Lecture 5 -Lung Function tests

Lung Function Tests (LFT)

• Course code: AHS2053H

• Compiled by: Dr. S. Manie

Purpose of Lung Function Tests

• Assess mechanical properties of the respiratory system

• Diagnose/define abnormalities in lung function

• Monitor progress of respiratory diseases

• Evaluate response to treatments (e.g., bronchodilator responsiveness)

• Risk assessment prior to surgical procedures

Categories of Tests

• Broadly classified into three categories:

  • Ventilatory Function

  • Gaseous Exchange Function

  • Exercise Testing

Factors Influencing Normal Values

• Height: Taller individuals typically have larger lung volumes

• Age: Lung capacity generally decreases with age

• Gender: Males usually have larger lung volumes compared to females

• Ethnicity: Variations can exist based on ethnic background

Contraindications to LFTs

• Recent abdominal or eye surgery

• Hemodynamic instability

• Recent hemoptysis (coughing up blood)

• Presence of a pneumothorax (collapsed lung)

• Acute severe illness

• Aneurysm

Terminology in Lung Function Tests

Key Measurements

• Peak Expiratory Flow Rate (PEFR):

  • Definition: Highest flow during forced expiration from full inspiration

  • Normal Range: 300-600 L/min; severe obstruction: <100 L/min

• Vital Capacity (VC):

  • Definition: Volume of gas expelled after full inspiration

  • Normal Range: 3-6 liters (±80% of Total Lung Capacity - TLC)

  • Adequate cough: >1 liter

• Forced Vital Capacity (FVC):

  • Similar to VC but a forced measurement

  • Observations: FVC may differ in conditions like COPD

• Forced Expiratory Volume in 1 Second (FEV1):

  • Definition: Volume of gas expelled in the first second of forced exhalation

  • Normal: 80% of VC; range: 2-4 liters

  • Conditions: <75% of VC suggest airway obstruction

• FEV1/FVC Ratio:

  • Normal: 70-80%

  • Indicates severity of obstruction:

    • Moderate: 50-60%

    • Severe: <30%

    • Restrictive diseases can show 100%

• Total Lung Capacity (TLC):

  • Definition: Total volume of gas in the lungs after maximum inspiration

  • Normal Range: 3-8 liters

Additional Lung Capacities

• Functional Residual Capacity (FRC):

  • Volume remaining in the lungs after a tidal expiration

  • Normal (standing): ±40% of TLC; 2.4 liters

  • COPD: up to 80% of TLC

• Tidal Volume (TV):

  • Amount inhaled/exhaled during one cycle

  • Normal: 300-800 ml (5-7 ml/kg ideal body weight)

• Inspiratory Reserve Volume (IRV):

  • Amount of air that can be inspired above normal tidal volume

  • Normal: 3.1 liters

• Expiratory Reserve Volume (ERV):

  • Additional amount exhaled after tidal expiration

  • Normal: 1.2 liters

• Residual Volume (RV):

  • Volume remaining in lungs after maximal expiration

  • Normal: 20-30% of TLC

• Minute Ventilation/Volume:

  • Volume of gas breathed in and out per minute

  • Normal: 5-7 L/min

Ventilatory Function Evaluation

• Total Lung Capacity (TLC):

  • Methodologies:

    • Total body plethysmography

    • Gas dilution with inert gas (e.g., Helium)

    • Nitrogen washout

Spirometry Testing

• Conducted through maximal forced expiration after maximal inspiration

• Patient Instructions: seated position, use nose clip, deep inhale, and forceful exhale

• Minimum of three acceptable test efforts encouraged

Acceptable Effort Criteria

• Absence of artifacts (no cough or glottic closure)

• No hesitation starting the test

• Sufficient exhalation

• Reproducibility within 0.2L of the highest FVC and FEV1

• Potential to repeat tests up to 8 times for validity

Spirometry Interpretation

Normal Spirogram

• Characterizes a plot of Volume vs. Time with majority of FVC expelled in the first second (FEV1)

• Normal FEV1/FVC ratio >75%

Obstructive Pattern

• Features:

  • Reduced FEV1 in relation to normal FVC

  • Ratios indicating severity: FEV1/FVC<75% (mild), <60% (moderate), <40% (severe)

Restrictive Pattern

• Characteristics:

  • Reduced VC and TLC due to increased stiffness or muscle function impairment

  • FEV1 may be less reduced than FVC, leading to potentially normal/increased FEV1/FVC ratio

Flow-Volume Loop Analysis

• Measurement Technique:

  • Plots Flow vs Volume, measuring airway resistance during inspiration and expiration

• Depicts normal vs obstructive vs restrictive patterns

  • Normal: Majority of flow dependent on effort

  • Obstructive: Flow reduction with respect to volume

  • Restrictive: Flow limitations due to reduced lung volumes

Diffusion Capacity - Gaseous Exchange Function

• Measured using: Carbon monoxide (CO)

• Factors Affecting Diffusion Capacity:

  • Surface area, blood volume, hemoglobin levels, diffusion gradient, membrane thickness

Sample Results in LFT

• Observed results can significantly diverge from predicted values based on factors such as age and height

  • Example: 38-year-old male with a FVC of 3.39 L (predicted 6.06 L = 56%)

  • Example: 70-year-old male with FVC of 1.46 L (predicted 3.78 L = 39%)

References

• Hough, A. (2014). Physiotherapy in Respiratory and Cardiac Care (4th edition)

• Heuer AJ (2021). Wilkins’ Clinical Assessment in Respiratory Care (9th edition)