Pulmonary Gas Mileage - The P:F Ratio


Mileage:Gas Ratio

Every week on Monday you put a full tank of gas into your car. Usually, a full tank of gas lasts you all week (you drive about 450 miles a week).


Then, you start to notice that full tank isn’t lasting you as long as it used to. You’re not able to make it all week without stopping at the gas station. Eventually, things get so bad that you’re getting less than 100 miles on your full tank. What gives? Your mileage to gas ratio (M:G Ratio) is less than a quarter of what it used to be. Clearly, something is wrong with your car, the way you’re driving, or both.


Just like you expect to get a certain number of miles out of your tank of gas, you should expect to get a certain PO2 from the amount of oxygen that you inhale - there’s a ratio for this too.


Pulmonary Gas Mileage

Our atmospheric air contains 21% oxygen (0.21). This is like the gas that we’re putting in the tank. How far do we expect that 21% oxygen to take our PO2? We expect our oxygen to take our PO2 about 4-5 times it’s own value (Cairo, 2014). A few examples:

  • We breathe 21% oxygen, and our PO2 should normally be between 80 and 100 (that’s ~4-5 times 21).

  • If we have someone on 50% oxygen, we should see their PO2 rise to ~200-250 (that’s 4-5 times 50).

  • Finally, if we have someone on 100% oxygen, we should see their PO2 rise to ~400-500 (that’s 4-5 times 100).

While this 4-5 times increase is useful in estimating what an ideal patients PO2 should look like, we actually use a different measurement to look at the effectiveness of oxygen therapy - the P:F Ratio.


The P:F ratio is exactly what it sounds like - a ratio of PO2 to FiO2. We simply divide whatever the current PO2 is by the current FiO2 (as a decimal) to get our ratio. For example:

  • 90 (PO2) / 0.21 (FiO2) = 428 (P:F Ratio)

  • 214 (PO2) / 0.50 (FiO2) = 428 (P:F Ratio)

  • 428 (PO2 / 1.0 (FiO2) = 428 (P:F Ratio)

There’s our normal PO2, and normal FiO2 that we breathe from the atmosphere, as well as what we would expect from 50 and 100% FiO2. However, everything we’ve talked about so far is physiologic - nothing has been wrong. Remember our car that kept getting worse and worse gas mileage? We were putting in the same amount of gas, but it wasn’t bringing us very far. And the worse the gas mileage got, the more we knew something was wrong with the car. With the body, we can estimate how bad things are by looking at this P:F Ratio - the less PO2 we get for a given FiO2, the sicker the patient is.


BTW... altitude can screw with PF ratios.

Sometimes you'll run into a test question that asks:

What is the percent of oxygen at the top of Mount Everest? (or any other random high altitude place.)

.

..

...

Answer: 21%.


Seems weird, right? The best way to explain why oxygen is still 21% anywhere you go in our atmosphere is that while the percent is the same, but the partial pressure isn't. As altitude increases, pressure decreases. All the molecules move apart from each other, but in equal proportions.


You can see from the picture above that while the oxygen may be in the same percent, it's not as concentrated. 21 is 21% of 100, but 2.1 is also 21% of 10. Being at altitude is kind of like adding water to our gas tank - oxygen and gas just are not as effective when they're diluted.


Harowitz Index for Lung Function (The P:F Ratio)

Now that we know the relationship between FiO2 and PO2, what a P:F ratio is, and how to calculate one, we should look at ranges to determine general levels of badness. While the Harowitz index does include on it the "severity of ARDS" the Berlin Definition (coming next) is now used for diagnosis. However, the P:F Ratio ranges remain the same.



Berlin Criteria

There is another method for determining ARDS that is more specific to the patient receiving some type of positive pressure therapy - the Berlin Criteria for ARDS.


This criteria still uses a P:F ratio to determine the severity of ARDS, but it also assumes the patient is getting 5 or more cmH2O of PEEP / CPAP as well. There are also some initial questions that need to be answered prior calculating a score.


It’s important not to take these calculations out of context. Just because your patient isn’t oxygenating doesn’t mean they automatically have ARDS. Sure, their P:F Ratio might suck, but what do you actually suspect is wrong with them?

  • Is this a CHF patient with massive amounts of pulmonary edema?

  • Is this a COPD and/or asthma patient who is in the middle of an acute exacerbation?

  • Is there a pulmonary embolism?

  • Is there a far right shift on the oxyhemoglobin desaturation curve?

  • Has the patient aspirated?

Sure, some of those conditions might lead to ARDS, but a poor P:F ratio by itself does not diagnose ARDS - which is one reason why the Berlin definitions include introductory questions for the clinician (JAMA, 2012).

What about SPO2 to FiO2 ratio?

Yeah, that's a thing too. We will have to do a separate blog on that. There are certain ranges where using your SPO2 and estimated FiO2 might gave you a quick, rough idea of where you're at as far as a pseudo P:F ratio. Here is a decent paper that digs into the correlation:


Adams JY, Rogers AJ, Schuler A, et al. Association between peripheral blood oxygen saturation (SpO2)/fraction of inspired oxygen (FiO2) ratio time at risk and hospital mortality in mechanically ventilated patients. Perm J 2020;24:19.113. DOI: https://doi.org/10.7812/TPP/19.113


What now?

You’ve concluded that your patient has a horrible P:F ratio, but now you’re wondering what to even do with this information. Everyone is going to have a different model for how to deal with a poor P:F ratio, but all of them should include increasing oxygenation by whatever means you can while still protecting the lungs. If you have already obtained a P:F Ratio, your patient is probably already on some sort of non-invasive ventilation like CPAP or BiPAP, or is already intubated. However, as I move through what my mental model looks like (yours might be different) there is value in starting from the ground up and assuming we're dealing with a fresh patient. Then, we can talk specifically about the sick intubated patient at the end. Here is how my brain works when it comes to the graduation of treatments.


Positioning always comes first.

Positioning is so often overlooked. Placing the slumped over, crunched up, side bent patient on oxygen is like trying to read a balled up piece of paper by shining a flashlight on it. It makes a lot more sense to unfold the paper. Sitting a patient up, letting their diaphragm have room to move, getting their spine straight, getting their guts out of their chest (increasing FRC), and letting fluid settle to the bases are all great ways to start oxygenating the patient more effectively.


In the ICU setting, placing a patient prone may also be an option. It's also worth mentioning that if the patient has an issue with the left lung, positioning the right lung slightly down might also increase oxygenation (and vice versa). Why? Gravity pulls blood to the lower lung. If the good lung is closer to the floor, it will get more perfusion.


Then... Flow, Flow and more Flow

Follow your local guidelines, but I'm going to ignore the upper flow limits on the nasal cannula and non-pressurized masks. A nasal cannula maxing out at 6 Lpm? No... maybe closer to 60 Lpm while I figure out what to do next (If you think their head will explode try it on yourself). 15 Lpm on a non-rebreather or Oxy-mask? Negative. I might go up to flush if I don’t think I’m matching their inspiratory flow demands (Driver et al, 2017). This is all especially true when pre-oxygenating prior to intubation - we should also be utilizing nasal cannulas at higher flow rates during intubation for apneic oxygenation (Gleason et al, 2018). Flow limits for nasal cannulas are meant to limit how dry the patients nasal passages get over time. Dry nasal passages is not our concern when we have a hypoxic emergency. Will I keep a patient on high-flow through devices that were not really designed for this purpose for very long? No. High flow that is not heated or humidified is a bridge while assessing or prepping the patient for something else. However, I'm never going to deprive my patient of the flow they need.

Pressure Comes Next

Sometimes pressure might come first, too. When? When we eyeball our patients work of breathing, it's pretty subjective. We go based off of the look on their face, their posture, demeanor, attentiveness, and so on. Sure, SPO2 and ETCO2 obviously come into play, but those never tell the whole picture. When the patient looks like a pile of trash trying to shovel air, are we really starting with with the least helpful thing and then slowly working up? Maybe you do, and that's your prerogative. Other may feel the best approach is to start with non-invasive ventilation like CPAP or BiPAP and then work your way back down. CPAP or BiPAP will provide pressure to recruit lung, provide decent flow and FiO2, and lessen work of breathing immediately. Do we really want to let the patient struggle while we see if each device on the way up to CPAP or BiPAP will be the key? If you were the patient, would you want something immediately that was more likely to work, or would you rather stay panicked and wear yourself out even more while the clinicians figure it out because they don't want to be 'too aggressive'? I'll leave that up to your interpretation, and to figure out the exceptions to that way of thinking (pneumothorax?).


The BVM is also a non-invasive device. If the patient doesn’t have a decent respiratory rate and tidal volume, and you don’t have BiPAP with a programmed rate to give it to them, you’ll need the BVM. Some people choose to place a nasal cannula under the BVM mask to provide constant flow and pressure under the mask (with a peep valve on the BVM). I can't imagine you don't have PEEP valves for your BVM in 2021, but if you don't, you need them.


This is a good point to stop and consider for a moment the trajectory of the patient. Before you go any further, you really have to ask yourself what you’re going to gain by getting any more invasive with your patient. If you’re already providing pressure, flow, and rate - is delivering all of that through a plastic straw going to change anything? Sure, it might protect the airway - you don’t want to be blowing air down the patients esophagus all day long (mostly that's happening due to BVM ventilation which would prompt intubation). But what about the right NOW issue of their oxygenation status? We certainly don't want to perform an airway procedure on someone who is hypoxemic, we know what happens when they're on the steep part of the curve. We have to optimize what we have going for us before we can get any more invasive.


Maybe I was a little dismissive of intubation... Airway protection isn't the only thing you're going to get from an advanced airway, I don't want to down play it. You get much more control. You gain the ability to set minute volumes, breath delivery, precise control of FiO2 and flow, and the ability to influence mean airway pressure. Those things are the light at the end of the tunnel, we just have to make sure they don't desaturate on the way there.


Finally, control

That's really what the advanced airway gives us - control. I mentioned earlier that the patient might already have an advanced airway because you have access to a P:F Ratio. So why did we go over all that other stuff on the way to the intubated patient? Because those items still apply. Positioning, flow, pressure - they're just in a different form of delivery here. So, how do we improve oxygenation in the intubated patient?


When your patient is hypoxemic, we're not trying to be stingy with oxygen - the FiO2 should already be all the way up to 1.0 (100%). Alright, we knew that already. What's next?


If you're using a ventilator, you actually might see an improvement because of the FiO2 delivery. Ventilators have a real ability to deliver 100% oxygen, and to deliver consistent tidal volumes and rates - the BVM is not nearly as reliable for FiO2, rate, or volume. But what if traditional settings and modes are not improving the oxygenation status?


Add pressure, but also time. Traditionally, the next step would be to prolong the inspiratory phase of breath delivery (Chang, 2014). The reasoning is that the longer you spend on inhalation, the more oxygen gets delivered. Remember, we bring in oxygen during inhalation, and expel carbon dioxide during exhalation. If you think of your inspiratory to expiratory ratio (I:E Ratio), it's how long we spend on inhalation versus exhalation. For example, if we spend one second inhaling and two seconds exhaling, that's a 1:2 I:E ratio. If we spend 2 seconds inhaling and 2 seconds exhaling, that's a 1:1 I:E Ratio. If we reverse the normal ratio and spend more time inhaling and less time exhaling, we call this an inverse I:E ratio. For example, if we spend 4 seconds inhaling and 2 seconds exhaling, this is a 2:1 I:E Ratio, and it's obviously the inverse of how we normally breath. What does an inverse I:E Ratio actually do?


Snap shots for out mechanical ventilation course:


1. This could be a half of a second in, and 2 sections out.

2. This could be 4 seconds in and 2 seconds out.

At first it seems simple - since we are spending more time on inhalation, we should be bringing in more oxygen. That's true, however, that's not the whole story. What we're actually doing is increasing the mean airway pressure. The more time spent at a higher pressure on average, the better our lung recruitment and oxygen delivery will be (Chang, 2014). So, it's not just the inspiratory time, but rather the average time spend at an increased pressure over the course of a minute (that's just how we measure it). This is what airway pressure release ventilation (APRV) does as well - it spends a long time at an increased pressure to attempt to improve oxygenation and maintain recruitment.


Quick stop.

What level of clinician do you assume are we talking to at this point? Paramedic? NP? EMT? Critical Care Paramedic? PA? MD/DO?


These principles are something I wish anyone able to use a BVM would know. There is nothing about I:E ratios and mean airway pressure that a brand new EMT can't understand. It's not as if you receive a critical care certificate and all of a sudden the time and pressure centers of your brain are now unlocked - that's just when someone decided to tell you it's a thing. I:E ratios, mean airway pressures, FiO2... they're just as real on a BVM as they are on a ventilator. It just depends how much time you decide to invest in knowing how to use your tools.


*steps down off of soap box*

Conclusion

P:F ratio isn't just a tool to rate your patient on a scale of oxygenation badness. The P:F ratio also tells you if you're being aggressive enough with your oxygenation strategy, and what you can probably expect from your patient when you try to correct things. If you do find that your patient has difficulty getting oxygen from their lungs to their body, what's your strategy? Map out what your process is for graduating from one strategy to another. And when all else fails... ECMO?


Oh yeah, and knowing how to oxygenate someone beyond squeezing a BVM doesn't have anything to do with what level of clinician you are ;)



Peer Review


Kristin Ireland, RRT, EMT

The partial pressure of oxygen is basic to understanding all things respiratory. It is considered elementary knowledge in respiratory therapy school, and this knowledge is essential to a member of an ICU team. ‘P:F Ratio’ is an ambient term repeated during ICU conversation. To understand the gradient created by hyperoxia in the lungs and where that oxygen goes is going to tell you a lot about the a/c membrane (alveolar/capillary membrane) you are dealing with. The gas mileage metaphor is a great way to explain this as I am sure we have all had a car that was breaking down at one point or another. ‘COPDers' also have lungs that begin to break down on them over time. If we are pumping these people full of 100% FiO2, we expect to see a direct response from their PO2. If we don’t, well, something is wrong. The big question is always, “why does this matter?” When attempting to breathe, normally we depend on time, pressure, and flow to keep us at a normal oxygenation status. When something internally goes awry, our bodies change to keep up with the demand. When we feel like we can’t breathe, we attempt to breathe faster and harder to keep our P:F Ratio normal.


This response is in line with positioning. I cannot tell you how important this is. If your patient is situated in a position that makes you think you’d be uncomfortable, they are probably uncomfortable themselves. I always ask, “are you comfortable? Because you are making me uncomfortable just looking at you and I am not the one laying here.” The position of life is sitting up. Anatomically, it makes sense on so many levels. Sometime patients don’t know what is good for them and a simple adjustment will increase their SPO2, thus increasing PO2. Proning is never a bad idea - especially if you can get your patient to at least lay in the recovery position. Increasing surface area and decreasing posterior atelectasis are two typically noninvasive ways to increase a PO2.


Pressure is also a consideration. While it is not an exact science, I have found by asking a patient to breathe in deeply and hold it, releasing slowly through pursed lips (creating PEEP), expecting a lag in the pulse ox an increased SPO2 will usually indicate they have a pressure deficit.


As an EMT myself, I have never gone wrong learning above my licensure level. I knew a majority of the time what was going on and I was able to offer assistance and advice when needed. This has followed me into the hospital setting. People begin to rely on your knowledge and not your name badge.




References


Rice, T. W., Wheeler, A. P., Bernard, G. R., Hayden, D. L., Schoenfeld, D. A., Ware, L. B., & National Institutes of Health, National Heart, Lung, and Blood Institute ARDS Network (2007). Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest, 132(2), 410–417. https://doi.org/10.1378/chest.07-0617


Driver, B. E., Klein, L. R., Carlson, K., Harrington, J., Reardon, R. F., & Prekker, M. E. (2018). Preoxygenation With Flush Rate Oxygen: Comparing the Nonrebreather Mask With the Bag-Valve Mask. Annals of emergency medicine, 71(3), 381–386. https://doi.org/10.1016/j.annemergmed.2017.09.017


The ARDS Definition Task Force*. Acute Respiratory Distress Syndrome: The Berlin Definition. JAMA.2012;307(23):2526–2533. doi:10.1001/jama.2012.5669


Kacmarek, R., Stoller, J., Heuer, A., Chatburn, R. & Kallet, R. (2017). Egan's fundamentals of respiratory care. St. Louis, Missouri: Elsevier.


Cairo, J. (2014). Mosby's respiratory care equipment. St. Louis, Mo: Elsevier.


Gleason, J. M., Christian, B. R., & Barton, E. D. (2018). Nasal Cannula Apneic Oxygenation Prevents Desaturation During Endotracheal Intubation: An Integrative Literature Review. The western journal of emergency medicine, 19(2), 403–411. https://doi.org/10.5811/westjem.2017.12.34699


Chang, D. (2014). Clinical application of mechanical ventilation. Clifton Park, New York: Delmar Cengage Learning.


Picture credit for the altitude / oxygen percent picture: https://theconversation.com/how-does-altitude-affect-the-body-and-why-does-it-affect-people-differently-95657