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 hemoglobin 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.

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Author's Bio: 

Dr (Major) R k Bhardwaj joined the armed forces medical college (afmc), Poona in 1978, and graduated as an MBBS in Oct 1982. From 1982 to 1997 he served in various capacities in the army medical corps including tours of duty at Siachen glacier and at Jaffna in Sri Lanka where he was intimately involved in the triage, management, and evacuation of battle casualties.
In 1987 he completed his diploma in ENT(DLO) from IGMC (mayo hospital) at Nagpur. From 1988 to 1990 he completed his ms in ENT from afmc at Poona where he was awarded the silver medal for standing first in ent in afmc. He was awarded the 'chief of army staff commendation card' for meritorious duty. From 1991 to 1993 he worked
as an ent specialist at Chandigarh where he performed a large number of ent surgeries and endoscopic procedures. In 1993 he was posted to army hospital (r&r hospital) the largest and most prestigious military medical establishment. He honed his surgical and diagnostic skills and was made a recognized postgraduate guide of the Delhi University.
In 1997 he left the army and has since then been in active practice. Apart from being attached to various prestigious hospitals of south Delhi including Sitaram Bhartiya institute, max med centre, Indian spinal injuries centre (Vasant Kunj) he has a well-equipped ent centre at Vasant vihar where he practices every evening.
Dr Bhardwaj is actively involved in his association affairs where he has been the editor of the news journal apart from being a member of the executive for many years. He was the secretary of the association in 2001-2002 and was the president of the All India association, Delhi branch from 2004 to 2005.
Dr Bhardwaj was the health consultant with the media group tv today network which includes "Aaj Tak and headlines today'. Presently he was health consultant with star news and was involved in bringing out the weekly health program called - 'Apna khayal rakhiyega'. Dr Bhardwaj was the doctor on a panel with the Shri ram school, Vasant vihar and has carried out the annual medical examinations at the heritage school and bloom public school. He contributes articles for the parenting magazine. the specialist at Chandigarh where he performed a large number of ent surgeries and endoscopic procedures.
In 1993 he was posted to army hospital (r&r hospital) the largest and most prestigious military medical establishment. He honed his surgical and diagnostic skills and was made a recognized postgraduate guide of the Delhi University.