How to Calculate Expiratory Reserve

The expiratory reserve volume (ERV) is the amount of air that can be expired after a normal exhalation. Measuring the ERV is a common component of a pulmonary function test and can assist in identifying early pulmonary disease. The ERV is one of a variety of measures for lung volume which we may generally describe in terms of inhalation and exhalation. A physician typically measures these lung volumes with a spirometer but there are also more sophisticated methods. The methods for measuring ERV are provided in the following steps.

    Define the ERV more completely. The ERV is the maximum amount of air that can be expelled from the lungs after a normal exhalation. The volume remaining in the lungs after a normal exhale is the functional residual capacity (FRC). The volume of air in the lungs that cannot be expelled is the residual volume, or RV. Therefore, ERV = FRC - RV.

    Exhale into a spirometer as completely as possible after a normal exhalation. This device measures the air volume and air flow that is exhaled into it over a specified time. It was invented in 1846 and is a standard piece of equipment for family doctors.

    Use body plethysmography to measure ERV. This method measures pressure changes and requires the patient to sit inside an airtight box and exhale a particular volume until a shutter closes the breathing valve.

    Calculate lung volumes with helium dilution. Patients breathe a known concentration and volume of helium for a specified period of time. The helium concentration in the exhaled air is then measured and the lung volumes may then be calculated.

    Interpret the values for the ERV. An ERV is typically about 1.2 liters for males and 0.93 liters for females. A significantly lower figure can indicate a pulmonary restrictive disease such as pulmonary fibrosis.


About the Author

Allan Robinson has written numerous articles for various health and fitness sites. Robinson also has 15 years of experience as a software engineer and has extensive accreditation in software engineering. He holds a bachelor's degree with majors in biology and mathematics.

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