Respiratory Mechanics & Ventilatory Monitoring
Respiratory Mechanics & Ventilatory Monitoring
Clay Freeman, DNP, CRNA
The mechanical components of ventilation
Pressure, flow, and volume: the only things the ventilator truly knows
- Pressure: force required to move gas into the respiratory system.
- Flow: rate at which gas moves into or out of the lungs.
- Volume: total amount of gas delivered or exhaled.
Everything else (compliance, resistance, work of breathing) is derived from these.
Pressure monitoring: what are you actually measuring?
Where pressure is measured matters
Airway pressure displayed on the ventilator is usually measured at the machine,
not in the alveoli. That pressure reflects:
- Breathing circuit compliance
- Endotracheal tube or LMA resistance
- Airway resistance
- Alveolar (elastic) pressure
Key pressure terms
- Peak Inspiratory Pressure (PIP)
- Highest pressure during inspiration.
- Reflects: airway resistance + inspiratory flow + alveolar pressure.
- Plateau Pressure (Pplat)
- Measured during inspiratory hold (zero flow).
- Reflects alveolar pressure + chest wall compliance.
- Best estimate of risk for barotrauma.
- PEEP
- Baseline pressure remaining at end expiration.
- Prevents alveolar collapse and improves oxygenation.
Clinical interpretation
- High PIP with normal Pplat → airway resistance issue (ETT kink, secretions, bronchospasm).
- High PIP and high Pplat → compliance problem (ARDS, pneumoperitoneum, obesity).
Flow monitoring: how gas moves
What flow tells you
- Inspiratory and expiratory timing.
- Presence of air trapping or auto-PEEP.
- Patient–ventilator synchrony.
Flow measurement limitations
- Flow is usually measured near the ventilator, not the patient.
- Affected by gas compression, circuit compliance, and humidity.
- Ventilators integrate flow to estimate tidal volume — small flow errors become larger volume errors.
Auto-PEEP detection
If expiratory flow does not return to zero before the next breath,
lung emptying is incomplete.
- Seen commonly in obstructive disease.
- Leads to dynamic hyperinflation and reduced venous return.
- Fix by increasing expiratory time, lowering RR or VT.
Volume monitoring: what was actually delivered
- Tidal volume is calculated by integrating flow over time.
- Subject to error from flow sensor drift.
- Delivered VT ≠ alveolar VT (dead space matters).
Compliance and resistance
Compliance
Compliance reflects how easily the respiratory system expands.
- Static compliance: measured at zero flow (uses plateau pressure).
- Dynamic compliance: measured during active flow.
- Low compliance → stiffer lungs or chest wall.
Resistance
Resistance reflects opposition to airflow.
- ETT diameter
- Bronchospasm
- Secretions
- High inspiratory flow rates
Ventilator loops: visual physiology
Pressure–volume loop
- Shows lung compliance.
- Flattening = overdistension.
- Shift right = reduced compliance.
Flow–volume loop
- Helpful for detecting obstruction.
- “Scooping” of expiratory limb suggests obstructive disease.
- Failure to return to baseline suggests air trapping.
Work of breathing
Work of breathing increases when:
- Compliance is low.
- Resistance is high.
- Trigger sensitivity is poorly set.
Poor synchrony increases oxygen consumption and fatigue.
SCOPE GUIDE
Strategies
- Look at trends, not single numbers. Sudden changes matter more than absolute values.
- Separate resistance from compliance using PIP vs plateau pressure.
- Use flow curves to detect auto-PEEP and patient–ventilator mismatch.
Clinical Optimization
- High inspiratory flow increases resistance and PIP.
- Added connectors increase mechanical dead space and resistance.
- Obesity and pneumoperitoneum primarily affect compliance.
- Auto-PEEP worsens hypotension by reducing venous return.
Pearls
- PIP tells you something is wrong. Plateau tells you where it’s wrong.
- If expiratory flow doesn’t hit zero, the lungs didn’t empty.
- Ventilators estimate — your job is interpretation.
References
- Principles and Practice of Mechanical Ventilation.
- Miller’s Anesthesia.
- Anesthesia Equipment: Principles of Application.
- Elisha S, Heiner JS, Nagelhout JJ. Nurse Anesthesia. 7th ed. Elsevier; 2023.
