Pulmonary function exam

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Pulmonary Function Testing

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Transcript of Pulmonary function exam

Page 1: Pulmonary function exam

Pulmonary Function Testing

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Goals

• Indications for PFTs• Know types of studies that can be

ordered• Understand how common tests are done• Interpretation of the data

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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

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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

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Normal Lung Volumes and Capacities

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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

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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

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The Spirogram

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Spirometry and Flows

FEF 25-75%: mean forced expiratory flow during middle half of FVC; sensitive to small airways disease

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Flow/Volume Loops

Includes inspiratory and expiratory flows

Instantaneous flows

Shape of curve restrictive vs. obstructive

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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

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Classification of Impairment

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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

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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

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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)

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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

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Intrathoracic Obstruction

Extrathoracic Obstruction

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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

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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)

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Helium Dilution

Point A: 2 L of 10% HePoint B: 5% He now present in system; FRC must be 2L!

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Plethysmography

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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

Mo

uth

Pre

ssure

(Pm)

Volume(V)(monitoredbybox pressure)

(Pm,V)

(Pm +DPm, V +DV)PV = (P + DP)(V + DV)

V = FRC

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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**

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Lung Volume Patterns

TLC

FRC

RVERV

IC

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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)

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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

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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

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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

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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

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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

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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

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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

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Flow-Volume Loops

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Flow/Volume Loops in Obstruction and Restriction

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Broncho -provocation

Agents

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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

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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

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Static Compliance

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Diffusion