ATS Pulmonary Function Laboratory Manual

chapter  8

Test Procedure for FRC N2

(Continued from previous page)

Step Action 6.

When N 2

concentration is ≤1.5%, have the patient perform an ERV maneuver and turn the patient out of

the system when RV is attained. Remove the mouthpiece and nose clip.

7. Perform at least one technically satisfactory measurement. If additional trials are performed, allow at least a 15-minute waiting period between trials. In patients with severe obstructive or bullous disease, the time between trials should be at least 1 hour (9). If only one FRC N2 measurement is made, caution should be used in interpretations. If more than one measurement is made, report the mean value of technically ac- ceptable results that agree within 10% rounded to two decimal places (e.g., 3.12 L).

Test Procedure for FRC He

Step Action 1.

Set-up and test preparation: • Turn equipment on, allow adequate warm-up time, and calibrate as instructed by the manufacturer. • Ask the patient if he/she has a perforated eardrum (if so, use an earplug). • Seat the patient comfortably; dentures need not be removed. Explain the procedure emphasizing the need to avoid leaks around the mouthpiece during the washout, and place the nose clip on the patient’s nose. • For young children, testing is best if done while the child watches a video and sits upright in the lap of the parent or someone the child trusts. For very young children, a small face mask that can be sealed with putty is preferred (8). 2. Have the patient breathe on the mouthpiece for approximately 30–60 seconds to become accustomed to the apparatus, and to assure a stable end-tidal expiratory level. 3. When breathing is stable and consistent with the end-tidal volume being at FRC, switch the patient into the closed breathing circuit. 4. Adjust the O 2 flow to compensate for O 2 consumption (significant errors in the calculation of FRC can result if O 2 consumption is not adequately accounted for). Note the He concentration every 15 seconds. 5. He equilibration is considered complete when the change in He concentration is less than 0.02% in 30 sec- onds. The test rarely exceeds 10 minutes, even in patients with severe gas or exchange abnormalities (10). 6. Once He equilibration is complete, turn the patient “out” of the system. If the measurements of ERV and IC are to be linked to the FRC measurement, assure that the spirometer has adequate volume for a full ERV and IVC maneuver. 7. Perform at least one technically satisfactory measurement. If only one measurement is made, caution should be used in interpretation. If more than one measurement is made, the value reported should be the mean of technically acceptable results that agree within 10%, rounded to two decimal places (e.g., 3.12 L). A minimum of 5 minutes should elapse before the test is repeated, with the patient breathing room air. Longer wait times between repeated tests may be necessary if there is significant maldistribution of in- spired air to allow elimination of test gas.

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