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 diaphragmatic dysfunction • 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 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
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Airway Pressure Paw= Po + Vt/C + RV
C = Vt / ∆P
and ∆P = P Plat - PEEP
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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 motion in pressure support ventilation Paw + Pmus = Vt/C + VxR + PEP • Pressure = pressure applied by the ventilator on the airway + pressure generated by respiratory muscles • Pmus is determined by respiratory drive and respiratory muscle strenght
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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)
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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 ventilator inspiratory and expiratory time time • Patient « fighting » with the ventilator
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-waveform • Importance of flow-pattern
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-Ineffictive triggering at 30 l/mn - Increase in flow rate - Subsequent increase of expiratory time - Decreased dynamic hyperinflation - Subsequent decrease in 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 Patient 1
Inspiratoy flow terminate despite continued Pes defelection
Patient 2
Double Trigerring
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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
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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 ramp flow • Inadequate pressure slope during presure controlled ventilation • Termination criteria that prolong expiratory time 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 avoid excessive effort and auto-triggering
