During health human blood maintains an Oxygen saturation of about 94-99 %. This is also called SPO2 ( the amount of blood saturated with oxygen or also the percentage of Oxygenated haemoglobin in the blood). This level is measured by a simple device that is clipped on to your finger and which gives an accurate reading in a few seconds ( a Pulse Oximeter - see picture above).

Hemoglobin is the Oxygen Carrier in the blood and it helps transport Oxygen to various tissues all over the body for their functioning. 

Our breathing is controlled by a very complex mechanism called the Respiratory Drive - it has many components including the involvement of Neural and Peripheral Chemoreceptors. The Central Chemoreceptors in the brain are very sensitive to Carbon Dioxide (CO2) levels.  Even a minor increase in CO2 levels brings about immediate changes in our breathing - a 5mmHg change in PaCo2 can double breathing rates in healthy persons.

Build up of excessive CO2 normally occurs along with depletion of Oxygen levels.

There are remarkable instances - as reported in publications as well as based on my conversation with Anesthesiologists in the USA - that patients with Covid-19 are able to tolerate very low SPO2 levels with great ease. Under normal circumstances, a person with SPO2 levels below 90 % starts feeling breathless and this increases with a proportionate reduction in the SPO2 levels.  

Surprisingly patients with Covid-19 are reporting to hospitals with very low SPO2 levels but still walking, talking on their cell phones, etc.  without the respiratory distress associated with it. This has caused clinicians to label it as "Silent Hypoxia" or "Happy Hypoxia".

So what's the problem?

The problem with the delay in symptoms of hypoxia is that patients are reporting late to Hospitals. This delays their treatment and chances of a successful outcome.

Why does it Happen?

Clinicians are not really sure why this phenomenon takes place - it has something to do with low Co2 levels in the blood- this prevents patients from developing the classical symptoms of breathlessness.

The reasons range from damage to the surfactant of the lungs to some blood clotting disorder or some other mechanism not yet understood.

The implication of this finding is that patients with lung injury may be more serious than thought initially - since symptoms appear late- checking SPO2 levels will help to pick up patients who need supplemental Oxygen earlier and thus help prevent further injury to the lungs.