Dead Space

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”.

• 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 (Vd) – Tidal volume (Vt) ratio is 0.30 (150:500).
• I.e., Equivalent to 1/3rd of tidal volume.

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.

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.

Exam Question

DEAD SPACE