What is the controlling variable?

Are we aiming to deliver a set pressure or a set volume?

Do we want each breath to be delivered to a maximum pressure of 20cm H2O for example or do we want the maximum volume to be delivered to be 500mls for example?

This is a value set by the clinician via the ventilators control panel.

What is triggering each breath?

Is it the patient who, by initiating a breath, changes the flow or the pressure. The ventilator then responds to this by delivering a breath.

Or is it time that triggers each breath? The clinician sets this by telling the ventilator to deliver a respiratory rate of 12 sec/min for example. So the ventilator will deliver a breath every 5 seconds. 

What is terminating each breath?

Is it a fall in the gas flow or pressure?

Or is it when a certain time period passes?

Then ask yourself if the mode you are using contains only one type of breath or several types of breath. Does it allow the patient to sometimes initiate a supported breath for example, whilst still delivering some mandatory breaths?

The next question should be is the controlled variable, (pressure (flow) or volume) , adjusted on a breath by breath basis or even within each breath?

What is the controlling variable?

Different modes based on controlling variable will include:

Volume controlled ventilation. (VCV)

Pressure controlled ventilation. (PCV)

Assist controlled ventilation. (ACV)

Pressure support ventilation (PSV)

Continuous positive airways pressure (CPAP)

What is the trigger variable?

Different modes based on trigger variable will include:

Supported intermittent mandatory ventilation. (SIMV)

Bilevel positive airways pressure. (BiPAP)

Airway pressure release ventilation (APRV)

What is the terminating each breath?

Different modes based on cycle variable will include:

Dual modes.

Adaptive support ventilation. (ASV)

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