This document discusses patient-ventilator asynchrony, which occurs when there is a mismatch between the patient's respiratory effort and the ventilator's breathing cycles. It describes different types of asynchrony that can occur during triggering, cycling, flow delivery, and expiration. The key factors that determine inspiratory flow during pressure support ventilation are also examined. Close monitoring of pressure and flow waveforms is emphasized to identify asynchrony and adjust ventilator settings appropriately to reduce the patient's work of breathing.

1. Patient – Ventilator Asynchrony
Dr Vincent Ioos
Medical ICU – PIMS
APICON 2008
Workshop on Mechanical Ventilation

2. Goal of mechanical ventilation
• Do you mechanically ventilate your patient to
reverse diaphragmatic fatigue ?
or
• Do you encourage greater diaphragm use to
avoid ventilator-induced diaphragmatic
dysfunction?

4. Patient triggered ventilation
• Assisted mechanical ventilation
• Avoid ventilator induced diaphragmatic
dysfunction
• Providing sufficient level of ventilatory support
to reduce patient’s work of breathing

6. Volume or pressure oriented?

7. Volume oriented modes
• Inspiratory flow is preset
• Inspiratory time determines the Vt
• The variable parameter is the airway peak and
plateau pressure

8. Equation of insuflated gases
in flow assist control ventilation
• Describes interactions between the patient
and the ventilator
• Pressure required to deliver a volume of gas
in the lungs is determined by elastic and
resistive properties of the lung
Paw = Vt/C +VR + PEP

9. Airway Pressure
C = Vt / P and P = P Plat - PEEP
Paw= Po + Vt/C + RV

10. Flow shapes

11. Pressure oriented modes
• Pressure in airway is the preset parameter
• Flow is adjusted at every moment to reach the
preset pressure
• The variable parameter is Vt

12. Equation of motion
in pressure support ventilation
• Pressure = pressure applied by the ventilator
on the airway + pressure generated by
respiratory muscles
• Pmus is determined by respiratory drive and
respiratory muscle strenght
Paw + Pmus = Vt/C + VxR + PEP

13. Determinant factors
of inspiratory flow in PSV
• Pressure support setting
• Pmus (inspiratory effort)
• Airway resistance
• Respiratory system compliance
• Vt directly depends on inspiratory flow, but
also on auto-PEEP (decreases the driving
pressure gradient)

15. Look at the curves !

16. A challenge for the intensivist
• Discomfort anxiety
• Increased work of breathing
• Increased requirement of sedation
• Increased length of mechanical ventilation
• Increased incidence of VAP


17. Patient-ventilator asynchrony
• Mechanical ventilation: 2 pumps
– Ventilator controlled by the physician
– Patient’s own respiratory muscle pump
• Mismatch between the patient and the ventilator
inspiratory and expiratory time time
• Patient « fighting » with the ventilator

20. Ventilation phases

21. Trigger asynchrony
• Ineffective triggerring: muscular effort without
ventilator trigger
• Double triggerring
• Auto-triggering
• Insensitive trigger: triggering that requires
excessive patient effort

22. Ineffective triggering

24. Double triggering
• Cough
• Sighs
• Inedaquate flow delivery

25. Auto-triggering
• Circuit leak
• Water in the circuit
• Cardiac oscillations
• Nebulizer treatments
• Negative suction applied trough chest tube

28. Flow asynchrony
• Fixed flow pattern (volume oriented)
• Variable flow pattern (pressure oriented)

29. Volume oriented ventilation
(fixed flow pattern)
• Inspiratory flow varies according to the
underlying condition
• If patient’s flow demand increases, peak flow
should be adjusted accordingly
• Usually, peak flow is too low
• Dished-out appearance of the presure-wave-
form
• Importance of flow-pattern

31. -Ineffictive triggering at
30 l/mn
- Increase in flow rate
- Subsequent increase of
expiratory time
- Decreased dynamic
hyperinflation
- Subsequent decrease
in ineffictive trigerring

32. Importance of flow pattern
Increase in peak-flow setting fron 60 to 120
l/mn eliminated scooped appearance of the
airway pressure waveform

33. Pressure oriented ventilation
(variable flow)
• Peak flow is depending on :
– Set target pressure
– Patient effort
– Respiratory system compliance
• Adjustement : rate of valve opening = rise time =
presure slope = flow acceleration

37. Termination asynchrony
• Ventilator should cycle at the end of the neural
inspiration time
• Delayed termination:
– Dynamic hyperinflation
– Trigger delay
– Ineffective triggering
• Premature termination

40. Set inspiratory time < 1 sec

41. PSV = 10 cmH2O
Inspiratoy flow terminate despite
continued Pes defelection
Double Trigerring
Patient 1 Patient 2

42. Expiratory asynchrony
• Shortened expiratory time:
Auto-PEEP  trigger asynchrony
– Delay in the relaxation of the expiratory
muscle activity prior to the next mechanical
inspiration
– Overlap between expiratory and insiratory
uscle activity
• Prolonged expiratory time

43. Auto-PEEP created by flow patterns
that increases inspiratory time
• Lower peak flow during control ventilation
• Switch from constant flow to descending ramp
flow
• Inadequate pressure slope during presure
controlled ventilation
• Termination criteria that prolong expiratory
time during PSV

47. Conclusion
• Look at your patient !
• Look at the curves !
• Have a good knowledge of the ventilation
modalities of the ventilator you are using
• Excessive ventilatory support leads to ineffective
triggering
• Do not forget to set trigger sensitivity, to avoid
excessive effort and auto-triggering