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PULMONARY FUNCTION TESTS
Pulmonary Function Tests – Learning Objectives
Essential Understand what a PFT measures: static lung volumes, spirometry, Flow Volume Loop, DLCO. Accurately recognise an obstructive pattern using asthma and COPD as clinical examples Accurately recognise a restrictive pattern using asbestosis as a clinical example Be able to perform spirometry on a patient (Pulmonary lab at Mater; opportunities on rural term) Accurately identify the major structures visible on a PA and lateral CXR (Diagnostic Imaging Pathways website)
Static Lung Volumes
Static lung volumes are determined using methods in which airflow velocity does not play a role. They reflect the elastic properties of the lungs and chest wall. The sum of two or more lung volumes constitutes a lung capacity. The subdivisions and capacities are expressed in liters. Four volumes are measured (tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume). Using these four measurements of volume, four capacities are calculated
Subdivisions of Lung Volume
Inspired and expired volumes during normal quiet breathing. Most lung volumes and capacities can be measured by spirometry. (TLC, FRC, and RV are not determined by spirometry.) ERV, expiratory reserve volume FRC, functional residual capacity IC, inspiratory capacity IRV, inspiratory reserve volume RV, residual volume TLC, total lung capacity VC, vital capacity VT, tidal volume Tidal volume (TV) is the volume of air that is inhaled or exhaled with each normal respiratory cycle. Inspiratory reserve volume (IRV) is the maximal volume of air that can be inhaled after a normal tidal inhalation. Expiratory reserve volume (ERV) is the maximal volume of air that can be exhaled after a normal tidal exhalation
The mean forced expiratory flow during the middle half of the FVC (FEF 25% – 75%) is the slope of the line that intersects the spirographic tracing at 25% and 75% of the FVC. The FEF 25% - 75% is less effort dependent than the FEV1 and is a more sensitive indicator of small airways obstruction (airways <2mm in diameter). The line approaches the horizontal due to prolongation of the expiratory phase in airways obstruction.
FLOW VOLUME LOOPS
The flow-volume loop is recorded by a spirometer during a forced inspiratory and expiratory VC manoeuvre. The shape reflects the status of the lung volumes and airways throughout the respiratory cycle. Peak expiratory flow rate is sometimes used to estimate degree of airways obstruction, but is very dependent on patient effort. The small airways (diameter < 2mm), constitute < 10% of the total airway resistance, but their surface area is large. Disease affecting primarily the small airways can be extensive, yet not affect tests such as FEV1. Expiratory flow rates over the lower 50% of the FVC (i.e. approaching residual volume) are sensitive indicators of small airways disease, such as early obstructive and interstitial lung disease. A: Normal Inspiratory limb of loop is symmetric and convex. Expiratory limb is linear. B. Restrictive disease (e.g. sarcoidosis, kyphoscoliosis) Configuration of loop is narrowed because of ↓lung volumes but shape is basically normal. At comparable lung volumes, flow rates are normal. (Actually ↑ because airways held open by ↑elastic recoil) C. COPD, asthma All flow rates ↓, but particularly expiratory flow rate is ↓↓.
Spirometric flow-volume loops.
A is an expiratory flow-volume loop of a nonasthmatic, without airflow limitation.
B to E are expiratory flow-volume loops in asthmatic patients with increasing degrees of
airflow limitation (B is mild; E is severe). Note the “scooped” or concave appearance of the
asthmatic expiratory flow-volume loops; with increasing obstruction, there is greater
“scooping.”
Causes of Restrictive Ventilatory Defects
Carbon Monoxide Diffusion Test (DLco)
Whereas spirometry measures the mechanical properties of the lungs, the lung diffusing capacity test (DLco) measures the ability of the lungs to perform gaseous exchange. Interstitial Lung Disease Interstitial pneumonitis Fibrosis Pneumoconiosis Granulomatosis e.g. sarcoidosis Pulmonary oedema Space-Occupying Lesions Tumour Cysts Pleural Diseases Pneumothorax Hemothorax Pleural effusion, empyema Chest-wall Diseases Injury Kyphoscoliosis Spondylitis
The single breath DLco test requires the patient to inhale a gas consisting of: Helium - 10% Carbon Monoxide-1000 ppm Air-balance The inhaled gas is held in the lungs for 10 seconds, during which time the carbon monoxide diffuses across the respiratory membrane into the pulmonary capillary blood. The helium does not diffuse. During exhalation, a portion of the breath representative of alveolar air is collected in a sample collection system, and the carbon monoxide concentration in this sample is determined. The difference between the inspired and expired carbon monoxide concentrations is calculated, and the diffusing capacity of the lungs determined. (The difference between the two values = the amount of carbon monoxide which has crossed from alveolus → pulmonary capillary blood) Average result for a young healthy male = 17 ml/minute This test can be of great diagnostic benefit in lung disorders not detectable by spirometry or chest Xray. The ability of the lungs to pass oxygen from alveoli → pulmonary capillary blood can be affected by damage to or loss of respiratory membrane as in emphysema, and by thickening of the membrane by fibrosis or inflammation (interstitial lung disease e.g. asbestosis) The DLco test is more sensitive than spirometric measurements and chest Xray for the detection of interstitial lung disorders. Causes of ↓ DLco Interstitial lung disease Emphysema Severe anaemia Smoking Causes of ↑ DLco Polycythaemia Early left ventricular failure
