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

Patient – Ventilator Asynchrony

Dr Vincent IoosMedical ICU – PIMS

APICON 2008Workshop on Mechanical

Ventilation

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?

Patient triggered ventilation

• Assisted mechanical ventilation• Avoid ventilator induced diaphragmatic

dysfunction• Providing sufficient level of ventilatory

support to reduce patient’s work of breathing

Volume or pressure oriented?

Volume oriented modes

• Inspiratory flow is preset

• Inspiratory time determines the Vt

• The variable parameter is the airway peak and plateau pressure

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

Airway Pressure

C = Vt / P and P = P Plat - PEEP

Paw= Po + Vt/C + RV

Flow shapes

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

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

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)

Look at the curves !

A challenge for the intensivist

• Discomfort anxiety • Increased work of breathing• Increased requirement of sedation• Increased length of mechanical ventilation• Increased incidence of VAP

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

Ventilation phases

Trigger asynchrony

• Ineffective triggerring: muscular effort without ventilator trigger

• Double triggerring• Auto-triggering• Insensitive trigger: triggering that

requires excessive patient effort

Ineffective triggering

Double triggering

• Cough

• Sighs

• Inedaquate flow delivery

Auto-triggering

• Circuit leak• Water in the circuit• Cardiac oscillations• Nebulizer treatments• Negative suction applied trough chest

tube

Flow asynchrony

• Fixed flow pattern (volume oriented)• Variable flow pattern (pressure oriented)

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

-Ineffictive triggering at 30 l/mn

- Increase in flow rate

- Subsequent increase of expiratory time

- Decreased dynamic hyperinflation

- Subsequent decrease in ineffictive trigerring

Importance of flow pattern

Increase in peak-flow setting fron 60 to 120 l/mn eliminated scooped

appearance of the airway pressure waveform

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

Termination asynchrony

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

• Delayed termination: – Dynamic hyperinflation– Trigger delay– Ineffective triggering

• Premature termination

Set inspiratory time < 1 sec

PSV = 10 cmH2O

Inspiratoy flow terminate despite continued Pes defelection

Double Trigerring

Patient 1 Patient 2

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

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

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