Ventilation distribution

Explain the factors influencing the distribution of ventilation during the inhalation of 500ml of air from Functional Residual Capacity in the erect posture (97B7)

Nunn p.163

Gravity, intrapleural pressure, and compliance

At dependent part of the lung, intrapleural pressure would be higher (or less negative) due to the weight of the lung tissues.

Combination of:

1. higher intrapleural pressure, and
2. gravity

Leads to:

1. alveoli less expanded at the base of lung
2. compliance at base is more optimal and is higher than apex

In turn, these two factors
=> base ventilates better than apex

 

[See diagram 20050306(4) - Regional difference in compliance]

 

Lung volume and recoil

Very low lung volume

At very low lung volume (lower than closing capacity),

e.g. residual volume

Because the lung base is so compressed that intrapleural pressure becomes positive,
AND/OR

Because small airways at base are closed,

=> initially, apex will expand first before intrapleural pressure at base becomes negative and air flows into base.

NB. Only the first part of inspired gas goes preferentially to apex.

Reduced recoil

Where the elastic recoil of lung is reduced (e.g. emphysema)

=> intrapleural pressure becomes less negative

=> at base, intrapleural pressure may even become positive

=> during inspiration, intrapleural pressure at lung base take a while to become negative and airflow occurs

Thus, lung base is only ventilated intermittently in emphysema.

 

Right vs left lung

Supine or erect position

Right lung ventilates better than left, because of its larger size.

 

Lateral position

Better ventilation on dependent side because:

1. Diaphragm on the lower side is higher and thus contract better
2. Alveoli on dependent side is less expanded

In anaesthetized patients, better ventilation in upper side.

Flow rate

Nunn p165

In erect position, in normal lung,

• during slow inspiration from residual volume
  => ventilation of apex:base = 1:3
• during slow inspiration from FRC
  => ventilation of apex:base = 2:3
• fast inspiration (flow rate > 1.5 L/sec) from FRC
  => ventilation of apex:base = 1:1 or 1+:1

NB: Normal tidal breathing is about 0.5L/sec

Time constant

Definition of time constant

Time constant is:

1. the time required to inflate to 63% of final volume if inflation is prolonged indefinitely;
   OR,
2. the time required to inflate the lung unit if the flow rate is constant throughout

Time constant is proportional to by resistance and compliance.

Alveoli with uniform time constant

• Initial flow is higher in alveoli with lower airway resistance
• Final volume is higher in alveoli with higher compliance
• Pressure built-up are identical
• Distribution of gas is independent of flow rate, duration, and frequency of inspiration
• Dynamic compliance is not affected by frequency, and doesn't differ much from static compliance
• If inspiration is checked by closure of upper airway, there will be no redistribution of gas

Alveoli with different time constant

• (same) Initial flow is higher in alveoli with lower airway resistance
• (same) Final volume is higher in alveoli with higher compliance
• Pressure raises faster and higher in alveoli with low time constant
• Distribution of gas is dependent on flow rate, duration and frequency of inspiration
• Increase in RR => reduced dynamic compliance
• Dynamic compliance would be different from static compliance
• After closure of upper airway, there will be redistribution of gas
  => from low time constant to high time constant

Additional notes

Examiner's comment

• Effect of gravity in erect posture on size of alveoli and traction on apical alveoli
• Compliance and how it relates to size
• (extra) time constant and resistance
• (extra) right vs left