Pulmonary function exam
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Pulmonary function tests (PFTs) are used to characterize and monitor pulmonary disease. Common PFTs include spirometry to assess airflow limitation, lung volume measurements to detect restriction, diffusion capacity to evaluate gas exchange, and bronchoprovocation tests to identify airway hyperresponsiveness. Spirometry yields values like FEV1, FVC, and their ratio that are used to classify obstructive and restrictive patterns. Lung volumes are directly measured using techniques like plethysmography and indirectly calculated from spirometry. Diffusion capacity reflects membrane thickness and surface area via carbon monoxide transfer. PFTs provide objective data to diagnose and manage respiratory conditions.
![Pulmonary Gas Exchange
• Evaluating Hypoxemia:
• Hypoxemia with normal A-a gradient: hypoventilation
• Hypoxemia with increased A-a gradient: V/Q
mismatch, right-to-left shunt, diffusion impairment
• P(A-a)O2 = [PiO2 –(PaCO2/R)] – PaO2
• P(A-a)O2 = [0.21(PB-47) – (PaCO2/0.8)] –PaO2
• P(A-a)O2 = 150 – (PaCO2/0.8) – PaCO2](https://image.slidesharecdn.com/pulmonaryfunctionexam-140501005202-phpapp02/85/Pulmonary-function-exam-29-320.jpg)
Pulmonary function exam
- 2. Goals • Indications for PFTs • Know types of studies that can be ordered • Understand how common tests are done • Interpretation of the data
- 3. Indications • Characterize known or suspected pulmonary disease (COPD screening) • Follow evolution of pulmonary disease • Assess effectiveness of therapy • Pre-op assessment of surgical risks • Assess need for surgical interventions • Assess impact of an occupational exposure
- 4. Pulmonary Function Tests • Spirometry/flow-volume loop • Lung volumes • Diffusion capacity • Arterial blood gas, shunt fraction measurement, dead space • Airway resistance • Inspiratory/expiratory muscle pressures • Airway reactivity (methacholine/exercise challenge) • Cardiopulmonary exercise test
- 5. Normal Lung Volumes and Capacities
- 6. Primary Lung Volumes • VT: tidal volume - air inhaled during quiet breathing • IRV: inspiratory reserve volume - maximal volume inhaled from quiet breathing • ERV: expiratory reserve volume - maximal volume exhaled from quiet breathing • RV: residual volume - volume remaining after maximal exhalation
- 7. Lung Capacities = Sum of Primary Lung Volumes • TLC: total lung capacity - sum of 4 primary volumes • VC: vital capacity - volume exhaled from maximal inspiration to maximal expiration • FRC: functional residual capacity – resting, end-expiratory volume • IC: maximal volume inhaled from FRC
- 9. Spirometry and Flows FEF 25-75%: mean forced expiratory flow during middle half of FVC; sensitive to small airways disease
- 10. Flow/Volume Loops Includes inspiratory and expiratory flows Instantaneous flows Shape of curve restrictive vs. obstructive
- 12. Normal Reference • Normal standards depend upon: – Height – Gender – Age – Race • Reproducibility criteria (3 trials examined) • Rate of decline: normal fall in FEV1 with age = 20-30cc/year; in COPD = 50- 80cc/year
- 14. Interpretation of Spirometry • Step 1: obstruction or not? – Low FEV1/FVC (<70%) = obstruction • Step 2: Interpret severity (based upon FEV1) • Restriction: FEV1 and FVC reduced in proportion (i.e. normal FEV1/FVC ratio) • Flow/Volume Loops – Obstruction – concave, scooped appearing – Restriction – decreased VC, normal shape – Upper airway obstruction: cut-off insp and/or exp limbs
- 15. Bronchodilator Response: • Response to inhaled bronchodilators: – Typical in asthma; some patients with COPD and CF have reversibility also – “Real response”: consists of a change in FEV1 by at least 12% (and 200cc) after inhalation of albuterol
- 16. Broncho-provocation testing • Reveals airway hyper-reactivity (asthma) • Useful to assess non-specific hyperrresponsiveness in a patient with symptoms c/w asthma, but without obstruction or bronchodilator reversibility (cough variant asthma, exercise-induced asthma) • Methacholine – 75% asthmatics will react • Histamine – 90-95% asthmatics will react • Decline in FEV1 by 20% at concentration of 8mg/ml or less (methacholine)
- 17. Flow-volume loops and upper airway obstruction • Extrathoracic obstruction – vocal cord dysfunction, goiter, cause flattening of inspiratory limb of flow/volume loop • Intrathoracic obstruction – bronchogenic cancer in right mainstem bronchus, flattening of expiratory limb of flow/volume loop
- 19. Lung Volumes • Spirometry measures volume differences between identifiable lung capacities (TLC, FRC, RV), but cannot measure the absolute volume of these key volumes • Lung volumes measure FRC and use spirometry to calculate TLC and RV • FRC can be measured by following techniques: – Closed circuit helium dilution – Open circuit nitrogen washout – Plethysmography or body box
- 20. Dilution Techniques • Closed circuit helium dilution – starting at FRC, patient breathes helium for 7 minutes (until equilibrium) from known volume system with known He concentration; measure helium concentration after maneuver • Open nitrogen washout – starting at FRC, begin inspiring 100% O2 and collect/measure all nitrogen exhaled from the lungs for 7 minutes (N2 essentially washed out). Given known initial concentration of nitrogen in the lungs (81%), use the measured concentration and volume of nitrogen in collected air to calculate the starting lung volume (FRC) at end of maneuver • Both techniques underestimate actual FRC if ventilation isn’t homogeneous (i.e. obstructive lung disease)
- 21. Helium Dilution Point A: 2 L of 10% He Point B: 5% He now present in system; FRC must be 2L!
- 22. Plethysmography
- 23. Plethysmography • Measures thoracic gas –performed at FRC • Underlying principle: Boyle’s Law – Patient sits in sealed box, patient pants against shutter that is closed at FRC – Alveolar pressure changes measured at mouth (presumes open glottis/equal pressures); – Box pressure changes measured with respiratory efforts – proportional to lung volume increases/decreases due to respiratory efforts PV = (P + P)(V + V) V = FRC
- 24. Lung Volume Determinants • FRC: – Lung and chest wall properties • TLC: – Lung and chest wall properties – Inspiratory muscle strength • RV: – Lung and chest wall properties – Expiratory muscle strength – Airway Closure**
- 26. Diffusion • Volume of gas transferred across alveolar/capillary membrane/per minute/mmHg of difference between the alveolar and capillary blood • Determined from CO uptake during 10 seconds of breath-holding • VCO = (Area/Thickness) x (Solubility/ MW) x (PA-PC)
- 27. Diffusion – use of CO • Rate of transfer of CO across respiratory membrane relates to hemoglobin affinity (240 fold higher than for O2) • CO transfer rate decreases in anemia and increases in polycythemia • DLCO is artificially low in smokers (have baseline CO in blood – i.e. concentration gradient working against CO uptake) • High altitude – increased transfer of CO
- 28. Diffusion Loss of membrane surface area; increase in thickness: • pneumonectomy, emphysema, interstitial disease, CHF Changes in Pulmonary Circulation • Pulmonary vascular disease Increases in DLCO • pulmonary hemorrhage, left-to-right intracardiac shunts, asthma
- 29. Pulmonary Gas Exchange • Evaluating Hypoxemia: • Hypoxemia with normal A-a gradient: hypoventilation • Hypoxemia with increased A-a gradient: V/Q mismatch, right-to-left shunt, diffusion impairment • P(A-a)O2 = [PiO2 –(PaCO2/R)] – PaO2 • P(A-a)O2 = [0.21(PB-47) – (PaCO2/0.8)] –PaO2 • P(A-a)O2 = 150 – (PaCO2/0.8) – PaCO2
- 30. Case 1 • FVC 75% pred; FEV1 60% pred; ratio 64% • TLC: 125% pred; FRC 120% pred; RV 160% • DLCO: 30% pred • ABG: 7.42/42/70 on RA
- 31. Case 2 • FVC 85% pred; FEV1 50% pred; ratio 55% • TLC: 95% pred; FRC 105% pred; RV 145% pred • DLCO: 85% • ABG: 7.37/48/58 on RA
- 32. Case 3 • FVC 65% pred; FEV1 68% pred; ratio 85%; FEF25-75% 120% pred • TLC 65% pred; FRC 65% pred; RV 70% pred • Normal flow-volume shape • DLCO 45% pred
- 33. Case 4 • FVC 85% pred; FEV1 71% pred; ratio 75%; FEF25-75%45% • TLC 85% pred; FRC 60% pred; RV 100% pred; ERV 10% pred • DLCO 85% pred
- 36. Flow/Volume Loops in Obstruction and Restriction
- 38. Mechanics • Neuromuscular disease with MIP, MEP • Pressure/volume – elastic properties of lung • Esophageal balloon to measure changes in expiratory lung volume with changes in transpulmonary pressure • Transpulmonary pressure = alveolar pressure measured at mouth and pleural pressure measured by balloon
- 39. Static Compliance • volume/ pressure • Mean compliance is 260cc/cm • Interstitial lung disease – increased elastic recoil, decreased compliance, less V/ P • COPD – decreased elastic recoil, increased compliance
- 41. Diffusion