Spirometry Reading

Introduction

Spirometry is an important, commonly used investigation in respiratory medicine. It is the most common type of Pulmonary Function Test (PFT).

It allows a narrowing of a diagnosis through the patterns the investigation produces. It also allows for progression of a disease to be monitored as it sets a baseline that can change over time through the worsening/management of a disease.

Spirometry involves the measurement of 2 factors by a computer while a patient blows through a mouthpiece:

• FEV₁ -Forced Expiratory Volume
  • This is the volume of air that a patient can forcefully exhale in 1 second
• FVC – Forced Vital Capacity
  • This is the total volume of air that a patient can forcefully exhale after a full inhalation

These results are presented as a ratio: FEV₁/FVC – and depending on the ratio form two broad patterns of spirometry:

• Restrictive
• Obstructive

Results can also be presented on a volume of air over time graph.

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Obstructive Pattern

An obstructive pattern of lung diseases shows a graph with a greatly reduced RATE of expiration (FEV₁) due to an obstructive process that limits the volume of air expelled every second.

The FVC can be NORMAL OR can be REDUCED due to a condition known as air trapping. This results in a volume of air that is difficult to expire in SOME with an obstructive process – such as asthma. This is due to the bottlenecking that happens as a result of constricted bronchioles.

Additionally the overall obstructive pattern in COPD has a restrictive element (reduced FVC) due to emphysema- destruction of alveoli.

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Restrictive Pattern

A restrictive pattern of disease shows a graph with an overall reduced total volume. This makes sense as restrictive diseases RESTRICT the volume of air that lungs can hold, through direct damage to the alveoli OR not allowing full inspiration to take place -reducing the volume of expiration.

The FEV₁ can be SLIGHTLY reduced or NORMAL.

Overall this gives a NORMAL or INCREASED FEV₁/FVC ratio: