Spirometry is a crucial tool for assessing lung function. It measures the volume and flow of air during breathing, providing key data on respiratory health. This technique helps diagnose and monitor conditions like asthma and COPD.

The spirometer's design and function are central to accurate measurements. It captures important parameters like forced vital capacity and forced expiratory volume, which reveal vital information about a person's lung performance and potential respiratory issues.

Spirometer and Measurements

Spirometer Design and Function

• Spirometer measures lung function by recording the volume and flow of air inhaled and exhaled
• Consists of a mouthpiece connected to a measuring device (pneumotachograph or flow sensor) that converts airflow into electrical signals
• Measures various respiratory parameters such as forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), and peak expiratory flow (PEF)
• Used for diagnosing and monitoring respiratory disorders like asthma, chronic obstructive pulmonary disease (COPD), and restrictive lung diseases

Key Spirometry Parameters

• Forced vital capacity (FVC) represents the maximum volume of air that can be forcibly exhaled after a maximum inhalation
  • Reduced FVC may indicate restrictive lung disorders (interstitial lung disease or neuromuscular disorders)
• Forced expiratory volume in 1 second (FEV1) measures the volume of air forcibly exhaled in the first second of the FVC maneuver
  • Reduced FEV1 suggests airflow obstruction (asthma or COPD)
• Peak expiratory flow (PEF) is the maximum flow rate achieved during a forced expiration
  • Reduced PEF may indicate airway obstruction or poor expiratory muscle strength
• FEV1/FVC ratio compares the amount of air exhaled in the first second to the total volume of air exhaled
  • Reduced FEV1/FVC ratio (<0.7) indicates airflow obstruction (asthma or COPD)
• Bronchodilator response assesses the change in FEV1 after administering a bronchodilator medication
  • Significant improvement in FEV1 (>12% and >200mL) suggests reversible airway obstruction (asthma)

Graphical Representation

Flow-Volume Loop

• Flow-volume loop is a graphical representation of airflow and volume during a forced expiratory maneuver
• X-axis represents volume (liters) and Y-axis represents flow rate (liters/second)
• Provides a visual assessment of airway obstruction, restriction, and upper airway abnormalities
• Obstructive patterns show a concave shape in the expiratory portion of the loop (asthma or COPD)
• Restrictive patterns demonstrate a reduced FVC with a relatively preserved shape of the loop (interstitial lung disease or neuromuscular disorders)
• Upper airway obstruction may present with a flattened inspiratory portion of the loop (tracheal stenosis or vocal cord paralysis)

Advanced Techniques

Pneumotachograph and Body Plethysmography

• Pneumotachograph measures airflow by detecting the pressure drop across a fixed resistance
  • Provides accurate flow measurements for spirometry and other pulmonary function tests
• Body plethysmography measures lung volumes and airway resistance by placing the patient in a sealed chamber
  • Determines total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC)
  • Useful for diagnosing air trapping and hyperinflation in obstructive lung diseases (COPD or emphysema)

Diffusing Capacity and Lung Volumes

• Diffusing capacity of the lung for carbon monoxide (DLCO) assesses the ability of the lungs to transfer gas from the alveoli to the blood
  • Reduced DLCO may indicate impaired gas exchange (emphysema, interstitial lung disease, or pulmonary vascular disorders)
• Lung volumes and capacities provide information about the static properties of the respiratory system
  • Total lung capacity (TLC) represents the maximum volume of air in the lungs after a maximal inspiration
  • Residual volume (RV) is the volume of air remaining in the lungs after a maximal expiration
  • Functional residual capacity (FRC) is the volume of air in the lungs at the end of a normal expiration
  • Increased RV and FRC suggest air trapping and hyperinflation (COPD or emphysema)