|And you always leave a note. (Hurry up Netflix!)|
Are we killing patients?
A recent study suggests that the old-time myth was true. In fact, if this study is corroborated, we may need to change our practice somewhat. The one-sentence summary of Effect of high flow oxygen on mortality in chronic obstructive pulmonary disease patients in prehospital setting: randomised controlled trial is:
That's a really high number.For high flow oxygen treatment in patients with confirmed chronic obstructive pulmonary disease in the prehospital setting, the number needed to harm was 14; that is, for every 14 patients who are given high flow oxygen, one will die.
By comparison, when we rush a STEMI patient to the cath lab, or give then thrombolytics, we have to treat 50 patients to save one life. This study suggests that we can save far more lives by using a nasal cannula instead of a face mask. Can this be true?
Let's first take a look at some older evidence, giving us some context. In the following post we'll go over the new article that has generated so much interest, and see if some practical conclusions can be drawn.
Two ICU studies
The authors of Influence of inspired oxygen concentration on deadspace, respiratory drive, and PaCO2 in intubated patients with chronic obstructive pulmonary disease wanted to study this (possible) myth of the hypoxic drive. They studied the effect of increasing oxygen delivery to a very sick bunch of COPD-ers, 12 patients who had already been intubated after a bad COPD episode. These patients were just starting to recover, being weaned from the vent, able to breathe or their own, and were likely to be extubated in the next day or so.
The researchers bumped up the oxygen level to 70% for 20 minutes, and checked what happened with the vent and the blood gas. Although 70% doesn't seem high, it is actually right about what we are delivering with a standard non-rebreather mask at 15 lpm (See Weingart's article for explanation; PDF if you prefer).
So what happened? Apnea? Bradypnea? Failure of the hypoxic drive????
They, uh, got more oxygen. That's it. In this population of sick-sick-sick patients, nothing happened.
Okay, you say, perhaps they were a delicate population, but they had been getting beta-agonists and steroids, and probably antibiotics for a few days. That's the whole reason, after all, that they were being weaned off the vent - they were now better.
Maybe we should instead look at patients who are not yet intubated, but might be if just one more thing tips them over.
|"Our study included one patient - this guy"|
The investigators titrated oxygen up with Venturi masks to two different levels, either a PaO2 of > 50 mm Hg in 17 patients, and > 70 mm Hg in another 17 patients. They got all all the usual meds, of course. As expected, some of these very sick patients had to be intubated, and some even died. The rates of intubation and death, however, were the same in each group. And no CO2 retention either!
So why all the fuss? As an editorial in Critical Care Medicine pointed out, the main evidence for the harm of oxygen in COPD exacerbations comes from the 70s, a time before noninvasive ventilation, routine steroid use for COPD, and even Atrovent was barely a year old! Another editorial from the same journal lays into the medical-education complex for perpetuating this lore:
"One sample of medical mythology is the commonly told story that the administration of oxygen to a patient with chronic obstructive lung disease will shut down the patient's hypoxic respiratory drive and lead to apnea, cardiorespiratory arrest, and the subsequent death of the patient. ... It is not clear where this fallacious information comes from, but it seems to enter the medical information database at an early age, at the medical student or resident level, almost like a computer virus corrupting the appropriate function of the equipment. In addition, this myth becomes very difficult to extinguish during the career of the physician, even with clear factual information of long standing. The danger here is that this medical mythology will inappropriately influence treatment decisions in patients."The Bottom Line
This is hardly a comprehensive review of all the literature out there on the topic, but most of the other studies are observational, and it's really hard to draw firm conclusions from that sort of data. I haven't bothered to review them here, because why waste our tine if higher-quality studies have been done?
With this background, I'll discuss the important EMS study from 2010 in the next post.