Intrinsic PEEP / Gas Trapping

Intrinsic PEEP is also known as Auto PEEP or PEEPi

• Develops when there is an inability to exhale the complete breath
  • Seen on Flow curve: patient still exhaling when next breath starts

• Contributory factors include
  • Increased expiratory resistance
    • Bronchospasm from Asthma/COPD
    • Circuit factors
      • HMEs
      • Kinked ETT
  • Too short expiratory time
    • when the Ti is set too high
    • when I:E ratio is too short
  • A combination of both

• Results in gas being trapped in the lungs
  • Increased pressure for the given volume can cause barotrauma
  • Trapped gas increases intra-thoracic pressure which reduces venous return, preload and cardiac output causing hypotension
  • The amount of gas that is unable to be expired progressively increases with each breath and can result in dynamic hyperinflation. Eventually it becomes difficult to inhale at all and tidal volumes and compliance progressively fall, with hypercarbia and resulting cardiorespiratory collapse

• Measured using end expiratory hold in a sedated/paralyzed patient
  • Occlusion of the expiratory flow for 3-5 seconds allows the pressures within the airways to equilibrate
• • The resulting measure is called a static PEEP or total PEEP and is in fact the alveolar pressure at end of expiration

PEEPi is the “extra PEEP” over the PEEP you had set.

PEEPtot = PEEPset + PEEPi

 
[az_toggle_section] [toggle title=”Remembering you last set PEEP at 8cmH2O and PEEP total is measure at 14cmH2O during the expiratory hold. Can you say how much PEEPi your patient has, why he has it, and what you are going to do to correct the problem?” id=”tgl-1″] PEEPi = PEEPtot – PEEPset = 14cmH2O – 8cmH2O = 6cmH2O
Your patient has 6cmH2O of intrinsic PEEP which he probably acquired because he has an undiagnosed chronic obstructive pulmonary disease and the RR that you set at 18/min the night before didn’t allow him enough time to exhale. That’s why when you increase a RR or decrease a I:E ratio, you should always take the time to observe properly the new curves to ensure you are still leaving enough time for flow to go back to zero.
To trigger a breath, your patient has to generate enough negative pleural pressure to overcome PEEP before being able to start moving air in and generating the flow or pressure required to trigger the ventilator.
To fix the problem, you need to get rid of the PEEPi. You will have to give him time to exhale properly to his needs, even if it means tolerating a higher PaCO2 than you would like (this is called permissive hypercapnea). The easiest way is to lower his RR. Until you get rid of the PEEPi, you can decide to put him back on a controlled mode which also means sedating him, or you can leave him on a support mode, but increase the set PEEP to more than 2/3 of PEEPi to help him overcome that PEEPi while avoiding resedation and permitting him to progress in his weaning. [/toggle] [/az_toggle_section]