Lung function tests

Lung function tests aid the diagnosis of numerous respiratory conditions. An understanding of these tests and what they show, both in health and disease, helps to underpin an understanding of pathological processes. 

Volumes and capacities reveal much about the physiology of the lungs. They have a tremendous reserve to be utilised in times of increased oxygen demand, not only by increasing respiratory rate but the volumes with which we breathe.

Volumes

Lung volumes during inspiration and expiration may be categorised into:

Tidal Volume (VT): During normal breathing we inspire and expire around 500ml of air.

• Minute volume, the amount we respire in a minute is calculated by multiplying VT by the respiratory rate.

Inspiratory reserve volume (IRV): The additional volume inspired with maximal effort (3L).

Expiratory reserve volume (ERV): The additional volume expired with maximal effort (1.2L).

Residual volume (RV): The volume that remains after maximal expiration (1.2L).

Capacities

Lung volumes may be combined to give capacities:

Functional residual capacity (FRC): This is a combination of the ERV and RV (2.4L).

Vital capacity (VC): Volume of gas on maximal inspiration and expiration (5L).

Total lung capacity (TLC): The total volume of gas in lungs at maximal inspiration. TLC helps define the difference between obstructive and restrictive lung disease:

• Reduced TLC: Restrictive lung disease may occur due to parenchymal disease, neuromuscular disease or chest wall deformities which lead to a reduced TLC.
• Normal or increased TLC: Obstructive lung disease may occur due to increased resistance in the conducting airways, typically with a normal TLC. In emphysematous disease, TLC may be increased.

A peak flow is a simple test of the peak flow rate during forced expiration following maximal inspiration.

Measurement

Peak flow demonstrates diurnal variation. Flow is highest in the afternoon and lowest in the early hours.

Patients are instructed to expire, as hard and fast as they can, into the device. They are normally given three attempts with the best value recorded. Results must be corrected to normal values for a patient's age and gender. Normal values peak in the third decade for men and women before falling as we age.

Peak flow is higher in men (blue line) than women (orange line). Peak flow also varies with height. A healthy young man may expect a peak flow of 700 L/min, while a woman may expect a value of 420 L/min.

Diagnostic value

The diagnostic value of peak flow is limited due to a wide range of normal values. Results may only drop after significant disease has developed.

However it may be used to:

• Monitor disease over a period of days or months through a peak flow diary.
• Stratify the severity of an asthma attack.

Spirometry measures the flow and volume of air, typically with a focus on exhalation. We measure two values:

• FVC: the forced (expiratory) vital capacity is a persons maximal expiration following full inspiration.
• FEV1: the forced expiratory volume in one second, i.e the volume of FVC expelled after one second.

Technically this simply a type of spirometry obtained from a vitalograph. However, the medical community frequently uses the term spirometry to mean a vitalograph. 

Obstructive

Obstructive lung disease includes (ABC): 

• Asthma 
• Bronchiectasis
• COPD (chronic bronchitis and emphysema)

In obstructive disease, there is an increase in airway resistance and a reduction in airway flow secondary to bronchoconstriction and inflammation.

The following changes are seen in obstructive lung disease (blue line - normal, orange line - obstructive):

• FVC: may be normal but often reduced due to air trapping
• FEV1: reduced
• FEV1/FVC: < 70%

Restrictive

Restrictive lung diseases are characterised by a reduced TLC. Causes are diverse (PAINT):

Intrinsic:

1. Pleural:

• Pleural effusions
• Chronic empyema

2. Alveolar:

• Oedema
• Haemorrhage

3. Interstitial:

• Idiopathic pulmonary fibrosis
• Sarcoidosis

Extrinsic:

4. Neuromuscular:

• Myasthenia gravis
• ALS

5. Thoracic/extrathoracic:

• Kyphoscoliosis 
• Obesity
• Ascites

The following changes are seen in restrictive lung disease (blue line - normal, red line - restrictive):

• FVC: reduced
• FEV1: reduced
• FEV1/FVC: > 80%

A flow-volume loop plots both inspiration and expiration on the y-axis while flow rate is plotted on the x-axis. Here plots show maximal inspiration and expiration.

Phases

Inspiration: A relatively symmetrical saddle-shaped curve that runs from the residual volume to the total lung capacity (below the x-axis).

Expiration: Rises to the peak expiratory flow rate (PEFR) before a relatively linear fall to residual volume (above the x-axis).

Vital capacity can be calculated as the maximal volume of air that can be inspired and expired in a breath. PEFR can be seen as the maximal flow rate during the expiratory phase.

Obstruction

Obstructive conditions are caused by obstruction of conducting airways.

Total lung capacity is normal and may be increased, vital capacity may be reduced. Residual volume may be increased due to trapping of air.

Restriction

In restrictive diseases the airways are normal but respiration is limited. Elastic recoil may be increased causing supra-normal expiratory flow.

Total lung capacity and vital capacity are reduced. Residual volume may be normal but is frequently reduced.