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Patient – Ventilator Asynchrony
Dr Vincent Ioos
Medical ICU – PIMS
APICON 2008
Workshop on Mechanical Ventilation
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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?
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Patient triggered ventilation
• Assisted mechanical ventilation
• Avoid ventilator induced diaphragmaticdysfunction
• Providing sufficient level of ventilatory support to reduce patient’s work of breathing
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Volume or pressure oriented?
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Volume oriented modes
• Inspiratory flow is preset
• Inspiratory time determines the Vt
• The variable parameter is the airway peak and plateau pressure
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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
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Airway Pressure
C = Vt / ∆P and ∆P = P Plat - PEEP
Paw= Po + Vt/C + RV
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Flow shapes
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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
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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
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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)
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Look at the curves !
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A challenge for the intensivist
• Discomfort anxiety
• Increased work of breathing
• Increased requirement of sedation
• Increased length of mechanical ventilation
• Increased incidence of VAP
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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
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Ventilation phases
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Trigger asynchrony
• Ineffective triggerring: muscular effort withoutventilator trigger
• Double triggerring
• Auto-triggering
• Insensitive trigger: triggering that requiresexcessive patient effort
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Ineffective triggering
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Double triggering
• Cough
• Sighs
• Inedaquate flow delivery
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Auto-triggering
• Circuit leak
• Water in the circuit
• Cardiac oscillations
• Nebulizer treatments
• Negative suction applied trough chest tube
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Flow asynchrony
• Fixed flow pattern (volume oriented)
• Variable flow pattern (pressure oriented)
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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
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-Ineffictive triggering at30 l/mn
- Increase in flow rate
- Subsequent increase ofexpiratory time
- Decreased dynamichyperinflation
- Subsequent decreasein ineffictive trigerring
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Importance of flow pattern
Increase in peak-flow setting fron 60 to 120 l/mn eliminated scooped appearance of the
airway pressure waveform
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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
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Termination asynchrony
• Ventilator should cycle at the end of the neural inspiration time
• Delayed termination:
–Dynamic hyperinflation
–Trigger delay
– Ineffective triggering
• Premature termination
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Set inspiratory time < 1 sec
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PSV = 10 cmH2O
Inspiratoy flow terminate despitecontinued Pes defelection
Double Trigerring
Patient 1 Patient 2
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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
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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
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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