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Thursday, April 24, 2008

The hypoxic drive theory: reality or simply a hoax?

(This is part one of a six post series.)

One of the theories that evolved as the field of respiratory therapy evolved was the THEORY of the hypoxic drive. That is where if a patient's oxygen level gets low enough, the patient's brain will signal the patient to breathe.

While CO2 is usually what causes people to breathe, when the levels of CO2 is chronically high, particularly in patients with chronic bronchitis who have developed a chronically elevated CO2 level (CO2 retainer) "that cannot be lowered significantly regardless of patient effort," according to Egan, Fundamentals of Respiratory care, (page 336 volume 6, 1995), CO2 no longer effects a patients drive to breathe, and low levels of oxygen (hypoxemia) "drives ventilation quite strongly."

Due to this theory, it is recommended that people who are CO2 retainers not be placed on oxygen greater than what is absolutely necessary. Generally, the accepted wisdom is that a PO2 level of 50-60 torr is the target PO2 we reach for with these patients, which is equivalent to an SpO2 of 80 to 90%.

And, if a patient is given too much oxygen, the theory suggests, this patient will lose his drive to breathe, his CO2 becomes much higher, he ultimately becomes lethargic, and will at some point stop breathing. Therefore, we medical workers in charge of the patient want to avoid using FiO2s greater than 40% for the most part.

However, also according to Egan, (page 707, volume 6, 1995) while hypoventilation is a hazard of oxygen therapy, "this harmful effect should never stop us from giving oxygen to a patient in need. Preventing hypoxia should always be the first priority."

So far I've stated the obvious that all of you wise RTs already know, but what happens if you have a chronic CO2 retainer patient on 100% oxygen to maintain a PO2 of 40%? Will that patient lose his drive to breathe?

In my professional experience, I have seen maybe two patients who became lethargic when the oxygen was turned up, but I have also seen many patients in this situation be placed on a 60% or greater FiO2s and never have his drive to breathe suppressed.

The point of this post is this: Is the hypoxic drive theory a hoax?

As I reported in a previous post, I accidentally mentioned that I am not necessarily a fan of the hypoxic drive theory to an RT student. I did not go into detail as I caught myself. However, the following week she told me she "in passing" mentioned this to her teacher, who said, "What are they filling your head with?"

Yes, it is true that we have to be careful what we tell our RT students, but at the same time we want them to be aware that what they learn in school is not exactly the same as what occurs in real life here at the hospital. And, the truth to the matter is, the hypoxic drive theory is just a THEORY.

In the next few weeks I will review briefly why people breathe, and then I will share some information I have that challenges the hypoxic drive theory. My goal here is not to convince you that the hypoxic drive theory is a hoax, but to inform you of other theories out there that might more accurately describe your patient's condition.

Stay tuned.

WARNING: As I stated earlier, the hypoxic drive theory is the gold standard theory of respiratory care. Most doctors live by it. If a doctor says lower the oxygen level on a patient because he is a CO2 retainer, it is your job to do what he says, even though you have proof that a) the patient is not a retainer, and b) the patient is a retainer and the oxygen is not harming the paitent. You may try to convince him otherwise, and he may also grumble and gripe.

WARNING #2: Since the hypoxic drive theory is the gold standard of respiratory care, it is strongly defended by RT teachers. And therefore, for you RT students out there reading this, know that any material opposing the theory will more than likely not be on your RT exam.

NOTE: Despite what I wrote in warning #2, I am now aware that some RT Programs no longer teach the hypoxic drive theory.

(To view part 2, click here.)

Post updated 10/24/2010)

1 comment:

Rick Goulet said...

Smashing article. In 27 years of paramedic practice I have never managed to put someone in Hypoxic drive. I tend too agree with you that it is a well accpted theory that has been put into practice;

Rick Goulet