Crit Bits: Auto-PEEP

Crit Bits: Auto-PEEP

Anand Swaminathan, MD and Haney Mallemat, MD

Take-Home Points

• Positive end-expiratory pressure (PEEP) may be intrinsic or extrinsic.
• Auto-PEEP is dangerous and can affect the pulmonary system, hemodynamics, and the ability to trigger breaths on a ventilator.
• Strategies to treat auto-PEEP include treating the underlying cause, decreasing the respiratory rate, increasing the inspiratory flow rate, and decreasing the tidal volume.

• In 2015, we did segments on ventilator management with Mallemat.
• What is PEEP? PEEP is positive end-expiratory pressure. This is a setting on the ventilator to keep the alveoli and distal airways open. We want to improve oxygenation. This can be done by increasing the fraction of inspired oxygen or keeping the alveoli open to allow better oxygenation.
• There are 2 types of PEEP. There is the extrinsic PEEP that we provide to patients, and there is intrinsic PEEP. Intrinsic PEEP is generated by the patients and the ventilator. Auto-PEEP is the phenomenon of generating intrinsic PEEP.
• When is auto-PEEP generated?
  • In patients who are not intubated and who present with respiratory distress, we classically see chronic obstructive pulmonary disease and asthma. These patients do not have enough time to get all their breath out at the end of expiration. This builds a positive pressure in the alveoli and the chest.
  • In patients who are intubated, these same populations can develop auto-PEEP on a ventilator. Other patient populations may generate auto-PEEP on a ventilator, for example, a patient who is not adequately sedated. Patients may become tachypneic because of anxiety and pain, and this can lead to auto-PEEP. We can also induce auto-PEEP. For example, if a patient has a metabolic acidosis, and we increase the respiratory rate and tidal volume, auto-PEEP can be generated by trying to fix their other metabolic problem.
• Why is auto-PEEP so dangerous? There are 3 problems that occur with auto-PEEP.
  • The first problem affects the pulmonary system. When you have a patient whose lungs are progressively inflated, this causes injury and overstretch to the lung. We try to target a tidal volume of 6 to 8 cc/kg/breath. If patients have auto-PEEP, their lungs are getting progressively larger and stretching. This violates the principle of overstretch. There may be elevations in the peak or plateau pressure. This may not be seen in the ED but later on in the ICU.
  • The second problem is hemodynamic. You will see this in the emergency department. As the inflation of the lung increases, positive pressure in the thorax increases. This elevated intrathoracic pressure decreases venous return, filling to the left ventricle, cardiac output, and systemic pressure. The patient could die as a result.
  • The third problem may not be seen in the ED, because our patients are deeply sedated and sometimes paralyzed.Patients who have severe auto-PEEP may not be able to trigger a breath on a ventilator. When you breathe, your diaphragm flattens, and you take a breath in. In patients with hyperinflation, the diaphragm is already flat, and they need to generate a lot more force to trigger the breath. The ventilator doesn’t give them that breath until they generate a lot more force.
    • Patients who aren’t ventilated need to chill out. Patients who have asthma feel as if they are going to die. This makes them anxious and tachypneic. Tachypnea means that there is less time in expiration. It sounds counterintuitive, but you want to decrease their respiratory rate a little bit so they aren’t causing air trapping.
    • There are a variety of ways to do this. Mallemat likes to use ketamine. It causes bronchodilation, and it also makes patients relax. He doesn’t use dissociative doses, although you can. He starts out with 0.3 to 0.5 mg/kg. If he needs more, he titrates up from there. Taking away that little bit of anxiety and slowing the respiratory rate gives patients more time for exhalation.
    • Some physicians use fentanyl or benzodiazepines, but Mallemat feels that ketamine is a much better agent.
• How can we identify mechanically ventilated patients at risk of developing auto-PEEP? There are some waveforms you just need to know if you are managing ventilated patients in the ED. The flow waveform is a horizontal line. Every deflection or rise above that baseline will be an inspiratory breath. Everything below the line will go from the patient back to the ventilator or expiration. During expiration, the waveform will come back to baseline and will often be zero flow for a few microseconds until the next breath. A person who is developing auto-PEEP will not have the expiratory waveform come back to baseline and will have a next breath initiated. There is a gap between the expiratory waveform and the baseline. Identifying this is gap vitally important, because it puts your patient at risk for auto-PEEP. If this goes on for a while, patients can develop some of the complications described above.
• Is there anything we can do to treat auto-PEEP in mechanically ventilated patients?
  • Fix the underlying problem. For example, after intubating a patient with asthma, make sure your respiratory therapist is hooking up continuous nebulizers. Make sure the patient is still receiving ketamine.
  • Decrease the respiratory rate to allow more time in expiration. This means there is a greater chance of the expiratory waveform reaching the baseline.
  • Increase the inspiratory flow rates. This is for volume ventilation. For example, go up to 80 L/min from 60 L/min. This will still deliver the same tidal volume, but it will go in faster. If you decrease the inspiratory time at a given respiratory rate, by default you will give more expiratory time.
  • You can decrease the tidal volume you are giving your patient. If you are giving 8 cc/kg/breath, decrease to 7 cc/kg/breath. You may need to go down to volumes of 5 cc/kg/breath.

Related Segments

EM:RAP 2015 October - Ventilators 101 – Part 1

EM:RAP 2015 October - Ventilators 101 – Part 2