Understanding the Oxyhemoglobin Dissociation Curve; An essential concept for Critical Care providers.
DECIDING BETWEEN CPAP OR BIPAP...
YOUR PATIENT IS IN RESPIRATORY DISTRESS, BUT YOU DON'T THINK THEY NEED TO BE INTUBATED. YOU'VE DECIDED TO GIVE NON-INVASIVE VENTILATION A TRY, BUT SHOULD YOU USE CPAP OR BIPAP?
THIS IS A SUPER EASY DECISION TO MAKE?
Setting up BiPAP - LTV1200
It's all about Oxygenation vs. Ventilation. Low O2 = CPAP, high CO2 = BiPAP, Both High CO2 and Low O2 =BiPap
Remember what improved Oxygenation..., PEEP and Higher FiO2. That's It. CPAP is essentially PEEP, but better.
PEEP, or Positive End Expiratory Pressure, is a degree of pressure maintained in the alveoli only at the very end of expirations. The rest of the time the pressure is normal physiologic airway pressure. This is OK for someone without lung disease, but for someone with lung disease this may be insufficient.
CPAP is continuous Positive Airway Pressure. You set the pressure you want, or that the patient needs, and they get that pressure throughout both the inspiration and expiration phase. So essentially their getting PEEP the whole time their breathing, not just at the end the respiratory cycle.
Now for BiPAP...
Remember how we improve Ventilation, or the blowing off of CO2? Remember as a kid when you'd stand in the front yard and breath as fast as you could, and as deeply as you could, until you fell over and nearly passed out? NO? Never mind...
We improved Ventilation by increasing the Minute Ventilation.
Let's recall the formula for Minute Ventilation...
Respiratory Rate x Tidal Volume.
So if someone isn't intubated, how can we manipulate either of these values?
Well, BiPAP, or Bi-level Positive Airway Pressure, delivers one pressure during the exhalation phase (Called EPAP or Exhalation Positive Airway Pressure), and a higher pressure during the inhalation phase (Called IPAP or Inhalation Positive Airway Pressure).
This higher pressure during inhalation makes it easier to take a deep breath, in essence forcing more air into the lungs than one would normally take.
Further more, this makes it easier for someone to take a breath in, and thus will allow someone who's trying to breath fast to do so with less effort.
More volume and easier to breath fast = increased Minute Ventilation!!!
"But what about those patients who are both hypoxic and hypercarbic?"
In the case where someone needs both better oxygenation and improved ventilation, use BiPAP. You'll be able to adjust your IPAP to help titrate you ventilation and your EPAP to improve oxygenation. You get the best of both worlds.
The Flight Service I work for uses the LTV 1200 ventilator.
Here's a video demonstrating how to set up the LTV 1200 for BiPAP.
I hope this helps explain when to use CPAP and when to use BiPAP.
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Fly Safe, and Live Well