Learning Objectives Evaluate the factors involved in lung compliance. Key Points A low lung compliance would mean that the lungs would need a greater-than-average change in intrapleural pressure to change the volume of the lungs.
A high lung compliance would indicate that little pressure difference in intrapleural pressure is needed to change the volume of the lungs. Persons with low lung compliance due to obstructive lung diseases tend to take rapid shallow breaths and sit hunched over to make exhalation less difficult.
Persons with high lung compliance due to restrictive lung diseases tend to have difficulty expanding and deflating the lungs. Two factors determine lung compliance: elasticity of the lung tissue and surface tensions at air water interfaces. Two factors determine lung compliance — elasticity of the lung tissue and surface tensions at air water interfaces. The ability of the lungs to expand is expressed using a measure known as the lung compliance.
Lung compliance is the volume change that could be achieved in the lungs per unit pressure change. Elastance, also known as the elastic resistance is the reciprocal of compliance, i. This is a measure of the resistance of a system to expand. Elastance is a measure of the work that has to be exerted by the muscles of inspiration to expand the lungs.
An increased elastance needs to be counteracted by an increased power of the muscles of inspiration, leading to an increased work of breathing work of breathing is the physical work that have to be carried out by the muscles of respiration to overcome the elastic resistance of the respiratory system and the non-elastic resistance of the airways. The elastance of the whole respiratory system depends on the elastance of the chest wall and that of the lungs. Since the chest wall and the lungs have a serial relationship, in forming the respiratory system, the elastance of the whole respiratory system can be calculated by the addition of the elastance of the chest wall and the lungs.
Since the elastance in each of the lungs and the chest wall is approximately 5 cmH 2 O, the elastance of the respiratory system is approximately 10 cmH 2 O.
Changes in the elastance and therefore the compliance of the chest wall are uncommon. In contrast, the elastance of the lungs is affected by many respiratory diseases. Thus, variations in the elastance of the respiratory system are mainly due to alterations of the elastance of the lungs, which is governed by two main factors:. In the lung, flow can change from purely laminar with little friction, to fully turbulent with instabilities, large frictional forces and high pressure dissipation.
In ETT and in upper airways flow is mostly turbulent, whereas laminar flow predominates in the more distant and small airways. Resistance is a dynamic force, acting only when flow is present. The effects of airway resistance can be visualized on dynamic flow—volume curves during inspiration and expiration. Analysis of this curve may be useful for identifying different clinical situations Fig.
The elastance of the respiratory system E RS reflects the capacity of the pulmonary system to return to its resting position and measure the recoil pressure over a given volume. The latter can be present during short expiratory times e. It can be measured with an occlusion manoeuvre at the end of the expiratory phase under controlled MV Fig.
C RS and the equation of motion should be written as:. C RS can vary with lung volume. It is analysed over the whole lung volume by plotting static pressure values over a large volume ranges, e. The slope of this curve represents the C RS and is not constant over lung volume. The patient must be passive for accurate results, though paralysis is not always mandatory [ 13 ].
The latter depends primarily on the aerated lung volume available to ventilation. In this phase, a larger change in P AW is necessary to produce a small change in volume. It indicates a region where the work of breathing is lower. In reality, alveolar recruitment occurs all over the curve [ 15 ]. In several situations e. If E CW is high compared with E L , then a large part of the elastic pressure will be used to distend the chest wall and much less than the total P AW to expand and distend the lung.
The pleural pressure P PL has to be estimated to calculate C CW , and in critical practice this is done using oesophageal catheters with a balloon that measure oesophageal pressure P ES Figs P ES is a very useful substitute in determining P PL , although there are some controversies about its absolute value. The C CW can be calculated with the following equation:. It represents the force really distending the lung parenchyma. Elastance or compliance of the lung can then be easily obtained as:.
The normal C STAT RS in a patient depends first on the size of the lung and, therefore, of the height of the patient and age for children. Because compliance has a direct relationship with the lung volume available for ventilation, loss of lung volume e. FRC, representing the amount of lung aerated at end expiration, is much smaller in ARDS patients than in patients with normal lungs [ 19 ].
In other words, C and E changes do reflect the amount of aerated lung volume available, which indirectly reflects the severity of the ARDS process in the whole lung. It is an important indication of the volume that can be safely delivered to the remaining lung and the risks of ventilation in terms of VALI. The specific elastance is the ratio of elastance to FRC. The concept of specific lung elastance reflects the ratio between the transpulmonary pressure stress and the change in lung volume relative to its resting volume during respiration strain.
Transitional flow occurs in places that branch within smaller airways, in which the air flow becomes in between laminar and turbulent flow and has moderate resistance. Laminar and Turbulent Flow : Laminar flow a has orderly layers and low resistance. Turbulent flow b has disorganized layers and high resistance. Privacy Policy. Skip to main content.
Respiratory System. Search for:. Factors Affecting Pulmonary Ventilation. Factors Affecting Pulmonary Ventilation: Surface Tension of Alveolar Fluid The surface tension of alveolar fluid is regulated by pulmonary surfactant, allowing efficient respiration. Learning Objectives Describe surfactant and its role in alveolar fluid surface tension. Key Takeaways Key Points Type II avleolar epithelial cells secrete pulmonary surfactant to lower the surface tension of water, which helps prevent airway collapse.
Reinflation of the alveoli following exhalation is made easier by pulmonary surfactant. The surfactant reduces surface tension within all alveoli through hydrophilic and hydrophobic forces. Insufficient pulmonary surfactant in the alveoli can contribute to atelectasis collapse of part or all of the lung. Key Terms atelectasis : The collapse of a part of or the whole lung caused by inner factors, rather than a pneumothorax. Surface tension : The inward force created by films of molecules that can reduce the area of a surface.
Examples Elective cesarean sections are becoming more common. Factors Affecting Pulmonary Ventilation: Compliance of the Lungs Lung compliance refers to the magnitude of change in lung volume as a result of the change in pulmonary pressure.
Learning Objectives Evaluate the factors involved in lung compliance. Key Takeaways Key Points A low lung compliance would mean that the lungs would need a greater-than-average change in intrapleural pressure to change the volume of the lungs.
A high lung compliance would indicate that little pressure difference in intrapleural pressure is needed to change the volume of the lungs. Persons with low lung compliance due to obstructive lung diseases tend to take rapid shallow breaths and sit hunched over to make exhalation less difficult.
Persons with high lung compliance due to restrictive lung diseases tend to have difficulty expanding and deflating the lungs. Two factors determine lung compliance: elasticity of the lung tissue and surface tensions at air water interfaces. Two factors determine lung compliance — elasticity of the lung tissue and surface tensions at air water interfaces. Key Terms Lung compliance : The ability of the lungs and pleural cavity to change in volume based on changes in pressure.
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