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Tidal volume is the first lung volume to know. Tidal volume, or TV, is the air moved in or out of the lungs during normal, quiet breathing. The average tidal volume in an adult is about 500 milliliters. Tidal volume is the baseline for other lung volumes and is included in all capacities that contain a normal breath. The mechanism behind tidal volume is the rhythmic contraction and relaxation of the diaphragm and intercostal muscles during resting breathing.
Inspiratory reserve volume, or IRV, is the next lung volume. Inspiratory reserve volume is the extra air that can be inhaled after a normal inspiration. The typical inspiratory reserve volume is about 3,000 milliliters in an adult. This additional inhalation happens when the inspiratory muscles contract more forcefully, expanding the thoracic cavity beyond usual resting levels.
Expiratory reserve volume, or ERV, is the third basic lung volume. Expiratory reserve volume is the extra volume of air that can be exhaled after the end of a normal expiration. The average ERV in adults is about 1,200 milliliters. Exhaling this extra air requires active contraction of abdominal and internal intercostal muscles, which push the diaphragm upward and decrease thoracic volume further.
Residual volume, or RV, is the fourth main lung volume. Residual volume is the air left in the lungs after a maximal exhalation. This air, around 1,200 milliliters in adults, cannot be expelled and plays a crucial role in preventing lung collapse. Residual volume cannot be measured with basic spirometry because it remains trapped in the lungs even after forced expiration.
Total lung capacity, or TLC, is a lung capacity formed by adding up all four primary lung volumes. Total lung capacity is the maximum volume of air the lungs can hold and is usually about 6,000 milliliters in an adult. The calculation of total lung capacity is tidal volume plus inspiratory reserve volume plus expiratory reserve volume plus residual volume. TLC reflects both the elastic properties of the lung tissue and the chest wall.
Vital capacity, or VC, is another key capacity. Vital capacity is the maximum amount of air that can be exhaled following a full, maximal inhalation. The average adult vital capacity is about 4,700 milliliters. Vital capacity is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume. It does not include residual volume, since that air remains in the lungs after maximal exhalation.
Functional residual capacity, or FRC, is the volume of air remaining in the lungs after a normal, unforced expiration. FRC averages about 2,400 milliliters in adults. Functional residual capacity is made up of expiratory reserve volume plus residual volume. FRC represents the equilibrium point where the elastic recoil of the lungs balances the outward pull of the chest wall.
Body plethysmography is considered the gold standard for measuring lung volumes, especially in patients with obstructive lung diseases. Body plethysmography uses pressure changes in a closed chamber to accurately calculate lung volumes, including those not measurable by basic spirometry, such as residual volume and functional residual capacity. Gas dilution methods are less accurate in patients with airway obstruction, because trapped air is not detected, which can lead to an underestimation of lung volumes.
Pulmonary function tests, or PFTs, are a group of noninvasive tests that assess lung volumes, capacities, flow rates, and gas exchange. PFTs are used both for diagnosis and monitoring of lung diseases. Common parameters measured in PFTs include forced vital capacity, or FVC; forced expiratory volume in one second, or FEV₁; and diffusing capacity of the lung for carbon monoxide, known as DLCO.
Forced vital capacity, FVC, measures the maximum amount of air that can be forcibly exhaled after a full inspiration. Forced expiratory volume in one second, FEV₁, measures how much of that air is exhaled in the first second of the forced breath. The ratio of FEV₁ to FVC helps distinguish between obstructive and restrictive lung diseases: in obstructive diseases, the FEV₁/FVC ratio decreases, while in restrictive diseases, the ratio is usually normal or increased.
Diffusing capacity of the lung for carbon monoxide, DLCO, measures how efficiently gases pass from the alveoli into the blood. Normal DLCO values range from 80% to 120% of the predicted value based on age, sex, and body size. A low DLCO can indicate diseases that affect the alveolar membrane or pulmonary vasculature, such as pulmonary fibrosis or emphysema. The measurement of DLCO involves having the patient inhale a small amount of carbon monoxide and measuring how much is absorbed during a breath-hold.
Pulmonary function testing was developed in the early 20th century with the invention of spirometry. Spirometry allowed clinicians to measure lung volumes and capacities for the first time. In the mid-20th century, the DLCO test was introduced to assess gas exchange efficiency, providing new insights into diseases affecting the alveolar-capillary membrane. By the late 20th century, PFTs became standardized and were incorporated into preoperative assessments for thoracic surgeries. In the 21st century, technological advancements have made PFTs more accurate and predictive of surgical outcomes.
Pulmonary function tests play a key role in preoperative assessment, especially before lung resection surgery. PFTs, including DLCO, help predict how a patient will tolerate the loss of lung tissue and what their postoperative function and exercise capacity will be. Studies have shown that one month after a lobectomy, the FEV₁ drops to approximately 79.5% of its preoperative value, and the DLCO drops to about 81.5% of its baseline level. These values help guide decisions about surgical eligibility and inform patients about expected changes in breathing after surgery.
In patients with chronic obstructive pulmonary disease, or COPD, lung resection can paradoxically improve FEV₁, DLCO, and exercise capacity in selected cases. The mechanism behind these improvements is the removal of hyperinflated or diseased lung segments, which can enhance the function of remaining healthier lung tissue. This effect is not seen in every patient, so careful preoperative evaluation with PFTs is essential.
Pulmonary function testing provides an objective method to assess the function of the respiratory system. However, PFTs do not always point directly to a specific diagnosis. Instead, they provide quantitative data that help clinicians better understand the patient’s clinical problem and guide further workup or management.
Functional residual capacity is especially important during anesthesia and mechanical ventilation. Because FRC is the lung volume at which no breathing effort occurs, decreases in FRC during anesthesia can lead to atelectasis and impaired oxygenation. Monitoring and optimizing FRC during surgery can reduce postoperative pulmonary complications.
The total lung capacity of about 6,000 milliliters is larger than some people expect—equivalent to the volume of three large soda bottles. This capacity is only achieved with maximal breathing effort and includes both the air that can be moved and the air that remains trapped in the lungs.
In PFT interpretation, a reduced forced vital capacity often points to restrictive lung disease, while a reduced FEV₁ with a low FEV₁/FVC ratio suggests obstructive lung disease. A low DLCO with preserved lung volumes may indicate a vascular or interstitial lung disease, whereas a low DLCO with low lung volumes and flow rates suggests more advanced parenchymal disease.
In the context of thoracic surgery, preoperative measurements of FEV₁ and DLCO are now standard practice. These measurements, along with clinical evaluation, help identify patients at increased risk for perioperative and postoperative pulmonary complications. For example, a patient with a preoperative DLCO below 80% of predicted is at greater risk for postoperative hypoxemia and prolonged recovery.
The largest measured value in standard PFTs is total lung capacity at 6,000 milliliters, while the smallest is tidal volume at approximately 500 milliliters. This wide range reflects the lung’s extraordinary ability to handle both resting and maximal respiratory demands.