2003B13 Describe the factors that affect airways resistance.

 

List:

·       Intro

·       Factors ↑lower airway resistance

·       Factors ↑upper airway resistance

 

Intro:

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:

↓Radius

·  ↓ Absolute lung size

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

o Female > male

o Shorter > taller

·  ↓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

↑Density

·  e.g. ↓ with heliox

↑Viscosity

·  e.g. ↓ with ↓ gas temperature

↑Length

·  Not under control

 

Factors ↑upper airway resistance:

Reduced calibre

·  Obesity -> OSA

Deformity

·  Upper airway tumour, surgery

Hypotonia

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

 

 

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