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Tuesday, April 19, 2011

Oxygen and asthma

I have been asked many times over the course of the years I've been blogging about respiratory therapy and asthma if it is true that asthma effects your bodies ability to diffuse oxygen. The answer is most of the time no, yet sometimes yes. Allow me to explain.

In the past I have ardently noted on this blog or my asthma blog that I have rarely seen an asthmatic with a low oxygen saturation. I also have memories of waking up from anesthesia and asking for my rescue inhaler, and the anesthesiologist saying, "You don't need your inhaler, your oxygen saturation is normal."

"I don't care what your numbers say, " I said back then, "What I feel is short of breath, and I need my inhaler." Of course I didn't say it like that, my response was more of a grunt and a certain look we asthmatics have that says, "Gimme my inhaler!"

So, does oxygen effect diffusion? If so, when? To find the answer to this question allow me to refer to the god of all respiratory books: "Egan's Fundamentals of Respiratory Care." The volume I paid $63.95 for is volume 6, or the 1993 version. Yet I think the answer would be unchanged in the newer versions. If you learn different please let me know.

According to my version, the answer comes on page 472. Egan notes that early on during an asthma attack hyperventilation (rapid breathing) occurs due to anxiety, and this actually causes your bodies ability to diffuse oxygen to increase. Your oxygen saturation might even go up.

I would say that most of the asthma attacks I have are mild in nature, mainly thanks to all the good preventative medicines I take. A simple puff of my rescue medicine and my breathing is back to normal (most of the time). My oxygen levels do not decrease. My oxygen saturation does not drop below 98%.

But that hasn't always been the case. I will explain in a moment. First we need some definitions.

So what is oxygen saturation? This is also known as oxygen sat, sat, SpO2, or pulse ox. It's also referred to as the 5th vital sign after heart rate, respiratory rate, temperature and blood pressure. It's the percentage of oxygen you inhale that makes it to your blood stream.

More specifically, in your blood you have red blood cells (RBC). In the center is a protein called hemoglobin that makes the RBC look kind of like a boat or a donut. When the RBC comes into contact with the lungs the oxygen jumps on board the boat and takes a ride to a cell somewhere in the body.

When an oxygen sits on the RBC your blood is red. When oxygen hops off the RBC your blood has a darker color. Regardless, the oxygen saturation basically is a percentage of these hemoglobin that are saturated with oxygen. So, if I say your sat is 98%, that means that 98% of the RBC boats in your lungs have an oxygen on board.

Your sat is measured by a probe gently and painlessly placed over your finger, or ear, or toe (although usually just a finger will suffice).

What is PO2?

Without going into too much detail, this is the partial pressure of oxygen. In your arteries, which carries oxygenated blood from your lungs to tissues of your body, a normal PO2 is about 100. If this drops to 60 you have hypoxemia, which means low oxygen in the blood.

So as your asthma worsens, your PO2 may decrease. The only way to know what your PO2 is to draw an Arterial Blood Gas (ABG). This is an invasive poke where an RT puts a needle into your radial artery, which runs right along your radial nerve on the thumb side of your wrist.

Thankfully, however, modern wisdom gave us pulse oximeters, because there is a direct link between your PO2 and yoru oxygen saturation. Generally speaking, your SpO2 is 30 higher than your PO2, so if your SpO2 reading is above 90, you know your PO2 is above 60. As your sat drops below 60 you know your PO2 is likewise dropping to significant lows.

(For more detail on this see my post on the oxyhemoglobin dissociation curve.)

Pulse oximeters did not exist until the early 1990s, so those who took care of me when I was a hardluck asthmatic in the 1970s and 1980s were not able to check my oxygen levels. However, a tel-tale sign of low oxygen in the blood is blue lips and fingerstips.

There were many times I had this sign.

So what does this have to do with asthma?

I had an attack in 1998 and I have the results of my blood gases, and back then my PO2 was 64 and my sat was 94. This was still acceptable, yet you can see why my doctors might have been getting worried, especially since nothing was making my asthma better.

In the early stages of asthma, or during a mild asthma attack, your sat will be normal or increased. If you stay calm it will continue to be normal. If you become anxious and hyperventilate (breathe fast) it may actually be higher than normal.

However, as your asthma attack progresses, and your air passages become increasingly narrowed and blocked, even though you are breathing fast and heavy your lungs ability to diffuse oxygen is diminished. It is at this time your sat begins to decline.

Oxygen therapy:

As your asthma becomes even worse, your sat may drop further.


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