Lung Structure and Ventilation

Lung Volumes

Spirometry measures or estimates tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume, vital capacity, and total lung capacity.

Spirometry is a test that assesses how well the lungs work. It measures the amount of air at rest and with forceful inspiration and expiration, and can easily be done in the doctor's office. Respiratory volumes are the measures of air at different points during the breathing process. Tidal volume is the amount of air that flows into and out of the lungs during quiet breathing, or eupnea. Expiratory reserve volume is the amount of air that can be forcefully expired from the lungs beyond the normal amount (or beyond tidal volume). Residual volume is the amount of air left in the lungs after forceful expiration of as much air as possible. Residual volume is what prevents the lungs from collapsing after forceful expiration. Inspiratory reserve volume is the amount of air that can be taken in during maximum deep breathing beyond the normal inspiration (or beyond tidal volume).

In general, spirometry readings in a 45-year-old healthy male patient are as follows: tidal volume is usually around 500 mL. Expiratory reserve volume can be up to 1,200 mL. Normal residual volume is usually around 1,200 mL. Normal inspiratory reserve volume is around 3,100 mL.

Inspiratory capacity is the sum of inspiratory reserve volume and tidal volume, or the total amount that can be inspired at one time. Vital capacity is the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume, or the total amount of air that can be expired and inspired maximally. Functional residual capacity is the combination of expiratory reserve volume and residual volume, or the amount of air left in the lungs during quiet breathing. Total lung capacity is the volume of air in the lungs during maximum inspiration. It is the combination of tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume.

Measuring Lung Volumes Using a Spirometer

During spirometry measurements, the patient breathes into a mouthpiece and a computer calculates the change in lung volume. The total lung capacity is the maximum amount of air that can be taken in and out during maximum inspiration and expiration plus the residual volume, or the air that remains in the airways following maximal expiration. The inspiratory capacity is the total amount that can be inspired at one time. The vital capacity is the total amount of air that can be maximally expired and inspired.
Typically, doctors order spirometry testing for patients either diagnosed with or suspected of having breathing conditions such as asthma, chronic bronchitis, pulmonary fibrosis, or chronic obstructive pulmonary disease (COPD). For patients already diagnosed with a particular breathing problem, spirometry performed over time tells the doctor if the patient's medications are working appropriately and if the breathing has gotten under control. During the test, a patient's nostrils are clipped closed, the patient takes a deep breath, and then expels the breath as hard as possible into a tube. The test is usually done three times to ensure the results are accurate. The most important spirometry measurements include the forced vital capacity (FVC) and the forced expiratory volume in one second (FEV1). The FVC is the greatest amount of air a person can forcefully breathe out after breathing in as deeply as possible. If the FVC is lower than normal, it means the patient's breathing is being restricted. The FEV1 is the amount of air a person can force out of the lungs in one second. If the FEV1 is lower than normal, then there is an obstruction involved in the patient's breathing. The FEV1/FVC ratio represents the percentage of a person's lung capacity that the person is able to expire in one second. The higher this percentage is, the healthier the person's lungs are.