13 CyclingCycling refers to how the ventilator ends the inspiratory phase of the breath
14 Cycling MechanismsVolume cycling – inspiration ends when a preset tidal volume is deliveredPressure cycling – inspiration ends when a preset pressure is reached on the airwayTime cycling – inspiration ends when a preset inspiratory time has elapsedFlow cycling – inspiration ends when a preset flow has been reached
15 The mechanism that starts the inspiratory phase TriggeringThe mechanism that starts the inspiratory phase
16 Trigger MechanismsPressure triggered – a drop in airway pressure triggers the ventilatorFlow triggered – a constant (bias) flow of gas passes through the ventilator circuit. When the patient starts to inhale the ventilator detects the drop in bias flow and triggersTypes of triggered breaths: patient = assisted; ventilator = controlled, operator = manual
28 Peak PressurePressure on manometer immediately at end of inspiratory phaseRepresents pressure needed to overcome both elastic and airway resistanceUsed to calculate dynamic complianceCdyn = VT/Peak pressurePEAK PRESSURE WILL CHANGE WHEN EITHER ELASTIC OR AIRWAY RESISTANCE CHANGES!
29 Plateau PressurePressure on manometer after inspiration has ended but before expiration has startedRepresents pressure needed to overcome elastic resistance onlyUsed to calculate static complianceCstat = VT/plateau pressurePLATEAU PRESSURE CHANGES ONLY WHEN ELASTIC RESISTANCE CHANGES
30 Clinical Analysis By Comparing Peak and Plateau Pressure Changes Remember – a change in elastic resistance will affect both peak and plateau pressureRemember – a change in airway resistance only affects the peak pressureCompare the change in plateau pressures first, then compare the changes in peak pressure
31 Resistance and Pressure Vary Directly Resistance and Pressure Vary Inversely With Compliance