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Henry's Law - Fixing Hypoxia?

 

 

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Understanding how to positively make changes with our patients suffering from refractory hypoxia is essential in the prehospital and emergency medicine realm. From the start of our clinical training, we were taught to apply oxygen to all of our patients. As we’ve progressed through the education process and broadened our careers, we’ve learned that oxygenation isn’t as simple as just applying a non-rebreather mask.

Many aspects have to be taken into consideration when making decisions on how to treat our patients suffering from hypoxia. We know the single most important player in oxygen delivery is our hemoglobin (we can discuss this more on another post). But what if we don’t have a way to change the Hgb concentration? How can we fix or improve these patients' refractory hypoxia and overall clinical course?

One of the concepts taught during course work related to HEMS Critical Care, are gas laws and how those gas laws affect our patients during transport. Henry’s Law has always been taught relating to decompression sickness and the nitrogen release seen when surfacing too quickly. However, how many patients have you treated or seen with this presentation? I haven’t seen any and have done this a long time. Obviously there’s a group of providers that live in areas where diving is prominent and they may see this. But how can we apply Henry’s Law in a manner that is useful to our every day practice? Yes, everyday!

Henry’s law is defined as, "the amount of gas dissolved is directly proportional to the pressure of the gas over the solution”. So imagine the picture above. Prior to opening the Heineken bottle, there’s pressure being exerted against the beer, holding the gases inside the beer. As we open the cap, the pressure is released and the carbon dioxide is released. This is the standard explanation used to show how Henry’s law plays into nitrogen release as it relates to decompression sickness. But what if I told you to look at this in the opposite way.

When we have patients that are refractory hypoxic they obviously need something that oxygen alone isn’t providing. They're suffering from some type of diffusion gradient shunt. Henry’s law describes the answer. We can apply 3 simple rules to combat the hypoxia and diffusion shunt that’s taking place.

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  1. Increase the concentration: This is simple. Apply 100% FiO2 via any source possible. If you have the patient intubated, place them on the ventilator and increase FiO2 to 100%. This will drive the PO2 up, and cause nitrogen washout from the alveolar sacs. Don't we do this prior to RSI?
  2. Increase the surface area: Imagine your alveolar sacs as small balloons. If those balloons are too thick, have mucus or fluid inside them, they can’t be utilized for gas exchange effectively and you’ll have poor diffusion. But imagine if you simply added more surface area to those alveoli. Imagine again blowing up a balloon. As the walls of that balloon get thinner, you can see through the balloon walls easier. Thus, the amount of space for diffusion gets bigger. We recruit and fill our alveoli via our tidal volume breath. So by giving good physiologic tidal volumes that recruit alveoli, and then by adding PEEP, that will maintain alveolar recruitment, you’ll immediately aid in promoting better diffusion and add more surface area. If using a BVM, add a PEEP valve and apply PEEP that way.
  3. Add pressure: By adding pressure you now complete the definition of Henry’s law. Now the amount of gas (Oxygen and CO2) that's dissolved and diffused through the alveolar/capillary membrane is optimized. By adding pressure via the ventilator, and using volume or pressure delivered breaths, or via the BVM using good full compression of the BVM, you’ll push the gas through the alveolar membrane and into the solution (blood and plasma), thus improving the original diffusion gradient shunt.

 

 

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