The Pressure Volume (PV) test allows the researcher to know the static lung properties.  Static lung properties are described by the pressure-volume relationship of the lung.  The slope of the Volume-Pressure curve is the Instantaneous Compliance (IC) at a given pressure.  See PFT Manual Section Quasistatic Pressure Volume FRC Test for list of common parameters reported. 

Common Reasons for Issues

In this test, you need to watch out for a few things:

• The flow was properly zeroed during calibration.
• The expiratory duration is long enough and without attempts from the subject to breathe on its own.
• There should be no leaks in the tracheal line.
• Dead space in the expiratory line affecting the upper side of the curve.

The inspiration to 30 cm H20 phase of the PV test is not triggered

• • This can happen if the end of the breaths cannot be determined during the breath history phase. The inspiration phase is triggered at zero pressure when the previous breath ends.
  • Make sure the calibration 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 anesthesia.

30 cm H20 of pressure cannot be achieved during the inspiration phase

• • • There could be a leak in the tracheal tube surgery. Check the surgery site and the tubing connection to the manifold.
    • The inspiration flow rate is too low. Increase in inspiration flow slightly and repeat the test (See the Setting Inspiration Flow section on page 25).
    • There could be a leak in the valves (See the Leaks section below)
    • If you have a test lung for your species type, connect it and attempt the PV test. If the test passes, the issue was probably with the tracheal surgery or the connection to the manifold. If it still fails, try the other items in this list.
    • 30 cm H20 must be reached within a unique specified time for each species, or the inspiration will time out. Keep in mind that the rate you need to achieve for a “good” test is usually less than the time out value (See the Setting Inspiration Flow section on page 25). For example, with mice the inspiration time out is 5 seconds. However, the best inspiration rate for the average sized mouse is to reach 30 cm H20 in 1-2 seconds.

The slow expiration phase was too slow or too fast

• After the PV test is complete, check the Te value and compare to the values listed for your species type (See the Setting Slow Expiration Flow section on page 26). If the Te is too high, lower the slow expiration flow rate slightly. If the Te is too low, increase the slow expiration flow rate slightly.

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.

Test Starts When Flow Is Zero

Just like the Functional Residual Capacity (FRC) test, the PV test must begin at FRC so that it can accurately determine IC.  If IC is not measured properly, your Total Lung Capacity (TLC) measurement will be off as well. Check that the test starts when the flow is at zero, and the volume is at a minimum.  The volume curve is reset at the start of the test, so you may see it step up, but you should look to ensure that the volume appears to be at a minimum.

At Total Lung Capacity (TLC), Volume Rises as Pressure Falls

If pressure drop is accompanied with a slight rise in the volume, then some air escaped from the trachea into the chamber. Check that you tied the tracheal cannula tightly.

Sources of leak to consider:

• where the tracheal cannula attaches to the manifold,
• where the cannula is tied to the trachea,
• in the cannula itself.

At TLC, Volume Drops as Pressure Falls

If the pressure drop is accompanied with a slight decrease in the volume, then the air has escaped from the trachea directly to atmosphere. This can only be due to a leak in the manifold, and most probably one of the following:

• The N2 Sample port,
• Around mouth pressure transducer,
• The FRC valve.

Almost Vertical Volume Drop Immediately after the Hold Period

With certain apparatus, it is possible for the dead space in the expiratory line to be excessive. This is particularly true if you are using a pressure panel that is meant for a rat, with a mouse. The result is that you will lose the higher-pressure portion of the PV curve. To correct the problem, try the following in the following order:

• Shorten the tubing to the pressure panel, if possible,
• Place a flow limiter or a crimping attachment near the slow expiration valve on the manifold,
• Reduce the negative pressure reservoir.

Expiratory Duration Should be Long Enough

The system requires that the expiratory duration must be at least 1 second, otherwise it will not attempt to compute results. What the duration should be depends upon the species.

• Mouse should be around 1.5 seconds
• Rat should be about 2.5 - 3.5 seconds
• Dog and monkey should be 3 - 5 seconds

As far as the test results are concerned, this duration cannot be too long. If it is too long, then the animal tries to breathe and ruins your data. To lengthen the expiratory duration, you must reduce the expiratory flow on the pressure panel.

Volume Should Rise Steadily During Inspire to TLC

If the rise of the volume during this period is diminishing near the top pressure, then it is likely that the positive pressure in the Pressure Panel is set too low. The reason is that as the mouth pressure increases, the pressure difference between what is in the lung and what is in the pressure reservoir diminishes. This results in a steadily decreasing flow. If the pressure difference between the mouth pressure and the reservoir pressure is very small at TLC, your flow will be too.

You can increase the positive pressure in the reservoir. The positive pressure in the reservoir should always be at least 40 cm H2O, but 50 cm H2O is certainly safe too.

Volume-Pressure Curve is Smooth

Particularly in the negative pressures, it is possible to observe little oscillations on the Volume-Pressure curve. These oscillations are generally due to the animal trying to breathe before the test is complete.

To verify that, simply select “PV Chart Options > Show Time Domain” and look at the Pressure curve to see if the animal attempted to breathe at the end of the slow expiration (in the negative pressures).

If that is the case, it may simply be that your expiratory duration is too long for the animal to tolerate. If it is, increase the expiratory flow on the pressure panel. However, if the duration is reasonable, try the following:

• Consider increasing the inspiratory flow to shorten the inspiration.
• Consider ventilating the subject prior to running this test.
• Consider adjusting (increasing) the anesthesia.

Overlays of Consecutive PV Tests Differ

When you are looking at PFT test results, you can select multiple rows in the PV table to overlay their results as illustrated below:

One test for repeatability of data is to overlay 2 consecutive tests. With the PV test you should get very consistent results, so an overlaid test should trace almost identically. If one particular curve is an outlier, right click and select “Reject Data”.

Additional Resources:

FinePointe Pulmonary Function Test (PFT) Manual

Functional Residual Capacity (FRC) Test Troubleshooting

PFT Flow Volume (FV) Test Troubleshooting

PFT Troubleshooting Leaks