21B12: Exam Report

Explain the physiological factors that affect airway resistance

31% of candidates passed this question.

It was expected candidates cover the breadth of the factors that affect airway resistance. Generally, as a concept the type of flow (laminar vs turbulent) was answered well by most candidates, however many failed to mention the other factors that affect airway resistance. Airway diameter as a primary determinant of airway resistance was commonly omitted. Better answers which covered the factors affecting airway diameter classified them broadly and included examples such as physical compression/external obstruction, broncho-motor tone and local cellular mechanisms. Some answers did not explain these factors in enough detail and often with incorrect facts.

16B06: Exam Report

Describe the factors that affect airways resistance.

47% of candidates passed this question.

Candidates who used a structured approach of using formulae that describe resistance fluid flow scored well. Poiseuille’s law describes the determinates of resistance to laminar fluid flow and provides a useful answer structure. The most common mistakes were confusion between resistance and compliance as well as failure to describe turbulent as well as laminar flow.

F3viii / 21B12 / 16B06: Describe factors that affect airway resistance.


  • Respiratory System Resistance = the pressure difference b/w 2 points for a given flow


  • Normal RAW = 0.5 – 2cm H2O/L/sec
  • Majority RAW in bronchi gen 7 – 8
    • Although further airway generations are smaller in diameter, their total cross-sectional area is huge
  • Respiratory System Resistance is due to:

Tissue Resistance

  • 20% of Respiratory System Resistance
  • Due to friction of lung parenchyma as the tissues slide over each other

Airway Resistance

  • The forces which have to be overcome for gas to flow in the airways
  • Influenced by:
    • Properties of inspired gas (viscosity + density)
    • Type of flow (influenced by Reynold’s No.)
    • Airway Calibre
    • Lung volume

1) Properties of Inspired Gas

  • Behaviour of gas in flow depends on 2 intrinsic properties of the gas:
    1. DENSITY = the ratio of mass to volume
    2. VISCOSITY = internal property of fluid which is a measure of resistance to flow
  • Density & viscosity of gas are related to Airway Resistance (RAW) through the HP equation & Reynold’s No.

2) Type of Flow

  • Flow = the volume of fluid (gas or liquid) passing a point in unit time

Laminar Flow

  • Particles of fluid flow in parallel lines
  • Flow greatest at centre
  • Described by Hagen-Poiseuille equation

V = flow

h = viscosity

L = length

  • Viscosity dependent (density independent)
  • Very efficient because ∆P varies directly with flow
  • Present in smooth tubes with low flow rates g. anaesthetic circuit

Turbulent Flow

  • Disorganised air flow moving in the same direction
  • More likely at high flow rates
  • Density dependent (viscosity independent)
  • Flow is not described by HP equation ∴Fanning Equation
  • It is less efficient because ∆P varies directly with V2
  • For a given flow, the necessary driving P is greater
  • The driving P is also proportional to the 5th power of tube radius

Transitional Flow

  • Flow is neither fully turbulent nor fully laminar
  • There is a gradual transition from laminar → turbulent

Reynold’s Number

  • A dimensionless number used to determine if flow is going to be laminar or turbulent
  • Low (<2000) = laminar flow because viscous forces predominate = smooth, consistent fluid motion
  • High (>2000) = turbulent flow = random eddies, vortices, flow fluctuations = dominated by inertial forces

3) Airway Calibre

  • All types flow affected by airway calibre
  • ↓radius of airway = ↑RAW
  • Calibre affected by:
    • Intraluminal factors
      • Tumor
      • Foreign Body
    • Intramural factors:
      • Mainly affected by bronchial smooth m. tone
      • Drugs
        • Adrenergic = bronchodilation
        • Cholinergic = bronchoconstriction
      • Innervation
        • Parasymp = bronchoconstriction
        • Symp = poorly innervated lung
      • MC products
        • Bronchoconstriction
      • Extraluminal factors
        • Kinking ETT

4) Lung Volume

  • Bronchi are supported by radial traction by lung tissue
  • As lung vol ↑ = ↓RAW

NB: if the reciprocal of Resistance (conductance) is plotted, it gives a linear relationship with lung volume

Measurement of Airway Resistance

  • P1 = mouth pressure → measured with manometer
  • P2 = alveolar pressure → body plethysmography
    • Body plethysmograph measures total volume of gas in lungs
    • Uses: BOYLE’S LAW: P x V is constant (at constant temp)

∴P1V1 = P2(V1 – ∆V)

    • ∴to deduce alveolar P:
      • On INSP ↑alveolar V & ↓alveolar P
      • ∆P in the body plethysmograph box
      • Can calculate the ∆ volume & deduce the alveolar P
  • Flow rate → measured with Pneumatochograph
    • Flow is derived from the pressure difference calculated against a known small resistance

If you blow into mouthpiece (with known Resistance) P1 & P2 are deduced and Flow can be calculated