dead space, ventilation

Question 1

DEAD SPACE (VD) = definition

Answer 1

• VD is the dead space ventilation = the volume of a breath/gas that does not participate in gas exchange.

Question 2

Anatomical dead space

Answer 2

= gas that remains in the conducting airways (approx 1/3 of tidal volume)

• In the anatomical dead space gas exchange can’t occur.
• Gas in the anatomical dead space cannot enter the blood → important for expired air resuscitation
• If the anatomical dead space increases, you need to move large volumes of gas through the respiratory system = the work of breathing and the tidal volume will increase

Question 3

Alveolar + physiological dead space

Answer 3

• Alveolar dead space – gas enters alveoli that are poorly perfused
• Physiological dead space = anatomical dead space + alveolar dead space.
• Because alveolar dead space in healthy individuals is small, anatomical + physiological dead space are identical

Question 4

Minute ventilation (VE)

Answer 4

• Minute ventilation (VE) is controlled to maintain gas pressures in the blood at the correct levels.
• Q = Stroke volume x heart rate
• VE = VT (tidal vol) x respiratory frequency (RF)

Question 5

alveolar ventilation (VA)

Answer 5

• The alveolar ventilation (VA) is the amount of gas reaching the alveoli - it is the key determinant of blood gas levels.
• VA = RF X (VT - VD)

Question 6

VE is set to match the needs to:
+
VE must match metabolic requirements:

Answer 6

VE is set to match the needs to:

• deliver oxygen to the tissues
• remove carbon dioxide

VE must match metabolic requirements

• when we exercise, need to eliminate CO2 increases so ventilation increases → hyperpnoea
• if ventilation is excessive and PCO2 decreases: this is hyperventilation
• If ventilation is insufficient and PCO2 increases: this is hypoventilation
• If VT falls, RF must rise → rapid shallow breathing is inefficient