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Let's Discuss The Impact Of Asthma On Oxygen Levels  

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By John Bottrell, RRT

4 min read

Interestingly, asthma is not considered a disease that lowers oxygen levels. This is true for most asthmatics, and it is true for most asthma attacks. However, there is one exception to this rule. So, let’s investigate the impact of asthma on oxygen levels.

Why doesn't asthma usually lower oxygen levels?

Air enters your mouth and nose. It then travels down your pharynx and larynx to your bronchial airways. From there, it travels to the alveoli. Alveoli are where gas exchange occurs. It is here where oxygen (O2) from the air enters your blood system, and it is here where a waste product called carbon dioxide (CO2) leaves your blood and enters your lungs.1,2

With each breath, you inhale O2 and exhale CO2. Through the process of normal breathing, your body maintains normal O2 and CO2 levels inside your blood. So, with good asthma control, it makes sense that your blood O2 and CO2 levels should remain normal.

Even during most asthma attacks, O2 and CO2 levels remain normal or at least in the normal range. Three theories explain this:

  1. Asthma Impacts Airways and not Alveoli
  2. Hypoxic Pulmonary Vasoconstriction
  3. Hyperventilation

Let’s discuss each of these individually.

Asthma impacts airways and not alveoli

Asthma attacks cause airways to become swollen and edematous. This, plus increased bronchospasm and mucus production, works to cause airways that are obstructed. Where the obstructions occur, airways are abnormally narrow. This causes a phenomenon where you can get air in past obstructed airways but have difficulty getting air out. This creates a feeling like you cannot catch your breath. This is what makes you feel shortness of breath (dyspnea).1

Still, because air can get in, oxygen can get in. Since oxygen can get past obstructed airways, it travels to alveoli. From there, it crosses over to your blood system. And in this way, your oxygen levels stay in the normal range even during most asthma attacks. The exception here is with acute, severe asthma attacks, which I will describe below.3

Hypoxic Pulmonary Vasoconstriction

Let's introduce you to a couple of terms here. Perhaps you've heard them before. Hypoxic is a term that means that not enough oxygen is available for your tissues. This is caused by hypoxemia. Hypoxemia is a term that means not enough oxygen is getting to your blood from your lungs.4

Another term I'd like to introduce you to is ventilation. Ventilation means that gas exchange is occurring inside your lungs. It means that you are able to inhale oxygen and get it into your blood. It means CO2 gets from your blood to alveoli and you are able to exhale it.4,5

During most asthma attacks, only some airways are obstructed. If some airways become severely obstructed, this of course may prevent oxygen from getting to alveoli. However, your lungs have a neat ability to maintain normal oxygen levels even while this is happening. It has an uncanny ability to constrict blood vessels leading to poorly ventilated regions of your lungs. So this shunts blood from poorly ventilated areas to well-ventilated areas. This works to keep your oxygen levels normal even during asthma attacks.5,6

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Hyperventilation

Most patients experiencing mild and moderate asthma attacks will hyperventilate (breath faster than normal). This may be due to receptors in your brain being stimulated by the attack. It may also be due to stress and anxiety caused by the attack. This makes sure that oxygen is quickly and constantly and rapidly resupplied inside alveoli. This can help maintain oxygen levels during mild and moderate asthma attacks.5

Breathing fast like this will also cause you to exhale lots of CO2. So, it can cause your CO2 levels to decrease as well. This is actually a good sign at this point, as it shows that your body is doing what it needs to do to maintain normal oxygen levels.5

What are the exceptions?

So, if you are admitted to an ER, you will have your oxygen saturation monitored. This gets monitored closely because an asthma attack with a saturation of less than 92 percent is considered a severe asthma attack. The number one cause of death due to asthma is hypoxia. So, it's very important to monitor oxygen levels in all asthmatics admitted to the ER. If your oxygen saturation drops below 92 percent, supplemental oxygen is needed to maintain a saturation of at least 90 percent. (To understand why our goal is 90% here, check out my article from our sister site "Interpreting Oxygen Levels.")6,8

This brings us back to CO2. At some point, all the efforts to breathe during a severe asthma attack become exhausting, and the asthmatic may begin to poop out. A classic sign of this is when CO2 levels begin to rise. It is at this point where doctors may begin to consider intubation and mechanical ventilation. This can help with both ventilation and oxygenation.3

So, oxygen may be necessary during severe exacerbations. It is rarely needed on a long-term basis by asthmatics. Home oxygen therapy is also rarely needed by asthmatics.6

What to make of this?

So, asthma typically does not cause low oxygen levels. This is generally because asthma occurs in the airways and not alveoli. During mild and moderate asthma attacks, our bodies have an uncanny ability to make adjustments to maintain normal oxygen levels.

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