Ventilator Dyssynchrony

Ventilator dyssynchrony is an uncoordinated interaction between the patient and the ventilator during a spontaneous (or partially spontaneous, e.g. SIMV) mode of breathing, that leads to:

Abdominal muscle contraction is highly sensitive for dyssynchrony.

Failure to Trigger

Failure of a ventilator to respond to a patients respiratory effort. May be due to:

  • Trigger setting too high
    • Reducing the trigger may help, but may also lead to over-triggering
    • Changing from a pressure to a flow trigger may be beneficial
      • Pressure trigger generally requires more work but is more controllable by the patient
      • Flow trigger is more comfortable but greater risk of over-triggering
        Flow triggers are more modern and used by default on most newer ventilators.
  • High iPEEP
    High intrinsic PEEP ↑ inspiratory threshold work, making it harder to reach the prescribed trigger. ↑ Extrinsic PEEP to 80% of iPEEP ↓ the gap and makes triggering a breath easier.

Over-triggering

Inappropriate delivery of a spontaneous breath. May be due to:

  • Trigger too low
  • Distorted measurements
    • Cardiogenic oscillations
      Movement of tracheal gas caused by cardiac activity may reach the triggering threshold.
    • Condensation
    • Circuit leak
  • Diaphragmatic pacing
    Inappropriate pacing of the diaphragm by a cardiac pacemaker (usually temporary epicardial leads).

Flow Starvation

A patient will expect an inspiratory flow proportionate to their work of breathing. If the prescribed flow rate is too low, then the patient:

  • Attempts to augment inspiration with further negative pressure effort
    The inspiratory pressure waveform becomes “scalloped”, with a negative deflection in the positive pressure breath.

Double Triggering

Delivery of two pressure-supported breaths during one inspiratory effort. This occurs when:

  • The prescribed inspiratory time is shorter than the neural inspiratory time
    Inspiratory effort continues whilst the inspiratory valve is closed, so there is negative pressure in the circuit but no flow.
  • Inspiratory effort continues once the next trigger window is reached
    • A second supported breath is delivered
      This breath is usually low-volume, because it is occurring with high baseline lung volume.

Reverse Triggering

Occurs when a controlled breath produces involuntary patient inspiratory effort.

References

  1. Dünser MW, Dankl D, Petros S, Mer M. Clinical Examination Skills in the Adult Critically Ill Patient. Springer International Publishing; 2018.