Intrinsic PEEP and dynamic hyperinflation

Questions on intrinsic PEEP are frequently reported. Usually, they take the form of a viva, and follow the "how'd you ventilate that" sort of format. Previous past paper SAQs include the following:

• Question 28 from the first  paper of 2014
• Question 7 from the second paper of 2006
• Question 4 from the first  paper of 2006 and the identical Question 11 from the first paper of 2002 

 LITFL has an excellent page on this topic.

Definition of intrinsic PEEP and dynamic hyperinflation

Brochard (2002) defines intrinsic PEEP as "unintentional or un-measured end-expiratory hyperinflation".

• Expiration usually occurs as a passive movement
• Resistance to air flow (eg. bronchospasm) results in decreased expiratory flow
• Decreased expiratory flow results in a longer time required to expire the full tidal volume
• If expiration is interrupted before its natural end by the next inspiration, some un-expired residual gas remains in the chest
• This gas exerts a pressure onto the respiratory circuit
• As a result the alveolar pressure at the end of expiration is higher than zero (zero being the atmospheric pressure).
• This process of incomplete emptying is called dynamic hyperinflation, and the positive alveolar pressure is called intrinsic PEEP or auto-PEEP.

Clinical features associated with dynamic hyperinflation

Only four features of this phenomenon are asked for in Question 7 from the second paper of 2006. One could produce many more.

• wheeze
• prolonged expiratory phase
• increased chest distension
• decreased chest expansion
• bilaterally decreased air entry
• increased autoPEEP in the expiratory hold manoeuvre
• increased peak airway pressures
• increased plateau pressures
• short expiratory time setting on the ventilator

Causes of intrinsic PEEP

Increased resistance to expiratory flow, due to:

• Machine factors:
  • Blocked or faulty expiratory valve of the ventilator
  • kinked expiratory limb of the ventilator tubing
  • rain-out in the expiratory limb
  • clogged water-sodden HME
  • kinked ETT
  • ETT clogged with sputum
  • ETT being chewed on by the patient
• Ventilator settings
  • Short expiratory time, eg. in very high respiratory rate
  • High I:E ratio
• Patient factors
  • Bronchospasm
  • Increased respiratory rate

Static  measurements of intrinsic PEEP

The expiratory hold manoeuvre

• An expiratory breath hold stops all flow in the airways; so you can eliminate the expiratory airway resistance (the flow dependent component of intrinsic PEEP).
• Thus you are able to measure the "static PEEP", the PEEP due to the elastic recoil of the lungs putting pressure on the gas trapped inside them.
• Ideally, this should be measured in a totally paralysed patient, at zero extrinsic PEEP.
• Under ideal conditions, the trapped gas will equilibrate in the circuit and one should see the true intrinsic PEEP appear after 1 second of expiratory hold.
• In reality, at expiration many of the smaller airways end up closed (particularly in bronchospasm) with the result that the expiratory hold manoeuvre may be ineffective. Only the most "open" (least bronchospastic) lung units will reveal their intrinsic PEEP by this method, and the really spastic lung units with the highest intrinsic PEEP will not be observed. 
• Question 28 from the first  paper of 2014 mentions this issue. The college answer mentions that plateau pressure is the best means of measuring the intrinsic PEEP.

• Plateau pressure is measured with the inspiratory hold manoeuvre
• The high pressure at the plateau ensures all the little airways are splinted open
• This allows the intrinsic PEEP to equlibrate across the entire respiratory circuit.
• The college, in their answer to Question 28 recommend to read the plateau pressure after a 2s pause; the ideal pressure is as usual, undr 25-30 cmH₂O

Consequences of intrinsic PEEP

• CO2 retention
• Increased work of breathing
• increased intrathroracic pressure, thus
  • decreased venous return, hemodynamic instability
  • decreased lymphatic return
  • decreased organ perfusion, organ oedema and decreased organ function

Management of intrinsic PEEP

• Reverse reversible patient factors; eg.  bronchospasm can be treated with bronchodilators and steroids
• correct machine factors, eg. empty the water out of the tubes, cheange the HME, change the ventilator valve
• Suction ETT, ensure patency
• Increase expiratory time by decreasing respiratory rate and decreasing I:E ratio
• Apply PEEP to counteract the increased work of breathing
• Decrease tidal volume
• Exotic last-line measures may be deployed:
  • Heliox, to reduce the viscosity of the respiratory gas mixture
  • ECCO2R, to manage the resulting hypercapnea
  • High-frequency oscuillation