). The blood saturation is nearly 100% when room air is inhaled (C). Room air has PO2 of  75mmHg.  Note that there is very little change in blood saturation (SaO2) by increasing the PO2 from 75 to 100 mmHg (the red line is horizontal between 75-100).

Another way to look at this: If you increase the PO2 from 25 to 50 (doubling)  the Saturation goes from 50 (A) to nearly 85(B). Whereas increaseing the PO2 from 50 to 100 (doubling) only mober the Saturation from 85(B) to 100(C). This shows the efficiancy of system to be able to deliver the most amout of oxygen to the tissue even with the low level of oxygen present in the lungs.

As the Green and the Blue lines demosntrate the balance can change by changes in CO2, Acidity (PH), DPG and temperature.

COVID-19

When it comes to COVID-19 illness there may be a number of factors in play. Most patients with pre-existing conditions already have changes that may shift the curve to the right (high fever and high Co2). Furthermore, obesity, asthma and other conditions may decrease the ability to clear the lungs of secretions and mucus may contribute to decreased oxygenation. Additionally, there is significant inflammation associated with the chemicals released in COVID-19 (cytokines). These can cause devastating changes to the ability to exchange oxygen in the lungs.

Oxygenation in COVID-19 severely symptomatic patient can deprive oxygen from organ. This can  progress to organ failure.  One of the most common organ systems to fail is the kidneys which may require dialysis.