# Dead Space

From pEx

## Contents

### What is dead space[edit]

- 150 mls of each inhaled tidal volume remains behind in anatomical dead space
- Alveolar ventilation is therefore (500 - 150) x 15 = 5250ml/min
- This is the amount of air available for gas exchange
- Total ventilation is easy to measure, however alveolar ventilation is more difficult
- Alveolar ventilation equation method:

- No CO
_{2}is in anatomical dead space, therefore all exhaled CO_{2}can be assumed to have come from alveolar gas - Therefore from a known concentration of alveolar CO
_{2}, a known expired volume of CO_{2}and a total expired volume, alveolar ventilation can be calculated

- No CO

### Fowler's method of measuring anatomic dead space[edit]

- Subject inhales a single breath of 100% O
_{2}, then expires with a nitrogen analyzer sampling gas at the lips - Initially no nitrogen, then a rise and plateau as pure alveolar gas is being sampled
- The volume of dead space is determined by plotting N
_{2}concentration against expired volume, then drawing a vertical line at the midpoint of the curve - This measures the volume output down to midpoint of the transition form dead space to alveolar gas

- Subject inhales a single breath of 100% O

### Bohr's method of measuring anatomical dead space[edit]

- All expired CO
_{2}comes from alveolar gas and none from dead space

- All expired CO

- Fowler's method measures the volume of the conducting airways down to where rapid dilution of inspired gas occurs - the
*anatomical dead space* - Bohr's method measures
*physiologic dead space*, as it measures the volume of the lung that does not eliminate CO_{2} - Anatomic dead space + Alveolar dead space = Physiologic dead space

- Fowler's method measures the volume of the conducting airways down to where rapid dilution of inspired gas occurs - the

### Factors Affecting Anatomical Dead Space[edit]

- Increases with body size, age, lung volume, erect posture, bronchodilation, emphysema
- Decreases with supine posture, bronchoconstriction, lung volume loss, intubation

### Factors Affecting Alveolar Dead Space[edit]

- Increases with age, decreasing pulmonary artery pressure (due to hypoperfusion), IPPV - via hydrostatic failure of perfusion, increasing tidal volumes, anaesthetic gases, lung disease

### Alveolar Gas Equation[edit]

- Used to calculated the relationship between the fall in P
_{O2}and the rise in P_{CO2}that occurs in hypoventilation - R = the
**respiratory exchange ratio**or**respiratory quotient**- the ratio of CO_{2}production to O_{2}consumption - R is determined by the metabolism of tissues in a steady state

- Used to calculated the relationship between the fall in P

- This formula assumes that alveolar dead space is negligible
- Also ignores the bulk flow of oxygen generated from the pressure gradient of more O
_{2}being taken up than CO_{2}excreted in the alveoli, drawing oxygen from the anatomical dead space - This effect is included in the following version of the alveolar gas equation:

- The alveolar gas equation assumes:

- Inspired and alveolar gases obey ideal gas laws (Boyle's law and others)
- There is no water or CO
_{2}in inspired gas - Nitrogen in inspired gas is in equilibrium with dissolved nitrogen in blood
- Alveolar gases are saturated with water