Functional residual capacity (FRC) is the volume of gas contained in the lungs at the end of a normal quiet expiration (see Figure 14.1). This measurement is used to evaluate both restrictive and obstructive lung defects. Changes in the elastic properties of the lungs are reflected in the FRC. FRC is the sum of the expiratory reserve volume (ERV) and residual volume (RV).

1. Fit the patient with nose clips. Instruct the patient to breathe normally through the mouthpiece/filter (bacterial/viral) combination that is attached to the lung volume apparatus. The patient is generally in the seated position.

2. Three methods may be utilized for measurement, depending on the instrumentation used:

   1. Nitrogen washout or open-circuit technique

   2. Helium dilution or closed-circuit technique

   3. Body plethysmography

3. Have the patient breathe normally for about 3–7 minutes.

4. Perform the test a second time. The FRC should vary by not more than 5%–10% (0.05–0.10). Report the average of the test values.

5. See Chapter 1 guidelines for intratest care.

1. A value <75% (<0.75) of the predicted is consistent with restrictive ventilatory impairment.

2. A value >125% (>1.25) of predicted demonstrates air trapping (hyperinflation), consistent with obstructive airway disease (e.g., emphysema, asthma, bronchiolar obstruction).

Pretest Patient Care

1. Explain the purpose and procedure of the test. Explain that this is a noninvasive test requiring patient cooperation. Assess the patient’s ability to follow verbal instructions.

2. Record the patient’s age, gender, weight, and height.

3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Normal

• Approximately 1800 mL (1.70 L) in women; 2300 mL (2.30 L) in men.

• Predicted values are based on age, height, weight, ethnicity, and gender. Women have an FRC that is generally 10% less than men.

• The observed value should be 75%–125% (0.75–1.25) of the predicted value.