Medical Health Encyclopedia

Using the results, the physician determines two important values:

• The forced vital capacity (FVC). FVC is the maximum volume of air that can be exhaled with force and is an indicator of the lung size, elasticity, and how well the air passages open and close.
• The forced expiratory volume in one second (FEV1). FEV1 is the maximum volume of air expired in one second. Airflow is considered to be limited if the outflow of forced exhalation is persistently low over the course of one second. Steady but faster than normal decline in FEV1 over time characterizes COLD.

Calculating a ratio of FEV1 to FVC is the best method for determining the presence and severity of COLD. The severity of airway obstruction may be graded by the percentage of the patient's predicted FEV1:

• Mild COLD is an FEV1/FVC ratio of 70% or higher, with a predicted FEV1 of less than 80%.
• Moderate is an FEV1/FVC ratio of 60% to 69%, with a predicted FEV1 of less than 80%.
• Moderately severe is an FEV1/FVC ratio of 50% to 59%
• Severe is an FEV1/FVC ratio of 34% to 49%
• Very severe is an FEV1/FVC ratio of less than 34%.

Spirometry is a painless study of air volume and flow rate within the lungs. Spirometry is frequently used to evaluate lung function in people with obstructive or restrictive lung diseases such as asthma or cystic fibrosis.

Tests for Measuring the Ability of the Lung to Exchange Gases

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Arterial Blood Gas. The physician may request an arterial blood gas test to determine the amount of oxygen and carbon dioxide in the blood (its saturation). Low oxygen (hypoxia) and high carbon dioxide (hypercapnia) levels are often indicative of chronic bronchitis, but not always of emphysema. A blood gas analysis that shows very low oxygen levels (measured as PO2) is useful for determining which patients would benefit from oxygen therapy. This procedure typically draws blood from an artery in the wrist, which can be painful.

Click the icon to see a depiction of arterial blood gas sampling.