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# Phase Variables A ventilator-supported breath can be divided into four phases: inspiration, the change from inspiration to expiration, and expiration. More detail can be learned by studying the changes in flow and time during these phases. When a particular phase variable is examined, it is ter...

# Phase Variables A ventilator-supported breath can be divided into four phases: inspiration, the change from inspiration to expiration, and expiration. More detail can be learned by studying the changes in flow and time during these phases. When a particular phase variable is examined, it is termed a phase variable. ## Trigger Variable The trigger variable is the variable that starts inspiration. This can be pressure, volume, flow, or time. Many ventilators use time or pressure as a trigger variable. ### Time-Triggered A time-triggered breath is initiated when a pre-set time interval has elapsed. The frequency of the ventilator controls the time interval for one complete respiratory cycle (inspiratory time and expiratory time). For example, if the frequency is set at 12 breaths per minute (60 seconds), the time-triggering interval for each breath is 5 seconds. The ventilator will deliver a breath every 5 seconds, regardless of the patient's breathing effort or requirement. (60 seconds/12 breaths = 5 seconds/breath) ### Pressure-Triggered A pressure-triggered breath is started when the ventilator senses the patient's spontaneous inspiratory flow gradient. The patient generates a pressure gradient or inspiratory effort, triggering the ventilator. (Figure 3-9). The amount of negative pressure (below the patient's baseline airway pressure or end-expiratory pressure) required to trigger the ventilator is the *sensitivity level*. Acceptable sensitivity levels for pressure-triggering vary from -1 to -5 cm H₂O below the patient's baseline pressure. For example, if the sensitivity for pressure triggering is set at -3 cm H₂O, the patient needs to generate a pressure of -3 cm H₂O at the airway opening to trigger the machine. If the sensitivity increases to -5 cm H₂O, the ventilator becomes less sensitive to the patient's inspiratory effort—more effort is needed to trigger inspiration. Increasing the sensitivity from -3 to -5 cm H₂O decreases the sensitivity setting on the ventilator. In situations with auto-PEEP, the triggering pressure may increase. The patient must overcome both the auto-PEEP level and the ventilator's sensitivity settings to successfully trigger the ventilator. (See Figure 12).