Lessons · Medicine foundations · dead space and alveolar ventilation
Dead space: the part of every breath that does no work
Part of every breath never reaches the gas-exchanging part of the lung, so the ventilation that counts is (tidal volume minus dead space) times the rate.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
This is the exam question that separates people who multiplied from people who understood. Two breathing patterns move the same six litres a minute; one of them does far less useful work, and the only thing that tells you so is subtracting the dead space before you multiply, not after.
How to think about it
Subtract the dead space from one breath, on its own line, and look at what is left. Multiply that remainder by the rate. The order matters enormously: the dead space is thrown away once per breath, so a fast shallow pattern throws it away many more times.
Worked example
Alveolar ventilation = (tidal volume − dead space) × rateSubtract first, then multiply.
Tidal volume 500 mL, dead space 150 mL: 500 − 150 = 350 mL of useful air per breathWhat actually reaches the alveoli each time.
350 × 12 = 4200 mL/minFour point two litres a minute of ventilation that does something.
The same person at 250 mL and 24 breaths: (250 − 150) × 24 = 2400 mL/minIdentical minute ventilation of 6000 mL/min, and not much more than half the useful ventilation.
Your turn
Tidal volume is 600 mL, dead space is 150 mL and the rate is 10 breaths per minute. Write the line that gives the alveolar ventilation.
(600 − 150) × 10 = mL/min
Solve one, graded on the server
The trap
Working out the minute ventilation first and then subtracting the dead space once. That treats the dead space as a single loss for the whole minute instead of a loss on every breath, and it flatters a fast shallow pattern enormously.