What is Pulmonary Function Test

The answer to the question what is Pulmonary Function Test should be known to every doctor. The Pulmonary function tests especially the bedside ones can be used by even a non medico to asses the function of the lung.

1. Simple Bed Side Test

Now what is Pulmonary Function Test that can be used at the bedside? They are the following:

  • Breath holding time: It is very simple and useful bedside test. Normal is> 25 seconds. Patients with breath holding time of 15 to 25, seconds are considered borderline cases and breath holding time < 15 seconds indicate severe pulmonary dysfunction.
  • Match test: Person is asked to blow off match stick from a distance of 15 cm. A person with normal pulmonary reserve will blow off the match stick from this distance.
  • Tracheal auscultation: If breath sounds are audible for more than 6 sec. it denotes significant airway obstruction.
  • Able to blow a balloon.
  • Spirometry by pocket size microspirometers can now be performed on bed side.

2. Spirometry

3. Forced Spirometry (Timed Expiratory Spiro gram)

4. Flow Volume Loops

5. Body Plethysmography, Helium Dilution, Nitrogen Washout

The answer to the question what is Pulmonary Function Test cannot be complete without a discussion on plethesmography. These techniques are employed for measuring functional residual capacity, residual volume and total lung capacity.

Helium dilution technique is also used to measure which is the volume at which airway closes. Normally it is 1 litre less than functional residual capacity. If functional residual capacity] falls below closing capacity there will be significant hypoventilation and V/Q abnormalities.

Body plethysmograph is also used to measure airway resistance (normal = 2.5 cm H2O/liter/second

6. Lung Compliance

It is volume change per unit of pressure.

Lung compliance : 0.2L/cm H20

Chest wall compliance : 0.2L/cm H20

Total compliance: 0.2L/cm H20 (chest wall compliance in opposite direction).

6. Pleural Pressures

Normal intrapleural pressure is -3 to -5 cm H2O.

During inspiration it becomes more negative up -7 cm H20.

During expiration it is +1 to +2 cm of H2O.


Before surgery patients with lung diseases should undergo tests for lung function. Again the surgeon may ask what is Pulmonary Function Test and why is it important before a surgery.

• Patients with FeV1<15ml require appropriate preoperative preparation before surgery like chest physiotherapy, antibiotics, bronchodilators etc.

• Patient with FeV1 <10 mI/kg and history of dyspnea at rest or on minimal activity should be subject to only life saving operations.

While answering the question what is Pulmonary Function Test the following criterion should also be mentioned.

Three most important criteria to indicate severe respiratory compromise are:

1. Dyspnea at rest or on minimal activity.

2. FeV1 <15 mI/kg (normal 65ml/kg).

3. pO2 <60 mmHg (or oxygen saturation <90%) room air.

Thus every physician as well as a surgeon should know what is Pulmonary Function Test and how to asses the functionality of the lung.

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