2003B13 Describe the factors that affect airways resistance.



·         Intro

·         Factors ↑lower airway resistance

·         Factors ↑upper airway resistance



Ohm’s law

·   Resistance (cmH2O/L/s) = (mouth pressure – alveolar pressure) (cmH2O) / air flow rate (L/s)

·   Air flow rate = mouth pressure – alveolar pressure / airway resistance

Normal airway resistance

·   Adult 2cmH2O/L/s

·   Neonate 48 hours: 25cmH2O/L/s

·   Highest at generation ~4

·   Small airways: “silent zone”

Reynolds number

·   Re = diameter x velocity x density / viscosity

·   Dimensionless index

·   Ratio of inertial forces to viscous forces

·   <2000 likely laminar flow

·   >2000 likely turbulent flow

·   Factors ↑likelihood of turbulent flow: ↑diameter, ↑velocity, ↑density, ↓viscosity

Laminar flow

·   Smooth flow in concentric rings. Fast at centre, slow at periphery.

·   Likely in small airways

·   R = (8 x length x viscosity) / (π x radius4)

·   (P1-P2) Q

Turbulent flow

·   Chaotic flow

·   Likely in large airways

·   (P1-P2) (length x density) / radius5

·   (P1-P2) Q2 i.e. higher R for a given Q

Transitional flow

·   Likely in middle sized airways


Factors ↑lower airway resistance:


·   ↓ Absolute lung size

o Neonate 25cmH2O/L/s > Adult 2cmH2O/L/s

o Female > male

o Taller > shorter

·   ↓Relative lung volume

o Pregnant

o Obese

o Supine

o Anaesthetised

·   Intraluminal obstruction:

o Mucus, foreign body, tumour

·   Luminal obstruction: bronchoconstriction, swelling

o SNS: β2 -> dilate

o PSNS -> mAChR -> constrict

o NANC: VIP -> NO dilation

o Physical/chemical: smoke, NO2 etc - >constrict

o Inflammatory: asthma, anaphylaxis -> constrict

o Bronchodilators: β2 agonists, antimuscarinics, PDEi, sevoflurane, ketamine

·   Extraluminal obstruction: especially dynamic airways compression (DAC)

o Pressure drop between alveolus and mouth due to airway resistance

o At equal pressure point (PIP = PAW), unsupported airways collapse -> ↓flow rate

o (i.e. Starling resistor effect)

o Cartilage absent after generation 11

·   Causes of DAC:

o Forced expiration

o Low lung volume

o Increased airway resistance -> exaggerated pressure drop

o Decreased lung elastic recoil -> ↓distending transpulmonary pressure


·   e.g. ↓ with heliox


·   e.g. ↓ with ↓ gas temperature


·   Not under control


Factors ↑upper airway resistance:

Reduced calibre

·   Obesity -> OSA


·   Upper airway tumour, surgery


·   Anaesthesia, sleep (REM > NREM) -> ↑resistance



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