PULMONARY FUNCTION TESTING. Purposes of Pulmonary Tests. General Categories of Lung Diseases. Types of PF Tests

Transcription

1 PULMONARY FUNCTION TESTING Wyka Chapter 13 Various AARC Clinical Practice Guidelines Purposes of Pulmonary Tests Is lung disease present? If so, is it reversible? If so, what type of lung disease is present? How bad is the problem? Is rehabilitation an option? Is there more than one lung disease present? General Categories of Lung Diseases Obstructive Diseases characterized by airflow limitations Asthma; Emphysema; Chronic Bronchitis Restrictive Diseases characterized by a decrease in lung volumes Pulmonary Fibrosis; Severe Kyphosciolosis; Obesity Types of PF Tests Tests of Lung Volumes & Capacities FRC; RV; TLC; IC; VC Tests of Airflow Spirometry measures flowrates of exhaled gas based upon a forced exhalation Flow-volume loops creates a visual picture showing the flowrate at any given lung volume Carbon Monoxide Diffusing Capacity Measures how well gas can diffuse across the alveolar-capillary membrane This value is decreased in Emphysema & in Pulmonary Fibrosis What is Considered Abnormal? Normal values for volumes, capacities, diffusion & flowrates are based upon a person s; Age Height Gender Nomograms provide an easy way of determining a person s expected values Example of a Nomogram

2 When is a value abnormal? Normal = % of predicted Mild impairment = 65 79% predicted Moderate impairment = 50 64% Severe impairment = % Very Severe = <35% How to calculate % predicted % predicted = measured value X 100 predicted value How Lung Volumes Change in Disease What causes Restrictive Changes? HOW VOLUMES & CAPACITIES CHANGE Restrictive Disorders Vt N or decreased FRC decreased RV decreased TLC decreased

3 VC decreased IC - decreased Obstructive Disorders Vt N or increased FRC increased RV increased N or increased VC N or decreased IC N or decreased Types of flowrates Simple Spirometry FVC forced vital capacity Usually expiratory maneuver FEV 1 amount of volume exhaled in 1 sec. FEF average speed of gas being exhaled after the first 200ml up to 1200 ml Looks at quality of airflow in the larger airways FEF 25-75% - average speed of gas coming out of the lungs after the first 25% up to 75% of the VC Looks at the quality of airflow from the smaller airways Good index of small airways disease seen in asthma and emphysema

4 Calculating Simple Spirometric Data What is FVC? What is FEV 1? What is FEV1/FVC ratio? To calculate; Observed FEV1 X 100% Observed FVC FEV1% is a good general index of presence of obstruction verses restrictive disorder Significance of FEV1% In obstructive disease; FEV1% is decreased In restrictive disease; FEV1% is increased or normal Flow-Volume Loops Obtained when a patient does an Expiratory FVC followed by an Inspiratory FVC Inspiration is below the x-axis & Expiration is above the x- axis Significance of Flow-Volume Loops Gives a unique visual picture of flows throughout the breathing cycle During the last 25% of exhalation (from 75% to 100% of expiratory flow), flowrate is effort independent Allows clinicians to identify obstruction on either inspiration OR exhalation Examples of Flow-Volume Loops As you can see, each patient type has a distinctive signature pattern.

5 Important Data Points on Flow-Volume Loop FEF25%; FEF50%; FEF75%; FIF25%; FIF50%; FEF75% These points give indications of flow at specific lung volumes Maximal Voluntary Ventilation Also called MVV or MBC Patient breaths rapidly using breaths greater than a normal Vt but less than IC or FVC Significance of MVV Is a test of overall function of respiratory system. Influenced by; Status of respiratory muscles Compliance of lung-thorax system Condition of ventilatory control mechanisms Resistance offered by airways Normal values in young people is l/m. Only decreases of >30% are significant Before & After Bronchodilator Tests Spirometric values (FEV1); FEF ; FEF25-75%; MVV & Flow-volume loops can be repeated after giving the patient an SVN tx or MDI with a bronchodilator Improvement is defined as a 20% increase in any observed values Diffusion Tests Carbon monoxide (CO) diffusing capacity DLCO. Most popular type is the single-breath method Patient breathes in to TLC a mixture of 0.3% CO + 10% He + 21% O2 and holds breath for about 10 seconds Patient exhales into a small bag where gas concentrations are analyzed DLCO Calculations Where; FaCOo = fraction of CO at beginning of breath-hold

6 FICO = fraction of CO in the reservoir FAHe = fraction of He at the end-tidal sample FIHe = fraction of He in inspired gas FACOo = FICO X FAHe FIHe Significance of DLCO Normals vary by as much as 30% Decreased in restrictive diseases (generally) Alveolar fibrosis Decreased by space occupying tumors and after lung resection Decreased in loss of lung tissue Emphysema Measuring RV, FRC and TLC FRC is measured indirectly using one of the three following techniques; 1. Body plethysmography (body box) 2. Nitrogen washout study 3. Helium dilution Once FRC is measured, RV and TLC can be calculated mathematically 1. Body Plethysmography Patient is sealed in a box & ventilates through a mouthpiece with a pressure transducer and shutter valve allowing obstruction at the mouthpiece Patient breathes gas from the box. At FRC, the shutter is closed; the patient pants for breaths/min against an obstruction As this occurs, pressure is measured at the proximal airway AND within the box at the same time Boyle s law is used to determine the final volume in the box: P1V1 = P2V2 Where; P1 = Original pressure in the box (760 torr) V1 = Original volume in the box (1,000L) P2 = Increased pressure in the box as a result of chest expansion V2 = Final volume in the box Example; (760 mmhg)(1000l)=(760.2 mmhg) x (760 mmhg)(1,000l) = x mmhg x = L Difference between V1 & V2 change in volume in the patient s thorax 1,000 L L =.263 L As the patient pants against an obstruction, the volume in the thorax increases & the pressure in thorax is decreased. Using Boyle s law again; Pa1Va1 = Pa2Va2 Where Pa1 = prox airway pressure at resting FRC(this equals 760 mmhg) Va1 = volume of FRC

7 Pa2 = pressure in airway after inspiring against an obstruction Va2 = Final volume in thorax (# from previous equation) (Pa1)(Va1) = (Pa2)(Va1 + V) (760 mmhg)(x) = 700 mmhg (x +.263) 760 mmhg x = 700 mmhg x mmhg x = mmhg L 60 mmhg 60 mmhg x= 3.07 L this is the patient s FRC Body Plethysmography is popular since it is the most accurate method for determining FRC Other methods do not accurately measure trapped gas commonly present in patient s with COPD 2. Nitrogen Washout for FRC Patient connected to breathing circuit & inspires 100% O2. Total volume of exhaled air is collected separately Test normally lasts for about 7 minutes or until expired N 2 is 1% to 2.5% Since nitrogen makes up about 80% of FRC on room air, the volume of nitrogen is the total exhaled gas will equal about 80% of FRC Total volume exhaled into bag is measured along with N 2 concentration in the bag Example; Total volume collected 50 L Measured N2 % 5% Volume of nitrogen in bag = 50 x.05 = 2.5 L Example continued; 2.5 L = x..80 FRC 1 FRC x = L (this is the patient s FRC) Problems with Nitrogen Washout Atelectasis may result from washout of nitrogen from poorly ventilated lung zones (obstructed areas) Elimination of hypoxic drive in CO 2 retainers is possible Underestimates FRC due to underventilation of areas with trapped gas 3. Helium Dilution for FRC Since helium is inert, as patients breaths the helium, it s volume does not decrease since it is not absorbed by the blood. Patient is connected to a rebreathing system and a known volume & concentration of helium is added to the system The patient breathes until the final He concentration is stable. (About 7 minutes) FRC is calculated using the following equation; FRC = (% He initial - % He final) x initial vol. % He final Where % He initial = 10% % He final = 3.5% Initial vol. = 200 ml = 2,000ml or 2.0 L

8 10% FRC = (% He initial - % He final) x initial vol. % He final FRC = (10% - 3.5%) x 3.5% 2.0 L = 3.7 L Problems with Helium dilution for FRC are similar to Nitrogen washout problems; Presence of severely obstructed airways as found in COPD patients tends to cause poor distribution of helium Leaks in the system or patient will cause erroneous values ADDITIONAL PULMONARY FUNCTION TEST Determination of Closing Volume Closing volume is the point during slow exhaled VC maneuver at which small airways begin to collapse In young, healthy adults is about 10% of VC Closing volume increases with age At age 60 may be as high as 40% Increases with lung disease (COPD) Is a good earlier indicator of small airways disease Single-breath Nitrogen washout study is used to determine Closing Volume Patient breathes in 100% O 2 from RV to TLC and slowly exhales back to RV Single-breath Nitrogen washout curve Single-breath Phases Phase 1 N 2 % is zero (gas from upper airways) Phase 2 Steep increase in N 2 % (gas from smaller airways & alveolar regions) Phase 3 Slow increase in N 2 % (gas from mostly alveolar regions Phase 4 Closing volume (gas from apices only is exhaled N 2 % is highest in apices)

9 CO 2 response curve Patients are connected to a rebreathing circuit and given a mixture of 93% oxygen while CO 2 % is gradually increased up to 7% Changes in minute ventilation are measured as CO 2 concentration is increased. CO 2 response is determined by the peripheral chemoreceptors Decreased response to CO 2 is seen in lung disease (COPD patients with severe airway obstruction)