Snapshot A 23-year-old man presents to the emergency department with weakness. He states his legs were weak yesterday but now he is also having trouble breathing. He recently recovered from a diarrheal illness. The patient's forced vital capacity is significantly reduced and he is subsequently intubated (Guillain Barre syndrome). Introduction Clinical definition spirometry measures volume of expired air during forceful and complete exhalation forced vital capacity (FVC) total exhaled volume after a maximal inspiration forced expiratory volume in 1 second (FEV1) the volume exhaled in the first second of maximal expiration FEV1:FVC ratio low = obstructive disease air can not get out quickly concerning for obstructive disease (such as asthma) normal/high = restrictive disease no obstruction of airway concerning for restrictive disease (such as pulmonary fibrosis) flow-volume loops plot out spirometry findings normal obstructive disease restrictive disease bronchoprovocation challenge administration of bronchospastic agent (such as methacholine) followed by spirometry can aid in diagnosis of asthma Pulmonary Function Tests MetricsDefinitionObstructiveRestrictiveElasticityEase of recoil after lungs have expanded↓↑ComplianceEase of alveoli expansion↑↓VolumesMinute volumeVolume of air expired in 1 minute=/↓↓Residual volume (RV)Air left in lung after max expiration↑↓Total lung capacity (TLC)All the air in the lung = vital capacity + residual volume↑↓Tidal volumeVolume of air breathed in and out for a normal breath↓↓Vital capacityTotal volume exhaled after full inspiration.↓↓SpirometryForced expiratory volume (FEV1) normal = 4LVolume expelled in one second↓=/↓Forced vital capacity (FVC)normal = 5LEntire volume that can be expelled =/↓↓(FEV1)/FVCnormal = 80%Considered best screen, values of less than 50% correlate with postoperative pulmonary complications.↓=/↑