What is pulmonary function (tests?(PFTsPulmonary function tests are a group of test thatmeasure how well the lungs take in and releaseair 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 ageb. 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 patients condition for weaning from a ventilator.• Documenting the effectiveness of therapeutic intervention
?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
…SPIROMETRYIn a spirometry test, you breathe into amouthpiece that is connected to aninstrument called a spirometer. Thespirometer records the amount and therate of air that you breathe in and out.over a period of time
• 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. Measurement Definition Average Adult Values (mL)Tidal volume (VT) Each normal breath 500Inspiratory reserve volume Maximal additional volume 3000(IRV) that can be inspired above V T Expiratory reserve volume Maximal volume that can be 1100(ERV) expired below VT Residual volume (RV) Volume remaining after 1200 maximal exhalationTotal lung capacity (TLC) RV + ERV + VT + IRV 5800Functional residual capacity RV + ERV 2300(FRC)
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 FEV1 FVC FEV1/ FVC Normal 4.150 5.200 80 % 1 COPD 2.350 3.900 60 % 2 FEV1Liter 3 COPD 4 FVC FEV1 5 Normal FVC 1 2 3 4 5 6 Seconds
Flow-Volume LoopsFlow volume loopsprovide a graphicalillustration of a patientsspirometric efforts. Flowis plotted against volumeto display a continuousloop from inspiration toexpiration. The overallshape of the flow volumeloop is important ininterpreting spirometricresults
?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 DilutionPoint 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 Mouth Pressue (Pm) (Pm,V) r PV = (P + ∆P)(V + ∆V) (Pm +∆Pm, V +∆V) V = FRC Volume (V) (monitored by box pressu re)
Diffusing capacity (Transfer(factorThe volume of a substance (CO) transferred acrossthe alveoli per minute per unit alveolar partialpressure. CO is rapidly taken up by haemoglobin; itstransfer is therefore limited mainly by diffusion. Asingle breath of 0.3% CO and 10% helium is held for20 seconds. Expired partial pressure of CO ismeasured. Normal value 17-25 ml/min/mmHg.Value is reduced with increased alveolar membranethickness (e.g. pulmonary fibrosis). May also bereduced with pneumonectomy (results in reducedalveolar membrane).