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Auto-PEEP: how to find it and what to do about it
It is the most common reason a ventilated asthmatic suddenly loses a blood pressure, and it is invisible unless you go looking for it.
What it is
Auto-PEEP, also called intrinsic PEEP, is the alveolar pressure still sitting above the set PEEP at the end of expiration. It happens when the next breath arrives before the last one has finished leaving. Volume accumulates breath on breath, which is why the other name for it is dynamic hyperinflation.
The ventilator does not display it. The pressure it shows you at end-expiration is the pressure in the circuit, and while the expiratory valve is open that reads as the set PEEP no matter what is happening in the alveolus. You have to close the valve to see the truth.
Finding it on the waveform
Look at the flow-time trace, not the pressure trace. In a patient with no trapping, expiratory flow returns to the zero line and stays there briefly before the next breath begins. In a patient with auto-PEEP, the next breath starts while expiratory flow is still negative: the trace is cut off mid-descent.
That single observation is the screening test, and it costs nothing. Every other sign is downstream of it.
Measuring it
Perform an end-expiratory pause of 0.5 to 1.0 seconds. Holding the expiratory valve shut lets circuit pressure equilibrate with alveolar pressure, and the number displayed during the hold is the total PEEP.
The measurement needs a passive patient. Active expiratory effort pushes the reading up and will overstate the trapping; a patient triggering through the hold invalidates it entirely.
Why it hurts
| Consequence | Mechanism |
|---|---|
| Hypotension, sometimes arrest | Raised intrathoracic pressure throughout the cycle cuts venous return and so cuts preload and cardiac output |
| Ineffective triggering | The patient must drop airway pressure by the auto-PEEP plus the trigger threshold before the ventilator sees any effort at all |
| Rising work of breathing | Every breath starts with an inspiratory threshold load that does no ventilation |
| Barotrauma | Regional overdistension raises the risk of pneumothorax |
The hypotension is the one that kills, and it is often misread. A hyperinflated patient who becomes hypotensive on the ventilator looks like they need fluid and pressors. Disconnecting them from the circuit for twenty to thirty seconds and watching the pressure come back is both the diagnostic test and the first treatment.
Unloading it
- Give expiration more time. Lower the respiratory rate, to 10 to 14 in severe obstruction, and shorten inspiratory time. Target an I:E of 1:3 to 1:5.
- Treat the obstruction: bronchodilators, suction a plugged tube, check the tube is not kinked or bitten.
- Accept the CO2. Permissive hypercapnia, with a PaCO2 of 80 to 100 and a pH at or above 7.15 to 7.20, is the correct trade against hyperinflation, unless there is cerebral edema, pulmonary hypertension or severe right-heart failure.
- In COPD, consider applying external PEEP at 75 to 85 percent of the measured auto-PEEP. This splints collapsing small airways open so the patient can trigger against a smaller gradient, without adding to the trapped volume. Going above the measured auto-PEEP makes the trapping worse.
Note the order. Rate comes first, because expiratory time is the variable that actually determines whether the lung empties, and it is the one the ventilator gives you direct control over.
The sanity check
Auto-PEEP of zero in a severe asthmatic ventilated at a rate of 30 is not a reassuring measurement. It is a measurement that was taken wrong.
Nobody ever mastered a ventilator by reading about one.
Set up an obstructive patient, shorten the expiratory time until the flow trace stops returning to zero, and watch the blood pressure follow. 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.
Keep reading
- Reading ventilator waveforms
- Pressure control vs volume control
- Ventilator glossary, for the terms in this article
- Every topic in the library