Body Plethysmography Explained | Pulmonary Function Testing Tutorial

Lung volumes are directly measurable air amounts (tidal volume ~500mL, inspiratory reserve volume ~3L, expiratory reserve volume ~1.1-1.2L, residual volume ~1.2L which cannot be directly measured), while lung capacities are calculated values combining these volumes: total lung capacity (TLC = ERV + TV + IRV + RV), vital capacity (VC = ERV + TV + IRV), inspiratory capacity (IC = TV + IRV), and functional residual capacity (FRC = RV + ERV); pulmonary diseases are classified into obstructive (exhalation problems), restrictive (inhalation problems), gaseous exchange (alveolar membrane damage), and mechanical (musculoskeletal or neural issues).

Respiratory volumes and capacities measure lung function: (1) Tidal Volume (TV) - air moved during normal breathing, (2) Inspiratory Reserve Volume (IRV) - additional air that can be inhaled forcefully (2500-3000 ml), (3) Expiratory Reserve Volume (ERV) - additional air that can be exhaled forcefully (1000-1200 ml), (4) Residual Volume (RV) - air remaining after forced exhalation. Capacities combine these volumes: Inspiratory Capacity (IC = TV + IRV), Expiratory Capacity (EC = TV + ERV), Functional Residual Capacity (FRC = ERV + RV), Total Lung Capacity (TLC = IRV + TV + ERV + RV), and Vital Capacity (VC = IRV + TV + ERV).

Four lung volumes: Tidal Volume (TV) = 500 ml (normal breathing), Inspiratory Reserve Volume (IRV) = 2500-3000 ml (additional air inhaled forcibly), Expiratory Reserve Volume (ERV) = 1000-1100 ml (additional air exhaled forcibly), Residual Volume (RV) = 1100-1200 ml (air remaining after maximum exhalation). Five lung capacities: Inspiratory Capacity (IC) = TV + IRV = 3000-3500 ml, Expiratory Capacity (EC) = TV + ERV = 1500-1600 ml, Vital Capacity (VC) = IC + ERV = 4500-5000 ml, Functional Residual Capacity (FRC) = ERV + RV = 2100-2300 ml, Total Lung Capacity (TLC) = VC + RV = 5600-6000 ml.

Lung volumes include tidal volume (~400-500 cc in 70 kg), inspiratory reserve volume, expiratory reserve volume, and residual volume (gas trapped at end of maximal exhalation). Lung capacities combine volumes: vital capacity (~4 L in 70 kg) and functional residual capacity (~2.5 L in 70 kg). FRC is critical during anesthesia induction as it represents oxygen reserve during apnea. Factors increasing FRC include obstructive disease, emphysema, PEEP, asthma, and aging. Factors decreasing FRC include restrictive disease, obesity, pulmonary edema, fibrosis, atelectasis, pregnancy, pleural effusion, and respiratory muscle weakness. Standard spirometry cannot measure residual volume or FRC directly; specialized tests like body plethysmography or nitrogen washout are required.

Respiratory volumes measure lung function: Tidal volume (TV) = 500 mL (normal breath), Inspiratory reserve volume (IRV) = 3000 mL (additional inspired air), Expiratory reserve volume (ERV) = 1000-1100 mL (additional expired air), Residual volume (RV) = 1100-1200 mL (air remaining after forced expiration). Capacities combine volumes: Inspiratory capacity (IC) = TV + IRV = 3500 mL, Expiratory capacity (EC) = TV + ERV = 1600 mL, Functional residual capacity (FRC) = ERV + RV = 2100-2200 mL, Vital capacity (VC) = TV + IRV + ERV = 4600 mL, Total lung capacity (TLC) = TV + IRV + ERV + RV = 5700-5800 mL. These measurements indicate maximum functional capacity of the respiratory system.

Boyle's Law states that for a fixed amount of gas at constant temperature, the pressure and volume are inversely related, meaning as pressure increases, volume decreases proportionally, and vice versa; this relationship is mathematically expressed as P1 × V1 = P2 × V2, where P represents pressure and V represents volume before and after a change.

Boyle's Law states that at constant temperature and amount of gas, the pressure and volume of a gas are inversely proportional, meaning that as pressure increases, volume decreases, and vice versa; this relationship can be mathematically expressed as P₁V₁ = P₂V₂, where P represents pressure and V represents volume for two different states of the same gas.

At constant temperature, the volume of a fixed mass of gas varies inversely with pressure (Boyle's Law: P₁V₁ = P₂V₂). When pressure increases, volume decreases proportionally, and vice versa. Graphically, this appears as a rectangular hyperbola when plotting P vs V, or a straight line through the origin when plotting P vs 1/V. The law applies only when gas amount and temperature remain constant. Real-world applications include bicycle tires—more air pumped in increases pressure, while larger containers require less pressure for the same gas quantity. Comparing different isotherms shows that higher temperature gases occupy larger volumes at identical pressures.

Boyle's Law establishes that pressure and volume of a gas are inversely proportional when temperature remains constant. As pressure increases, volume decreases proportionally. The mathematical equation P₁ × V₁ = P₂ × V₂ allows calculation of unknown values. To apply this law, identify variables by their units: pressure uses atm, mmHg, or psi; volume uses liters, milliliters, or cubic centimeters. Rearrange the formula to isolate the unknown variable, substitute known values, cancel repeating units, and verify results align with the inverse relationship principle.

Robert Boyle discovered in 1662 that at constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume. Mathematically, P ∝ 1/V or PV = k (constant). When plotted on a pressure-volume graph, each constant temperature curve is called an isotherm. When plotted as P vs 1/V, straight lines passing through the origin are obtained. This law explains why gases compress - reducing volume increases collision frequency with container walls, thereby increasing pressure. The inverse relationship holds true for any ideal gas at constant temperature, forming the foundation for understanding gas behavior under pressure changes.

Spirometry cannot measure residual volume because it requires the patient to exhale completely, which is impossible when residual volume is present. Therefore, spirometry cannot measure functional residual capacity, vital capacity, or total lung capacity directly. Alternative methods like helium dilution or nitrogen washout are needed to measure residual volume and derived capacities.

Inspiratory capacity (IC) is maximum air inhaled after normal expiration (~3000-3500 ml). Expiratory capacity (EC) is maximum air exhaled after normal inspiration (~1600 ml). Total lung capacity (TLC) is total air volume after maximum inspiration (~4600-5800 ml). Spirometry measures lung function by recording air movement during breathing. The resulting spirogram shows tidal volume, IRV, and ERV. However, spirometry cannot measure residual volume because it cannot be voluntarily expelled, limiting measurement of FRC and TLC.

Spirometry can measure all lung volumes and capacities except those containing residual volume. The three entities that cannot be measured by spirometry are: (1) Residual Volume (RV), (2) Functional Residual Capacity (FRC), and (3) Total Lung Capacity (TLC). These cannot be measured because they include air that remains in the lungs after maximal expiration. To measure RV and related capacities, specialized methods are required including helium dilution method, radiographic plan imagery, body plethysmography, and nitrogen washout technique.

Classic spirometry cannot measure residual volume (RV) because RV is air remaining after maximal exhalation. This prevents measurement of functional residual capacity (FRC = RV + expiratory reserve volume) and total lung capacity (TLC = sum of all lung volumes). Three techniques exist: helium dilution, nitrogen washout, and body plethysmography. The helium dilution technique uses a patient with a shutter, helium meter, and known spirometer volume. Helium is inert and insoluble in blood, preventing transfer to bloodstream. The calculation formula is: (Concentration_initial × Volume_spirometer) = (Concentration_final × (FRC + Volume_spirometer)). Normal values are determined through statistical analysis of healthy populations without respiratory disease or risk factor exposure, considered normal within 80-120% of predicted values.

Spirometry cannot measure residual volume (RV), which is the only static lung volume that cannot be determined by dynamic spirometry. Thoracic gas volume (TGV) requires both expiratory reserve volume (ERV) from spirometry and RV from plethysmography. Since spirometry alone cannot determine RV, it cannot calculate TGV or TLC. This limitation means spirometry alone cannot diagnose restrictive lung disease, which requires TLC determination for confirmation. Spirometry measures dynamic lung function but misses the static component essential for diagnosing restrictive processes.