Before I release my vodcast about some of the non-invasive experiments I've been working on, I wanted to do a back-to-basics podcast about positive pressure ventilation. In this quick vodcast we will cover why we give extra pressure behind our oxygen, a little about V/Q mismatch, and how titrating different pressures will yield different results!
I'll attach some of the pictures from the Vodcast so that you can see them in still motion without having to pause the video.
Why give positive pressure in the first place? Positive pressure can reverse the ventilation side of a ventilation/perfusion (V/Q) mismatch. By opening up alveoli that were collapsed (atelectasis) we can use more of the lung surface area to oxygenate the blood - more lung takes part in gas exchange.
Here are the lung volumes from the vodcast. We can influence these in several ways, depending on how much PEEP and PSV (pressure support ventilation) we give. We can also influence the ERV (expiratory reserve volume) and therefore the FRC (functional residual capacity) by how we choose to position our patient.
PEEP will increase our ERV (expiratory reserve volume). This can aid in fixing a V/Q mismatch on the ventilation side, as I mentioned before. You can see how the Vt is moved up the graph because it is working off of a higher baseline pressure. This will help with oxygenation.
Bi-level positive airway pressure adds an additional support pressure to the inhalation portion of the respiratory cycle. This is usually represented by two numbers, such as '10 over 5' - which means that when they inhale, they get 10, and when they exhale (and the rest of the time) there is 5 of pressure in the airway.
Different ventilators will achieve this in different ways. Your ventilator will either be ADDITIVE, and ABSOLUTE.
ADDITIVE: This is where you ventilator will have a baseline PEEP, and then ask you to place a PSV (pressure support ventilation) on top of the PEEP. For example, the picture below shows 5 of peep, and then 5 of PSV. This makes 10/5.
ABSOLUTE: Absolute ventilators will simply ask you to dial in the pressures as stated. This means that when someone says: 'Hey let's put this patient on 10 over 5' you would dial your IPAP (inspiratory positive airway pressure) to 10, and your EPAP (expiratory positive airway pressure) to 5.
Whatever type of ventilator you have, you can double check what it is by looking at your PIP (peak inspiratory pressure). Make sure your PIP matches whatever you THINK your inspiratory support should be.
IPAP will not only increase the size of your Vt, but it will reduce the work of breathing for the patient as well.
IPAP will only assist during inhalatory effort. This means that the pressure you have set to assist the breath in will start when the ventilator is triggered, and continue until the patient begins to exhale. (the gun is my way of showing when the patient triggered)
Titrating your PSV (the difference created between PEEP and PIP) will determine how much ventilation your will get from each breath. When explaining this I always tell people to think about a BVM. If you have a PEEP valve on a BVM and you set it to 5, that would be like your EPAP. Then, imagine you used a pressure manometer to monitor how hard you squeezed the bag - and squeezed it until it hit 10, or 15. Would you get a bigger Vt if you squeezed to 10, or 15? Obviously 15. Therefore, when you want a bigger Vt, create a bigger difference between your IPAP and your EPAP.
This difference is called your Delta, or your Delta-P (delta pressure). Some people also call it a driving pressure (since it's the difference between two pressures). Increase your Delta (the difference between the two pressures) to maximize ventilation.
Increasing the EPAP - or baseline pressure - will increase your oxygenation. This is just like the FiO2 and PEEP scale we use with the ARDSNET settings. There is a reason ARDSNET recommends that you increase PEEP to help with oxygenation - you will likely overcome atelectasis (open the lungs up) and get more gas exchange. Not to mention the oxygen is also under pressure now, being driven in.
Both PEEP and PSV will drive your lung volumes up towards the max. The PEEP will increase your ERV, and the PSV / IPAP will increase the size of the
Now check out the Vodcast!
Also check out this page:
This was a Podcast Tyler did with Josh Dillman - great stuff about NPPV! There is also a link to a video of me doing a little more explaining on additive BIPAP on that page. Enjoy!