Explained: What is silent hypoxia, and why has it puzzled doctors?

As medical practitioners around the world are busy treating people for Covid-19, many have reported a condition called ‘silent’ or ‘happy’ hypoxia, in which patients have extremely low blood oxygen levels, yet do not show signs of breathlessness.

The condition has puzzled medical practitioners, and many are now advocating for its early detection as a means to avoid a fatal illness called Covid pneumonia.

What is hypoxia?

Hypoxia is a condition wherein there is not enough oxygen available to the blood and body tissues. Hypoxia can either be generalised, affecting the whole body, or local, affecting a region of the body.

According to the Mayo Clinic, an American non-profit organisation, normal arterial oxygen is approximately 75 to 100 millimetres of mercury (mm Hg), and normal pulse oximeter readings usually range from 95 to 100 per cent. Values under 90 per cent are considered low.

When levels fall below 90 per cent, patients could begin experiencing lethargy, confusion, or mental disruptions because of insufficient quantities of oxygen reaching the brain. Levels below 80 per cent can result in damage to vital organs.

What is silent hypoxia?

According to an opinion piece in The New York Times by physician and inventor Dr Richard Levitan, Covid pneumonia — a serious medical condition found in severe Covid-19 patients — is preceded by ‘silent hypoxia’, a form of oxygen deprivation that is harder to detect than regular hypoxia.

In ‘silent’ or ‘happy’ hypoxia, patients appear to be less in distress. Many Covid-19 patients, despite having oxygen levels below 80 per cent, look fairly at ease and alert, according to multiple reports. This phenomenon has puzzled several medical practitioners.

In emergency wards, doctors have reported patients having oxygen levels below 80 or 70 per cent, in some cases even lower than 50 per cent. Those with such low levels of oxygen would ordinarily appear extremely ill, but not in silent hypoxia cases; patients have been reported sitting up in bed talking or operating mobile phones.

In many cases, Covid-19 patients with silent hypoxia did not exhibit symptoms such as shortness of breath or coughing until their oxygen fell to acutely low levels, at which point there was a risk of acute respiratory distress (ARDS) and organ failure.

Silent hypoxia: What explains this phenomenon?

According to a report in The Guardian, the reason why people are left feeling breathless is not because of the fall in oxygen levels itself, but due to the rise in carbon dioxide levels that occur at the same time, when lungs are not able to expel this gas efficiently. This response does not appear to be kicking in in some Covid-19 patients, the report says.

According to Dr Levitan, this happens because in patients with Covid pneumonia, the virus causes air sacs to fall, leading to a reduction in levels of oxygen. However, the lungs initially do not become stiff or heavy with fluid, and remain “compliant” — being able to expel carbon dioxide and avoiding its buildup. Thus, patients do not feel short of breath.

In the NYT piece, Levitan has said that a medical device called a pulse oximeter– used to detect oxygen level in the blood– could help in the early detection of silent hypoxia.

Using the device, those who have Covid-19 or those suspected of having it, can check their oxygen levels early on. A fall in oxygen levels, caused by the silent hypoxia, can serve as a signal for seeking additional treatment immediately, and not wait for a coronavirus test.

Others have expressed concern against this, saying that frequent use of the device would only lead to increased anxiety among many.