Physiological dead space

Write short notes on physiological dead space (1990)

Dead space

Definition - the volume occupied by gas which does not participate in gas exchange in lung.

A few different types, including:

1. anatomical dead space
2. physiological dead space
3. alveolar dead space
4. apparatus dead space

Anatomical dead space

Anatomical dead space is the volume of the conducting airways.

=> about 150mL in an average adult

=> or 2.2mLs/kg

Anatomical dead space is constant regardless of circulation.

Physiological dead space

Physiological dead space is the part of the tidal volume which does not participate in gas exchange.

Includes:

• anatomical dead space
• alveoli with no perfusion (i.e. infinite V/Q) (e.g. West's zone 1, alveolar dead space)

The difference between anatomical dead space and physiological dead space is alveolar dead space.

With increased cardiac output (e.g. during exercise), physiological dead space is reduced (due to reduction in alveolar dead space).

Alveolar dead space

Alveolar dead space is the part of the inspired gas which passes through the anatomical dead space to mix with gas at the alveolar level, but does not participate in gas exchange. (i.e. infinite V/Q)

Apparatus dead space

When using mask or anaesthetic circuit tubing, this adds to the conducting zone.

Measurement

Measurement of anatomical dead space

By using Fowler's method.

Fowler's method

Based on rapid dilution of gas already in lung (N2 or CO2) by inspired gas (100% O2).

1. Single breath of 100% O2
2. During the following expiration, [N2] increases from 0% (pure dead space gas) to equilibrium (pure alveolar gas) (i.e. plateau)
   => as per [N2] vs time graph
3. Using [N2] vs expired volume graph, anatomical dead space is taken to be at the mid-point of the transition from conducting zone to gas exchange zone.

[Diagram 20050228(1) - Fowler's method]

Measurement of physiological dead space

By using Bohr's equation and Bohr's method

Bohr's method

Based on "all expired CO2 comes from alveolar gas", and dead space doesn't eliminate CO2.

V_T x F_ECO2 =  V_A x F_ACO2

Also, V_T = V_A + V_D 

=> V_T x F_ECO2 =  (V_T - V_D) x F_ACO2

=> V_T x (F_ACO2 - F_ECO2) = V_D x F_ACO2

=> V_D/V_T = (F_ACO2 - F_ECO2)/F_ACO2

• V_A = ventilated alveolar volume
• V_T = tidal volume
• V_D = dead space volume
• F_ECO2 = fractional concentration of CO2 in mixed expired air
• F_ACO2 = fractional concentration of CO2 in alveolus

Bohr equation: V_D/V_T = (P_ACO2 - P_ECO2)/P_ACO2

Normal value: 0.2~0.35

NB: P_ECO2 is the partial pressure in MIXED expired gas, NOT end-tidal gas

Enghoff modification - using measured arterial PaCO2 as an estimate of the ideal alveolar PACO2

=> modified: V_D/V_T = (P_aCO2 - P_ECO2)/P_aCO2

 

Additional notes

Factors influencing anatomical dead space

• Size of subject
  => increases with body size
• Age
  => at infancy, anatomical dead space is higher for body weight (3.3mL/kg)
• Posture
  => sitting 147mL, supine 101mL
• Position of neck and jaw
• Lung volume at the end of inspiration
  => anatomical dead space increases by 20mL for each L of lung volume
• Drugs
  e.g. bronchodilator will increase dead space

Factors influencing alveolar dead space

• Low cardiac output can increase alveolar dead space (increasing West's zone 1)
• Pulmonary embolism

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