L3 Pulmonary Function Tests PDF

Biomedical Eng. Dep 5rth year, Lec. 3 Pulmonary Function Tests INTRODUCTION: Pulmonary function testing is a group of tests that provide objective data on a patient's lung function. These tests must be interpreted within the context of the patient's history and phy...

Biomedical Eng. Dep 5rth year, Lec. 3 Pulmonary Function Tests INTRODUCTION: Pulmonary function testing is a group of tests that provide objective data on a patient's lung function. These tests must be interpreted within the context of the patient's history and physical examination, though their patterns can suggest different categories of respiratory disease. PULMONARY FUNCTION MEASUREMENT: Three basic types of measurements are made in pulmonary clinic: 1. VENTILATION: This is performed using device called a spirometer that measure volume displacement and amount of gas moved in a specific time. Usually this requires the patient to take a deep breath and then exhale as a rapidly and completely as possible. Called the forced vital capacity, this gives an indication of how much air can be moved by the lungs and how freely this air flows. 2. DISTRIBUTION: Measurement quantity degrees of lung obstruction and also determine the residual volume, which is the amount of air that cannot be removed from the lungs by the patients effort. 3. DIFFUSION: Measurements identify the rate at which gas is exchanged with the blood stream. Pulmonary Function Tests page1 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 The pulmonary function can be assessed by means of two major classes of tests. These are: (i) Evaluation of the mechanical aspects of pulmonary function, which affects the bulk gas transport into and out of the lungs. (ii) Evaluation of gas exchange or diffusion at the alveoli. The ability of the pulmonary system to move air and exchange oxygen and carbon dioxide is affected by the various components of the air passages, the diaphragm, the rib cage and its associated muscles and by the characteristics of the lung tissue itself. Among the basic tests performed are those to determine the volumes and capacities of the respiratory system. These are defined in (figure 1) below: Figure 1. Volumes and capacities of lungs. Pulmonary Function Tests page2 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 THE RESPIRATORY SYSTEM The primary functions of the respiratory system are to oxygenate the blood; that is to dissolve oxygen into the blood; also to remove carbon dioxide from the blood. If the blood is not oxygenated sufficiently due to failure of the circulatory system, then the oxygen content of the blood decreases rapidly. After 60 to 90 minutes, the subject will become unconscious, death occurring in 4 to 5 minutes. Inspiratory reserve volume (IRV) is the maximum additional volume that can be accommodated by the lung at the end of inspiration. Expiratory reserve volume (ERV) is the maximum additional expiration, as measured from lung volume at the end of expiration. Residual volume (RV) is the amount of gas remaining in the lungs at the end of maximal expiration. Total lung capacity (TLC) is the amount of gas contained in the lung at the end of maximal inspiration. Vital capacity (VC): The maximal volume of gas that can be forcefully expelled after maximal inspiration. Inspiratory capacity (IC): The maximal volume of gas that can be inspired from the resting expiratory level Tidal volume (TV) is normally considered to be the volume of air entering the nose and mouth with each breath. Alveolar ventilation volume, the volume of fresh air that enters the alveoli during each breath, is always less than tidal volume. Functional residual capacity (FRC): The volume of gas remaining after normal expiration. It will be noted that functional residual capacity (FRC) is the same as the resting volume. Dead space is the functional volume of the lung that does not participate in gas exchange. Pulmonary Function Tests page3 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 SPIROMETRY The instrument used to measure lung capacity and volume is called a spirometer. Basically, the record obtained from this device is called a spirogram. Spirometers are calibrated containers that collect gas and make measurements of lung volume or capacity that can be expired. By adding a time base, flow-dependent quantities can be measured. The addition of gas analyzers makes the spirometer a complete pulmonary function testing laboratory. There are two basic classes of spirometers: laboratory units, which are either desktop consoles or cabinet-size machines operated by trained technicians; and portable spirometers, which are either compact desktop units or handheld devices intended for general-practice and home use. Pneumotachometers Pneumotachometers are devices that measure the instantaneous rate of volume flow of respired gases. Basically, there are two types of pneumotachometers, which are: (i) Differential manometer: Differential pressure sensors are sometimes used in place of turbine transducers. Commonly referred to as pneumotachs, these designs can measure low flow rates with high accuracy. An added advantage is cost: because they are relatively inexpensive, pressure transducers enable the implementation of disposable pneumotachs. It has a small resistance, which allows flow but causes a pressure drop. This change is measured by a differential pressure transducer, which outputs a signal proportional to the flow according to the Poiseuille law, assuming that the flow is laminar. The unit is heated to maintain it at 37°C to prevent condensation of water vapour from the expired breath. Pulmonary Function Tests page4 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 Figure 1 Differential manometer (ii) Hot-wire anemometer: It uses a small heated element in the pathway of the gas flow. The current needed to maintain the element at a constant temperature is measured and it increases proportionally to the gas flow that cools the element. Pneumotachometer is commonly used to measure parameters pertaining to pulmonary function such as forced expiratory volume (FEV), maximum mid-expiratory volume, peak flow and to generate flow-volume loops. Although these devices directly measure only volume flow, they can be employed to derive absolute volume changes of the lung (spirometry) by electronically integrating the flow signal. Conventional mechanical spirometers, though more accurate than pneumotachometers, have limitations due to their mechanical inertia, hysteresis and CO2 build-up. Pneumotachometers, on the other hand, are relatively nonobstructive to the patient and this makes them suitable for long-term monitoring of patients with respiratory difficulties. Pulmonary Function Tests page5 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 Figure 2 Hot wire Anemometer PULMONARY FUNCTION ANALYSERS The assessment of pulmonary function is important in the diagnosis and evaluation of obstructive and restrictive pulmonary diseases. Obstructive lung disease is clinically identified by a decrease in expiratory flow rates, the anatomical basis of which is airway narrowing, whereas restrictive lung disease is clinically identified by decreased lung volumes. Pulmonary functions utilized in the diagnosis and prognosis of these diseases include Static Lung Volumes (VC), Flow Rate (FEV1), Flow-Volume Loops and Maximal Ventilatory Volume (MVV). Pulmonary functions are not the only means of evaluating lung disease. Other assessments include diffusion capacity and arterial blood gases. Vital capacity maneuver measures dynamic and static lung volumes. Static lung volumes are single volumes whereas dynamic volumes or capacities are combinations of static lung volumes. Each component of the forced vital capacity maneuver is divided into different pulmonary functions. Pulmonary Function Tests page6 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 Dynamic pulmonary functions are Inspiratory Capacity, Vital Capacity, functional Reserve Capacity and total Lung Capacity, whereas the static pulmonary functions are Inspiratory Reserve Volume, Expiratory Reserve Volume, Tidal Volume and Residual Volume. A complete pulmonary function analyser contains all the equipment necessary for testing the above mentioned various parameters. It comprises a nitrogen analyser, a vacuum pump, an X-Y recorder, pneumotachs, a digital display, plumbing and valves and other electronic circuits. A simplified block diagram of the system is shown in figure 3. Modern instruments are designed to completely automate the measurements of ventilation, distribution and diffusion. The systems are designed around computers which control the procedures by opening and closing appropriate valves, measuring flow rates and the concentrations of various gases, and calculating and printing the results. An analog-to-digital converter supplies the measurement data to the computer. Inputs to the A-D converter are from various measurement devices, which include a pneumotach that provides a signal proportional to the air flow for various measurements and carbon monoxide and helium analyzers for diffusion measurements. The software controlled pulmonary measurement procedure allows new programs to be added or existing ones to be modified. Pulmonary Function Tests page7 / 8 Biomedical Eng. Dep 5rth year, Lec. 3 Pulmonary Function Tests page8 / 8