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Lecture 11

Lecture 11 Notes.pdf

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Biological Sciences
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Stephen Reid

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1    Lecture 11: Pulmonary Function Tests and Alveolar Ventilation 1. Spirometry and Lung Volumes A spirometer is a device used to measure lung volumes. The old-fashioned models consisted of a drum of air upside down in water. A tube allows the subject to breathe in and out of the air drum. As the person inspires, air is removed from thedrum causing it to movedownward thereby pulling the pen upward and creating a positive deflection on the chart paper. When expiration occurs air is blown into the drum causing it to move upward and the pen to move downward, giving a negative deflection. 2. Lung Volumes and Capacities The volume of a breath is referred to as tidal volume. The average value for tidal volume is approximately 500 ml. At a normal breathing frequency of 12 breaths per minute, this translates into 6 000 ml of air taken into the lungs every minute.However, not all of thisair enters the alveoli and is involved in gas exchange.A portion of the inspired air remains in the anatomical dead space of the conducting zone, where no gas transfer occurs (see below). The maximum amount of air exhaled following a maximal inspiration is called vital capacity. However, no matter how hard we expire, there will always be approximately 1.2 L of air left in the lungs that cannot be exhaled (residual volume). Standard spirometry techniques which can measure breath volume can't be used to measure this residual volume of air - but there is another technique, the inert gas technique,that can measure this. The residual volume, plus our expiratory reserve volume, is termed our functional residual capacity. The entire air capacity of the respiratory system, including the reserve volumes and residual volume, is referred to as total lung capacity. 3. Pulmonary Function Tests Lung volumes are measured by pulmonary function tests which can also be used to diagnose various pathological conditions or lung disorders. Generally, these disorders can be classified as either restrictive or obstructive. A restrictive pulmonary disease interferes with lung expansion, and thus inspiration, and may be caused by damage to the chest wall, the lungs or the pleura. Examples of restrictive pulmonary diseases include pulmonary edema, where the expansion of the alveoli is inhibited by fluid in the lungs and pulmonary fibrosis. The other type of lung diseases assessed using these pulmonary function tests are classified as obstructive. These conditions hinder expiration. Since air does not leave the lungs efficiently, the lungs over-inflate. Diseases that damage the alveoli, such as asthma or emphysema, are classic examples of obstructive disorders. When we perform a lung function test, we are often trying to measure forced vital capacity, by breathing in as hard as one can, andthen exhaling as hard as one can. This gives us then values for both our maximal inspiration and our maximal exhalation. Forced vital capacity is determined by 2    several factors, including the strength of the chest and abdominal muscles, airway resistance (which can be altered by bronchitis or asthma), lung size (which can be altered by the size of the person, or by diseases such as tuberculosis), and the elasticity of the lung tissues also plays a factor (this can be altered by diseases, or simply by age). A second variable that is measured by lung function tests is forced expiratory volume, which is the proportion of forced vital capacity that can be exhaled in a given time, for example in one second (what is termed FEV ). 1enerally within five or six seconds a person can breathe out their entire vital capacity, so forced expiratory volume over five or six seconds (FEV 5 or FEV )6would approximate FVC. However, the vast majority of air (about 80%) is breathed out within the firstsecond - in a healthy person. Under disease conditions, this proportion changes (lower FEV/FVC 1atio with obstructive lung diseases and slightly higher ratio with restrictive) and this is why both forced vital capacity and forced expiratory volume are important in these tests. We can make these measurements using various spirometry techniques, from the old-fashioned air-filled drum in water, or more modern calibrated electronic spirometers. The one thing that can't be measured with these spirometry techniques is residual volume. However, this too can be measured via a different process called the inert gas technique. 4. The Inert Gas Technique When using the inert gas technique, the subject is connected to a spirometer containing 10% helium in air. We know the volume of the spirometer (V1), and the concentration of the helium in the air (C1). Helium is insoluble, and so it is nottaken up in the blood - thus it will stay in the lungs, and not diffuse throughout the body during the test. There is a valve in between the spirometer and the subject, and the lung volume we will be measuring is the lung volume at the time the valve is opened. When the valve is opened, the helium previously contained in the spirometer will be taken into the lungs, and over time the concentration ofhelium in the spirometer and in the lungs will be equal (since no helium is leaving the lungs and entering the blood). Thus, when we measure the concentration of helium in the spirometer following equilibration it will be the same as the concentration of He in the lungs (called C2). V2 is the volume of the lungs. Since we know the volume of the spirometer, by using the equation: C1V1 = C2 (V1 + V2) Or by rearranging it as: C2 = V1 (C1 - C2)/C2 We can find out what lung volume is - including the normally hidden residual volume,
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