Phase Variables

Breath Design and Delivery

  • Ventilator TCT: Time frame from inspiration start to expiration end.
  • 4 Phases of Breath:
    • Trigger Variable: Initiates inspiratory phase.
    • Limit Variable: Stops lung inflation.
    • Cycle Variable: Allows exhalation.
    • Baseline Variable: Determines baseline level.

Phase Variables

  • Breath Definition: One cycle of positive flow (inhalation) and negative flow (exhalation).
  • 4 Phase Variables:
    • Trigger
    • Limit
    • Cycle
    • Baseline

Trigger Variable

  • Trigger Definition: Begins the inspiratory phase. Can be:
    • Patient-Initiated Breath: Triggered by a change in pressure or flow by the patient.
    • Mandatory Breath: Time-triggered by the ventilator.
    • Neurally Adjusted Ventilatory Assist (NAVA): Uses diaphragm's electrical activity to trigger a breath.

Types of Breaths

  1. Mandatory (Controlled) Breaths: Ventilator controls the breath.
  2. Assisted Breaths: Patient controls timing; ventilator provides remaining support.
  3. Spontaneous Breaths: Patient controls most aspects with minimal support.

Trigger Variable Types

  • Time Triggered: Ventilator triggers a breath after a set time, e.g., rate set at 12 breaths/min = 1 breath every 5 seconds.
  • Patient Triggered: RT sets sensitivity; breath is initiated based on patient effort.
  • Manual Triggering: RT can manually trigger a breath via a button.

Sensitivity Settings

  • Patient Triggering: Should be set to minimize effort without causing auto-triggering (ventilator activating without effort).
    • Trigger settings too sensitive can lead to unwanted breaths.
    • Monitor for increased Work of Breathing (WOB).

Pressure Triggering

  • Mechanism: Initiates a breath when patient’s inspiratory effort drops airway pressure.
  • Typical Sensitivity Setting: -0.5 to -1.5 cmH2O; sensitivity at 2.0 cmH2O means patient must create a drop of ≥ 2 cmH2O.

Limit Variable

  • Definition: Maintains a preset level before inspiration ends without terminating inspiration.
  • Possible Types:
    • Pressure: Limits pressure within a certain range.
    • Volume: Limits maximum volume delivered.
    • Flow: Limits maximum flow during breath.

Case Study Discussion

  • Patient Parameters: VT = 467 ml, RR = 15, PEEP 8, Trigger = -1 LPM, FiO2 = 0.30
    • Identify triggers and limits for breaths.
  • Concerns: Low exhaled volume alarm, low pressure alarm; monitor waveform abnormalities for insights into ventilatory issues.

Examples of Triggering and Limiting

  • Time Triggering Example: Breath is triggered after a set time, e.g., every 5 seconds.
  • Patient Pressure Triggering: Pressure triggering adjusts slightly based on PEEP levels; auto-PEEP can complicate this.
  • Flow Triggering: Triggered when flow decreases significantly, generating a breath based on set parameters.