Those of us who are in the ICU, especially those of us taking care of the patients caught up in this pandemic have learned that certain patients can be quite challenging to oxygenate and ventilate. For the uninitiated, the whole purpose of mechanical ventilation is to be a band-aid of sorts for patients who we can’t get oxygen into them or carbon dioxide out of them. We are quite precise with our wording here. When we cannot ventilate someone, again, cannot get the CO2 out of them, they go into hypercapnic respiratory failure. When we cannot oxygenate them and their O2 is low, then these patients are hypoxemic respiratory failure. Here you say you cannot oxygenate your patient. There are combinations of the two, of course. Some patients are intubated because of hypercapnia and some develop hypercapnia while on the ventilator. Long story short is that it gets quite tricky and we need to be very intentional with our methods on these patients because we can actually end up causing them further harm if we make a mistake. I will disclose that I cannot go into every single nuance and detail of mechanical ventilation here. This is meant to be a quick guide and a jump off point for your own personal deeper dives. This is not medical advice.
The problem with hypercapnia is that it induces a respiratory acidosis. Patients try to compensate for this respiratory acidosis due to the increase in carbon dioxide by increasing their serum bicarb. For this, you need some kidneys to sort this out. It also does not happen overnight. As a side pointer, many of you may already know this but to those of you who don’t, want to know how to recognize a patients with chronic hypercapnic respiratory failure? Note their serum bicarbonate level on the date of admission. Normal serum bicarb is usually between 22 to 26. These patients arrive at the hospital with a serum bicarb in the 30’s. That means they are likely chronic retainers. I suspect they have underlying COPD, obstructive sleep apnea, or obesity hypoventilation syndrome amongst other differentials at baseline.
We are seeing a lot of hypercapnic respiratory failure, people who we cannot ventilate, in our current pandemic population because we are using ARDS strategies to oxygenate and ventilate our patients. We have learned that using low tidal volumes are beneficial for ARDS patients. We try to use 4-6cc/kg of ideal body weight at the set tidal volume. We set the respiratory rate accordingly to hit a desired minute ventilation. Minute ventilation is respiratory rate times tidal volume. Normal minute ventilation is usually 5-8L per minute. Wow, you’re getting a bunch of nuggets of information in here. We can also titrate the respiratory rate periodically based on blood gas results. If the patient is found to have hypercapnia, being that we cannot blow off enough carbon dioxide, also stated that we aren’t ventilating these patients enough, we have various strategies that we can attempt to correct the hypercapnic respiratory failure.
In my practice I allow for a term called permissive hypercapnia. To me this means that I allow the CO2 to climb up to a point where the body develops a respiratory acidosis. Each clinician has their own threshold but I have no qualms with the pH hitting 7.25 on these patients. Obviously hemodynamic stability is key. Some patients will not tolerate that pH. Others will tolerate pH’s less than 7.25. Remember, every patient is different. This is not medical advice. This is called a “lung protective strategy” in the literature. Going back to those landmark papers in critical care, we need to go back to the ARDSnet papers where they found that using 6cc/kg of ideal body weight is crucial to prevent ventilator induced lung injury. We also need to keep the plateau pressures under 30. You butt kicking RT can teach you how they do this. Permissive hypercapnia came to be when they realized that trying to obtain the perfect blood gas by increasing the pressures necessary to ventilate the patient actually ended up causing harm. Now there are schools of thought where they say that the hypercapnia is actually beneficial but I won’t get into that at this point. Let’s dig in to the strategies mentioned in the cited paper with my own takes on them.
Ventilator Changes for Hypercapnic Respiratory Failure
Earlier, I mentioned minute ventilation. Some ventilators will have this listed on the screen so you can eyeball the number. Makes life easier. But again minute ventilation is respiratory rate times tidal volume. The number is hopefully between 5-8 as mentioned before. For ARDS patients you really don’t have much wiggle room regarding tidal volume, as this is set to 4-6cc/kg of ideal body weight, so respiratory rate is a knob you can turn. When I say you I mean the respiratory therapist. They are the owners of the vent. Although I must admit that I love turning knobs and pushing buttons.
Notice how I am not talking about ventilator modes here. That is a conversation for a different day. That would take another several hours. I am not going to get into the waveforms and all that jazz. Just know that you can adjust the RR and that could be a quick way to increase your minute ventilation and therefore correct the hypercapnic respiratory failure.
But there is some nuance that goes with this. You also have to take a look at your I:E ratio. That is your inspiratory time to expiratory time ratio. Typically speaking, our I:E ratio at this exact moment is 1:3. One second to inhale, 3 seconds to exhale. In patients who are hypercapnic, they are no exhaling as much. You may need to shorten the inspiratory time to allow for some more expiratory time so the patient can blow off some more CO2. It is important in these patients for the respiratory therapist to check if the patient is developing something called Auto-PEEP. This is also called breath stacking. In other words their E-time is not long enough. This question always shows up on the boards in asthmatic patients who all of a sudden crash while they’re on the vent. It asks what should you do. The answer is to disconnect them from the vent. This process allows for all the trapped pressure in the patients chest to escape. Before I diverted to that board question, there are other tricks regarding sedation and sometimes even reaching for a paralytic (after deep sedation of course) to help you ventilate your patient better and set them up on the vent properly. Remember, no one wants to be on a vent.
Increasing the PEEP is another strategy to improve ventilation. It’s okay if you didn’t think of this one but try to follow along. I am also not going to go into driving pressure and lung compliance here. You can look that up yourself. When you look at basic ventilator settings, we all know that the ways to increase the oxygenation is to turn up the PEEP and FiO2 and the ways to increase the ventilation is to titrate the respiratory rate and tidal volume. There is obviously a lot more to that but bear with me. PEEP helps recruit alveoli by popping open atelectatic or consolidated alveoli. All of a sudden, those alveoli from which the patient wasn’t able to ventilate from before suddenly become open! When I place patients on ARPV, short for airway pressure release ventilation, a mode that I have discussed before on my website but not on this podcast, one of the things you need to be careful with is hypercapnia. I typically do not put patients on APRV if I am having a very challenging time ventilating them because this tends to get worse before it gets better. But there are patients who you place on APRV with a baseline pCO2 in the 70’s and then after several hours on APRV their pCO2 is now in the 50’s. Well what happened? Here, ARPV helped recruit those alveoli that weren’t recruited before and now the patients hypercapnic respiratory failure has improved.
At this point in time proning is not something new to us. The PROSEVA trial showed that we can improve oxygenation as well as ventilation in ARDS patients by using this positioning of patients.
Things I do not personally like and may be completely wrong in in Hypercapnic Respiratory Failure: Bicarb
Some people like to put patients who have hypercapnic respiratory failure with a subsequent respiratory acidosis on bicarb drips. As my knowledge in physiology has increased, I have begun to think that this concept does not make sense. Here’s why: bicarbonate is metabolized into carbon dioxide and water. In order for the patient to be able to ventilate, we need to get rid of the carbon dioxide. If we cannot get right of the carbon dioxide via the lungs, then are we really just worsening the acidosis? I haven’t found any data to either support my thought process or go against it. Please feel free to prove me wrong. I have asked other clinicians who are smarter than I am about this and no one seems to know the answer.
ECMO and Extracorporeal CO2 removal devices
Other ways of correcting hypercapnic respiratory failure when all these strategies have failed include making the call for the patient to be placed on ECMO and other extracorporeal CO2 removal devices. If you have these at your institutions then chances are you know all about them. I won’t get into them too much here but you can check out more in the cited article.
Wrapping things up in Hypercapnic Respiratory Failure
Hope you were able to learn a thing or two about how to manage patients with hypercapnic respiratory failure who are mechanical ventilation. Please check the cited article and don’t trust me. It’s completely free for you to download.
CLICK HERE to check out other posts I’ve created on Mechanical Ventilation
Citation for Hypercapnic Respiratory Failure Management
Tiruvoipati R, Gupta S, Pilcher D, Bailey M. Management of hypercapnia in critically ill mechanically ventilated patients-A narrative review of literature. J Intensive Care Soc. 2020 Nov;21(4):327-333. doi: 10.1177/1751143720915666. Epub 2020 Mar 30. PMID: 34093735; PMCID: PMC8142102.
Link to Article
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