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?

3

4

Patient triggered ventilation

• Assisted mechanical ventilation

• Avoid ventilator induced diaphragmaticdysfunction

• Providing sufficient level of ventilatory support to reduce patient’s work of breathing

5

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

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 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 factorsof 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 drivingpressure gradient)

14

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 ventilatorinspiratory and expiratory time time

• Patient « fighting » with the ventilator

18

19

20

Ventilation phases

21

Trigger asynchrony

• Ineffective triggerring: muscular effort withoutventilator trigger

• Double triggerring

• Auto-triggering

• Insensitive trigger: triggering that requiresexcessive patient effort

22

Ineffective triggering

23

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

26

27

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 theunderlying condition

• If patient’s flow demand increases, peak flowshould be adjusted accordingly

• Usually, peak flow is too low

• Dished-out appearance of the presure-wave-form

• Importance of flow-pattern

30

31

-Ineffictive triggering at30 l/mn

- Increase in flow rate

- Subsequent increase ofexpiratory time

- Decreased dynamichyperinflation

- Subsequent decreasein 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

34

35

36

37

Termination asynchrony

• Ventilator should cycle at the end of the neural inspiration time

• Delayed termination:

–Dynamic hyperinflation

–Trigger delay

– Ineffective triggering

• Premature termination

38

39

40

Set inspiratory time < 1 sec

41

PSV = 10 cmH2O

Inspiratoy flow terminate despitecontinued Pes defelection

Double Trigerring

Patient 1 Patient 2

42

Expiratory asynchrony

• Shortened expiratory time:

Auto-PEEP� trigger asynchrony

–Delay in the relaxation of the expiratorymuscle activity prior to the next mechanicalinspiration

–Overlap between expiratory and insiratoryuscle 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 rampflow

• Inadequate pressure slope during presurecontrolled ventilation

• Termination criteria that prolong expiratorytime during PSV

44

45

46

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 avoidexcessive effort and auto-triggering