# How to Calculate Expiratory Reserve

by Allan Robinson, Demand Media

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.

Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

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#### 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.