DEAD SPACE

Types: 

• Anatomical dead space.

• Physiological dead space.

ANATOMICAL DEAD SPACE:

• Inspired air reaching alveoli alone involves in gas exchange.

• Air in airways (trachea, bronchi) is expired is unchanged.

Since no gaseous diffusion.

– Hence airway space is “Functionally dead”.

• Referred as “Anatomical Dead Space”.

Metrics:

• In a normal adult, anatomical dead space – 2.2 cc/kg approx.

• Volume  – 150 ml.

• Inspired air – 500 ml with each breath (tidal volume).

• First 350ml mixes with alveolar air at rest.

Conversely, on each expiration:

• First 150 ml expired is dead space gas.

• Only last 350 ml is alveolar gas.

• Hence, Normal Dead Space (V_d) – Tidal volume (V_t) ratio is 0.30 (150:500).

• I.e., Equivalent to 1/3rd of tidal volume.

FACTORS INFLUENCING: 

Increased anatomical dead space:

• Seen during inspiration.

• Halothane.

• Atropine.

• During neck flexion.

Decreased anatomical dead space: 

• Associated with endotracheal intubation.

PHYSIOLOGICAL DEAD SPACE:

• Normally, 350 ml of inspired air enters alveoli.

• Yet, less air volume participates in alveolar gas exchange.

In disease states:

• No diffusion/gaseous exchange.

• Also overventilated alveoli.

• Such alveolar spaces constitute “Alveolar” dead space.

• Sum of anatomical & alveolar dead space referred as “Physiological” dead space.

• I.e., Volume of inspired air in airways (anatomical dead space) & in alveoli (alveolar dead space), that does not participate in gaseous exchange.

In healthy individuals, anatomical dead space & physiological dead space are identical.

• Alveolar diffusion, ventilation & perfusion are normal in healthy individuals.

• Because entire air entering alveoli participates in gaseous exchange.

FACTORS INFLUENCING:

Increased physiological dead space:

• In lung diseases.

MEASUREMENT OF DEAD SPACE:

1. FOR ANATOMICAL DEAD SPACE:

• Measured by Single-breath N₂ method.

• Involves nitrogen analysis in expired air.

• Following a single deep breath of 100% oxygen.

• At end of quiet expiration, subject breaths deeply pure O₂ & slowly & steadily breaths out.

• Initial air volume free of N₂ free gives anatomical dead space.

2. FOR PHYSIOLOGICAL DEAD SPACE:

• Determined by,

• Simply measuring tidal volume.

• CO₂ concentration in alveolar air & mixed expired air.

• Calculated by using “Bohr’s Equation”.

RELATIONSHIP OF DEAD SPACE WITH ALVEOLAR & PULMONARY VENTILATION:

• “Respiratory minute volume” – 

• Amount of air breathed in/out by lungs in one minute.

• Minute volume = Tidal volume x Respiratory rate.

• Considered as “Measure of pulmonary ventilation”.

Yet, dead space reduces volume of air reaching alveoli participating gas exchange.

• Alveolar ventilation –

• Amount of inspired air entering alveoli per minute during quiet breathing

• Alveolar ventilation – (Tidal volume – Dead space) x Respiratory Rate.

• Normal alveolar ventilation = (500 – 150) x 12 = 4200 ml/minute.

VARIATIONS WITH ALVEOLAR VENTILATION:

1. Increased alveolar ventilation:

Results in, 

Decreased alveolar PCO₂  → More CO₂ diffused out from blood to alveoli.

Increased alveolar PO₂ → More O₂ diffused in from alveoli to blood.

2. Decreased alveolar ventilation – Opposite effect.