Mechanics of Ventilation

Prof. K. Sivapalan

Mechanics of Ventilation 2

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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Recording Chest Movements

• Stethograph is used to record chest movements.

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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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• Compliance is the change in lung volume per unit change in airway pressure [∆V/ ∆P]- a measure of stretchability.

• The stretchability varies with volume as shown in the curve.

• Pressure is zero [equal to atmospheric pressure] at the end of quite expiration.

• The compliance is higher when measured by deflation than by inflation. [measured with saline shows the contribution of surface tension.

• Compliance is reduced in pulmonary congestion and fibrosis and increased in emphysema.

Compliance

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Work of Breathing

Work is performed by respiratory muscles for:

• Stretching the elastic tissues of the lungs and chest wall [65%]

• Moving other [in-elastic] tissues [7%]• Against air way resistance [28%]• Any of the above change- the work

changes.

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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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Measuring Dead Space

• CO2 Method-• TV x FeCO2 = AVxFaCO2

• AV= TV x FeCO2 / FaCO2

• DV= TV-AV• Nitrogen Method-• Pure oxygen inspired.• Monitor nitrogen in

expired air.• VD= GrayxVE /

Pink+Gray2013

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Ventilation

• Pulmonary ventilation- volume of air moved into the respiratory passages per unit time- measured by Douglass bag or Maxplang respirometer.

• 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.• When erect, the difference of blood pressure

between the apex and the base would be about 30 cm. blood [23 mmHG]

• Blood can flow through the capillaries only when the intra vascular pressure is higher than intra alveolar pressure.

• Arterioles constrict in reduced oxygen and increased carbondioxide.

• Pulmonary capillary pressure is 7 mmHg.• Pulmonary oedema is accumulation of fluid in

alveoli.2013

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

• The relative size of alveoli increase towards apex but the blood flow increases toward the base.

• 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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