Past Papers · SAQ

Airway Resistance

Current · V5 (2025) → C3.vi Historical · V4 (2023) → F3.viii 4 exam appearances

2021B Q12

Exam question

Explain the physiological factors that affect airway resistance.

CICMWrecks answer

Master answer

Airway Resistance

Resistance is the impedence to flow.
Normal Airways Resistance (AWR):
- Adult: ~2 cmH₂O/L/s
- Newborn: ~20 cmH₂O/L/s – declines markedly
Main Site of AWR:
- Mid-sized bronchi 7th/8th generation
- comparatively smaller cross-sectional area
- (note that in neonates, a greater proportion of Raw comes from the smaller peripheral airways)
In the airway flow can be laminar or turbulent
- Flow depends on Depends on Reynolds number

Re\;={\;2\;r\;ρ\;v\;\over η}={diameter\;\times\;density\;\times\;velocity\;\over viscosity}

where
Re is Reynold’s number
r is radius
ρ is density
v is velocity
η is viscosity

Laminar Flow

Ordered flow occuring in concentric layers within a tube
Flow in the center is fastest and flow in the most peripheral layer is the slowest
Resistance to laminar flow obeys the Poisuille-Hagen Equation

R\;=\;{8\;η\;l \over {π\;r^4}}

where
R is vessel resistance
η is viscosity
L is length of vessel
r is radius of vessel

Increases in viscosity of gas, or length of the tube increase resistance
Increases in radius of the tube, decreases resistance by a power of 4

Turbulent Flow

In turbulent flow, due to the disorganized flow and increased likelihood of friction
with the static airway wall, resistance is markedly increased.
Resistance to turbulent flow obeys the following equation

R ∝ {ρ\;l \over {π\;r^5}}

where
R is Resistance to flow
ρ is density
l is length of vessel
r is radius of vessel

Increases in density of gas, or length of the tube increase resistance
Increases in radius of the tube, decreases resistance by a power of 5

Factors affecting Airway Resistance

Physics factors (see above):
- Location in the airway (see above): Mid-sized bronchi are the location of greatest airway resistance, resistance progressively declines with successive airway generations
- Flow:
  - Laminar Flow vs turbulent flow
  - Depends on Reynolds number factors
    - density more important than dynamic viscosity, velocity (flow rate) as Main Site is Turbulent.
- Flow rate:
  - Flow related airway collapse
    - Airways beyond generation 11 have no structural rigidity
    - High flows can reverse transmural pressure gradient and cause airway collapse
- Newborn: ~20 cmH₂O/L/s – declines markedly with age to ~2 cmH₂O/L/s
Radius changes
- Muscular control of airway diameter
  - Neural:
    - Parasympathetics important in bronchomotor tone → airway constriction via ACh and muscarinic receptors
    - Sympathetic system virtually absent in lung
  - Hormonal:
    - Although no symppathetic innervation, abundance of β₂ adrenoceptors → airway dilation
- Smooth mm tone:
  - (↓r) bronchospasm, Musc antag (PSNS), LTs, PGF2-alpha
  - (↑r) β₂-agonists, adrenaline neb, SNS
- ↓intramural radius:
  - oedema, ↑mucous, wall hypertrophy
- External compression:
  - tumour, haemorrhage, PTX
  - dynamic airways compression with forced expiration
Lung Volume
- ↑ lung vol →
  - ↑ radial traction → ↓ AWR
  - ↑ -ve intrapleural → ↑ patency of small airways
- ↓ lung vol → ↑ AWR
- CPAP or PEEP → ↑ FRC → reduces flow-related airway collapse at low lung volumes

Airway resistance and conductance versus lung volume — CICMWrecks original diagram

Measurement of Airway Resistance

AWR \; = \; {{\Delta \, P} \over {Q}}

TGV = (P_B – P_{H_2O}) \frac{\Delta V}{\Delta P}

Body Plethysmography:

Q measured with flow meter
Lung Volume is measured with Plethysmography
∆P via Plethysmography and Boyle’s Law
- (A) box pressure is atmospheric
- (B) inspiration
  - ∆V allows for ∆P measurement for given Q since
  - PV = constant
Then AWR can be calculated from 1

Interrupter Technique (direct from AWR eqn)

Method
- Manometer distal to shutter
- Used to measure mouth and alveolar pressure
- Flow during inspiration or expiration in interrupted for 50–100 ms repeatedly throughout the respiratory cycle.
Measure
- Q – flow rate (measured before interruption)
- P2 – mouth pressure (measured before interruption)
- P1 – pressure in alveoli (measured at the end of the interruption at te level of the mouth after equilibration)
Apply Ohm’s Law (AWR equation above).
Adequate for normal lungs not diseased.

Gladwin / Sakurai 2016

Reusable content

Formulae used in this answer

Airway resistance — pressure-flow relationship

Airway resistance from ΔP / flow

CWF-0058

AWR \; = \; {{\Delta \, P} \over {Q}}

Poiseuille resistance

CWF-0056

R\;=\;{8\;η\;l \over {π\;r^4}}

Reynolds number

CWF-0028

Re\;={\;2\;r\;ρ\;v\;\over η}={diameter\;\times\;density\;\times\;velocity\;\over viscosity}

Thoracic gas volume — body plethysmography

Plethysmographic lung volume

CWF-0059

TGV = (P_B - P_{H_2O}) \frac{\Delta V}{\Delta P}

Turbulent flow — pressure-drop relationship

Turbulent resistance relationship

CWF-0078

R ∝ {ρ\;l \over {π\;r^5}}

Answer by Gladwin, Sakurai

Past papers

Exam appearances

4 appearances

Exam	Exact exam wording	Candidate success
2021B Q12	Explain the physiological factors that affect airway resistance.	31%
2016B Q06	Describe the factors that affect airways resistance.	47%
2013B Q23	What factors affect airway resistance? (80% of marks) Briefly outline how it may be measured and/or changes in flow are detected. (20% of marks)	—
2009A Q18	Describe the factors that affect airway resistance.	30%