Spirometer traces and exercise comparisons

What you’ll learn

• Calculate breathing rate, tidal volume and minute ventilation.
• Interpret oxygen-consumption trends from a stated trace model.
• Evaluate repeated measurements and participant-related variation.

Understand the idea

Core practical 17 uses spirometer traces to study exercise-related changes. Tidal volume is the volume moved in one normal breath, measured from the peak-to-trough difference for a complete breathing cycle. Breathing frequency is complete cycles per minute. Respiratory minute ventilation is tidal volume multiplied by breathing frequency, so a change in either factor can alter the total air moved each minute.

In a specified closed-system record where carbon dioxide is absorbed, a gradual fall in the gas-volume baseline can be used to estimate oxygen uptake. Compare like points in the breathing cycle or an appropriate mean trend over a sufficient interval. Do not confuse the oscillation amplitude with the baseline gradient, and check the time-axis units before converting to a per-minute rate. Leaks and temperature changes can invalidate a simple oxygen-consumption interpretation.

Comparing the same participant before and after an agreed activity reduces some between-person variation, but order, recovery time and previous activity still matter. Several participants are needed for a broader conclusion, with privacy, consent and the right to stop respected. These questions use supplied traces; any actual human exercise or breathing-apparatus activity requires appropriate supervision, hygiene and participant suitability checks.

Key formulae

$$f=\frac{\mathrm{complete\ breaths}}{\mathrm{time\ in\ minutes}}$$

$$\dot V_E=V_Tf$$

$$r_{\mathrm{O_2}}=\frac{\mathrm{baseline\ volume\ fall}}{\Delta t}\quad\text{under the stated trace conditions}$$

Worked example

A supplied trace shows 10 complete breaths in $30\ \mathrm{s}$, each with a tidal volume of $0.80\ \mathrm{dm^3}$. In the stated closed-system model with carbon dioxide absorbed, the mean gas-volume baseline falls by $0.60\ \mathrm{dm^3}$ over $120\ \mathrm{s}$. Calculate minute ventilation and oxygen uptake per minute.

1. Ten breaths in half a minute gives $20\ \mathrm{breaths\,min^{-1}}$.
2. Minute ventilation is $0.80\times20=16\ \mathrm{dm^3\,min^{-1}}$.
3. The baseline fall occurs over $2.0\ \mathrm{min}$, so oxygen uptake is $0.60/2.0=0.30\ \mathrm{dm^3\,min^{-1}}$ under the stated model.

Answer: Minute ventilation $16\ \mathrm{dm^3\,min^{-1}}$; oxygen uptake $0.30\ \mathrm{dm^3\,min^{-1}}$.