Pulmonary function tests

Pulmonary function tests or lung function tests are useful in assessing the functional status of the respiratory system both in physiological and pathological conditions. Lung function tests are based on the measurement of volume of air breathed in and out in quiet breathing and forced breathing. These tests are carried out mostly by using spirometer.

TYPES OF LUNG FUNCTION TESTS

Lung function tests are of two types:

1. Static lung function tests

2. Dynamic lung function tests.

Static Lung Function Tests

Static lung function tests are based on volume of air that flows into or out of lungs. These tests do not

depend upon the rate at which air flows. Static lung function tests include static lung volumes and static lung capacities.

Dynamic Lung Function Tests

Dynamic lung function tests are based on time, i.e. the rate at which air flows into or out of lungs. These tests include forced vital capacity, forced expiratory volume, maximum ventilation volume and peak expiratory flow. Dynamic lung function tests are useful in determining the severity of obstructive and restrictive lung diseases.

LUNG VOLUMES

Static lung volumes are the volumes of air breathed by an individual. Each of these volumes represents

the volume of air present in the lung under a specified static condition (specific position of thorax).

Static lung volumes are of four types:

1. Tidal volume

2. Inspiratory reserve volume

3. Expiratory reserve volume

4. Residual volume.

TIDAL VOLUME

Tidal volume (TV) is the volume of air breathed in and out of lungs in a single normal quiet respiration. Tidal volume signifies the normal depth of breathing.

Normal Value

500 mL (0.5 L).

INSPIRATORY RESERVE VOLUME

Inspiratory reserve volume (IRV) is an additional volume of air that can be inspired forcefully after the end of normal inspiration.

Normal Value

3,300 mL (3.3 L).

EXPIRATORY RESERVE VOLUME

Expiratory reserve volume (EVR) is the additional volume of air that can be expired out forcefully, after

normal expiration.

Normal Value

1,000 mL (1 L).

RESIDUAL VOLUME

Residual volume (RV) is the volume of air remaining in lungs even after forced expiration. Normally,

lungs cannot be emptied completely even by forceful expiration. Some quantity of air always remains in the lungs even after the forced expiration Residual volume is significant because of two

reasons:

1. It helps to aerate the blood in between breathing and during expiration

2. It maintains the contour of the lungs.

Normal Value

1,200 mL (1.2 L)

LUNG CAPACITIES

Static lung capacities are the combination of two or more lung volumes.

Static lung capacities are of four types:

1. Inspiratory capacity

2. Vital capacity

3. Functional residual capacity

4. Total lung capacity.

INSPIRATORY CAPACITY

Inspiratory capacity (IC) is the maximum volume of air that is inspired after normal expiration (end expiratory position). It includes tidal volume and inspiratory reserve volume .

IC = TV + IRV

= 500 + 3,300 = 3,800 mL

VITAL CAPACITY (VC)

Vital capacity (VC) is the maximum volume of air that can be expelled out forcefully after a deep (maximal) inspiration. VC includes inspiratory reserve volume, tidal volume and expiratory reserve volume.

 VC = IRV + TV + ERV

= 3,300 + 500 + 1,000 = 4,800 mL

Vital capacity is significant physiologically and its determination is useful in clinical diagnosis as explained

later in this chapter.

FUNCTIONAL RESIDUAL CAPACITY

Functional residual capacity (FRC) is the volume of air remaining in lungs after normal expiration (after normal tidal expiration). Functional residual capacity includes expiratory reserve volume and residual volume.

FRC = ERV + RV

= 1,000 + 1,200 = 2,200 mL

TOTAL LUNG CAPACITY

Total lung capacity (TLC) is the volume of air present in lungs after a deep (maximal) inspiration. It includes all the volumes.

TLC = IRV + TV + ERV + RV

= 3,300 + 500 + 1,000 + 1,200 = 6,000 mL

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