Flow Volume Loops Describes change in lung volume and air flow during a respiratory cycle Curve begins just before expiration when there is no flow through the airways lung volume is at a maximum Curve then proceeds through an organized set of steps including expiration with increasing flow and decreasing lung volume peak expiratory flow where expiration is the fastest end expiration with decreasing flow and decreasing lung volume cessation of expiration at residual volume with no flow inspiration with increasing flow and increasing lung volume peak inspiratory flow where inspiration is the fastest end expiration with decreasing flow and increasing lung volume return to the initial point of no flow and maximal volume Analyzing the structure of these curves can provide important insights into mechanisms of disease upper airway obstruction will present with blunting of peak expiratory and inspiratory flow curves will be flatter rather than proceeding to peaks total tidal volumes may remain the same obstructive lung disease will present with diminished expiratory flows and increased lung volumes relatively smaller changes during inspiration restrictive lung disease will present with smaller lung volumes during each respiratory cycle curve will not reach as large volumes as normal Compliance Describes distensibility of respiratory system Describes change in lung volume for a given change in pressure (C = V/P) ↑ compliance in emphysema and aging ↓ compliance in pulmonary fibrosis, pulmonary edema, ARDS, and chest wall disease Elastance Describes elastic properties (inverse of compliance, elastance = P/V) Lungs tend to collapse inward Chest wall tends to expand outward Pressure-Volume Curve V = FRC (functional residual capacity) FRC = volume in lungs at end of normal tidal expiration airway pressure = atmospheric pressure = no airflow collapsing force from lungs = expanding force from chest wall resting volume when there is no airflow at the end of tidal expiration combined lung and chest wall system is at equilibrium V < FRC e.g., end forced expiration ↓ volume in lungs → ↓ collapsing force on lungs and ↑ expanding force on chest wall combined lung and chest wall system "wants" to expand V > FRC e.g., inspiration ↑ volume in lungs → ↑ collapsing force on lungs and ↓ expanding force on chest wall combined lung and chest wall system "wants" to collapse