Variable—Ending Inspiration

Inspiration ends when one of the variables (flow, pressure, volume, or time) has reached a preset value (1). Flow and time are the most common variables used to end inspiration for pressure-limited modes and volume and time are the most common variables for 

In pressure ventilation, most ventilators cycle from inspiration to expiration when inspiratory flow decreases to a predetermined percent of its peak flow or when a preset inspiratory time is reached. It has become increasingly clear that cycling plays an important role in assuring patient-ventilator synchrony (11). This has been quite well studied in pressure support ventilation and several authors have shown that a fixed cycle value can lead to premature cycling in patients with restrictive diseases and delayed cycling in those with obstructive disease (12,13). 

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The variable that limits inspiration is the one that attains a constant value before inspiration ends. If one or more variables are not allowed to rise above a preset value during the inspiratory time, it is referred to as a limit variable. This is not necessarily the variable which will end inspiration (1), for example pressure support (PS), where pressure is limited but inspiration is ended by a drop in inspiratory flow. 

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