mechanics

Airway Resistance

Raw = (PIP − Pplat) ÷ Flow

Normal: 0.5–2.5 cmH₂O/L/sec in intubated patients; < 5 cmH₂O/L/sec (normal, non-intubated)

Variables

Raw

Airway resistance(cmH₂O/L/sec)

Normal: 0.5–2.5 cmH₂O/L/sec (intubated)

PIP

Peak inspiratory pressure(cmH₂O)

Normal: 15–30 cmH₂O typically

Pplat

Plateau pressure(cmH₂O)

Normal: < 30 cmH₂O

Flow

Inspiratory flow rate(L/sec)

Normal: 0.5–1.0 L/sec (30–60 L/min)

What It Measures

Airway resistance measures how much pressure is required to move gas through the airways at a given flow rate. It reflects the diameter and length of the airways — narrower airways have exponentially higher resistance (Poiseuille's law: resistance ∝ 1/r⁴). The ETT itself contributes significantly to resistance in intubated patients.

When to Use

Evaluate when PIP is rising without a rise in Pplat, suggesting an airway resistance problem rather than compliance. Useful for monitoring bronchospasm response to bronchodilators and for detecting secretion accumulation or ETT obstruction.

Interactive Calculator

Peak Inspiratory Pressure (PIP)(cmH₂O)

Plateau Pressure (Pplat)(cmH₂O)

Inspiratory Flow(L/sec)

Worked Examples

Normal ventilated patient

→ 2.0 cmH₂O/L/sec

Normal airway resistance

Acute bronchospasm

→ 33 cmH₂O/L/sec

Severely elevated resistance — treat with bronchodilators

Clinical Significance

Elevated Raw (> 5 cmH₂O/L/sec) is seen in bronchospasm (asthma, COPD exacerbation), secretion retention, kinked or partially obstructed ETT, and small ETT size. Bronchodilator therapy reduces Raw. Suctioning clears secretion-related resistance. ETT diameter critically affects resistance — a 7.0 mm tube has substantially higher resistance than an 8.0 mm tube.

Memory Tips

💡Resistance = Pressure OVER Flow — resistance opposes flow, like friction in a pipe
💡The Poiseuille law: resistance increases by the FOURTH power as radius decreases — cutting radius in half = 16× resistance!
💡PIP − Pplat = the pressure used to overcome resistance during flow; the rest (Pplat − PEEP) overcomes elastance

NBRC Pearls

High PIP, normal Pplat = airway resistance problem. High Pplat = compliance (alveolar) problem
Bronchodilators target resistance; adjust PEEP/TV for compliance problems
Auto-PEEP (intrinsic PEEP) is caused by air trapping in high-resistance states (asthma, COPD) — prolongs expiratory time

← All Formulas

What It Measures

Clinical Significance

Memory Tips

💡Resistance = Pressure OVER Flow — resistance opposes flow, like friction in a pipe

💡The Poiseuille law: resistance increases by the FOURTH power as radius decreases — cutting radius in half = 16× resistance!

💡PIP − Pplat = the pressure used to overcome resistance during flow; the rest (Pplat − PEEP) overcomes elastance