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Pulmonary Function Test • Publication 014181-001 Rev 02 • www.datasci.com ©2023 Data Sciences International
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:
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