Viva F6(v)

What is dead space?

• Dead space is the fraction of tidal volume which does not participate in gas exchange.

What are the components which make up dead space?

• Dead space is composed of apparatus dead space and physiological dead space.
• Physiological dead space is composed of anatomical dead space and alveolar dead space

What is apparatus dead space?

• Apparatus dead space is the dead space in an artifical breathing circuit.
• For an intubated patient, the apparatus dead space is represented by the volume of the endotracheal tube, tube connectors, HME and the Y-piece of the ventilator circuit
• For a patient  ventilated non-invasively, the apparatus dead space is the volume of the NIV mask which is occupied by gas

What happens to the total dead space when the patient is mechanically ventilated?

• Apparatus dead space can increase the total dead space:
  • Mechanical ventilation using a large NIV mask
  • Large circuit components, eg. a big HME
• Apparatus dead space can also reduce the total dead space
  • Use of ETT (smaller volume than the upper airway)
  • Tracheostomy (bypass the upper airway altogether)

How do you measure physiological dead space?

• Using the Enghoff modification of the Bohr equation:
  
• I.e. it is the fractional difference between arterial CO₂ and expired CO₂
• The Enghoff modification was to use  arterial CO₂ (PaCO₂) instead of alveolar CO₂, because arterial CO₂ is the mixed product of all the lung units and therefore should theoretically be representative of the average CO₂ of all the alveoli put together.

What is anatomical dead space?

• Anatomical dead space is "the volume of the air passages through which the gas is conducted to the alveoli", and "the volume of gas exhaled before the CO2 concentration rises to its alveolar plateau" (Nunn's; 8th ed)
• Alternatively: "Anatomical dead space is the portion of dead space which is external to the alveoli, consisting of mainly conducting airways, and represented by Phase I and half of Phase II in the single-breath nitrogen washout test"

How do you measure anatomical dead space?

• Using the Fowler method:
  
   
• The subject takes a breath of 100% oxygen.
• Oxygen floods into the respiratory tract, completely displacing the other gases from the respiratory tract, so that at the end of inspiration the anatomical dead space is completely filled with oxygen.
• Then, the subject exhales.
  • Phase I: dead space, i.e. pure oxygen.
  • Phase II: half dead space, half alveolar ventilation
  • Phase III: alveolar ventilation
    

What factors influence the volume of the anatomical dead space?

• Those factors are:
  • Body size and age (2ml/kg ideal body weight; higher in the infant)
  • Posture (slightly smaller with the subject in a supine position)
  • Airway manoeuvres: a jaw thrust and chin lift adds an extra 40ml)
  • Lung volume: because as the lung inflates, neighbouring alveoli will pull on the smaller airways by traction, and increase their diameter. 
  • Bronchospasm (this is probably only imaginary, but it is mentioned in textbooks)
  • Tidal volume: anatomical dead space appears to decrease in proportion to the tidal volume.

How does tidal volume affect anatomical dead space?

• When measured by Bohr's method, the anatomical dead space appears to decrease in proportion to the tidal volume.
• The threshold for this is around 350ml
• With low enough volumes, anatomical dead space appears to be zero ml
• This is because of laminar flow and expiratory gas mixing
  • With low flow rates, there is less turbulent flow in the airways, and a relatively small central column of gas can move in and out of the alveoli while the peripheral gas in the conducting airways remains undisturbed, clinging to their walls
  • because of the relatively slow movement of gas out of the alveoli, there is more chance for the alveolar gas to mix (by diffusion) with the gas of the conducting airways, making the exhaled gas concentration more homogeneous and alveolar-like

What is alveolar dead space?

• "the part of the inspired gas that passes through the anatomical dead space to mix with gas at the alveolar level, but which does not take part in gas exchange." (Nunn's)
• It is the difference between physiological dead space and anatomical dead space.
• Wests' Zone 1 contain alveolar dead space.
• Under normal circumstances, this volume is minimal. 

How do you measure alveolar dead space?

• Alveolar dead space can be calculated from the difference between the physiological and anatomical dead space volumes. 

What factors increase the alveolar dead space?

• Poor cardiac output: eg.haemorrhagic shock
• Parenchymal lung disease: destructive processes such as emphysema 
• High positive pressure ventilation: increasing the positive pressure pushes blood out of the well-ventilated regions and therefore increases the alveolar dead space
• Pulmonary vascular occlusion: eg. venous thromboembolism
• Posture: decreases when patient is supine
• Extremes of gravity or acceleration: eg. in weightlessness, it decreased by 30% 
• Open chest and one lung ventilation, if the lung in the open hemithorax is not deflated

How might shunt mimic dead space?

• The presence of a large enough shunt can give rise to the appearance of increased physiological dead space.
• Where shunt is substantial, it can create the illusion of alveolar dead space because it increases the difference between the alveolar and arterial CO₂.
• This is because with a large enough shunt, even CO₂ will not be cleared effectively 
• This will behave very much like dead space, in the sense that increasing the ventilation will have little additional effect on decreasing the PaCO₂.
• This requires a large shunt: with a normal cardiac output, a shunt fraction of 60% would be expected to produce an alveolar dead space fraction of around 21%

What happens when your dead space is suddenly increased?

• The effect on gas exchange is the same as the effect of decreasing the tidal volume
  • Decreased CO₂ clearance
  • Decreased oxygenation due to increased alveolar CO₂ 
• Decreased efficiency of ventilation
  • For any given minute volume, CO₂ clearance will be decreased
  • Thus, there will be increased minute volume requirements
  • Thus, work of breathing is increased