Spirometry: Lung Volumes and Capacities
Spirometry is the most common pulmonary function test (PFT). It measures the volume of air an individual can inhale or exhale as a function of time. Clinically, it is the gold-standard diagnostic tool used to assess lung function, differentiate between obstructive and restrictive ventilatory defects, and monitor chronic respiratory conditions like asthma and Chronic Obstructive Pulmonary Disease (COPD).
Static Lung Volumes and Capacities
A standard spirometry tracing maps out the baseline architectural air spaces within the lungs. These are divided into non-overlapping static volumes, which can be combined to define specific lung capacities:
1. Four Core Lung Volumes
Tidal Volume (TV): The volume of air inspired or expired during normal, quiet, baseline breathing. In a healthy adult, this is approximately 500 mL.
Inspiratory Reserve Volume (IRV): The maximum extra volume of air that can be forcefully inspired above a normal tidal inhalation.
Expiratory Reserve Volume (ERV): The maximum extra volume of air that can be forcefully expired after a normal tidal exhalation.
Residual Volume (RV): The volume of air remaining in the lungs after a maximal, forced exhalation. Crucial Clinical Note: Residual volume cannot be measured by direct spirometry because a person cannot physically exhale this air. It must be measured using indirect methods like helium dilution or body plethysmography.
2. Four Core Lung Capacities
Vital Capacity (VC): The maximum volume of air a person can expel from the lungs after first filling the lungs to their absolute maximum (calculated as TV + IRV + ERV).
Inspiratory Capacity (IC): The maximum amount of air that can be breathed in starting from a normal expiratory level (calculated as TV + IRV).
Functional Residual Capacity (FRC): The volume of air remaining in the lungs at the end of a normal, passive exhalation (calculated as ERV + RV). This represents the baseline equilibrium point of the respiratory system.
Total Lung Capacity (TLC): The total volume of air contained within the respiratory system after a maximal inhalation (calculated as VC + RV).
Dynamic Spirometry: The FVC and FEV1 Framework
To diagnose respiratory diseases, clinicians use dynamic spirometry, which forces the patient to exhale as hard and as fast as possible. This generates a Volume-Time Curve and a Flow-Volume Loop, yielding three vital operational parameters:
Forced Vital Capacity (FVC): The maximum total volume of air that the patient can forcefully exhale after a full, deep inhalation.
Forced Expiratory Volume in 1 Second (FEV1): The specific volume of air exhaled during the very first second of the forced FVC maneuver.
FEV1/FVC Ratio: The fraction of total vital capacity exhaled in the first second. In a healthy adult, this value is typically between 70% and 80%.
Differentiating Ventilatory Patterns
Spirometry results cleanly divide abnormal lung mechanics into two primary categories based on characteristic shifts in volumes and ratios:
1. Obstructive Defect (e.g., Asthma, COPD, Bronchiectasis)
The Problem: Airway resistance is increased, making it difficult to empty the lungs. Exhalation is slowed down.
Spirometry Signature: FEV1 drops drastically, while FVC drops only mildly or remains near normal. Consequently, the FEV1/FVC ratio drops below 70% (or below the lower limit of normal).
Loop Configuration: The expiratory curve on a flow-volume loop demonstrates a characteristic "scooped-out" or concave appearance.
2. Restrictive Defect (e.g., Idiopathic Pulmonary Fibrosis, Sarcoidosis, Scoliosis)
The Problem: Lung tissue is stiff or the chest wall cannot expand, restricting total air volume during inhalation.
Spirometry Signature: Both FEV1 and FVC drop equally. Because they drop in tandem, the FEV1/FVC ratio remains completely normal or may even increase (greater than 80%).
Verification: A restrictive pattern suggested on spirometry must be formally confirmed by measuring Total Lung Capacity (TLC). A true restrictive defect is defined by a TLC below 80% of the predicted value.
Spirometry References
Miller, M. R., Crapo, R., Hankinson, J., et al. (2005). Standardisation of spirometry. European Respiratory Journal, 26(2), 319-338.
Graham, B. L., Steenbruggen, I., Miller, M. R., et al. (2019). Standardization of spirometry 2019 update. An official American Thoracic Society and European Respiratory Society technical statement. American Journal of Respiratory and Critical Care Medicine, 200(8), e70-e88.
West, J. B., & Luks, A. M. (2016). West's Respiratory Physiology: The Essentials (10th ed.). Philadelphia: Wolters Kluwer.
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