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Setting PEEP in ARDS
PEEP is not there to open alveoli so much as to stop them closing again, and to even out the stress between the ones that are open.
What PEEP is for
A lung with ARDS is not uniformly stiff. It is a patchwork: collapsed regions next to aerated ones. Healthy alveoli adjacent to collapsed tissue are pulled on from one side and can see something close to twice the pressure applied globally. That local amplification, not the average pressure, is what injures lung.
PEEP works by holding open the units that would otherwise collapse and reopen with every breath, which removes the interface doing the amplifying. Redistributing stress is the point. Recruiting volume is the mechanism.
The ARDSNet ladders
PEEP and FiO2 are set together, moving up the table as oxygenation demands it. The lower table is the usual default.
| FiO2 | Lower PEEP table | Higher PEEP table |
|---|---|---|
| 0.30 | 5 | 5 to 14 |
| 0.40 | 5 to 8 | 14 to 16 |
| 0.50 | 8 to 10 | 16 to 20 |
| 0.60 | 10 | 20 |
| 0.70 | 10 to 14 | 20 |
| 0.80 | 14 | 20 to 22 |
| 0.90 | 14 to 18 | 22 |
| 1.00 | 18 to 24 | 22 to 24 |
The higher table is worth considering in obesity and in abdominal hypertension, where a large part of the applied pressure is spent on the chest wall rather than on the lung.
A cruder heuristic that will not get you into much trouble: 5 to 10 cmH2O in mild disease, 10 to 15 in moderate, 15 to 20 in severe.
Judging whether it worked
Oxygenation is a slow and noisy signal. Compliance is faster and more honest. Change PEEP, wait, then measure the plateau pressure and work out the driving pressure.
If driving pressure fell, the PEEP recruited lung: the same breath now goes into more of it. If driving pressure rose, the PEEP overdistended what was already open. Oxygenation can improve in both cases, which is why it is the wrong thing to steer by alone.
What PEEP costs
- It raises pressure inside the chest throughout the cycle, which cuts venous return and so cuts cardiac output. The effect is largest in a patient who is underfilled.
- It raises central venous pressure by roughly 0.5 mmHg per cmH2O applied, which is a reading artifact rather than a real rise in filling.
- Overdistended alveoli compress the capillaries beside them, raising pulmonary vascular resistance and loading the right ventricle.
- Overdistension increases dead space, so the CO2 rises and the end-tidal to arterial gradient widens.
When more PEEP is the wrong answer
Right ventricular failure, massive pulmonary embolism and pulmonary hypertension are the clearest cases: the right ventricle is already struggling against its afterload and PEEP adds to it. Keep it at or below 10 if you can.
In obstructive disease the relevant pressure is the one the patient generated themselves. Applied PEEP in COPD is a trigger-assist measure, set at 75 to 85 percent of the measured auto-PEEP, not a recruitment measure.
In severe disease that is not responding, prone positioning and neuromuscular blockade act on the stress distribution more effectively than another few cmH2O, and both should be considered before accepting a driving pressure that will not come down.
Nobody ever mastered a ventilator by reading about one.
Raise PEEP in steps and watch compliance improve and then reverse as recruitment gives way to overdistension. The Ventilator Workbook puts a live ventilator in front of you with a simulated patient behind it, so you can break things that cannot be harmed. Three modules and time on the simulator are free, and no card is needed.