In one breath
Dead space is the part of each breath that never takes part in gas exchange: of a 500 mL breath, about 150 mL only fills the conducting airways (the anatomical dead space). Ventilated alveoli that get no blood add alveolar dead space, and the two together make the physiological dead space, which in health is about the same as the anatomical. Your pages cover the classic way to measure the anatomical part, the single-breath nitrogen test: after a breath of pure O₂, the first N₂ in the expired air marks the arrival of alveolar gas, and the volume breathed out before that point is the dead space.
Your pages start partway through this topic
Your PDF opens in the middle of measuring dead space. The start of the topic, what dead space is, its anatomical and physiological types with their normal values, and the first steps of the nitrogen test, is on the book’s previous pages.
Leads to: Alveolar ventilation · Ventilation–perfusion ratio · Lung volumes, capacities and spirometry
The basics, before your pages
Worth knowing, though not in your pages: this section is standard physiology, added because the opening of the topic is missing from your PDF. Where your pages do give a fact, the line says so.
- Dead space is the volume of each breath that does not exchange gas with blood: wasted ventilation.
| Kind | What it is | Normal |
|---|---|---|
| Anatomical | Conducting airways, nose to terminal bronchioles | about 150 mL |
| Alveolar | Ventilated alveoli with little or no blood | almost nil |
| Physiological (total) | Anatomical + alveolar | about 150 mL |
- Your pages use the same 150 mL in the worked example on Alveolar ventilation, and ch114 gives it too (Lung volumes, capacities and spirometry). Ch114 runs the airways down to the respiratory bronchioles; the usual answer is the terminal bronchioles, because respiratory bronchioles already carry a few alveoli.
- Rule of thumb: the anatomical dead space is about 2 mL per kg of body weight (1 mL per pound).
- In health almost every ventilated alveolus is perfused, so physiological ≈ anatomical. In lung disease the physiological dead space can grow many times over (Guyton: up to about 10 times, 1–2 L).
- How each is measured:
- anatomical: the single-breath nitrogen test, Fowler’s method, which is the part your pages cover below;
- physiological: Bohr’s equation, from CO₂: VD ÷ VT = (PaCO₂ − PECO₂) ÷ PaCO₂, where PECO₂ is the PCO₂ of mixed expired air. Normally about 0.3, which is 150 of a 500 mL breath.
- What changes it:
- anatomical ↑: a deep breath (the airways stretch), neck extension, bronchodilators such as atropine;
- anatomical ↓: neck flexion, bronchoconstriction, tracheostomy;
- alveolar ↑: anything that cuts the blood supply of ventilated alveoli, such as a pulmonary embolus or a fall in pulmonary arterial pressure (your pages describe this as zone 1 in Pulmonary circulation).
Single-breath nitrogen test
- The subject has just breathed in pure O₂, so when expiration starts, the dead space holds nothing but O₂.
- During expiration, the N₂ concentration of the expired air is recorded against the volume breathed out.
- N₂ can only come from the alveoli, so the first N₂ marks the arrival of alveolar air.
- Worth knowing, though not in your pages: the N₂ is what was already in the alveoli; the one breath of O₂ dilutes it but cannot wash it out.
The equivalent boundary
- The switch from dead-space air to alveolar air is not sudden. N₂ rises along an S-shaped curve, because some alveolar gas mixes with dead-space gas on its way out.
- So the S is replaced by an equivalent sharp boundary: a vertical line, placed so that the N₂ in the dead-space section matches the N₂ in the alveolar section.
- Dead space = the volume expired up to that line, read straight off the volume record.
- Worth knowing, though not in your pages: on the usual drawing, this means the shaded area to the left of the line, under the S, equals the shaded area to its right, above the S.
Planimetry
- A planimeter, an instrument for measuring the area of a drawn shape, measures two areas on the graph:
- the area that stands for the dead-space volume;
- the whole area, which stands for the total volume expired (VE).
- The dead space then follows by simple proportion: dead space = VE × (dead-space area ÷ total area).
- Illustration (made-up areas): a dead-space area of 3 units out of 10, with VE 500 mL, gives 500 × 3 ÷ 10 = 150 mL.
- Your book’s figure adds that the real alveolar volume is larger than the part shown on the graph.
Applied: ventilators add dead space
The tubing of a mechanical ventilator adds its volume to the dead space; your book’s key point is that dead space rises in artificial ventilation. If minute ventilation is not raised to match, less air reaches the alveoli. See Alveolar ventilation and Artificial respiration.
Which method measures which dead space
Worth knowing, though not in your pages: the nitrogen test measures the anatomical dead space, the volume of the airways. Bohr’s CO₂ equation measures the physiological dead space. Examiners like to swap them.
Draw it: the single-breath nitrogen curve
- Axes: volume expired (mL) on x; N₂ concentration (%) on y.
- A flat stretch at zero N₂: pure O₂ from the dead space.
- An S-shaped rise: dead-space and alveolar gas mixed.
- A nearly flat plateau: alveolar gas.
- A vertical dashed line through the S, with the small area left of it (under the curve) and the small area right of it (above the curve) shaded as equal. Label the volume up to the line “dead space” and the whole volume “VE”.
Exam-answer skeleton: "Dead space: types, normal values and measurement" (short note)
- Definition: the part of each breath that exchanges no gas (wasted ventilation).
- Anatomical, alveolar and physiological dead space, with values: 150 mL, and physiological ≈ anatomical in health.
- Anatomical dead space by the single-breath N₂ test: the principle and the S-shaped curve.
- The equivalent boundary, then planimetry and simple proportion (draw the curve).
- Physiological dead space by Bohr’s equation.
- What raises or lowers dead space, including ventilator tubing.
- Why it matters: alveolar ventilation = (tidal volume − dead space) × rate.
Asked in exams
- Dead space: anatomical and physiological: Jul/Aug 2014, 2 marks · Jan 2021, 3 marks · Jan/Feb 2022, 4 marks · May 2022, MCQ