Interpretation of flow-volume curves

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BACKGROUND

Flow volume curve is a graphic plot that provides useful information about lung functions and the relationship between lung volume and maximal rate of airflow.¹

Maximal expiratory flow-volume curve- This is the plot of maximal expiratory flow (Vmax) against volume during the performance of a forced expiratory vital capacity (FVC) maneuver. The inspiratory part of F-V curve depends on factors like airway resistance and strength of inspiratory muscles. It is lesser influenced by airway obstruction than expiratory flow. The reduction in maximal expiratory flow indicates airway obstruction from extrathoracic reasons.²

The aim of the experiment is interpretation of flow-volume curves.

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REQUIREMENTS

Subjects: Human

Apparatus: Spirometry

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PROCEDURE

The subject is asked to breathe into the spirometer. The subject is asked to take a deep breath and exhale with complete forced expiration. The different flow –volume curves are interpreted based on the values.

Basic parameters:

VC (vital capacity): This is the maximum volume of air exhaled or inspired during maximally forced (FVC) or slow (VC) manoeuvre, VC is normally equal to FVC. If the airflow obstruction is present, the VC is higher than FVC.

FEV1 (forced expiratory volume in 1 second): The volume expired in the first second of maximal expiration after a maximal inspiration. This indicates how fast the full lungs becomes empty.

FEV1/FVC: This gives a clinically useful index of airflow limitation

FEF25-75: This is average expired flow over the middle half of the FVC maneuver. This is the sensitive indicator of narrowing of small airways.

FEF50 and FEF75 (forced expiratory flow at 50% or 75% FVC): This is maximal expiratory flow measured at the point where 50% or 75% of the FVC has been expired (FEF50, FEF75)

PEF (peak expiratory flow): This is maximal expiratory flow rate achieved (8-14 L/s)

MEF25 (maximum expiratory flow at 25% VC): This is maximum expiratory flow at 25 % of vital capacity (1.5-2.5 L/s)

MEF50 (maximum expiratory flow at 50% VC): This is maximum expiratory flow at 50 % of vital capacity (3.5-5.5 L/s)

MEF75 (maximum expiratory flow at 75% VC): This is maximum expiratory flow at 75 % of vital capacity (4.5-8.5 L/s)

FVC6 (forced expiratory volume during the first 6 seconds): This value is alternative of FVC. This expiratory manoeuvre after 6 s is easier to perform in patients with airflow obstruction and elderly patient

PIF (peak inspiratory flow): This is the maximum flow of inspired air

MIF25 (maximum inspiratory flow at 25% VC): This is the maximum inspiratory flow at 25 % of Vital Capacity

MIF50 (maximum inspiratory flow at 50% VC): This is the maximum inspiratory flow at 50 % of Vital Capacity

MIF75 (maximum inspiratory flow at 75% VC): This is the maximum inspiratory flow at 75 % of Vital Capacity.

MIF50/MEF50 (maximum inspiratory: expiratory flow at 50% VC).²

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CONCLUSION

These flow-volume curves help in diagnosis and treatment of various lung diseases. The shape of the flow volume curves gives a clue whether the curves are normal or abnormal. The abnormality can be due to either obstructive or restrictive ventilatory defects.

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REFERENCES

http://docstarsdoctors.com/4.%20SPIROMETRY%20%20FLOW%20VOLUME%20CURVES.pdf. Accessed 19 June 2018.
https://www.jfmed.uniba.sk/fileadmin/jlf/Pracoviska/ustav-fyziologie/3ZH-Deter.oop.pdf. Accessed 19 June 2018.