Lung Volumes Lecture Notes PDF

Lecture Two Dr. Gerd Xuereb’s Lecture Notes Lung Volumes 1 of 11  Lecture Two Air Composition The air we breathe is a mixture of gases (mainly nitrogen [N2], Oxygen [O...

Lecture Two Dr. Gerd Xuereb’s Lecture Notes Lung Volumes 1 of 11  Lecture Two Air Composition The air we breathe is a mixture of gases (mainly nitrogen [N2], Oxygen [O2] and carbon dioxide [CO2]) and water vapour. In dry air at an atmospheric pressure (PATM) of 760 mmHg (normal atmospheric pressure at sea level), 79% of the total pressure is due to N2, 21% to O2 and 0.04% to CO2. Dalton’s Law & Partial Pressures Dalton’s law of Partial Pressures states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted by the individual gases, which is known as partial pressure (Pgas). P(total) = PA + PB + PC… The partial pressure of a gas is the pressure exerted by that individual gas in a mixture of gases. Finding the partial pressures of any one gas in a sample of air entails multiplying the gas’s relative contribution (%) by the Atmospheric Pressure PATM: PGAS = % of gas x PATM At sea level, in dry air, 25’C PO2 = 21% x 760mmHg PO2 = 160mmHg The partial pressure of an individual gas is determined only by its relative abundance in the mixture and is independent of the molecular size or mass of the gas. The partial pressures of gases in air vary depending on how much water vapour is present in the air. This water vapour ‘dilutes’ the contribution of other gases to the total pressure. In 37oC air that is 100% humid (typical of inhaled air, remember the 3 functions of the airways), the water vapour pressure is 47 mmHg. 2 of 11  Lecture Two We can calculate Pgas by subtracting the water vapour pressure from the total pressure. Therefore the partial pressure in our lungs is as follows: PO2 = % of gas x (Patm-PH20) PO2 = 21% x (760-47) 37’C PO2 = 0.21 x 713 100% Humidity PO2 = 150mmHg Sea Level PH2O is influenced by temperature and not by the barometric pressure. Hence, at high altitudes, although the barometric pressure decreases, PH2O remains 47 mmHg since inhaled air is still 37oC. Spirometry What happens in lung disease? Air volumes change! Therefore, through lung function tests, we can assess the function of the lung. A single respiratory cycle consists of an inspiration followed by an expiration. Inspiration is the movement of air into the lungs whilst expiration is the movement of air out of the lungs. Assessment of a person’s pulmonary function involves measurement of how much air the person moves during quiet breathing and with maximum effort. 3 of 11  Lecture Two Pulmonary function tests make use a spirometer, an instrument that measures the volume of air moved with each breath. Because air volumes change markedly in lung disease, their measurement is very helpful. When a subject is attached to the traditional water-filled spirometer through a mouthpiece and the subject’s nose is clipped closed, the subject’s respiratory tract and the spirometer form a closed system. When a subject breathes in, air moves from the spirometer into the lungs, and the recording pen, which traces a graph on a rotating cylinder, moves up. When the subject exhales, air moves from the lungs back into the spirometer and the pen moves down. Today, we use modern digital spirometers. The spirometer divides the air moved into the lungs into lung volumes: Tidal Volume (VT): the volume of air that moves during a single inspiration or expiration (ask patient: ‘breathe quietly’). Average VT during quiet breathing is ≈500ml for a 70kg man (approximately 1 pint of beer). Expiratory Reserve Volume (ERV): the volume of air forcefully exhaled after the end of a normal expiration (ask patient: ‘now stop at the end of a normal exhalation, then exhale as much air as you possibly can’). Average ERV in a 70kg man is ≈1100ml. 4 of 11  Lecture Two Inspiratory Reserve Volume (IRV): the additional volume that one inspires above the tidal volume (ask patient: ‘now, at the end of a quiet inspiration, take in as much additional air as you possibly can’). IRV in a 70kg man is ≈3000ml (6x increase over normal VT). Residual Volume (RV): volume of air in the respiratory system after maximal exhalation (even if you blow out as much air as you can, air still remains in lungs and airways). Average RV in a 70kg man is ≈1200ml. RV cannot be measured with a simple spirometer. Forced Expiratory volume (FEV1.0): the volume of air forcefully exhaled in the first second after maximal inspiration (ask patient: ‘take in as much air as possible, then blow it all out as hard and completely as you can’). Lung Capacities The sum of 2 or more lung volumes is called a capacity. Lung capacities include: Vital Capacity (VC): the sum of the IRV, ERV and VT. Vital capacity represents the maximum amount of air that can be voluntarily moved into or out of the respiratory system with one breath (ask patient: ‘take in as much air as possible, then blow it all out as completely as you can’). In Forced Vital Capacity (FVC) the procedure involves forceful exhalation (ask patient: ‘take in as much air as possible, then blow it all out as hard and completely as you can’). Total Lung Capacity (TLC): the sum of VC and RV. Functional Residual Capacity (FRC): sum of ERV and RV (this is the amount of gas remaining in the lungs at the end of a normal expiration). Inspiratory Capacity (IC): the sum of VT and IRV (the amount of air that a person can breathe beginning at the normal expiratory level and expanding the lungs to the maximum amount) 5 of 11  Lecture Two Measuring the Functional Residual Capacity Remember that residual volume (RV) cannot be measured by a simple spirometer and hence neither can FRC. FRC can be determined by either the helium dilution method or by body plethysmography. 1. Helium Dilution V1 at [C1] V2 at [C2] This method works as Helium is insoluble in blood. A spirometer of known volume (V1) is filled with air mixed with helium at a known concentration (C1). Before breathing from the spirometer, the subject expires normally. At the end of this expiration, the remaining volume in the lungs is equal to the functional residual capacity (FRC) (remember: FRC is the volume remaining in the lung after normal expiration) At this point, the subject immediately begins to breathe from the spirometer, and the gases of the spirometer mix with the gases of the lungs. As a result, the helium becomes diluted by the functional residual capacity gases, and the volume of the functional residual capacity can be calculated from the degree of dilution of the helium as follows: 6 of 11  Lecture Two Note that V2 is made up from the volume of the lung before (i.e. FRC) + the volume of the box, as these two volumes mix together once the valve is removed. Hence, the Helium molecules will now be distributed into a larger volume (V2). 2. Body Plethysmography A gas in a sealed container exerts a pressure created by collisions of moving gas molecules with the walls of the container and with each other. If the size or volume of the container is reduced, the collisions between gas molecules and the walls become more frequent and thereby the pressure rises. Boyle’s Law P1 x V1 = P2 x V2 Boyle’s law states that if the volume of a container of gas changes, the pressure of the gas will change in an inverse manner (assumes that temperature and the number of gas molecules remain constant). 7 of 11  Lecture Two The body plethysmograph is a large airtight box in which the subject sits. At the end of a normal expiration, a shutter closes the mouthpiece and the subject is asked to make respiratory efforts. When the patient makes an inspiratory effort against a closed airway, because the subject inhales a very small volume of air, the following happens: 1. Patient’ s lung volume increase and airway pressure decrease 2. Box volume decreases and box pressure increases Remember that in this case, there are two systems, the lungs and the box. Therefore, Boyle’s Law must be applied for each of the 2 systems: P1Box x V1Box = P2Box x V2Box P1Lungs x V1Lungs = P2Lungs x V2Lungs Step 1 We will use the ‘box system’ to calculate the change in volume. We know that Boyle’s Law states: P1Box x V1Box = P2Box x V2Box And that the volume after is equal to the (volume before - change in volume). Note: we subtract since the patient is inspiring air and hence air is moving out of the box: V2Box = V1Box - ΔV Therefore P1Box x V1Box = P2Box x (V1Box - ΔV) Rearranging the equation to make ΔV the subject: 8 of 11  Lecture Two Step 2 We will now look at the ‘lung system’ to calculate the FRC. Remember that at the start of the experiment, when the patient has expired normally, the FRC remains in the lung. Therefore, the volume before (V2) is equal to the FRC. We know that Boyle’s Law states: P3Lungs x V2Lungs = P4Lungs x V3Lungs If: We know that volume in the lungs (V3Lungs) after the inspiration is equal to the (volume before + the change in volume). In this case we are adding since the air is flowing into the lungs, and hence the volume in the lungs is increasing: V3 = V2 + ΔV P3 x V2 = P4 x V3 Therefore: P3 x V2 = P4 x (V2 + ΔV) Note that we will use the ΔV we calculated in step 1 in this equation to calculate the V2 (which we previously said is the FRC; volume of lung before) Rearranging to make FRC (V2) the subject of the formula: 9 of 11  Lecture Two Measuring other lung volumes & capacities Now that we measured the Functional Residual Capacity, FRC, we can easily measure the Total Lung Capacity and the Residual Volume: Total Lung Capacity, TLC: TLC = FRC + IC Residual Volume, RV: RV = FRC - ERV Obstructive vs Restrictive Diseases The FEV1/ FVC ratio gives us the percentage fraction of the vital capacity that can be expired in the first second. This can be used to distinguish between diseases. Note that the FEV1 depends on how ‘fast’ air can exit the lungs in the first second. If there is an obstruction in the airways (e.g. bronchoconstriction in asthma), the flow rate will be decreased and hence the forced expiratory volume, FEV1 will also be decreased. The FVC depends on how much air volume can enter the lungs, and hence in restrictive diseases, there is restriction ( ) and the lungs cannot inflate fully. Therefore, spirometry can help us differentiate between different diseases: Normality: In a normal person, FEV1/FVC% is approximately 80%; this means that in the absence of disease, 80% of the vital capacity can be forcibly expired in the first second. Obstructive disease: In a patient with an obstructive lung disease such as asthma, both FVC and FEV1 are decreased, but FEV1 is decreased more than FVC is. Thus, FEV1/FVC% is also decreased, which is typical of airway obstruction with increased resistance to expiratory airflow. Restrictive disease: In a patient with a restrictive lung disease such as fibrosis, both FVC and FEV1 are decreased, but FEV1 is decreased less than FVC is (FVC is decreased more than FEV1). Thus, in fibrosis, FEV1/FVC% is actually increased. 10 of 11  Lecture Two Jake vs. Salvu vs. Asthma Fibrosis Patient presents with wheezing, difficulty People with restrictive lung disease cannot breathing and chest thightness. fully fill their lungs with air. Their lungs are restricted from fully expanding. Asthma is characterized by a chronic inflammation of the pulmonary airways. The Restrictive lung disease most often results bronchiolar smooth muscle becomes from a condition causing stiffness in the lungs hyperresponsive to allergens, irritants, or other themselves. In other cases, stiffness of the agents. Exposure to these substances triggers a chest wall, weak muscles, or damaged nerves strong bronchoconstriction. The symptoms of may cause the restriction in lung expansion. an asth- matic attack result from the bronchoconstriction and are characteristic of an In Salvu’s case, the pulmonary fibrosis was “obstructive” pulmonary disease. The secondary to his hobby of breeding pigeons. bronchoconstriction, inflammation, and excess mucus all act to obstruct the lumen of the bronchioles. The increased resistance to airflow in the bronchi- oles accounts for the wheezing, abnormal spirometry volumes, and shortness of breath. The airway narrows secondary to: - Smooth muscle contraction - Thickening of the airway by inflammation - Presence of secretions in lumen 11 of 11