Airway resistance

Describe the factors that affect airways resistance. (99B3, 03B13)

 

Definition

Normal value of airway resistance are variable because of large changes with lung volume and methodological differences.

According to Nunn p.75, normal total respiratory resistance (include airway resistance and pulmonary resistance) at FRC
is < 0.84kPa.L^-1.sec

Location of resistance

Most pressure drops (i.e. greatest resistance) occurs in the medium-sized bronchi up to Z7.

<20% of the pressure drop can be attributed to smaller (<2mm) airways (i.e. Z8 and below)

Factors affecting airway resistance

1. Lung volume
2. Density and viscosity of inspired gas
3. Contraction of bronchial smooth muscles
4. Small airway calibre

1. Lung volume

The bronchi are supported by radial traction of the surrounding lung tissues, and their calibre is increased as lung expands.

Conversely, at low lung volumes, compression of the airways and increased airway collapse lead to increased airway resistance.

=> At residual capacity, airway resistance is at its greatest

=> At total lung capacity, airway resistance is at its lowest

=> i.e. airway resistance is inversely proportional to the lung volume

Alternatively, 

Conductance (1/resistance) is directly proportional to the lung volume.

[See diagram Nunn p67 fig4.5]

NB: because of this inverse relationship between airway resistance and lung volume
=> the lower the volume,
=> the lower the flow rate
(adding the effect dynamic compression of airway)
=> the maximum flow rate is determined by lung volume

NB: The curve of Lung volume vs AWR is downward curve

NB: The curve of Lung volume vs PVR is a U curve

2. Density and viscosity of inspired gas

The higher the density
=> Re increases
=> turbulent flow more likely

The higher the viscosity
=> Re decreases
=> laminar flow more likely

In practice, density is a lot more significant

=> flow is not very laminar at medium-sized airway (where most resistance is)
(but may be more laminar at very small airways)

=> The higher the density of gas, the greater the resistance

NB. At diving, Helium-O2 mixture is used to reduce the density, and thus reduced the AWR-increasing effect of breathing compressed air.

3. Contraction of bronchial smooth muscles

Innervation of bronchial smooth muscles:

• autonomic nervous system
• vagus nerve (motor)

Relaxation (i.e. decreasing AWR)

• adrenergic receptor stimulation (mainly beta2)
• epinephrine
• isoproterenol

Contraction (i.e. increasing AWR)

• irritation e.g. smoke
• parasympathetic stimulation
  via: acetylcholine, M3-receptor
• acetylcholine
• decreasing PACO2
• histamine (act on smooth muscle in alveolar ducts)

4. Small airway calibre

Past Z10

Additional notes

[Need to add: dynamic compression]

Additional notes

Need to add dynamic compression.

Examiner's comments

• Definition of airway resistance, plus Ohms law, correct units (SI units or cmH2O/L/sec), and normal value
• Relative contribution of upper and lower airway
• Relationship between airway resistance and volume
• Concept of laminar and turbulent flow and their effect on airway resistance (include Hagen-Poiseuille equation and Reynold's number)
• Relationship between AWR, driving pressure, flow and airway calibre in both turbulent and laminar flow
• Example of physiological factors affecting AWR - bronchomotor tone, respiratory rate, dynamic airway compression)
• common errors - incorrect units, incorrect Hagen-Poiseuille equation and Reynold's number
• common errors - confusing the relation of AWR-lung volume with PVR-lung volume
• common errors - ignore the role of upper airway
• common errors - measurement was not asked and not required.

To be added later

Dynamic compression

Details on small airway calibre