Basic Ventilator Waveforms

The three primary ventilator waveforms are:

Pressure-Time Waveform
Flow-Time Waveform
Volume-Time Waveform

1. Pressure-Time Waveform

Y-Axis: Airway Pressure (cm H₂O)
X-Axis: Time (seconds)

Volume Mode

Pressure Mode

Key Features:

Peak Inspiratory Pressure (PIP):
- The maximum pressure reached during inspiration.
Plateau Pressure (Pₚₗₐₜ):
- Pressure measured during an inspiratory pause when airflow is zero, reflecting alveolar pressure.
PEEP (Positive End-Expiratory Pressure):
- The baseline pressure during exhalation.

2. Flow-Time Waveform

Y-Axis: Flow Rate (L/min)
X-Axis: Time (seconds)

Volume Mode

Pressure Mode

Key Features:

Inspiratory Flow:
- Positive flow indicating gas delivery to the patient.
Expiratory Flow:
- Negative flow indicating gas leaving the patient.
Shape of the Curve:
- Depends on the flow pattern (constant, decelerating, sinusoidal).

3. Volume-Time Waveform

Y-Axis: Volume (mL)
X-Axis: Time (seconds)

Volume Mode

Pressure Mode

Key Features:

Tidal Volume (Vₜ):
- The volume of gas delivered during each breath.
Inspiratory and Expiratory Phases:
- Represented by the rise and fall of the curve.

Advanced Waveforms

Some ventilators also display:

Pressure-Volume Loop
Flow-Volume Loop

Pressure-Volume Loop

Flow-Volume Loop

These loops provide additional information about lung mechanics and are helpful in detecting certain pathologies.

Interpreting Waveforms

A. Pressure-Time Waveform

Normal Characteristics:

Square or Decelerating Shape: Depends on the mode and flow pattern.
PIP and Pₚₗₐₜ: The difference between them indicates airway resistance.

Common Abnormalities and Causes:

i. Elevated PIP with Normal Pₚₗₐₜ:

Cause: Increased airway resistance (e.g., bronchospasm, secretions).

Troubleshooting:

Suction airway to remove secretions.
Check for kinks or biting of ETT.
Administer bronchodilators.

ii. Elevated PIP and Elevated Pₚₗₐₜ:

Cause: Decreased lung compliance (e.g., ARDS, pulmonary edema.

Troubleshooting:

Evaluate for conditions causing reduced compliance.
Adjust ventilator settings (e.g., reduce tidal volume).

iii. Auto-PEEP (Intrinsic PEEP):

Feature: Baseline pressure does not return to set PEEP before the next breath.

Cause: Air trapping due to inadequate exhalation time.

Troubleshooting:

Increase expiratory time (reduce RR or tidal volume).
Decrease inspiratory time.

B. Flow-Time Waveform

Normal Characteristics:

Inspiratory Flow: Usually a square (constant) or decelerating pattern.
Expiratory Flow: Should return to zero before the next inspiration.

Common Abnormalities and Causes:

i. Expiratory Flow Does Not Return to Baseline Before Next Breath:

Cause: Auto-PEEP due to air trapping.

Troubleshooting:

Increase expiratory time.
Reduce RR or tidal volume.

ii. Sawtooth Pattern on Inspiratory Flow:

Cause: Secretions or condensation in the circuit causing turbulence.
Troubleshooting:
- Suction patient and clear ventilator circuit.

iii. Scooped Expiratory Flow:

Cause: Obstructive airway disease (e.g., COPD, asthma).
Troubleshooting:
- Administer bronchodilators.
- Adjust ventilator settings to prolong exhalation.

C. Volume-Time Waveform

Normal Characteristics:

Symmetrical Rise and Fall: Reflects consistent tidal volumes during inspiration and exhalation.

Common Abnormalities and Causes:

i. Incomplete Exhalation (Volume Does Not Return to Baseline):

Cause: Air trapping or leak in the system.

Troubleshooting:

Check for leaks in the ventilator circuit or around the ETT cuff.
Ensure complete exhalation by adjusting settings.

ii. Sudden Drop in Delivered Volume:

Cause: Disconnection, leak, or malfunction.
Troubleshooting:
- Check all connections.
- Inspect ETT cuff inflation.

D. Pressure-Volume (P-V) Loop

Features:

Compliance: Slope of the loop represents lung compliance.
Hysteresis: The difference between inflation and deflation curves.

Common Abnormalities and Causes:

i. Decreased Compliance (Stiffer Lungs):

Loop Becomes Flattened.

Causes: ARDS, fibrosis, pulmonary edema.

Troubleshooting:

Adjust tidal volumes and pressures.
Treat underlying cause.

ii. Increased Airway Resistance:

Beaking of the Loop: Indicates overdistension.

Troubleshooting:

Reduce tidal volume.
Lower PIP.

E. Flow-Volume Loop

Features:

Peak Inspiratory and Expiratory Flow Rates.
Shape: Provides information on airway patency.

Common Abnormalities and Causes:

i. Obstructive Pattern:

Scooped Out Expiratory Curve.

Cause: COPD, asthma.

Troubleshooting:

Administer bronchodilators.
Adjust I:E ratio.

ii. Restrictive Pattern:

Narrowed Loop with Decreased Volumes.
Cause: Fibrosis, ARDS.
Troubleshooting:
- Adjust ventilator settings to accommodate lower volumes.

Common Ventilator Problems:

1. High Peak Airway Pressures

Causes:

i. Increased Airway Resistance:

Bronchospasm
Secretions
ETT obstruction

ii. Decreased Lung Compliance:

Pulmonary edema
Pneumothorax
Atelectasis
ARDS

Actions:

i. Assess Pressure-Time Waveform:

Determine if Pₚₗₐₜ is also elevated.

ii. Suction Airways:

Remove secretions.

iii. Administer Bronchodilators:

If bronchospasm suspected.

iv. Check ETT and Circuit:

For kinks or obstructions.

v. Obtain Chest Imaging:

If pneumothorax or other pathology suspected.

2. Auto-PEEP (Intrinsic PEEP)

Signs:

Expiratory flow does not return to baseline.
Increased work of breathing.
Hypotension due to decreased venous return.

Actions:

i. Adjust Ventilator Settings:

Increase expiratory time
- Reduce RR
- Decrease Iratio (1:3, 1:4)
Decrease Tidal Volume.
Set PEEP at 50-80% of measured auto-PEEP

ii. Sedation:

Reduce patient effort that may contribute to air trapping.

iii. Monitor Hemodynamics:

If hypotension present - disconnect the ventilator and for manual chest decompression.

3. Patient-Ventilator Asynchrony

Signs:

Ineffective efforts.
Double triggering.
Auto-triggering.
Use of accessory muscles.

Actions:

i. Adjust Sensitivity Settings:

Ensure the ventilator detects patient efforts appropriately.

ii. Modify Ventilator Mode:

Consider changing to a mode that better suits patient needs (e.g., pressure support).

iii. Sedation:

Optimize to improve synchrony.

iv. Check for Causes:

Pain, anxiety, acid-base imbalance.

4. Leak in the System

Signs:

Volume-Time waveform does not return to baseline.
Low exhaled tidal volumes.
Alarms indicating low volume or pressure.

Actions:

i. Inspect Circuit and Connections:

Tighten loose connections.

ii. Check ETT Cuff:

Inflate if underinflated or consider cuff leak.

iii. Examine Patient:

For air escaping from mouth or nose.