Oxygen induced hypercapnea in COPD: morning report MythBusters

Incredible report this morning from Christina Wang with knowledge flying around in all directions. Really impressive display.

One cool talking point was about whether oxygen given to patients with COPD can worsen hypercarbia. There does exist some data to support maintaining an O2 sat between 88-92% in patients w/ COPD as opposed to higher O2 levels. Doing so causes less hypercarbia and improved outcomes.

Shravan shared a great article on the subject and I’ll highlight the main thrust:

Fact:

1: Giving patients with COPD oxygen supplementation does increase the pCO2. In one study, increasing the PaO2 from 4.9 kPa to 29 kPa increased to PCO2 from 8.4 to 11.4 kPa. This effect may be exacerbated in patients with the worst hypoxemia at baseline.

Myth:

1: The hypercarbia seen with O2 administration is due to decreased respiratory drive and drop in minute ventilation. ONLY PARTIALLY TRUE! Below is a graph looking at what happens when oxygen is given to patients with COPD. Note that minute ventilation drops initially (red line), but then improves almost to the baseline between 5-10 minutes later. PCO2 keeps rising however! What is causing this?!?!

o2 administration COPD

The role of VQ mismatch in precipitating hypercarbia in COPD w/ O2 administration!

Remember that the lung vasculature acts opposite to the rest of the body and you have arterial vasoconstriction with hypoxia as opposed to vasodilatation elsewhere. Data from several studies support a hypothesis that the worsening hypercarbia with supplemental O2 in COPD patients is due to V/Q mismatch mechanisms!  (as an aside, V/Q mismatch always seems to be the right answer in ICU report)

In the alveoli of patients w/ COPD, alveoli that have impaired ability to partake in effective ventilation, oxygen therapy leads to vasodilatation. This results in V/Q mismatch, higher proportion of dead-space, and an increase in pCO2!

Here’s the link to the article:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3682248/pdf/cc11475.pdf

 

 

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