Ventilation and Dead Space

Question 1

What is Tidal Volume (VT) and how is it divided?

Answer 1

VT is the air inspired in a normal breath. It is divided in:
✤ Alveolar volume VA
✤ Dead space VD

➔ VA (alveolar volume): the volume of air which reaches perfused alveoli.

➔ VD (dead space): the air remaining in either the conducting airways or non-perfused alveoli, that plays no part in gas exchange.

► VT=VA + VD

Question 2

What are the different types of dead space?

Classification of dead space.

Answer 2

❖ Anatomical dead space: VD (Anat) is the volume of the upper airways and first 16 generations of the tracheobronchial tree, which form the conducting airways.

❖ Alveolar dead space: VD (Alv) is the total volume of
the ventilated alveoli that are unable to take part in gas exchange due to insufficient perfusion.

❖ Physiological dead space: is the total dead space; that is, the sum of anatomical and alveolar dead space:
VD (Phys) = VD (Anat) + VD (Alv)

Question 3

How is anatomical dead space
measured?

Answer 3

VD (Anat) is measured using Fowler’s method.

Question 4

What factors affect anatomical dead
space?

Answer 4

❖ Size of patient:
VD(anat) increases as the lung size increases.

❖ Lung volume: at high lung volumes the airway diameter increases thus increasing VD(anat).

❖ Posture: in supine position the lung volume decreases which reduces airway diameter and decreases VD(anat).

❖ Bronchoconstriction:
There’s a reduction in airway diameter therefore reduces the VD(anat).

❖ Bronchodilation: airway diameter is increased therefore VD(anat) is increased.

Question 5

How is VD(alv) measured?

Answer 5

It cannot be measured directly.

It can be calculated from the formula:

❖ VD(phys) = VD(anat) + VD(alv)

❖ VD(phys) and VD(anat) have to be known.

❖ VD(phys) is measured using the Bohr equation.

❖ VD(anat) is measured using Fowler’s method.

Question 6

When is VD(alv) increased?

Answer 6

• In normal lungs alveolar ventilation and perfusion are equilibrated so VD(alv) is negligible.

It is increased in:

❖ Upright posture:
• Due to gravity the apical alveoli are well ventilated but not adequately perfused. This increases the VD(alv).

❖ Low pulmonary artery pressure:
• As a result to low RV output, leading to insufficient perfusion of the lung apices (high V/Q ratio) thus increasing VD(alv).

❖ PEEP:
• In the lung apices the increase in alveolar pressure will result in compression of the pulmonary capillaries leading to reduction of alveolar perfusion.
• The increased intrathoracic pressure reduces the venous return to the RV which in turn causes a reduction in pulmonary artery pressure.

❖ Pulmonary artery obstruction:
• Embolus (thrombus, gas, fat, amniotic fluid) results in the alveoli being ventilated but not perfused thus increasing VD(alv).

❖ COPD:
• There is destruction of alveolar septa which creates enlarged air spaces.
• Surface area for gas exchange ir reduced thus increasing VD(alv).

Question 7

Define what is Minute ventilation.

Answer 7

❖ Minute ventilation (VE):
• It’s the volume of air inspired per minute.
• VE = VT x RR

Question 8

What is alveolar ventilation?

Answer 8

❖ Alveolar ventilation:
• Is the proportion of VE (minute ventilation) that takes part in gas exchange.
• Alv. Ventilation = (VT-VD) x RR

Question 9

What is Dead space ventilation?

Answer 9

❖ Dead space ventilation:
• Is the proportion of minute ventilation (VE) that cannot participation in gas exchange.
• Dead space vent = (Vt - VA) x RR

Question 10

Describe the relationship between alveolar ventilation and PaCO2.

Answer 10

• Alveolar ventilation is the proportion of VE that takes part in gas exchange.
• As Alv vent increases it also increases the exchange of alveolar gas with atmospheric air.
• The portion of CO2 in atmospheric air is low therefore PACO2 will fall.
• This will facilitate the diffusion of CO2 across the alveolar-capillary barrier leading to a decrease in PaCO2.