The Functional Residual Capacity (FRC) tests measures the FRC lung volume by using Boyle’s Law.  This is the volume of air that remains in the lung at the end of a normal expiration.  This lung volume is important because by knowing this lung volume, it is easy to know all the other lung volumes.

See FinePointe Pulmonary Function Test (PFT) Manual Section FRC for list of common parameters reported. 

Common Reasons for Issues

Occlusion is not triggered

For the FRC test to successfully return good data, FinePointe must occlude the subject at the end of expiration (at FRC).

• • This can happen if the end of the breaths cannot be determined during the breath history phase. Occlusion is triggered at zero pressure when the previous breath ends.
  • Make sure the calibration zero is good for all leads.
  • Check the system for leaks (See the Leaks section below).
  • If breaths are erratic during the breath history phase, check the anaesthesia.

No breathing efforts during occlusion

In addition, the subject must attempt to breathe while its airway is occluded.  This can happen if the subject has been ventilated and is hyperventilated and is not trying to breath. See the sections below for more details

Leaks

Finally, there can be no leaks in the manifold.  Leaks in the manifold with violate a basic assumption which is made in this test that the volume is constant.

Animal Breathes During Tests

If your animal is breathing throughout the tests, consider the following possibilities:

• Inspiratory Flow Rate is too low.
• Expiratory Flow Rate is too low.
• Anesthesia dose should be increased.

Occlude at FRC

One of the most challenging objectives to meet with this test is to ensure that when the occlusion takes place, the animal is at FRC.

In the figure above, you can decide if FinePointe occluded the subject appropriately at FRC.

If it does not look correct, right click on the line corresponding to the test and select Reject Data.

Rejected data will be excluded from reports.

No Efforts During FRC Occlusion

If you ventilate the subject prior to running the FRC test, it is possible that when FinePointe occludes the airway, the subject is hyperventilated and does not try to breathe. The data returned may look something like the following:

The volume appears to drift away. FinePointe actually uses the Pressure to determine if an effort is present. The data above does technically have an effort or two, but this data should be rejected since the subject did not try to breathe past those couple of effort. To correct this, turn off the ventilator until the subject begins to breathe again on its own. Once the subject is breathing regularly again, you can start your test.

Data Assumptions Met

You can also get an indication of how well the pressure and volume data agrees with the model by looking at the R2 parameter.  FinePointe attempts to compute FRC on as many samples as it can. Not every pressure and volume pair yields the same FRC result. This parameter is a number between 0 and 1 representing the how well each pair agrees.  If all the pairs agree precisely, the result will be 1. If something has gone wrong, making them all differ, then the result can start to fall. Experience will tell you what is a good R2, and certainly if you intend to reject the worst one, criterion which can help you decide is the R2 result. Achieving R2 better than 0.95 should not be difficult.

If you see R2 results below 0.9, you should look for a leak around the tracheal tube or possibly in the manifold.

FRC Test is not triggering properly

Always make sure that the system has been powered on for at least 20 minutes before calibrating to allow the analog preamplifiers to warm up and stabilize the signals.  

Make sure the 3 flow signals have a good zero on each signal.  Any adjustments to the gain on the flow or high flow signal to achieve a good calibration, require the user to go back and re-calibrate the previous flow signals.  For example, if you adjust the gain on the high flow signal, you must repeat the calibration of the FRC Flow and Flow calibrations with the new gain setting.  

Additional Resources:

FinePointe Pulmonary Function Test (PFT) Manual

PFT Pressure Volume (PV) Test Troubleshooting

PFT Flow Volume (FV) Test Troubleshooting

PFT Troubleshooting Leaks