Pulmonary & Critical Care | Clinical reasoning, teaching, and synthesis

[
[
[

]
]
]

MEchanical Ventilation

Scalars

Ventilators commonly display three scalars:

  • Pressure–time scalar (pressure waveform)
  • Flow–time scalar (flow waveform)
  • Volume–time scalar (volume waveform)

Ventilator Waveforms trouble shooting

Air trapping

Major Ventilator Troubleshooting: High-Yield Summary

1. Air Trapping (Auto-PEEP)

Where to look

  • Flow–time waveform
  • Expiratory flow does not return to baseline before the next breath

Who gets it

  • COPD, asthma, emphysema
  • Obstructive physiology

Core problem

  • Inadequate expiratory time → trapped air

Fix (key principle: ↑ expiratory time)

  • ↓ Inspiratory time
    • Pressure control: shorten Ti
    • Volume control: ↑ inspiratory flow
  • ↓ Respiratory rate (if needed)
  • ↓ Tidal volume (last resort)
  • Give bronchodilators
  • Avoid unnecessary increases in minute ventilation

2. Breath Stacking / Double Triggering

Where to look

  • Flow waveform: two inspirations back-to-back
  • Volume waveform: double volume spikes in quick succession

What it means

  • Patient’s inspiratory demand > delivered tidal volume
  • Common during low tidal volume ventilation

Why it matters

  • ↑ Risk of volutrauma and barotrauma

Fix

  • Increase tidal volume (within safe limits)
  • Consider adjusting inspiratory time or mode
  • Improve patient comfort/synchrony (analgesia, sedation if needed)

3. Air Leak

Where to look

  • Volume–time waveform
  • Inspired and expired volumes do not match (volume doesn’t return to baseline)

First step

  • Check flow waveform
    • If flow doesn’t return to baseline → air trapping
    • If flow does return → true leak

Common causes

  • Endotracheal tube cuff leak
  • Circuit disconnection
  • Bronchopleural fistula
  • Pneumothorax

Fix

  • Check cuff pressure
  • Inspect circuit
  • Evaluate patient (lung exam, CXR)

4. Flow Hunger

Where to look

  • Pressure–time waveform
  • “Camel hump” or scooped pressure curve instead of smooth plateau

What it means

  • Patient wants more inspiratory flow

Fix

  • Increase inspiratory flow
  • Adjust rise time
  • Consider switching to pressure control if appropriate

5. Flow–Volume Loop Abnormalities

Normal

  • Smooth inspiratory and expiratory limbs

Abnormal patterns

  • Jagged / saw-tooth → excessive secretions
  • Inspiratory irregularities → condensation in tubing
  • Scooped expiratory limb → obstructive lung disease (COPD/asthma)

Fix

  • Suction secretions
  • Drain tubing condensation
  • Treat obstruction (bronchodilators, steroids)

6. Pressure–Volume Loop Abnormalities

What PV loops tell you

  • Compliance
  • Resistance
  • Work of breathing
  • Overdistension

Patterns

  • More upright loop → improved compliance
  • Flatter loop → worsening compliance
  • Wider loop → increased airway resistance

Danger signs

  • Fishtail appearance
    • Increased work of breathing
    • Trigger too insensitive
      → Fix trigger sensitivity
  • Bird-beak appearance
    • Alveolar overdistension
      → ↓ PEEP, ↓ driving pressure, ↓ tidal volume

One-Line Mental Checklist (ICU-Ready)

Flow not back to baseline? → Air trapping
Two breaths in a row? → Increase VT
Volume mismatch? → Leak vs air trapping
Camel hump pressure? → Increase flow
Bird-beak PV loop? → Overdistension

Leave a comment