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Thursday, August 18, 2022
COVID-19: Dr. Brown: Pulse oximeter can be key pandemic tool

Dr. William Brown

Special to The Lake Report

Without widespread COVID-19 testing designed to detect the virus in actively infected people and antibodies in those who have recovered, it’s nigh impossible to get a handle on the prevalence of the active disease and the magnitude of any “herd” effect provided by the latter group.

The lack of those numbers has been the biggest failing for most countries except South Korea and Germany, compared to less well-prepared and resourced countries, such as Italy, Spain and now the United States, with probably far worse to come in the undeveloped countries which have few, if any resources.

But beyond the statistics and modelling of the disease, there is the challenge of managing COVID-19. Some infected patients, especially those with known risk factors, go on to develop symptoms sometime between three and 14 days after becoming infected.

Roughly 5 to 20 per cent of those with early symptoms go on to develop a more serious illness called ARDS (Acute Respiratory Syndrome), which in its clinical features is analogous to what was observed in earlier SARS and MERS outbreaks.

The chief manifestations are increasing shortness of breath and tightness in the chest. In COVID-19 there have been other serious complications, including clotting disorders, kidney failure, liver failure, myocarditis, heart attacks or even ischemic strokes, some of which may be the presenting feature and even cause of death.

A common feature in the early stages of ARDS is that some patients develop hypoxemia (lower than normal oxygen content in the blood) without shortness of breath, despite X-ray and CT scan evidence of COVID-19 related pneumonia. Given that some of these patients go on to develop a much more serious, extensive pneumonia requiring supplemental oxygen and even assisted ventilation, surely it makes sense to track blood oxygen levels from the outset of symptoms in the disease to detect developing pneumonia as early as possible.

That’s precisely what’s been suggested using relatively cheap and widely available pulse oximeters. In their most simple form, the pulse oximeter is battery operated, clips on the tip of the middle finger and provides a digital readout of the blood oxygen level as well as the pulse rate.

But why the paradox of significant hypoxemia yet no shortness of breath? It all has to do with the pathology of the disease and a little physiology. The primary target of COVID-19 in the lungs are the many thousands of tiny sacks at the end of the network of bronchial tubes, called alveoli.

Here the air is separated from nearby fine capillaries by a thin layer of lining cells, which proximity normally makes transfer of oxygen and carbon dioxide between the pulmonary tract and the blood possible. Oxygen is taken up by the capillaries and carbon dioxide passes in the opposite direction from the capillaries into the lungs to be expired.

The problem with ARDS is that those normally wide-open alveolar sacs become plugged with cellular debris, inflammatory cells and fluid, which makes gas exchange impossible in the most severely affected alveoli.

However, blood continues to circulate through the affected alveoli, with the result that the overall blood oxygen level drops – hypoxemia. The lack of shortness of breath is probably related to the fact that the primary engine driving ventilation is not oxygen, but carbon dioxide, which diffuses better through inflamed alveoli than oxygen.

The body monitors blood oxygen and carbon dioxide levels closely and does so through an array of chemoreceptors in the carotid arteries in the neck, aorta in the chest and neurons in the brainstem, the last of which, control the rate and depth with which we breathe. Should the blood carbon dioxide level rise too much, the necessary cues trigger the brainstem network to increase the rate and depth of respiration through a variety of muscles.

In ARDS, it’s probably the case, at least early on in the pneumonia, that enough carbon dioxide gets through to keep the blood level relatively normal, even in the face of significant drops in the blood oxygen level. That’s one explanation.

The takeaway point is that monitoring blood oxygen levels – three to four times a day – in patients with symptoms suggestive of COVID-19, makes sense as an early warning sign to what might become a significant pneumonia at a stage when it can be best treated. Testing at home with a pulse oximeter makes sense to me.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the Infohealth series held on the second Wednesday of each month at the Niagara-on-the-Lake Public Library.