This document discusses pulmonary function tests, which are used to evaluate respiratory mechanics and gas exchange. Spirometry and tests of diffusing capacity are described. Spirometry measures volumes like forced vital capacity and flows like forced expiratory volume in 1 second. Pattern recognition can determine if a patient has an obstructive defect from conditions like asthma or a restrictive defect like fibrosis. Gas exchange is assessed through measures of oxygenation and carbon dioxide like arterial blood gases and pulse oximetry. Causes of hypoxemia include ventilation-perfusion mismatching and right-to-left shunts.

1. Pulmonary function test
Tests of respiratory mechanics
&
Tests of pulmonary gas exchange

2. Indications
 Evaluation of dyspnea, cough, abnormal CxR
 Quantitative assessment of severity of
pulmonary disease
 Screening for early disease
 Longitudinal surveillance of OLD
 Follow-up of disease activity
 Assessment of bronchodilator response
 Prediction of surgical risk
 Quantitate disability

3. Tests of respiratory
mechanics
Spirometry (PFT)
Body plethysmography or
He dilution method- FRC

4. Spirometry
 Deep inspirationForced expiration Deep
inspiration
 Requires patient cooperation
 Adequate test-
 Smooth flow-volume loop
 Exhalation x ~6 seconds
 At least 3 readings, maximum 8
 2 largest FVC & FEV1 within 0.2 L

5. Definitions
 Total lung capacity- TLC-
 Volume of gas after maximal inspiration
 Residual volume- RV-
 Volume of gas after maximal expiration
 Vital capacity- VC-
 TLC - RV
 Forced vital capacity- FVC-
 Total volume exhaled
 Forced expiratory vol. in 1st
sec.- FEV1-
 Volume exhaled during 1st
second of expiration

6. Diagram
 Lung volumes
 Flow-volume loop

7. Results & patterns
 Results-
 Absolute numbers- in litres or litres per second
 % predicted for height, age, sex, weight- >80% is normal
 Obstructive- asthma, COPD
 FEV1 decreased due to increased airway resistance
 FVC may be decreased due to air trapping
 Ratio FEV1/FVC is decreased
 Restrictive- fibrosis, ILD
 TLC reduced
 FEV1 & FVC reduced proportionally
 Ratio FEV1/FVC remains normal

8. Tests of gas exchange
 Diffusing capacity for CO- DLco-
 Low in pulmonary vascular diseases- PHT, PE
 Low in diffuse lung diseases- fibrosis, emphysema
 Pulse oximetry- correlates with SaO2
 ABG- for PaO2, PaCO2, pH
 Hypoxemia- PaO2 <60 mm Hg ~ SaO2 <90%
 Hypercapnia- PaCO2 >45 mm Hg = hypoventilation
 Hypocapnia- PaCO2 <35 mm Hg = hyperventilation

9. Hypoxia
 V/Q mismatch- asthma, COPD, ILD,
pneumonia, pulmonary vascular disease
 Commonest cause
 Elevated (A-a)DO2, normal PaCO2
 Responds to supplemental O2
 RL shunt- intracardiac or intrapulmonary
 Elevated (A-a)DO2
 Doesn’t respond to supplemental O2
 Hypoventilation- CNS/neuromuscular causes
 Increased PaCO2, normal (A-a)DO2
 Reduced inspired oxygen (PiO2)- high
altitude

10. Hypoxia
 V/Q mismatch- asthma, COPD, ILD,
pneumonia, pulmonary vascular disease
 Commonest cause
 Elevated (A-a)DO2, normal PaCO2
 Responds to supplemental O2
 RL shunt- intracardiac or intrapulmonary
 Elevated (A-a)DO2
 Doesn’t respond to supplemental O2
 Hypoventilation- CNS/neuromuscular causes
 Increased PaCO2, normal (A-a)DO2
 Reduced inspired oxygen (PiO2)- high
altitude