Pulmonary Function Measurements

Lung Volumes

• Tidal volume (VT)

• Inspiratory reserve volume (IRV)

• Expiratory reserve volume (ERV)

• Residual volume (RV)

Lung Capacities

• Vital capacity (VC)

• Inspiratory capacity (IC)

• Functional residual capacity (FRC)

• Total lung capacity (TLC)

Normal Values and Approximations

• Normal lung volumes and capacities for healthy individuals are approximated in figures/tables (e.g., Table 3-1; Figure 3-1) for healthy men and women aged 20 to 30 years.

Obstructive vs Restrictive Lung Disorders – General Concept

• Obstructive disorders affect airflow out of the lungs and alter lung volumes/capacities due to airway narrowing and air trapping (e.g., increased RV and FRC in some cases).

• Restrictive disorders primarily reduce lung volumes/capacities due to restrictions of lung or chest wall expansion (e.g., reduced TLC, FRC).

• Both types can be illustrated with changes in volumes and capacities (Figure references: 3-2 for obstructive, 3-3 for restrictive).

Indirect Measurements of Residual Volume and Capacities Containing Residual Volume

• Closed circuit helium dilution

• Open circuit nitrogen washout

• Body plethysmography (body box)

Pulmonary Mechanics – Core Principles

• Can measure volumes and capacities

• Can also measure rate at which gas flows in and out of the lungs

• Expiratory flow rate measurements provide data on airway integrity and severity of airway impairment

• Also indicate whether a patient has a large- or small-airway problem

• Tests for measuring expiratory flow rates are collectively referred to as pulmonary mechanics measurements

Overview of Pulmonary Mechanics Measurements

• Forced vital capacity (FVC)

• Forced expiratory volume timed (FEVT)

• Forced expiratory volume in 1 second / forced vital capacity ratio (FEV1/FVC) or FEV1%

• Forced expiratory flow 25%–75% (FEF25%-75%)

• Forced expiratory flow 200–1200 mL (FEF200-1200)

• Peak expiratory flow rate (PEFR)

• Maximum voluntary ventilation (MVV)

• Flow-volume loop

Forced Vital Capacity (FVC)

• Definition: Maximum volume of gas that can be exhaled as forcefully and rapidly as possible after maximal inspiration

Forced Expiratory Volume Timed (FEVT)

• Definition: Maximum volume of gas that can be exhaled within a specified time period

• Derived from the FVC maneuver

• Most frequently used time period: 1 second

Normal FEVT Values (percent of FVC exhaled by time period)

• FEV₀.₅ = $60\%$ of FVC

• FEV₁ = $83\%$ of FVC

• FEV₂ = $94\%$ of FVC

• FEV₃ = $97\%$ of FVC

FEV1/FVC Ratio (FEV1%)

• Definition: Comparison of the amount of air exhaled in 1 second to the total amount exhaled during the FVC maneuver

• Commonly referred to as forced expiratory volume in 1 second percentage: \text{FEV1}\% = \frac{FEV1}{FVC} \times 100\%

FVC, FEV1, and FEV1% – Clinical Use

• Collectively, the most commonly used pulmonary function measurements

• Used to distinguish between obstructive and restrictive lung disorders

• Used to determine the severity of a patient’s pulmonary disorder

Obstructive vs Restrictive Lung Disorders – Specific Patterns

• Obstructive lung disorders: both FEV1 and FEV1% are decreased

• Restrictive lung disorders: FEV1 is normal or increased relative to FVC

• These distinctions help guide diagnosis and management

Forced Expiratory Flow (FEF) 25%–75% (FEF25-75%)

• Definition: Average flow rate that occurs during the middle 50% of the FVC measurement

• Figure references: Figure 3-7 (illustrative)

• Conceptual example: Similar to measuring and averaging flow when a container has 1 L and 3 L of water accumulated in a 4 L container (metaphor used in Figure 3-8)

FEF200–1200

• Definition: Average flow rate that occurs between 200 mL and 1200 mL of FVC

• Figure references: Figure 3-9 and Figure 3-10 (illustrative)

• Conceptual example: Similar to measuring and averaging flow when 200 mL and 1,200 mL of water have accumulated in a container

Peak Expiratory Flow Rate (PEFR)

• Definition: Maximum flow rate that can be achieved during the FVC maneuver

• Normal values: Men ≈ $10\,\text{L/s}$; Women ≈ $7.5\,\text{L/s}$

• PEFR decreases with age and with obstructive lung disease

• Figure references: Figure 3-11

Maximum Voluntary Ventilation (MVV)

• Definition: Largest volume of gas that can be breathed voluntarily in and out of the lungs in 1 minute

• Figure references: Figure 3-13

Flow-Volume Loop

• Graphic representation of the FVC maneuver followed by a forced inspiratory volume (FIV) maneuver

• Normal loop (Figure 3-14)

• Obstructive pattern (Figure 3-15)

• Restrictive pattern (Figure 3-16)

Factors Affecting Predicted Normal Values

• Height: Taller subjects have greater pulmonary function values

• Weight: Generally, as weight increases, lung volumes decrease

• Age: After age 25, lung volumes, expiratory flow rates, and diffusing capacity values decline

• Gender: Males typically have greater lung volumes, expiratory rates, and diffusing capacities

• Race: Blacks and Asian subjects tend to have lower pulmonary function values than those of European-descent origin

Dynamic Compression and Expiratory Flow Rates

• Dynamic compression: effort-dependent portion of the forced expiratory maneuver

• Occurs during roughly the first 30% of the FVC maneuver

• Primarily involves large airways; greater patient effort yields higher values of FEF_{200-1200} and PEFR

• Effort-independent portion: flow rate during roughly the last 70% of the FVC maneuver

• Once maximum flow rate is attained, further muscular effort cannot increase the flow

• Illustration: Effort-dependent vs effort-independent portions (Figure 3-17)

Diffusion Capacity of Carbon Monoxide (DLCO)

• Definition: Measures the amount of carbon monoxide (CO) that moves across the alveolar-capillary membrane

• Resting male DLCO average value: DLCO_{rest} \approx 25\ \frac{\text{mL}}{\text{min}\cdot \text{mm Hg}}

Maximum Inspiratory and Expiratory Pressures

• Table 3-4 (reference resource for pressures; values not listed here)

Obstructive Lung Disorders – Characteristics and Examples

• Characterized by bronchial secretions, mucus plugging, bronchospasm, and distal airway weakening

• Result: reduction of gas flow out of the lungs and air trapping

• Flow of gas most notably reduced during forced exhalation

• Common obstructive disorders (CBABE): Cystic fibrosis, Bronchitis, Asthma, Bronchiectasis, Emphysema

• FEV1/FVC ratio and FEV1 are used in assessment and management of COPD; GOLD framework stages: Stage I (mild), Stage II (moderate), Stage III (severe), Stage IV (very severe)

Restrictive Lung Disorders – Characteristics

• Defined by pathologic restriction of lungs or chest wall, leading to decreased lung volumes and capacities

• Common restrictive disorders: Pneumonia, Pulmonary edema, Flail chest, Pneumothorax, Pleural effusion, Chronic interstitial lung disease

• Other causes: Lung cancer, Acute respiratory distress syndrome (ARDS), postoperative alveolar collapse (atelectasis)

Differentiating Obstructive vs Restrictive Disorders – Practical Approach

• Clinically, use FVC, FEV1, and FEV1% to distinguish obstructive vs restrictive patterns

Asthma Action Plan – Green, Yellow, and Red Zones

• Often recommended for children with asthma; plan developed with parents and physician

• Zones correspond to color categories similar to traffic lights: Green, Yellow, Red

• Visual aid: Asthma Action Plan Figure 3-12 (illustrative)

Ventilatory Mechanics Used to Predict Mechanical Ventilation Weaning Success

• Rapid Shallow Breathing Index (RSBI) ratio: RSBI = \frac{f}{VT} where f is respiratory rate (breaths per minute) and VT is tidal volume (in liters)

• Normal RSBI range: 60–105 breaths/min per liter

• Interpretation: RSBI < 105 is a predictor of weaning success from mechanical ventilation