Short Attention Span Summary
Treating CO poisoning
Hyperbaric oxygen therapy (HBOT) for carbon monoxide (CO) poisoning decreased mortality compared to patients who did not receive it, according to Taiwanese poison center data. Overall NNT = 10 (mortality rate with HBOT 19.62%; mortality rate without HBOT 29.64%). And there was a dose-response; two HBOT sessions reduced mortality more than one. Results were adjusted for “age, sex, underlying comorbidities, monthly income, and concomitant conditions.” Improved mortality was most pronounced in those under 20 years and those with respiratory failure. I believe these results. If anything, sicker patients would have been selected for HBOT, making the results either tend toward no effect or worse outcome with HBOT. For your boards, half-life of CO on hemoglobin is 320 minutes in room air, 80 minutes on 100% NRB, and 20 minutes with HBOT. Divide 320 by 4 twice.
Spoon Feed
Hyperbaric oxygen therapy for carbon monoxide poisoning decreased mortality, NNT = 10.
Source
Hyperbaric oxygen therapy is associated with lower short- and long-term mortality in patients with carbon monoxide poisoning. Chest. 2017 Apr 17. pii: S0012-3692(17)30723-7. doi: 10.1016/j.chest.2017.03.049. [Epub ahead of print]
Clay Smith, MD, Twitter
Peer Reviewer Comments
This isn’t my favorite article in large part because I feel like we know almost nothing about how to really treat CO poisoning, and I don’t know if this article moves us toward clarity other than saying we need a good RCT (which we already knew). My other take-away from this study is that maybe much of the research is focused on the wrong outcome. Generally, studies have measured delayed neurologic sequelae (DNS), probably because it’s easy to measure and relatively common in the the first month. However, the mortality signal which occurs primarily in the first month may suggest that there are some other inflammatory processes caused by HbCO (via new data on reactive oxygen species and ion channel issues) that increase risk of death. A 2015 review in Toxicology by Roderique is very interesting. It discusses that the problem with CO poisoning may have less to do with HbCO and hypoxic injury and more to do with downstream cellular effects. Maybe we should be trying other therapies like methylene blue, hydroxycobalamin, or insulin/calcium based on a cellular mechanistic understanding.
Here are my issues with this study. They don’t describe how patients were selected for HBOT. They don’t use much data to describe how sick they were acutely, other than acute hepatitis, renal, myocardial and respiratory failure–all of which occurred in a very low percentage of patients. And they never describe patients’ presenting HbCO levels (although this may not actually suggest severity of illness). Finally, they don’t describe the cause of death, which would have been insightful.
The strength of the study is its size. It shows a mortality benefit and argues that maybe the null effect of preventing DNS is due to the decreased mortality and subsequent increase in DNS.
At the end of the day:
Don’t give up hope on hyperbaric oxygen. We need a RCT to figure this out.
Thomas Davis, MD, Twitter
"If anything, sicker patients would have been selected for HBOT, making the results either tend toward no effect or worse outcome with HBOT." That is a pretty big assumption to make, especially when given the reported comorbidities of "hypertension, diabetes, hyperlipidemia, malignancy, stroke, dementia, coronary artery disease, congestive heart failure, COPD, liver disease, kidney disease, and alcoholism were less prevalent in the patients who received
HBOT." Also the HBOT group was significantly younger and age <36 is one of the factors reported to worsen cognitive outcome after COP. Just by random chance you wouldn’t expect a higher prevalence of the large majority of the comorbidities looked for to be in the non-HBOT group. It is a big ask to statistically adjust for these very different groups.
No question this is low quality evidence and very limited as a retrospective study. And you’re right, there is no way to eliminate inherent bias regarding which patients got the treatment. But it was interesting that more patients with respiratory failure and AKI got HBO yet still had better mortality overall. One would think that would have made the results tend toward the null. Maybe I was a little too optimistic in my initial summary.