17.4: Spirometry and Measuring Lung Capacity

Spirometry and Lung Capacities

Pulmonary function is tested to detect lung diseases or to monitor the progress of a disease. The most common pulmonary function test is done with a spirometer. The spirometer measures the volume of air expired as a function of time with normal quiet breathing or forced breathing. Different lung volumes can be measured to determine lung capacity.

Figure \(\PageIndex{2}\) The Spirometer: This tool is used to measure lung capacity by exhaling normally and forcefully into the device. (Image by Harmony Folse, licensed under CC BY-SA 4.0)

The following is a list of lung volumes and capacities:

1. Tidal volume (VT): volume of air inspired or expired during each normal (unforced) ventilation cycle.
2. Inspiratory reserve volume (IRV): maximum volume of air that can be forcefully inhaled after a normal (tidal) inhalation.
3. Expiratory reserve volume (ERV): the maximum volume of air that can be forcefully exhaled after a normal (tidal) exhalation.
4. Residual volume (RV): the volume of air remaining in the lungs after a maximum (forced) exhalation.
5. Vital capacity (VC): maximum volume of air that can be exhaled after maximum inhalation.
6. Total lung capacity (TLC): total volume of air in the lungs after a maximum (forced) inhalation.
7. Inspiratory capacity (IC): maximum volume of air that can be inhaled after a normal (unforced) exhalation.

Figure \(\PageIndex{2}\) Respiratory Volumes and Capacities: These two graphs show (a) respiratory volumes and (b) the combination of volumes that results in respiratory capacity. (Image and caption credit: "Respiratory Volumes and Capacities" by OpenStax is licensed under CC BY 4.0)

Instructions for Measuring Lung Capacity with the Spirometer

1. Place a disposable mouthpiece on the stem of the spirometer.
2. Note the spirometer has a calibrated dial from 0 to 7000 ml.
3. Now turn the bezel on the dial so the pointer is at the 1000 ml mark. See the image below for an example.

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4. Pinch your nose close with one hand and practice normal breathing with your mouth 1 – 2 minutes until you feel comfortable.
5. Keep your nose pinched with one hand and hold the spirometer with the other hand.
6. Make sure that when you are holding the spirometer, you do not cover the holes on the side of the spirometer with your hand. If these holes are covered, you will not get an accurate measurement. See the image below for an example on how to hold the spirometer properly.

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7. Inhale a normal breath (DO NOT INHALE FROM THE SPIROMETER) and purse your mouth around the mouthpiece of the spirometer. Blow out your breath NORMALLY! DO NOT FORCE IT OUT! YOU WILL HAVE TO MAKE A CONSCIOUS EFFORT NOT TO EXCEED YOUR NORMAL VOLUME.
8. Remove your mouth from the mouthpiece after the normal breath out. Check the number the pointer points to on the dial.
9. Calculate the volume difference between your recorded value in Step 8 and initial value (1000 ml). This is the Tidal Volume (VT).
   • Tidal Volume (VT) = _______ ml – 1000 ml = _______ ml (USE THIS VALUE FOR YOUR CALCULATIONS LATER! ALWAYS USE THE VALUE YOU FOUND AFTER YOU SUBTRACTED 1000 FROM IT!)
     
10. Have your lab partner count your respiratory cycles for 1 minute while you are seated at rest.
    • Respiratory cycles for 1 minute = _______ breaths
      
11. Calculate respiratory minute volume:
    • VT (from step 8) x respiratory cycles/min (from step 9) = _______ x _______ = _______ ml/min
      
12. Now reset the pointer back to 1000 ml.
13. Pinch your nose close with one hand and practice normal breathing with your mouth 1 – 2 minutes until you feel comfortable.
14. Inhale a normal breath (DO NOT INHALE FROM THE SPIROMETER) and exhale normally, then purse your mouth around the mouthpiece of the spirometer immediately. Blow out your breath FORCEFULLY! Blow hard into the spirometer until you run out of air!
15. Remove your mouth from the mouthpiece after the forced expiration. Check the number the pointer points to on the dial.
16. Calculate the volume difference between your recorded value in Step 14 and initial value (1000ml). This is the Expiratory Reserve Volume (ERV):
    • Expiratory Reserve Volume (ERV) = _______ ml – 1000 ml = _______ ml (USE THIS VALUE FOR YOUR CALCULATIONS LATER! ALWAYS USE THE VALUE YOU FOUND AFTER YOU SUBTRACTED 1000 FROM IT!)
      
17. Now reset the pointer back to 1000 ml.
18. Pinch your nose close with one hand and practice normal breathing with your mouth 1 – 2 minutes until you feel comfortable.
19. Inhale maximally (DO NOT INHALE FROM THE SPIROMETER) and purse your mouth around the mouthpiece of the spirometer immediately. Blow out your breath FORCEFULLY! Blow hard into the spirometer until you run out of air!
20. Remove your mouth from the mouthpiece after the forced expiration. Check the number the pointer points to on the dial.
21. Calculate the volume difference between your recorded value in Step 19 and initial value (1000ml). This is the Vital Capacity (VC):
    • 1st measurement: Vital capacity (VC) = _______ ml – 1000 ml = _______ ml
      
22. Repeat Steps 17 – 20 two more times to obtain 2 more Vital Capacity measurements.
    • 2nd measurement: Vital Capacity (VC) = _______ ml – 1000 ml = _______ ml
    • 3rd measurement: Vital Capacity (VC) = _______ ml – 1000 ml = _______ ml
      
23. For the questions and calculations in the results section that involve Vital Capacity, of your three Vital Capacity measurements, choose the HIGHEST measurement after subtracting 1000ml from it. That will be your vital capacity used for all calculations and questions.
24. Discard the mouthpiece when you are done with the experiment.

Attribution

All images of the spirometer on this page were taken and edited by Harmony Folse and are licensed under CC BY-SA 4.0.