I’ve been lecturing in the greater Houston area the past year for the Southeast Texas Regional Advisory Council’s Cardiac and Respiratory Symposium. One of my favorite slides in my airway management presentation is the oxygen-hemoglobin dissociation curve. The sigmoid-shaped curve is likely to be in every EMS textbook. It’s likely to be taught in every paramedic classroom and is no doubt recognizable, to some extent, by every paramedic on the street.
Despite the fact folks have seen it and learned about it at some stage, it’s not uniformly as familiar as it should be to our EMS providers. This is one of those tidbits of knowledge that’s dispersed as a part of the curriculum–there may be a question or two on an exam about why it’s S-shaped, with some answer about the affinity of oxygen to hemoglobin. But although the facts are imparted on the student, I’m constantly reminded that it’s useless to know the facts if you don’t also know how to use them.
UNDERSTANDING WHY
Over and over again throughout the past several years, I’ve reviewed EMS records at one agency or another regarding desaturations during intubation attempts. During case reviews, I ask the question about how the desaturation occurred and why there wasn’t an intervention to prevent it. Sometimes the answer is “I couldn’t see the monitor,” which is followed by me blowing a pupil, and is followed by an analogy about flying a plane at night with no altimeter.
The other common answer I get in many of these cases is: “I don’t know what happened “¦ one second they were at 92% and the next time I looked up they were at 60%!” I then reintroduce them to the curve.
So why is it that we make this mistake over and over again in clinical practice? The answer is likely in relation to the “availability heuristic.” To understand this, we have to appreciate what nearly all providers experience with the oxygen-hemoglobin dissociation curve. Usually, their experience is an oxygen saturation from 93—99%. In this range, desaturation occurs in a linear fashion–100, 99, 98, 97, 96, 95. So while they may have learned at some stage about the “s” shape, the “available” experience tells them the desaturation will continue to be linear. Of course, in reality, the curve changes dramatically at the 90—92% saturation range, and then it falls off the cliff. As I explain to the crews and to my lecture students, this is human physiology and will always be human physiology. It’s not going to change!
It’s useless to know the facts if you don’t also know how to use them.
UNDOING CONVENTIONS
We, as medical directors and EMS physicians, also have to work to undo the silly conventions we grew up with as paramedics. One great example is the duration of an intubation attempt.
When I ask “How long should your intubation attempt be?” in my lecture series, the answer is always “30 seconds” or “as long as you can hold your breath.” I then ask them how many times they saw an anesthesiologist or emergency medicine physician take a big gulp of air prior to intubation attempts. Trust me when I say that if my emergency medicine interns only had 30 seconds to attempt intubation, the first-pass success rate would be shocking.
We have to appreciate that this convention was developed in a time before pulse oximetry. In that day, we assumed everyone was hypoxic and gave them oxygen for whatever their complaint was because we couldn’t measure oxygenation. Now that we can monitor these things, we shouldn’t be compelled to follow these silly guidelines any longer. Intubation attempts should be limited by hypoxia and not by some artificial time limit.
Once they’re able to “walk among the facts” of the oxygen-hemoglobin dissociation curve, they’re able to understand preoxygenation and passive oxygenation use as adjuncts to buy more time for an intubation attempt, which may allow them minutes instead of seconds to intubate. It allows them to “pull up” their plane before they crash by understanding where the curve changes so they can become anticipatory clinicians instead of reactive.
CONCLUSION
The EMS physician must be engaged in their EMS service in order to make sure providers are practicing practical and evidence-based medicine and utilizing current technology in their critical decision-making. We have to make sure critical knowledge such as the oxygen-hemoglobin dissociation curve isn’t just recognizable on an exam, but that our providers understand how to apply that knowledge in clinical practice.
