Wednesday, August 22, 2012

Are all COPD patients CO2 retainers?

Your question:  I have observed that many doctors treat all COPD patients as though they were susceptible to oxygen induced carbon dioxide retention.  I think this prevents many patients from getting the oxygen they need.  Do you have any idea what percent of COPD patients are actually at risk for oxygen induced hypercarbia?  Do you know of any articles I can show our doctors?

My humble answer:  Great question.  I think a better method of tackling this question is to turn it on the doctor.  Ask him where his proof is.  He'll site that it's common practice among the medical community, yet this is not proof.   It's easy for something to become common practice just because it sounds good.  Yet "sounds good" is not proof.

Now I wouldn't recommend doing this to just any doctor, lest you'll get on his bad side.  Yet a wise doctor would enjoy engagement in such a discussion.  I will also post some links to other articles I've written about the hypoxic drive theory you can read.  In the first two links you will find several outside links to studies and articles on the subject.

In my post "A history of the myth" I write the following:

Dr. EJM Campbell gave a lecture to pulmonologists in 1960 about the dangers of giving too much oxygen to COPD patients.  This essentially gave birth to the hypoxic drive theory.  Of interest here is that the study Campbell referred to in his lecture consisted of only four COPD patients.  Because the hypoxic drive theory provided the first best example of why a COPD patient might stop breathing in the presence of oxygen, the theory has been taught from one generation to the next.  Despite scientific evidence, it has become the gold standard when treating COPD patients.

Yet this in no way proves the hypoxic drive theory nor that all COPD patients are CO2 retainers susceptible to oxygen induced hypercarbia.  It should also be noted that this study was performed before ABG testing was the norm, before oxygen saturations were monitored, and before any adequate studies scientific studies were ever performed on the subject. 

The truth is, there has never been any study to prove the hypoxic drive.

So what percentage of COPD patients are CO2 retainer.  Will Beachy wrote a good editorial on this subject, "Breathing Control in Chronic Hypercapnia," RT: for decision makers in respiratory care, June/ July, 2000.  He writes:

" The ubiquitous diagnosis of COPD does not automatically imply the presence of chronic hypercapnia or the potential for oxygen-induced carbon dioxide retention. Health care personnel often uncritically assume that patients diagnosed with COPD are 'carbon dioxide retainers,' and susceptible to 'O2-induced hypoventilation,' when in fact, this is true of only a small percentage of end-stage COPD patients. Therefore, concern about O2-induced hypercapnia is not warranted in most patients with a diagnosis of COPD.

"Before we can rationally evaluate the merits of a debate about the control of breathing in chronic hypercapnia, we must understand the chemical control of ventilation in normal, healthy humans. At issue is not the existence of a hypoxic drive in hypercapnic COPD patients, but whether oxygen administration suppresses this drive sufficiently to account for the commonly observed rise in PaCO2. An additional pertinent question is whether an acute rise in PaCO2 still stimulates the medullary chemoreceptors in these patients."

He then concludes his article by writing:

"The important clinical fact remains that regardless of the mechanisms involved, oxygen therapy in chronically hypercapnic COPD patients can induce further hypercapnia and acidemia. However, tissue oxygenation is of overriding importance; oxygen must never be withheld from an exacerbated, hypoxemic COPD patient. This means one must be prepared to mechanically support ventilation if necessary."

So, I don't think you can word it much better than Beachy does.  There are very few COPD patients who are truly retainers susceptible to oxygen induced hypercapnea, although very few is as close to an accurate number we can get.  I like to say that 10 percent of COPD patients are retainers, and 10 percent of retainers are susceptible to oxygen induced hypercapnea. 

Campbell, American Reveiw of Respiratory Disorders, 1967 (96, 126) noted that 90 percent of patients in severe respiratory distress were at risk for worsening CO2 retention and decreased level of consciousness.  However, his study included only four patients.

However, it was his study and his review that resulted in the hypoxic drive theory become so popular, and even despite several studies proving Campbell wrong, it was almost too late to turn the tide.  Despite facts, many doctors still believe the report of Campbell.

Maloney, Kiely, and McNicholas, "Controlled oxygen therapy and carbon dioxide retention during exacerbations of chronic obstructive pulmonary disease," Lancet, 2001, reported on a study of 24 patients with severe airflow obstruction who received 24-40% oxygen by venturi mask, only three had a significant rise in PaCO2 (8-26 mmHg). 

In the study the patients with the greatest degree of retention were more severely hypercapneic.  When all was said and done, only 13 percent of the patients suffered from oxygen induced hypercapnea.  This falls right in line with the numbers I estimated earlier. 

Further reading:

1.  Articles that disprove the hypoxic drive theory (coming soon)
2.  The history of the hypoxic drive
3.  The hypoxic drive theory: why do we breathe
4.  Siobal, Mark, BS RRT, "Hypoxic Drive in COPD: Is the fear of oxygen therapy based on fact or myth?" UCSF Sanfransisco General Hospital, slide show
 (12-17-2012 my hypoxic drive post will be published)

2 comments:

Anonymous said...

"I like to say that 10 percent of COPD patients are retainers"

I like to say? Seriously?

Rick Frea said...

Agreed. And just because someone is a retainer does not mean they should be kept hypoxic.