Describing patient-ventilator interactions


For getting an optimal synchronization between patient’s breathing cycle and the pressure cycle delivered by the ventilator, there are two triggerings,
- i) one that induces the pressure rise from the low pressure Pl to the high pressure Ph
- and ii) one that provokes the pressure release from Ph to Pl.
When the two cycles are phase synchronized, pressure rise is triggered at the beginning of the inspiratory effort, that is, at the beginning of the decrease of the ventilatory muscular pressure Pmus, and pressure release is triggered at the beginning of the pressure release associated with the beginning of the relaxation of the muscular pressure. Consequently, most of asynchrony events can be described using two parameters as follows (Fig. 1).

  • the delay \delta_{\rm h} between the beginning of the decrease of the muscular pressure and the onset of the pressure rise
  • the delay \delta_{\rm b} between the beginning of the relaxation of the muscular pressure and the pressure release to its low value.

Depending on the value of these two parameters we may have :

  • \delta_{\rm h} >0 : delayed pressure rise cycle (delayed inspiratory triggering) ;
  • \delta_{\rm h} <0 : advanced pressure rise cycle (advanced inspiratory triggering) ;
  • \delta_{\rm l} >0 : delayed pressure release cycle (delayed cycling) ;
  • \delta_{\rm l} <0 : advanced pressure release cycle (premature cycling).
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Fig. 1. Definition of the parameters used for characterizing a ventilation cycle.

We here proposed our own terminology for two main reasons. First, a ventilator is not breathing, so it cannot trigger inspiration. Second, the term "cycling" has no intuitive meaning and does not reflect the symmetry existing between pressure rise and pressure release. Moreover, there are two triggers in the functionning of the devices. We therefore believe that our terminology is closer to what actually occurs in patient-ventilator interactions. There are also other types of asynchrony events as

  • non-triggered cycles resulting from inefficient inspiratory efforts for which there is no pressure rise (neither pressure release, of course) :
  • double-triggerings corresponding to two pressure cycles for a single breathing cycle ;
  • driven cycle (backup cycle) when the ventilator triggers a pressure rise according to a backup frequency when the patient is not breathing to often ;
  • self-triggered cycle when the ventilator is apparently triggering a pressure rise without any reason (this type of asynchrony events is not yet fully understood and could depend on the ventilator used).

In order to describe the various situations we encounter in actual situations, we introduced a color map (Fig. 2) as follows [1].

  • phase Synchronized Cycles (CS) : \delta_{\rm h}  \approx 0, 
\delta_{\rm l} \approx 0 ;
  • cycles with a correct triggering of the pressure rise (\delta_{\rm h} >0) but with a significant delay of the pressure release (| \delta_{\rm h} |  > \tau_c) and designated by Dab for advanced pressure release and Drb for delayed pressure release ;
  • double-triggered cycles (DD) ;
  • non-triggered cycles (ND).

Cycles can then be (more or less objectively) ranked from the best (purple) to the worst (red) quality using the color map as shown in Fig. 2.

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Fig. 2. Color map corresponding to various quality of patient-ventilator interactions.

[1] E. Fresnel, J.-F. Muir & C. Letellier, Realistic human muscle pressure for driving a mechanical lung, EPJ Nonlinear Biomedical Physics, 2, 7, 2014. Online

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