Lung Volumes-Notes PDF

WSUSOM Medical Physiology – Respiratory Physiology Page | 1 of 6 Lung Volumes LUNG VOLUMES Learning Objectives 1. Define the following terms: anatomic dead space, physiologic dead space, minute ventilation an...

WSUSOM Medical Physiology – Respiratory Physiology Page | 1 of 6 Lung Volumes LUNG VOLUMES Learning Objectives 1. Define the following terms: anatomic dead space, physiologic dead space, minute ventilation and alveolar minute ventilation. 2. Draw a normal spirogram, labeling the four lung volumes and four capacities. List the volumes that comprise each of the four capacities. Identify which volume and capacities cannot be measured by spirometry. 3. Define the factors that determine total lung capacity, functional residual capacity, and residual volume. Describe the mechanisms responsible for the changes in those volumes that occur in patients with emphysema and pulmonary fibrosis. 4. Draw a spirogram resulting from a maximal expiratory effort. Label the forced vital capacity (FVC) and forced expiratory volume in 1 second. 5. Differentiate between the two broad categories of restrictive and obstructive lung disease, including the spirometric abnormalities associated with each category. 6. Describe the regional differences in alveolar ventilation in healthy and diseased lungs and explain the basis for these differences. Lecture Outline I. Dynamic Lung Volumes  Movement of tidal volume, alveolar volume and anatomical deadspace volume during inspiration and expiration. II. Static Lung Volumes  Inspiratory reserve volume  Tidal volume  Expiratory reserve volume  Residual volume  Total lung capacity  Inspiratory capacity  Functional residual capacity  Vital capacity III. Changes in static lung volumes in lung disease A. Restrictive Disorders  Diseases of thoracic cage  Diseases of nerve supply to respiratory muscles  Abnormalities of pleura and pleural space  Lung pathology B. Obstructive Disorders  Bronchoconstriction  Structural changes in airway (e.g. bronchitis)  Obstruction within the airway (e.g. foreign bodies)  Vital capacity WSUSOM Medical Physiology – Respiratory Physiology Page | 2 of 6 Lung Volumes Dynamic Lung Volumes  The above diagram outlines the dynamic changes in lung volume that occur on a breath to breath basis.  The starting point to explain the diagram is the end of expiration (Panel 3). Note that on average approximately 2200 ml of air is located in the respiratory zone of the tracheo-bronchial tree and 150 ml of stale air from the alveoli are located in the conducting zone of the tracheo-bronchial tree.  Once inspiration is initiated, 500 ml (purple shading) of fresh air is inspired from the atmosphere (Panel 4). This volume of air is known as tidal volume. Tidal volume (i.e. volume associated with a given breath) is comprised of deadspace volume and alveolar volume. Tidal volume (VT) = dead space volume (VD) + alveolar volume (VA).  Dead space refers to anatomical dead space which are structures that do not contribute to gas exchange (i.e. the conducting zone) and alveoli that are not perfused or poorly perfused (i.e. alveolar deadspace), leading to wasted ventilation. The combination of anatomical and alveolar deadspace is known as physiological deadspace. In a healthy individual anatomical and physiological deadspace are similar.  350 ml of the 500 ml travels to the respiratory zone of the tracheo-bronchial. This is the alveolar volume.  Accompanying the 350 ml of fresh air is the stale air that was in the conducting zone at the end of expiration. The stale air in the conducting zone is replaced by 150 ml of fresh air.  At the end of inspiration (Panel 1) the volume of air in the respiratory zone has increased to 2700 ml and is comprised of a mix of stale and fresh air. 150 ml of fresh air is also located in the conducting zone.  At the onset of expiration (Panel 2), 500 ml of air is expired. 150 ml of the 500 ml is located in the conducting zone and is initially expired followed by 350 ml of stale air. Also note that 150 of stale air remains in the conducting zone at the end of expiration.  The volume of air inspired or expired into the lung is often reported as the volume of air inspired or expired over a minute time period. If this is the case, the number of times an individual breaths (breathing frequency) is required along with the average tidal volume. Ventilation (VE) = frequency (f) * Volume (VT) or VT * f = VD * f + VA * f WSUSOM Medical Physiology – Respiratory Physiology Page | 3 of 6 Lung Volumes Static Lung Volumes  Simple measures of static lung volume can be obtained by having a subject completely fill their lungs and then expire forcefully into a spirometer (figure on the left). The volume of air expired during such a maneuver is known as the forced vital capacity and the volume of air expired in the first second of the maneuver is the known as FEV1.0 (see spirographic record). These simple measures allow clinicians to diagnose preliminarily the existence of pulmonary disease. In addition to measures of vital capacity the spirometry tracing will also allow for the measurement of other lung volumes and capacities which are shown in the diagram.  If more sophisticated equipment is employed measures of residual volume (RV) can be obtained. Once RV is measured total lung capacity (TLC) and functional residual capacity (FRC) can be calculated. As noted below, measures of these volumes and capacities are important because they are altered in response to lung disease (e.g. emphysema and bronchitis).  A plethysmograph is required in order to measure residual volume (right hand figure). The procedure to measure residual volume is the following. A patient enters the plethysmograph and the door is shut to seal the structure. As a result, the volume in the plethysmograph can be measured (see pneumotach 2) and is known (V2). Once inside, the patient inserts a mouthpiece into their mouth and their nose is clipped. Thereafter, the patient breaths quietly. The patient is then instructed to expire out as much air as possible. If the patient follows the instructions, the volume of air left in the lungs is residual volume. At the end of the forced expiration, a shutter comes down in front of the patient’s mouth. The patient then pants against the shutter.  This maneuver allows for a measurement of mouth pressure that is equivalent to alveolar pressure (P1). As the patient pants the pressure in the box oscillates and can be measured with a transducer. Thus, the box pressure (P2) is determined. As a result, the only unknown is the volume of air in the lung. In other words, V1P1 = V2P2 or V1 = V2P2/P1. V1 is the calculated residual volume.  WSUSOM Medical Physiology – Respiratory Physiology Page | 4 of 6 Lung Volumes Static Lung Volumes and Lung Disease  The figure above (left hand side) and table on the next page shows changes in lung volume that occur in response to restrictive and obstructive lung disease.  Note in the above figure, if measures of forced vital capacity are less than normal this reduction in volume may be indicative of lung disease. However, this reduction will not allow you to determine whether the disease is restrictive or obstructive in nature.  This is also the case if FEV1.0 is examined solely to detect the presence of lung disease.  However, obstructive and restrictive lung disease can be differentiated by examining the FEV1.0/FVC ratio concurrently with FVC and FEV1.0. More specifically, in obstructive lung disease FVC and FEV1.0 are reduced. In addition, the FEV1.0/FVC is reduced because it is difficult to expire a significant portion of the FVC in the initial second because of the presence of obstructed airways.  Conversely, although the FVC and FEV1.0 are reduced in individuals with a restrictive disease they are capable of expiring a significant percentage of this volume (80 % – 90 %) within the initial second because the airway is not obstructed.  Other lung volume alterations will also aid in differentiating between an obstructive and restrictive disease. Note that total lung capacity, residual volume and functional residual capacity are all increased in individuals with chronic obstructive pulmonary disease.  These lung volumes are increased in part because of the loss of lung elasticity and because air becomes trapped in the obstructed airways. In contrast, total lung capacity, residual volume and functional residual capacity is reduced in individuals with restrictive lung disease (e.g. pulmonary fibrosis) because the lung is unable to inflate adequately. WSUSOM Medical Physiology – Respiratory Physiology Page | 5 of 6 Lung Volumes Static Lung Volumes and Lung Disease WSUSOM Medical Physiology – Respiratory Physiology Page | 6 of 6 Lung Volumes  The table above shows examples of restrictive and obstructive lung disorders.  The restrictive disorder may be mechanical in nature (disease of the thoracic cage) or the result of the inability to properly active muscles of respiration (disease of nerve supply).  Lung volume restrictions might also be the result of abnormalities associated with the pleura or pleura space and as a result of lung pathology.  Obstructive disorders are typically characterized by bronchial irritation and inflammation, destruction of alveolar walls, loss of lung elasticity, edema and mucous accumulation.