3.1 Waveform Analysis & Dyssynchrony

Introduction to Waveform Analysis

Advanced waveform analysis is a core competency for the RRT-ACCS. Visualizing mechanics in real-time allows for immediate bedside adjustments. You must confidently analyze both scalars (parameter vs. time) and loops (parameter vs. parameter).

Flow-Volume Loops

The flow-volume loop plots flow on the Y-axis and volume on the X-axis. The inspiratory limb is above the baseline, and expiratory is below.

• Airway Obstruction: A classic 'scooped' or coved expiratory limb indicates expiratory airflow limitation, such as in asthma or COPD.
• Fixed Airway Obstruction: A box-like shape affecting both inspiration and expiration (e.g., tracheal stenosis).
• Variable Extrathoracic Obstruction: Truncated inspiratory flow with normal expiration (e.g., vocal cord paralysis).
• Variable Intrathoracic Obstruction: Truncated expiratory flow with normal inspiration (e.g., tracheomalacia).
• Secretions or Condensate: A jagged or sawtooth pattern in the expiratory and/or inspiratory flow limb.

Pressure-Volume Loops

The pressure-volume loop plots volume on the Y-axis and pressure on the X-axis. In positive pressure ventilation, the loop progresses counterclockwise.

• Overdistension ('Beaking'): A flattening of the upper right portion of the loop. Volume increases minimally despite significant increases in pressure, signifying overdistension. The clinical solution is to reduce tidal volume (Vt) or peak inspiratory pressure (PIP).
• Changes in Compliance: As compliance decreases (stiffer lungs, ARDS), the loop shifts downward and to the right, becoming flatter. Conversely, as compliance improves, the loop stands more upright.
• Lower Inflection Point (LIP): Represents the pressure at which dependent alveoli are recruited. Setting PEEP slightly above the LIP can prevent atelectrauma.
• Upper Inflection Point (UIP): Represents the transition to overdistension.

Scalars: Flow-Time and Pressure-Time Curves

Scalars plot variables against time (X-axis). They are vital for identifying timing-related dyssynchrony.

Detecting Auto-PEEP (Intrinsic PEEP)

Auto-PEEP is identified on the flow-time scalar when the expiratory flow does not return to the zero baseline before the next inspiratory breath is delivered. This indicates air trapping.

• Clinical Solution: Decrease respiratory rate, decrease inspiratory time (which increases expiratory time), or increase expiratory flow. In severe cases like status asthmaticus, permissive hypercapnia may be necessary to allow for prolonged expiratory times.

Identifying Dyssynchrony

Patient-ventilator dyssynchrony increases work of breathing and mortality.

Flow Starvation (Flow Asynchrony)

Occurs primarily in volume control ventilation when the set inspiratory flow is inadequate to meet the patient's demand.

• Waveform Presentation: The pressure-time scalar exhibits a "scooped-out" or concave appearance during inspiration, instead of a normal convex or linear rise.
• Clinical Solution: Increase the inspiratory flow rate. You can also change to a decelerating flow pattern or switch to pressure control ventilation, which provides a variable, demand-based flow.

Double Triggering

Occurs when the patient's neural inspiratory time is longer than the ventilator's set inspiratory time. The ventilator cycles to exhalation, but the patient continues to demand flow, immediately triggering a second breath.

• Waveform Presentation: Two breaths delivered consecutively without full exhalation in between.
• Clinical Solution: Increase the set inspiratory time or increase the tidal volume to better match the patient's neural demand.

Ineffective Triggering (Missed Triggers)

Occurs when the patient makes an inspiratory effort, but the ventilator fails to deliver a breath. This is most commonly caused by auto-PEEP, which creates an inspiratory threshold load the patient must overcome before the ventilator senses the trigger.

• Waveform Presentation: A downward deflection in the pressure-time scalar or an upward deflection in the expiratory flow-time scalar that is NOT followed by a breath.
• Clinical Solution: Reduce auto-PEEP (prolong Te). Applying extrinsic PEEP (set PEEP) to roughly 80% of the measured auto-PEEP level can help reduce the pressure gradient required to trigger a breath, without worsening air trapping (the "waterfall effect"). Check trigger sensitivity and ensure it is set appropriately (e.g., flow trigger 1-2 L/min).

Cycle Asynchrony (Early or Late Cycling)

• Early Cycling: The ventilator cycles to exhalation before the patient's neural inspiration is complete. Often seen as a spike in the pressure waveform at the end of inspiration, or double triggering.
• Late Cycling: The ventilator maintains inspiration longer than the patient's neural demand. Often seen in Pressure Support Ventilation (PSV) with COPD patients.
• Clinical Solution for Late Cycling in PSV: Adjust the Expiratory Sensitivity (ESENS) or flow cycle criteria. Increasing the flow cycle percentage (e.g., from 25% to 40% or 50% of peak flow) will cause the breath to terminate sooner, allowing for a longer expiratory time.