Oxygen Associated Hypercapnia

[chbare]

I received a PM some time ago asking me to provide evidence regarding statements I made about the "hypoxic" drive scarecrow. I am sorry that I have taken so long to look at my PM box, life has been crazy with my wife taking a job out of country and my new full time teaching job and part time EMS associated moonlighting. No excuses however.

If you are willing to put down some money, I highly recommend a respiratory textbook known as Egan's Fundamentals of Respiratory Care ninth edition or above. I will warn you that it does not hold hands and it does not pull punches, therefore it can be a difficult read if you are not at least familiar with physiology and chemistry. The acid-base balance chapter is rather liberal with basic equations.

There is actually a dedicated chapter on the regulation of breathing and the physiology of Oxygen associated hypercapnia is discussed. Conventionally, people say that the hypoxic drive causes the PaCO2 to increase in COPD patients; however, newer thoughts paint a different story. While the hypoxic drive theory is not totally dead, many issues must be appreciated such as the alleviation of hypoxic pulmonary vasoconstriction and changes in V/Q matching among other thoughts.

[BEorP]

I think that was from me! I'll need to try to get my hands on Egan's. The reason for my question was that I've read many various things from people like yourself who are obviously experts in this that make reference to the hypoxic drive theory largely being a myth. I've also read a bit about the importance of alleviation of hypoxic vasoconstriction as you mention here as well. What I haven't come across though is a nice review article or something similar that in a single document from an irrefutable source that outlines specifically what is wrong with the theory as it is traditionally taught. I was hoping that there was something that in a single PDF would pick apart the traditional hypoxic drive theory, but I guess I might be asking for something that does not exist. Maybe the chapter in Egan's is the closest thing to that that I will find! Thanks for the info!

[eb1040]

I think this summary by a couple of physicians and their references might be of some help to you.

http://cmbi.bjmu.edu...hypercapnia.htm

It also gives an explanation as to why NIV is useful which you can also find numerous studies from the manufacturers of these devices who have described how NIV offsets the deadspace and even when used as high FiO2, minute volume is restored to normal unless there is profound fatique or other disease processes which require intubation.

The summary also explains why a low flow device like a simple mask is not the best device and how a venturi mask which is a high flow device is better. A nonrebreather is also a low flow device which can not always meet ventilatory demand and can increase work of breathing regardless of what has been taught about the bag. Once the proper definition of low flow and high flow devices is understood, it is relatively easy to see the importance of the proper delivery device and not just the FiO2 being given. Textbook liter flows also do not ensure the FiO2 for low flow devices. Those quoted are for what is considered a normal individual with normal lungs breathing a normal nondistressed tidal volume and minute volume.

When looking at a book like Eagan or any source, it is a great idea to look at the references to see where they pulled their data from. You might find a study or article which provides the explanations you are seeking.

Also, when considering the SpO2, you must look at all the factors which will shift the Oxyhemoglobin curve. 92% does not give the same PaO2 in different situations especially at that slope of the curve where the patient's PaO2 can easily fall into a critical low. Carrying capacity must also be considered and the factors that inhibit the transport of oxygen to the tissues. Unless you have a good control over cardiac output, perfusion and oxygen uptake, just focusing on oxygen by itself could be a potentially fatal mistake.

Edited September 15, 2012 by eb1040