Lung Function Testing: Respiratory Module

• Respiratory Module: Session 3 focuses on the mechanics of breathing, specifically lung function testing.
• Lecture 2 is about lung function testing.

Objectives of Lung Function Testing

• Explain common lung function tests, including simple spirometry.
• Describe forced vital capacity (FVC) and forced expiratory volume in one second (FEV1.0) measurements.
• Explain obstructive and restrictive patterns in spirometry.
• Explain expiratory and inspiratory flow volume loops and their response to upper and lower airway obstruction.
• Describe the principles of measuring residual volume and transfer factor.
• Explain the nitrogen washout curve.

Spirometry and Lung Volumes

• Spirometry is used in lung function testing.
• Vital capacity is the most important thing in determining lung volumes.
• Vital capacity can be predicted using tables based on age, height, and sex.
• Spirometry is done with old and recent spirometers.

Goals of Spirometry

• Vital capacity measurements are important to predict pulmonary dysfunction.
• Spirometry can determine the functional nature of a disease (obstructive or restrictive).
• Spirometry is used to assess the severity and progression of a disease.
• Spirometry is used to assess the response to treatment.
• Spirometry is used to identify patients at increased risk of morbidity and mortality undergoing pulmonary resection.

Lung Volume Measurements

• Tidal Volume (TV): The amount of air inhaled or exhaled with each breath during resting conditions, averaging 500 ml for both adult males and females.
• Inspiratory Reserve Volume (IRV): The amount of air that can be forcefully inhaled after a normal tidal volume inhalation, averaging 3100 ml for males and 1900 ml for females.
• Expiratory Reserve Volume (ERV): The amount of air that can be forcefully exhaled after a normal tidal volume exhalation, averaging 1200 ml for males and 700 ml for females.
• Residual Volume (RV): The amount of air remaining in the lungs after a forced exhalation, averaging 1200 ml for males and 1100 ml for females.
• Total Lung Capacity (TLC): The maximum amount of air contained in the lungs after a maximum inspiratory effort, calculated as TLC = TV + IRV + ERV + RV, averaging 6000 ml for males and 4200 ml for females.
• Vital Capacity (VC): The maximum amount of air that can be expired after a maximum inspiratory effort, calculated as VC = TV + IRV + ERV, should be 80% TLC, averaging 4800 ml for males and 3100 ml for females.
• Inspiratory Capacity (IC): The maximum amount of air that can be inspired after a normal expiration, calculated as IC = TV + IRV, averaging 3600 ml for males and 2400 ml for females.
• Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal tidal volume expiration, calculated as FRC = ERV + RV, averaging 2400 ml for males and 1800 ml for females.

Forced Vital Capacity (FVC) Curve Measurements

• FEV1: The volume exhaled during the first second of the FVC maneuver.
• FEF 25-75%: The mean expiratory flow during the middle half of the FVC maneuver, reflecting flow through small airways (less than 2 mm in diameter).
• FEV1/FVC: The ratio of FEV1 to FVC multiplied by 100, expressed as a percentage, important because a reduction from expected values is specific for obstructive rather than restrictive diseases.

Obstructive vs. Restrictive Spirometry Patterns

• Maximal filling of the lungs is determined by the balance between maximum inspiratory effort and the force of recoil of the lungs.
• A restrictive deficit occurs if the lungs are unusually stiff or inspiratory effort is compromised by muscle weakness, injury, or deformity.
• An obstructive deficit occurs when small airways are compressed during expiration, particularly when forced, increasing flow resistance.
• If airways are narrowed, expiratory flow is compromised earlier in expiration, producing an obstructive deficit.
• Restrictive and obstructive deficits may be separated by having the patients breathe out from maximal inspiration on a spirometer.
• The spirometer plots expired volumes per unit time as a 'vitalograph'.
• A time-volume plot generated during spirometry is often referred to as a vitalograph tracing

Obstructive Defect Characteristics

• FVC is nearly normal.
• FEV1 is reduced markedly.
• FEV1/FVC ratio is less than 70%.

Restrictive Defect Characteristics

• FVC is reduced.
• FEV1 is reduced proportionately.
• FEV1/FVC ratio is normal or even higher (≥ 70%).

Flow Volume Curves

• Obstructive deficits may be more sensitively revealed by deriving an expiratory flow volume loop where the expiratory flow rate is plotted against lung volume.
• The peak expiratory flow occurs early on in expiration.
• As expiration continues, the small airways are narrowed of the lungs where there is small airway obstruction, producing a characteristic early fall in expiratory flow rate.
• Peak Expiratory Flow Rate (PEFR) can be measured using a peak flow meter.

Peak Flow in Airways

• In normal individuals, peak flow is affected most by the resistance of the large airways.
• In exacerbations of asthma with severe obstruction of the smaller airways, peak expiratory flow rate will be affected.
• Mild obstruction of the small airways produces 'scalloping' of the flow volume curve.
• More severe obstruction also reduces PEFR.

Residual Volume Measurement Principles

• Compliance of a patient's lungs is measured directly using a whole body plethysmograph.
• A plethysmograph may also be used to estimate the resistance of the airways more directly.
• The volumes of air remaining in the lungs after expiration (residual volume) may be measured by the Helium dilution test.
• Dead space may be measured by the nitrogen washout method.
• The nitrogen washout is used to measure the dead space in the lungs.