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Last updated date: 08th Dec 2023
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# A spirometry cannot be used to measure?

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Hint: Spirometer is used to measure the respiratory volumes. A spirometer measures ventilation, the movement of air into and out of the lungs. A healthy human breathes $12 - 16$ times/minute. We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen. The description of different volumes and capacities is given below.

Tidal Volume (TV): It is the volume of air inspired or expired during a normal respiration. It is approx. $500ml$. ($6000 - 8000ml$ of air per minute).
Inspiratory Reserve Volume (IRV)- It is the additional volume of air that can be inspired through forcible inspiration. ($2500 - 3000{\text{ }}ml$).
Expiratory Reserve Volume (ERV): It is the additional volume of air that can be expired through forcible expiration. ($1000 - 1100{\text{ }}ml$).
Residual Volume (RV): Even after forcible expiration, some amount of air is left in lungs called RV. ($110 - 1200{\text{ }}ml$).
Various pulmonary capacities can be derived by adding these volumes-
Inspiratory Capacity (IC): After normally expiring, the total volume of air one can inspire is IC. After expiration one can inhale normally and also via forcible inhalation. It includes TV and IRV.
Expiratory Capacity (EC): It is the volume of air one can expire after normal inspiration. After normal inspiration, the total volume that can be exhaled out normally as well as through forcible exhalation. It includes TV and ERV.
Functional Residual Capacity (FRC): This is the volume of air that remains in lungs after normal expiration. After expiring normally, the volume of air left in lungs ERV and RV.
Vital Capacity (VC): It is the maximum volume of air one can breathe after forcible expiration. So, after forcible expiration the air that can be inhaled is by normal inspiration, forcible inspiration, and also the air we exhale forcibly can be inhaled again. Or it can also be defined as the maximum volume of air one can breathe out after forced inspiration. After forcible inspiration the volume that can be exhaled includes the normal expiration, forcible expiration and the volume inhaled forcibly can also be exhaled back. This includes $TV + {\text{ }}IRV + {\text{ }}ERV$ or $\left\{ {VC = {\text{ }}TLC - {\text{ }}RV} \right\}$
Total Lung capacity (TLC): After forcible expiration, the volume of air our lungs can accommodate is called TLC. It is $RV + {\text{ }}VC$or$\left\{ {TV + {\text{ }}IRV + {\text{ }}ERV + {\text{ }}RV} \right\}$.
Dead Space Air: It is the volume of air that is present in the trachea-bronchial tree and does not take part in gaseous exchange. It is approx. $150ml$.

So, A spirometer cannot be used to measure residual volume (RV) because it’s not possible to measure the remaining amount of the volume left in lungs after maximum expiration. It also cannot directly measure FRC, TLC and dead space air.

Note:
Spirometry is also useful for evaluating disease progression- whether it is getting better, worse, or staying the same. This can help determine if a treatment is working or needs to be modified.
Spirometry may also be used before lung cancer surgery to predict how well a patient will tolerate the operation and manage once a portion or lobe of a lung is removed.