Pulmonary Function Testing
Goals
• Indications for PFTs• Know types of studies that can be
ordered• Understand how common tests are done• Interpretation of the data
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
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
Normal Lung Volumes and Capacities
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
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
The Spirogram
Spirometry and Flows
FEF 25-75%: mean forced expiratory flow during middle half of FVC; sensitive to small airways disease
Flow/Volume Loops
Includes inspiratory and expiratory flows
Instantaneous flows
Shape of curve restrictive vs. obstructive
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
Classification of Impairment
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
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
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)
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
Intrathoracic Obstruction
Extrathoracic Obstruction
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
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)
Helium Dilution
Point A: 2 L of 10% HePoint B: 5% He now present in system; FRC must be 2L!
Plethysmography
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
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**
Lung Volume Patterns
TLC
FRC
RVERV
IC
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)
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
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
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
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
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
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
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
Flow-Volume Loops
Flow/Volume Loops in Obstruction and Restriction
Broncho -provocation
Agents
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
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
Static Compliance
Diffusion