Pulmonary function tests

Jaber A. Manasia

What is pulmonary function tests?(PFTs)

Pulmonary function tests are a group of test that measure how well the lungs take in and release air and how well they move oxygen into the blood.(it is a non invasive)

Pulmonary Function Tests are used for the following reasons :

• Screening for the presence of obstructive and restrictive diseases

• Evaluating the patient prior to surgery – for patients who :

a. are older than 60-65 years of age b. are known to have pulmonary diseasec. are obese (as in pathologically obesity)d. have a history of smoking, cough or

wheezinge. will be under anesthesia for a lengthy

period of timef. are undergoing an abdominal or a

thoracic operation.

•Evaluating the patient's condition for weaning from a ventilator.

•Documenting the effectiveness of therapeutic intervention

pulmonary function tests• Examples include:• Spirometry• Lung volumes• Blood gases• Exercise tests• Diffusing capacity• Pulse oximetry

What is Spirometry?

• a simple and safe test• that measures lung volumes• with a graphical display• gives an estimation of lung function• Allows for diagnosis of airflow obstruction• Permits good follow-up for asthma and COPD

How the test is performed?

SPIROMETRY…

In a spirometry test, you breathe into a mouthpiece that is connected to an instrument called a spirometer. The spirometer records the amount and the rate of air that you breathe in and out over a period of time.

Hand-held spirometer

Spirometer in the fifties..

In the lung function lab

Spirobank

Simplicity

SpiroPro

SpiroStar

Datospir 70

Many types are available..

MicroLoop

What do we measure

with a spirometer?

• Lung volume measurement can be performed in two ways:

• The most accurate way is for a person to sit in a body plethysmograph, a sealed, transparent box that resembles a telephone booth, while breathing in and out against into a mouthpiece. Changes in pressure inside the box allow determination of the lung volume.• Lung volume can also be measured when a

person breathes nitrogen or helium gas through a tube for a specified period of time. The concentration of the gas in a chamber attached to the tube is measured, allowing estimation of the lung volume.

Table 22–1. Lung Volumes and Capacities. 

MeasurementDefinitionAverage Adult Values (mL)

Tidal volume (VT) Each normal breath500

Inspiratory reserve volume (IRV)

Maximal additional volume that can be inspired above VT 

3000

Expiratory reserve volume (ERV)

Maximal volume that can be expired below VT 

1100

Residual volume (RV)Volume remaining after maximal exhalation

1200

Total lung capacity (TLC)RV + ERV + VT + IRV 5800

Functional residual capacity (FRC)

RV + ERV2300

Two important parameters• FVC - Forced Vital Capacity. This

is the total amount of air that you blow out in one breath.

• FEV1 - Forced Expiratory Volume in one Second. This is the amount of air you can blow out within one second. With normal lungs and airways you can normally blow out most of the air from your lungs within one second.

• Normal FEV1/FVC ~ 80% 

Restrictive (fibrosis) ratio normal or increased Obstructive (asthma, COAD) usually low• Normal values vary, depending on

gender, race, age and height.

Spirometry: Normal and COPD

0

5

1

4

2

3

Lit

er

1 65432

FVC

FVC

FEV1

FEV1

Normal

COPD

3.900

5.200

2.350

4.150 80 %

60 %NormalCOPD

FVCFEV1 FVCFEV1/

Seconds

Flow-Volume Loops

Flow volume loops provide a graphical illustration of a patient's spirometric efforts. Flow is plotted against volume to display a continuous loop from inspiration to expiration. The overall shape of the flow volume loop is important in interpreting spirometric results

How is a flow-volume loop helpful?

• Helpful in evaluation of air flow limitation on inspiration and expiration

• In addition to obstructive and restrictive patterns, flow-volume loops can show provide information on upper airway obstruction:• Fixed obstruction: constant airflow limitation on inspiration

and expiration—such as in tumor, tracheal stenosis• Variable extrathoracic obstruction: limitation of inspiratory

flow, flattened inspiratory loop—such as in vocal cord dysfunction

• Variable intrathoracic obstruction: flattening of expiratory limb; as in malignancy or tracheomalacia

• 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• 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

Diffusing capacity (Transfer factor)

The volume of a substance (CO) transferred across the alveoli per minute per unit alveolar partial pressure. CO is rapidly taken up by haemoglobin; its transfer is therefore limited mainly by diffusion. A single breath of 0.3% CO and 10% helium is held for 20 seconds. Expired partial pressure of CO is measured. Normal value 17-25 ml/min/mmHg.

Value is reduced with increased alveolar membrane thickness (e.g. pulmonary fibrosis). May also be reduced with pneumonectomy (results in reduced alveolar membrane).