Topic Progress:

Clinical Case

The repeat ABG you ordered came back and looks improved and the rest of the night was uneventful.

In the morning, the team decides that given the patient is oxygenating better on FiO2 60%,  its time to assess if they can move the patient towards a gradual weaning from mechanical ventilation. They start by lowering sedation while on VC-SIMV with a pressure support of 12cmH2O, but they are rapidly concerned by the curves they see and the patient looks uncomfortable.  

Ventilator Dyssynchrony

Looking at the ventilator wave-forms below, lets see if we can figure out what’s driving this patient’s ventilator dyssynchrony… 

Dyssynchrony often occurs when the ventilator settings don’t match the patient’s requirements. It can cause

  • Increased work of breathing and muscle fatigue
  • Increased oxygen requirements

While management often requires increased sedation (which may prolong the duration of ventilation), in the first instance, an attempt should be made to adjustment ventilator settings or transition to a more comfortable mode; many argue that PC modes, or VC modes with auto-flow are more comfortable and less likely to induce dyssynchrony.

Types of Dyssynchrony 

The trigger may be inappropriate in a mandatory mode with no ability to trigger spontaneous breaths (VERY uncomfortable!) or in spontaneous mode, as outlined below:

Trigger set too high

A weak patient will be unable to initiate a spontaneous breath as they won’t sufficiently change the flow or pressure in the circuit


  • negative inflections on the PAW curve
  • often due to intrinsic PEEP too high to overcome, so signs of high intrinsic PEEP might be present

Trigger set too low

Other actions for example hiccups will trigger a breath


  • usually fast RR without physiologic explanation
  • extra breaths matching the external triggering event (hiccups..)

Flow set too low

results in insufficient TV being delivered and the patient may appear to be sucking or gasping


  • “pull down” on PAW curve upstroke during inspiration

Flow set too high

results in either an excessively large breath or the patient tries to initiate expiration early and works against the ventilator


  • most likely you won’t see it…but patient will be uncomfortable

Ti set too short

Prematurely ends inspiration and can result in inadequate TV

Patient discomfort as still trying to inspire when expiration begins


  • since still wanting more air, patient will sometimes be able to trigger a second breath right at the end of the ventilators breath. It will look like 2 stacked breaths

Ti set too long

Can result in excessive TV

In obstructive lung disease the resulting lack of expiratory time can result in gas trapping


  • most likely you won’t see it…but patient will be uncomfortable
  • signs of gas trapping if patient at risk and TE too short

Looks like a trigger dyssynchrony. Patient tries to trigger a breath (negative inflections in PAW curve), but can’t seem to reach the set trigger to start a breath. It would be unusual for your patient to have become too weak to breathe overnight, so it must be that the effort he needs to generate to reach the set trigger is too high…

Expiration phase of the flow curve doesn’t go back to zero before the next inspiration begins.  This can put your patient at risk of building high intrinsic PEEP…and maybe that’s why he can’t trigger a breath..

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 for the set RR
    • 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 (image 8.3.2)
  • 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

You last set PEEP at 10cmH2O and PEEP total is measured at 14 cmH2O during the expiratory hold. 


= PEEPtot – PEEPset

= 14cmH2O – 10cmH2O

= 4cmH2O

Your patient has 4cmH2O 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 eliminate 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 may 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 allowing him to progress in his weaning.

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