Mechanics of Ventilation Prof. K. Sivapalan. Introduction 20132Mechanics of Ventilation.
-
Upload
erik-curtis -
Category
Documents
-
view
219 -
download
1
Transcript of Mechanics of Ventilation Prof. K. Sivapalan. Introduction 20132Mechanics of Ventilation.
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.
2013
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.
2013 3Mechanics of Ventilation
Mechanics of Ventilation 4
• 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
2013
Mechanics of Ventilation 5
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
Mechanics of Ventilation 6
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
Mechanics of Ventilation 7
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.
2013
Mechanics of Ventilation 8
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
Mechanics of Ventilation 9
• 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
2013
Mechanics of Ventilation 10
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
2013
Mechanics of Ventilation 11
• 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
2013
Mechanics of Ventilation 12
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
2013
Mechanics of Ventilation 13
• 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
2013
Mechanics of Ventilation 14
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.
2013
Mechanics of Ventilation 15
• 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
2013