Lecture 8 Factors affecting Pulmonary Ventilation By Dr. Khaled Khalil

By the end of this session, the student should be able to: o Outline the various factors affecting airway resistance and correlate it to changes in pulmonary ventilation. o Describe the metabolism of surfactant, discuss its significance and relate its deficiency to clinical conditions. o Define compliance of the lung and chest wall, illustrate and discuss the compliance curve and describe the effect of surfactant on it. o Discuss work of breathing and relate it to clinical conditions. o Outline the various factors affecting airway resistance and correlate it to changes in pulmonary ventilation. o Describe the metabolism of surfactant, discuss its significance and relate its deficiency to clinical conditions. o Define compliance of the lung and chest wall, illustrate and discuss the compliance curve and describe the effect of surfactant on it. o Discuss work of breathing and relate it to clinical conditions.

SurfactantSurfactant - When the water forms a surface with air, the water molecules on the surface (in contact of air) are strongly attracted to each other trying to reduce the surface area of contact. This is called "surface tension". - In the lungs, a thin water film lines the alveoli. Since, it is in contact with air, it tends to contract to force air out of alveoli and cause the lungs to collapse. Fortunately, this does not occur due to presence of surfactant.

- Surfactant is a surface active agent that decreases surface tension of the fluid lining the alveoli. - Surfactant is a complex mixture of phospholipids, proteins and ions.  The most important phospholipid is dipalmitoyl phosphatidyl-choline (DPPC) which is surface active agent.  Surfactant proteins as: SP-A, SP-B and SP-C.  Calcium ions.

Functions of surfactant: 1.It decreases the surface tension of the fluid lining the alveoli through: a)It scatters among the fluid molecule decreasing the attraction between them. b)It also spreads over the fluid preventing air-fluid interface. * This decreased surface tension:  Helps lung expansion during inspiration.  Protects against pulmonary edema as it decreases the filtration forces for the fluid from pulmonary capillaries into alveoli.  Prevents lung collapse during expiration.  Decreases the work of breathing needed.

2.Surfactant activates alveolar macrophages. 3.It has a bacteriolytic effect, making the bacteria susceptible to alveolar macrophages. 4.The phospholipids of surfactant may act directly on T- lymphocytes cell membrane to prevent excess immune response. 5.It assists cilliary movement in upper respiratory tract (so, helps to remove any particles and mucus away from air passage).

- Surfactant decreases in the following conditions: 1. Respiratory distress syndrome (RDS): - In fetal life, surfactant is not produced in sufficient amounts until about the eighth month. Thus, premature babies lacking sufficient surfactant are born with collapse alveoli. - Because of the high surface tension, plasma fluid leaks into the alveoli producing pulmonary edema. Due to “glistening membrane” appearance, the condition is also called "hyaline membrane disease".

- RDS babies can be saved by mechanical ventilators that keep them alive until their lung can manufacture sufficient surfactant. Otherwise, bovine or synthetic surfactant can be administrated. elective caesarean section - When premature delivery (before 8 months) is life-saving for the mother, elective caesarean section may result in baby with collapsed lungs. It is better in this condition to inject the mother with cortisol hormone which is essential for surfactant formation.

2. Cigarette smoking decreases surfactant. It leads to destruction of type II alveolar cells. 3. After cardiac surgery during which the pulmonary circulation is interrupted, alveolar epithelium fails to secrete surfactant. 4. Long-term inhalation of 100% O 2.

Pulmonary Compliance The compliance is the ratio between the change in volume to the change in distending pressure (transmural pressure). i.e., C = ∆V / ∆P.

-Compliance of the lung can be defined as the extent to which the lung expands for each unit increase in its transmural pressure. -For the lungs, transmural pressure = intrapulmonary pressure – intrapleural pressure. It is called "transpulmonary pressure". -The normal compliance of the lungs in the average adult human is approximately 200 ml/cm of water pressure. That is every time, the transpulmonary pressure increase by one centimeter of water, the lungs expand 200 milliliters.

Dynamic Lung Compliance Curve

- From the curve, one can conclude:  The compliance of the lung is more during expiration (BZA) than during inspiration (AXB). This is known as "hysteresis" (i.e., forward path is different from the reverse path). Why????????  The compliance of the lung is determined by the diagonal line (BYA) connecting the two ends of the curve (average lung compliance)

Factors affecting compliance: 1. Factors in the lung: - It is affected by elastic forces of the lung (1/3 of compliance) and surface tension of the fluid lining the alveoli (2/3 of the compliance). - Lung compliance is decreased in pulmonary congestion, pulmonary fibrosis and pulmonary edema whereas it is increased in emphysema. - It is affected by the elastic properties of the thorax caused by elasticity of the muscles, tendons and connective tissues in the chest wall. -Chest wall compliance is decreased in case of: * Deformities of spine. *Arthritis of vertebra. *Skeletal muscle disease e.g. poliomyelitis. * Obesity. 2. Factors in the chest wall:

Work of breathing - During normal quiet respiration, work is done only during inspiration (during which respiratory muscles contract), whereas expiration is entirely a passive process. - At rest, work of breathing is about 1-2% of the total body energy expenditure. Even during heavy exercise, work is only about 3- 5% of the total energy expenditure of the body.

Work of breathing can be divided into: 1- Compliance work (elastic work) 2- Tissue resistance work 3- Airway resistance work 65% of total work7% of total work28% of total work - It is the work required to expand the lung against its elastic force. - This can be calculated as follow: Compliance work = volume x pressure needed to inflate this volume - It is the work required to overcome the resistance of non-elastic tissue of the lungs and thoracic cage. - It is the work required to overcome airway resistance during the movement of air into the lung. - The major factor, affecting airway resistance is the diameter of the passage ways. The larger the diameter of the airway, the less work done.

Conditions associated with increased work of breathing: 1- Increased compliance work: -Decreased elasticity (fibrosis). -Lung congestion (heart failure). 2- Increased tissue resistance work: - Diseases affecting thoracic cage e.g. kyphoscoliosis. - Muscle disease. 3- Increased airway resistance work: -Obstruction of air way. -Chronic bronchitis. -Asthma.

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