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Charles Gomersall
First posted June 2008
Definition
Uncoupling of the mechanical delivered breath and neural respiratory effort.
Types
Dysynchrony can occur in each phase of the breath:
- expiratory-inspiratory cycling (triggering)
- inspiration
- inspiratory-expiratory cycling
Expiratory-inspiratory cycling
Autotriggering
- 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
Triggering delay & ineffective efforts
- excessive delay between inspiratory muscle contraction and delivery of
breath
- patient factors
- dynamic hyperinflation resulting in intrinsic PEEP
- low respiratory drive
- weak inspiratory muscles
- ventilator factors
- high level of pressure support
- high tidal volume
- 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)
Inspiration
- dissociation between patient's respiratory effort and ventilatory assist
- 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.
- inspiratory asynchrony may be reduced by use of proportional assist
ventilation (PAV) or NAVA
Inspiratory-expiratory cycling
Premature opening of exhalation valve
- 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
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
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