Non-Invasive Estimation of Optimal PEEP
/“Non-Invasive Estimation of Optimal Positive End-Expiratory Pressure (PEEP) for Mechanically Ventilated Obese Patients.” Poster and oral presentation at 64th International Respiratory Congress Annual Meeting, December 2018.
As a general rule, mechanical ventilation of obese patients is more complex and difficult than in those with a normal body habitus. Obese patients have decreased chest wall compliance due to increased truncal adiposity, amongst other factors. They are also predisposed to other comorbidities that can lead to more involved physiologic challenges.
Critical Care Transport teams commonly transport patients who are intubated and mechanically ventilated. These patients are intubated for a variety of reasons, from altered mental status to hypercapnea to hypoxia. One of the more common challenges we face in our patients who are mechanically ventilated is difficulty with oxygenation – whether the patient is suffering primary or secondary hypoxemic respiratory failure. These challenges tend to be more pronounced in patients with concomitant obesity.
We should all be very familiar with the main parameters we can adjust to try to increase oxygenation – namely FiO2 and PEEP. In the transport environment, we often will default to an FiO2 of 100% in these patients, and then begin working on increasing the PEEP. Based off the ARDSNet trials, there are specific FiO2/PEEP tables that have been successfully used in the ICU environment, which some will adapt to the prehospital or interfacility environment.
As you can see, even the “Lower PEEP/higher FiO2” scale is still quite aggressive in regard to PEEP application, with a low threshold (FiO2 of just 50%) to increase PEEP to 10 and then above. In my anecdotal observations of most teams’ practice, I suspect we would be more liberal in our PEEP usage following a table such as this. Beyond just FiO2 and PEEP, we can also start to adjust Inspiratory:Expiratory ratios, as well as ventilator modes – but that is beyond the scope of this specific article discussion.
This discussion is a little unusual for this series, as this was actually an abstract/poster presented at a Respiratory Therapy conference late last year. However, I hope it will generate good discussion as well as make us ponder our initial ventilator settings more carefully.
This paper adds a further wrinkle to the more common PEEP table, with attempting to provide a structure to estimate initial PEEP requirements in obese patients. The authors surmised that many obese patients were not receiving adequate PEEP. In my experience, this is accurate, as I struggle to think of an obese patient with oxygenation difficulties who I was able to successfully manage on just 5 of PEEP.
Currently, optimal PEEP can be invasively calculated using esophageal balloons. This paper used esophageal balloons to actually calculate optimal PEEP in mechanically ventilated patients. They did this by finding the PEEP level at which the end-expiratory transpulmonary pressure (Ptp exp) was equal to 0. They then used these values to calculate the relationship between BMI and optimal PEEP, and used this to approximate a formula to provide a non-invasive method of estimating optimal PEEP.
The study involved a retrospective chart review of obese adult patients, who were mechanically ventilated for more than 48 hours. The time period covered 3 hours, and 91 patients met the inclusion criteria. Mean BMI was 43.
Results
As mentioned, a total of 91 patients were included. Using some complicated statistics which I won’t cover here, they found that optimal PEEP increased 1 mm Hg per 0.79 kg/m2 increase in BMI/4, with a p value of 0.001.
Essentially, they found that you could estimate the initial PEEP required by taking the BMI and dividing by 4. Importantly, they found that if anything, this consistently underestimated the optimal PEEP as determined by esophageal manometry, meaning that there is less danger of accidentally overshooting your target. This formula can therefore be used as a conservative guide for initial settings.
Limitations
Obviously this is quite a small, limited study – hence why it’s an abstract/poster presentation initially, and not a New England Journal of Medicine article. There was a small sample size, and it was within a single health system. They also only included patients who had an esophageal balloon placed, but do not clarify which patients were more likely to have these placed, and so there is potential for significant bias there.
Discussion
Although limited, this article has made me reconsider my initial ventilator settings with many of our patients, especially those who suffer from obesity. Most ventilators are preset to start at a PEEP of 5. However, based upon this model, that would only be recommended for patients with a BMI of 20, which is a small subset of our population. Even someone with a BMI of just 30 (ex: 6 feet tall at body weight of 107 kg/235 lb’s) could benefit from an initial PEEP of 8! Unlike the LTV1200, our Hamilton T-1 allows up to a PEEP of 25, and so in theory we could follow this formula all the way to a BMI of 100 (5 foot tall person at weight of 232 kg’s/512 lb’s).
However, there are obvious downsides to increasing PEEP values. Firstly, increasing PEEP will increase intrathoracic pressure, possibly reducing venous return and causing hypotension/hemodynamic compromise. PEEP can also increase the risk of barotrauma. Finally, we often must carefully consider the impact of increased PEEP on Peak Inspiratory Pressures and Plateau Pressures. As a reminder, a good general goal is to try to maintain Plateau pressures under 30.
Conversely, research is increasingly showing that we should also be trying to minimize Driving Pressure in these patients, which is calculated as the Plateau Pressure minus the PEEP. Most new literature suggests attempting to keep the Driving Pressure under 13-15 cm H2O. As you can see, there is considerable complexity here, and so each individual patient requires careful attention in order to find their optimal PEEP levels.
It would be remiss not to mention one of the simpler methods to improve oxygenation and reduce intrathoracic pressures in obese patients – sit them up! Especially in the EC-145 and our ground ambulances, it is quite easy to keep the head of bed at 30 degrees or above, even during the loading and unloading stage. In smaller helicopters, we have to lower the head in order to load/unload the patient, but please remember to elevate the head back up as soon as possible if permissible.
Of course, we must be careful to distinguish between Ideal Body Weight, and Total/Actual Body Weight. Example patient: 6 feet tall (72 inches), male patient. Total/Actual body weight of 107 kilograms.
This gives him an Ideal Body Weight of approximately 78 kilograms. Meaning that an initial tidal volume of 6 cc/kg of IBW would yield a tidal volume of approximately 470 cc’s. But we could then use his Total/Actual Body Weight to calculate a BMI, which would be 32. Using our proposed formula of BMI/4, our initial PEEP would then be 8. I hope that brief example is helpful.
In conclusion, although this is a limited study, I do think it will affect my practice. If nothing else, it’s a good reminder to be more aggressive with up-titrating PEEP and to re-familiarize yourself with the ARDSNet FiO2/PEEP tables. It will also help give me a conservative number to start with for a base level of PEEP, based upon the patient’s BMI, without just defaulting to “5” for every patient.
Authorship
Andrew Cathers, MD - Dr. Cathers is an Emergency Medicine Physician as well as Flight Physician, and Assistant Medical Director of University of Wisconsin Med Flight with a focus on Education and Training in their Program. He is kind enough to share recaps of recently published HEMS literature which should be posted approximately quarterly here on TamingtheSRU
