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Up Dysynchrony e-lectures Figure 5a & 5b Modes of ventilation Non-invasive ventilation Physiological effects Specific ventilators

Charles Gomersall

First posted June 2008, updated November 2009


Uncoupling of the mechanical delivered breath and neural respiratory effort.


Dysynchrony can occur in each phase of the breath:

  • expiratory-inspiratory cycling (triggering)
  • inspiration
  • inspiratory-expiratory cycling

Expiratory-inspiratory cycling


  • triggering of the ventilator in the absence of inspiratory muscle contraction
  • may result from:
    • random noise in circuit
    • water in circuit
    • circuit leak
    • cardiogenic oscillations
  • most common cause for this is a circuit leak. This results in a fall in circuit pressure (figure 1) or a discrepancy between flow leaving the ventilator and returning to the ventilator.

    Figure 1 Genuine patient triggering (middle) and auto-triggering (right) and with the ventilator set to pressure trigger and PEEP>0


Triggering delay & ineffective efforts

  • excessive delay between inspiratory muscle contraction and delivery of breat. This may be detected from the presure and flow waveforms (figure 2)

Figure 2. Trigger delay indicated by a negative deflection in pressure waveform prior to delivery of breath and a change in slope of flow waveform. Shaded area on pressure waveform represents work of triggering

  • in more extreme cases may be detectable on inspection of the patient with a visible delay between activation of inspiratory muscles and delivery of breath (allow your browser to run scripts in order to see animation below)

  • in most extreme cases triggering fails (figure 3)

Figure 3. Trigger failure

  • patient factors
    • low respiratory drive

    • weak inspiratory muscles

    • partially blocked ETT or tracheostomy

    • dynamic hyperinflation resulting in intrinsic PEEP

    • triggering requires flow of gas into the lungs. This requires a positive pressure gradient between the ventilator circuit and the alveoli

    • intrinsic PEEP reflects a positive end-expiratory alveolar pressure. If this exceeds set PEEP this will result in a negative pressure gradient between the circuit and the alveoli

    • the inspiratory muscles have to overcome this negative pressure gradient and change it into a positive pressure gradient in order to generate inspiratory flow

    • this results in a delay in delivering the breath or ineffective efforts

    • applying or increasing set PEEP will reduce this problem

  • ventilator factors
    • high level of pressure support or high tidal volume causing gas trapping and intrinsic PEEP (see above)
    • expiratory asynchrony with delayed opening of exhalation valve
  • triggering delay and ineffective efforts may be reduced by the use of neurally adjusted ventilatory assist (NAVA)


  • dissociation between patient's respiratory effort and ventilatory assist
    • profile
      • the dissociation between the profile of the patient's effort and the ventilatory assist may be reduced by adjusting the pressure rise time in patients receiving pressure support ventilation. However it should be noted that changes in rise time may have an impact on inspiratory to expiratory cycling. Increasing the rise time will decrease the peak inspiratory flow rate. As inspiratory to expiratory cycling in pressure support is usually dependent on flow falling to a set percentage of peak inspiratory flow a change in rise time will affect the absolute flow rate at which cycling occurs
      • in constant flow modes (eg volume preset assist control) the inspiratory flow rate may be inadequate to match the patient’s attempted inspiratory flow rate (figure 4)

        Figure 4. Inadequate inspiratory flow
    • amplitude
  • inspiratory asynchrony may be reduced by use of proportional assist ventilation (PAV) or NAVA

Inspiratory-expiratory cycling

Premature inspiratory-expiratory cycling

  • inspiratory muscle contraction continues into mechanical expiratory phase
  • associated with:
    • modes of ventilation with fixed (and short) inspiratory time
    • in pressure support mode:
      • low levels of pressure support
      • short respiratory time constant (eg ARDS)
      • relatively high cycling off threshold
      • dynamic hyperinflation
  • may result in a characteristic change in flow waveform (figure 5)

Figure 5. Change in flow waveform characteristic of premature inspiratory to expiratory cycling in pressure support mode

Delayed opening of exhalation valve

  • mechanical inspiration continues into neural expiration
  • may result in dynamic hyperinflation, particularly in patients with COPD which may in turn increase expiratory-inspiratory cycling dysynchrony
  • may also decrease the patient's spontaneous breathing frequency, possibly mediated by the Hering-Breuer reflex
  • associated with:
    • long set inspiratory time
    • in pressure support mode:
      • long respiratory time constant (eg COPD)
      • high pressure support level
      • low cycling off threshold

Further reading

Kondili E et al. Modulation and treatment of patient-ventilator dysynchrony. Curr Opin Crit Care, 2007; 13:84-9

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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