l U N G COMPLIANCE ? Physiology Unit

1.1.Lung Compliance: (stretchabitity). Compliance refers to the distensibility of the lung. Def: “ The change in volume of the lung produced by a change in pressure across the lung during inflation or deflation” (Transpulmonary pressure: The difference between the pressure of the respired gas at the mouth and the pleural pressure around the lung.)

1.2.Lung Compliance : (Stretchability) *It is important to understand that the lung will not increase in size if the pressure within it and around it are increased equally at the same time.

Explain FEV1/FVC as an index of airways resistance; Lung Compliance : (Stretchability) Learning Objectives: Explain the term lung compliance, thoracic compliance respiratory system compliance and normal compliance Explain lung compliance and discuss the factors affecting the compliance Define air way resistance and explain the mechanical and physiological factors that determine airway resistance. Explain the function of the liquid and the lining of alveoli and the role of lung surfactant in preventing collapse of the alveoli. Explain FEV1/FVC as an index of airways resistance; Discuss the clinical utility of PEFR and MMEFR as indices of airways resistance.

Lung Compliance : (Stretchability) i.e. “ volume change per unit of pressure change” expressed in (mL/cmH2O) Normal value of compliance of intact lung  = 200mLs/cmH2O. *Lower compliance = more effort of breathing needed.

Lung Compliance : (Stretchability)

Lung Compliance : (Stretchability)

Emphysema? Emphysema is a disease of the lung tissue caused by destruction of structures feeding the alveoli. Smoking is one major cause of this destruction, which results in the collapse of small airways in the lungs during forced exhalation

Pulmonary compliance is calculated using the above equation, where, ΔV is the change in the lung volume, and ΔP is the change in pleural pressure.

For example : if a patient inhales 500 mL of air from a spirometer with an intrapleural pressure before inspiration of – 5 cm H2O and -10 cm H2O at the end of inspiration. Then his lung compliance is calculated as follows:

Compliance can also change in various disease states. For example, in fibrosis the lungs become stiff, making a large pressure necessary to maintain a moderate volume. Such lungs would be considered poorly compliant. In emphysema, where many alveolar walls are lost, the lungs become so loose and floppy that only a small pressure difference is necessary to maintain a large volume. Thus, the lungs in emphysema would be considered highly compliant.

1. Lung elastic recoil 2. Lung volume 3. Disease 2.Factors affecting compliance: 1. Lung elastic recoil 2. Lung volume 3. Disease

2.Factors affecting compliance: Elastic recoil of the lung: Due to: Surface tension in the alveoli Stretched elastic fibres in the lung parenchyma Surface tension accounts for 70% of the elastic recoil

2.Factors affecting compliance: Recoil pressure of the lung (transpulmonary)  = alveolar pressure - intrapleural pressure Recoil pressure of the chest = intrapleural pressure - atmospheric pressure

2.Factors affecting compliance: 2.2. Lung volume At high lung volume : --> Elastic fibres already stretched  --> Greater pressure is required to inflate lung  --> Reduced compliance At very low volumes:  --> Alveoli radius reduced  --> pressure required to inflate alveoli is increased  --> Reduced compliance

Conditions that REDUCE compliance: 1.Fibrosis 2.Pulmonary hypertension/congestion  --> Increases stiffness of lung Alveolar atelectasis  * e.g. after prolonged period of ventilation 3.Reduced surfactant (prematurity, artificial ventilation.)   Conditions that INCREASE compliance: Normal ageing (alteration in elastic tissue) Asthma & emphysema.

3.Airways Resistance: R = ∆P/ V R = resistance, ∆P= the pressure difference V= flow.

Airways Resistance: Resistance to air flow into and out of the lungs. Molecules of air require energy in order for them to move forward. To begin flow, an opposing energy, resistance, must be overcome.

The total resistance to air flow in and out of the lungs has the following components: Inertia of the respiratory system, 2) Tissue resistance of the lungs and chest wall 3) Airways resistance. Tissue resistance encountered as the lung expands contributes to about 20% and airways resistance to about 80% of total resistance to flow of air in and out of the lungs.

4.Pulmonary surfactant It is a chemical secreted by the Type II alviolar calls: Eg. 1.Dipalmitoylnlecithin 2.Phosphatidyl glycine.

Surfactant Function? Reduces the surface tension. Prevents collapse of alvioli

5. FVC : (Forced vital capacity) The determination of the vital capacity from a maximally forced expiratory effort

FEV1 FEV1 “Volume that has been exhaled at the end of the first second of forced expiration”.

6. PEFR &MMEFR  PEFR  - Peak expiratory flow rate & MMEFR  - Mid-maximal expiratory flow rate

Peak expiratory flow rate (PEFR) is a person's maximum speed of expiration, as measured with a peak flow meter, a small, hand-held device used to monitor a person's ability to breathe out air. It measures the airflow through the bronchi and thus the degree of obstruction in the airways.

Mid-maximal mid expiratory flow rate (MMEFR): Maximum mid expiratory flow is the average rate of airflow measured between exhaled volumes of 25 and 75 per cent of the vital capacity during a forced exhalation. Maximal mid expiratory flow : (FEF25–75) The maximum rate of airflow measured between expired volumes of 25 and 75 per cent of the vital capacity during a forced expiration; It is represented graphically as the slope of the line connecting the points on the forced expiratory volume curve at 25 and 75 per cent of the forced vital capacity. 

Static Compliance (Cstat) Static compliance represents pulmonary compliance during periods without gas flow, such as during an inspiratory pause. It can be calculated with the formula: Where, VT = tidal volume, Pplat = plateau pressure, PEEP = Post end expiratory Pressure.)

Dynamic Compliance (Cdyn) Dynamic compliance represents pulmonary complaince during periods of gas flow, such as during active inspiration. Dynamic compliance is always less than or equal to static lung compliance. It can be calculated using the following equation, where Cdyn = Dynamic compliance; VT = tidal volume; PIP = Peak inspiratory pressure; PEEP = Positive End Expiratory Pressure:

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