Patient ventilator synchrony

Ideally, when a patient is connected to a mechanical ventilator, the patient and the ventilator are synchronized. This means breaths are given at the right time and done so with the correct amount of support. When there is a mismatch between what the patient wants or needs, and what ventilator is doing (or not doing), asynchrony may occur.

There are three main forms of interaction between the patient and ventilator, each of may be problematic and managed differently.

1. Trigger Asynchrony – related to breath initiation
2. Phase Asynchrony – related to breath duration
3. Flow and Volume Mismatching – related to ventilator settings of inspiratory flow and/or tidal volume

Trigger asynchrony occurs when the patient attempts to trigger a breath but is unable to, either because of inspiratory muscle weakness, inappropriate trigger sensitivity set on the ventilator or dynamic hyperinflation (auto-PEEP or intrinsic PEEP). Phase “cycle” asynchrony occurs when the inspiratory phase of the ventilator-delivered breath does not match the patient’s neural impulse for ventilation (respiratory rate or inspiratory time). To initiate a breath the patient’s respiratory effort must exceed a threshold (trigger) set in the machine. When this fails to happen this can be due to three factors:

1. Inspiratory muscle insufficiency
2. Inappropriate trigger sensitivity set on the ventilator
3. The presence of “intrinsic PEEP” (also known as “auto-PEEP” or dynamic hyperinflation)

Phase “cycle” asynchrony occurs when the inspiratory phase of the ventilator does not match the patient’s neural impulse for inspiration, such as when:

1. The set RR exceeds the spontaneous rate (without suppressing respiratory drive)
2. The set inspiratory time is either too long or too brief

Flow mismatching occurs in 3 conditions:

1. When the peak inspiratory flow rate is below patient peak flow.
2. Transient flow discrepancies between ventilator and patient flow pattern.
3. When the set flow rate greatly exceeds patient flow demand.

Tidal volume mismatching occurs when ventilator VT < patient VT and is most commonly encountered during lung-protective ventilation. It also may occur as a patient undergoes changes in activity/metabolism and/or response to pain, discomfort, regardless of ventilation strategy.

Content in this article originally was written for Open Critical Care and first appeared on COVIDprotocols.org, a collaborative project of Open Critical Care.