Mechanics of Ventilation

Prof. K. Sivapalan

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Introduction

• Gas exchange occurs in alveoli• Atmospheric air must go to alveoli

and expelled after exchange.• Negative pressure is created in the

lungs for sucking air by the muscles.• The lung recoils like a balloon and

expels the air in it- elastic fibers.

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Pleural Cavity.• The lungs are separated

from the chest wall and the diaphragm by the pleural cavity containing a thin film of fluid.

• The surface tension allows sliding but keeps both surfaces attached to each other.

• Expansion- chest muscles out words and the diaphragm downwards.

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• Diaphragm – contraction pulls the lung downwards.

• Increase diameter of the chest cavity-– Elastic fibers in chest wall– External intercostals– Sternocleinomastoid– Anterior serrati– Scalini

• Decreasing the chest cavity- – Elastic recoil of lungs– Internal intercostals– Abdominal muscles

Respiratory Muscles

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Changes in Pressure and Volume in Quiet Inspiration

• Diaphragm and external intercostals contract.

• Elasticity of the wall facilitates but the elasticity of lungs and surface tension in alveoli opposes

• Pleural pressure decreases to -7.5 cm H2O

• Intrapulmonary pressure falls to -1

• Air flows in.2013

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Changes in Pressure and Volume in Quiet Expiration

• Diaphrgm and external intercostals relax.

• Pleural pressure increases to -5 cm H2O – not to zero.

• Elasticity of the wall is over powered by the elasticity of lungs and surface tension in alveoli.

• Intrapulmonary pressure rises to +1

• Air flows out2013

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Surface Tension in Alveoli

• The surface tension in alveoli tends to collapse the alveoli.

• Surfactant secreted by type II alveolar epithelial cell reduces the surface tension from 72 dynes/cm to 5-30.

• Surfactant is a mixture of protein, phospholipids and ions.

• Respiratory distress syndrome- failure of alveoli to open in premature babies – secretion of surfactant after 7 months or later.

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Pleural pressure• The pleural cavity is under negative pressure

during quite breathing.• Forced inspiration creates more negative

pressure.• Positive pressure is observed in forced

expiration, cough and sneeze.• Any communication with atmosphere either

through lung or chest wall will suck air and results in Pneumothorax- lung collapses and chest wall expands.

• Closed, open and tension pneumathorax.• Loss of elasticity of lungs- barrel shaped chest2013

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• Lung volumes are measured by a spirometer.• Tidal volume- inspired or expired with normal breath-

500 ml in adult male• Inspiratory Reserve Volume- extra volume inspired by

maximal inspiration. [3 L]• Inspiratory Capacity = TV+IRV• Expiratory Reserve volume- extra volume expired by

maximal expiration.[1.1 L]• Residual Volume- Volume remaining after maximal

expiration.[1.2 L]• Functional Residual Capacity= RV+ERV• Vital Capacity=ERV+TV+IRV- maximal expiration after

maximal inspiration.

Lung Volumes

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Forced Vital Capacity

• FVC is the Vital capacity obtained by forced expiration after maximal inspiration.

• FEV1 is the fraction of the FVC expelled in the FIRST SECOND.[>80%]. Also peak flow.

• MVV- maximal voluntary ventilation- 125-170 L/m

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• Dead space is the non functional – not participating in gas exchange- space in the respiratory tract

• Anatomical- nose, pharynx, trachea, ..up to terminal bronchiole.

• Equals in ml to approximately the weight in pounds [150 ml]

• Physiological- includes nonfunctional alveoli as well

Dead Space

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Ventilation

• Minute ventilation = – average TV x average RR– 500x12= 6000 ml/minute

• Alveolar Ventilation- volume of air moved into alveoli per unit time– [TV-DV]x RR = [500-150]x12 = 4200 ml/min– Effects of increasing and decreasing dead

space

• Ventilation to different alveoli depends on gravity and expansion

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• Volume of blood flowing through the lungs per minute [5 L /min at rest]

• Perfusion of different area within the lung is influenced by gravity.

• When erect, more blood flows to the base.

• Lying causes venous congestion

Perfusion

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Features of Pulmonary Circulation

• Pulmonary artery ressure- 25/8 mmHg.

• Arterioles constrict in reduced oxygen and increased carbondioxide.

• Pulmonary capillary pressure is 7 mmHg.

• Pulmonary oedema is accumulation of fluid in alveoli.

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• Ratio between [alveolar] ventilation and perfusion- V/P = 4.2/5.5 =0.8

• The V/P of local areas in the lung vary due to gravity and disease.

• If not compensated adequately- defective gas exchange.

Ventilation Perfusion Ratio

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