Patient – Ventilator Asynchrony

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 gasesin 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 Paw= Po + Vt/C + RV C = Vt / P and P = P Plat - PEEP

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 motionin 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 Patient 1 Patient 2 Inspiratoy flow terminate despite continued Pes defelection Double Trigerring

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