Patient – Ventilator Asynchrony Dr Vincent Ioos Medical ICU – PIMS APICON 2008 Workshop 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 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

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

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 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

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

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