Measurement of breathing volumes, called spirometry, is the most important test for COPD. A bronchodilator (a medication that helps to relax airway muscles and open the bronchioles) may be given to see if a person's spirometric measurements improve. Arterial blood gas sampling (blood taken from an artery, not a vein) shows the levels of oxygen and carbon dioxide in the bloodstream. Pulse oximetry, a noninvasive tool used routinely in intensive care units and during any type of anesthesia, measures the oxygen saturation of hemoglobin, which is a reflection of the amount of oxygen in the bloodstream. Chest x-ray is used to determine the amount of lung damage done by COPD and can also indicate the presence of pneumonia (lung infection) or other types of lung disease processes.