Oximeters - should we use them?

Our esteemed colleague Dr William Haseltine, emeritus professor at Harvard Medical School, has been doing a great job of reminding us how to mitigate the risk of COVID-19.

I would like to drill down a bit on a comment he made today on CNN about home monitoring of oxygen levels via what is known as a pulse oximeter, and to sound a note of caution here.

Let’s start by explaining what a pulse oximeter is attempting to measure, and how it is both relevant, but also a risky device to rely upon, especially in the context of COVID-19. The pulse oximeter uses infrared light to attempt to measure the oxygen content of blood. The number generated is an approximation to the actual oxygen content and can be a few percentage points different from the true content. Now, while “normal” levels are approximately 96% and above, that doesn’t mean that the body cannot cope when oxygen levels are subnormal, nor that treating a subnormal level is good.

This last fact is possibly the hardest concept in medicine for both patients and many trainees and even qualified doctors to grasp. How can it be good to have subnormal levels of anything, particularly this vital substance oxygen? Well, the way I would put it is to say that it isn’t “good” per se, but rather that firstly the human body has extremely good adaptive mechanisms to compensate for significant deviation from normal levels of almost any crucial substance and secondly, just as importantly, there can be extremely negative side effects of trying to correct the abnormal.

We have numerous examples in the medical literature of well-intentioned efforts to achieve “normal” which actually result in harm. A classic example is within the world of trauma medicine where we have proven that supplementing the hemoglobin levels of a patient who has massive bleeding beyond approximately half of their normal levels is harmful.

In the pulmonary world, we know that patients live happily – forever – when they have long periods asleep with very low oxygen intake – sometimes far lower than 75% - due to sleep apnea (periods of cessation of breathing when asleep). They don’t die during these episodes and their cells and tissues don’t apparently have significant short-term dysfunction. Yes, there are some long-term consequences over years of untreated sleep apnea, but there appear to be no short-to-medium term consequences. In the case of sleep apnea, the correct treatment is to correct the cause of the apnea, NOT to give oxygen therapy.

In another example, we have millions of patients with moderate to severe emphysema whose normal resting oxygen levels are either left alone at approximately 88-90%, or augmented with supplemental oxygen to achieve the same level, BUT NO HIGHER. They don’t suffer because of allowing the oxygen levels to stay subnormal. These patients may have exercise limitation but that is due to their dysfunctional mechanics of breathing due to destroyed lung tissue rather than the oxygen level itself.

We also know that treating that low oxygen level with supplemental oxygen may in fact be harmful. It used to be routine to give supplemental oxygen to patients having heart attacks, regardless of their oxygen level. This turned out to be harmful and we no longer do it.

So, we know that oxygen requirements are not just slightly less, but often substantially less at rest than so-called normal. Exactly how much less will depend on whether someone has concomitant disease for which oxygen requirements are elevated, for example severe illness due to infection known as sepsis. 

In my hospital, we have not been targetting an oxygen level of anything more than 88% unless there are concomitant diseases. Even that may be a very generous level. During the height of the first COVID-19 peak, due to our – perhaps excessive - fear that use of certain open-circuit oxygen delivery devices would aerosolize the virus causing greater risk to staff, in order to achieve oxygen saturation levels of 88% or more, we ended up intubating patients and placing them on closed-circuit ventilators. Unfortunately, we fear we may have caused significant harm to these patients thereby. 

I want to go into a bit more detail on what are the adaptive mechanisms that we have to overcome low oxygen levels. There are two innate adaptive mechanisms for almost all patients: resting and hyperventilation. Resting reduces oxygen consumption and thereby oxygen needs. Hyperventilation is a combination of either increasing our rate of breathing and/or increasing the depth of our breathing. Principally this results in expulsion of more carbon dioxide with resultant augmentation of oxygen levels. This innate mechanism is often not experienced by patients as it can be quite subtle.

There are other adaptive mechanisms - which must be taught or learned – in which patients adopt a posture or manner of breathing which improves the efficiency of oxygen absorption by the lungs. The best known of these is to adopt a prone position (lying on one’s belly) in order that we improve the efficiency of oxygen uptake by the vast majority of the lungs which are located at the back of the chest. When we lie on our backs – or even on our sides – we are reducing the effectiveness of the lung.

There is a significant danger in recommending patients decide to go the hospital based on their own measurement of oxygen levels, particularly at arbitrary, unproven trigger points. Firstly, it creates additional fear and thereby hospital admissions in patients who would otherwise be either asymptomatic or minimally symptomatic, potentially overwhelming hospital systems with patients who would otherwise not come to the hospital. Secondly, once patients are admitted to the hospital, with the best will in the world, unfortunately physicians do not always make the best judgements. They are not all equally comfortable with, or knowledgeable about, pulmonary physiology which is abnormal, yet which can be tolerated without intervention. In addition, once patients are admitted to hospitals, they tend to have higher rates of secondary infection, i.e. in addition to COVID, as well as clots in the legs and lungs.

Instead of the focus on oxygen measurements, I would like us to start thinking about three strategies:

1. Encouraging patients to adopt prone positioning at home if they are symptomatic

2. Focusing on improving our telemedicine capability so that we keep as many patients safely at home, possibly giving dexamethasone (a steroid) at home in certain cases

3. Improving our knowledge of the potentially deleterious effects of oxygen supplementation in COVID19