Waveform of the week: Drive pressure too low

Driving pressure or set pressure during mechanical ventilation utilizing a pressure targeted mode (PC-CMV, PC-IMV, PC-CSV) may be inadequate for patients' inspiratory flow demands. 

A peak pressure of 15 to 20 cmH2O is generally needed to provide significant support if the goal is to off-load work of breathing and/or resting the patient.

By evaluating the flow waveform the operator can identify a appropriate pressure setting to meet patients inspiratory efforts. The flow waveform should have a constant linear deceleration for the inspiratory phase & a   constant acceleration to baseline during the expiratory phase. 

During the inspiratory phase once flow starts to decelerate it should not rise again. If the flow rises again this is a sign of an increased respiratory drive & that the pressure setting is too low. The below picture provides a example of both a normal flow waveform pattern & the dis-synchrony related to a high respiratory drive. 

Inadequate driving/set pressure during PC-CMV as evidence by 'camel backing' flow rising after  deceleration.

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A Quick & Easy Way to Set "T-Low" During Airway Pressure Release Ventilation

A quick & easy way to initially set "T-Low" during Airway Pressure Release Ventilation is to use the "Expiratory Time Constant (RCexp)". The RCexp indicates alveolar emptying time and it takes at least 4 time constants for adequate alveolar emptying (~99%).

References state set T-Low to obtain a "Peak Expiratory Flow Rate Termination Point (T-PEFR)" at 50 to 75% of the measured "Peak Expiratory Flow Rate"