1. Mechanics of Ventilation
Prof. K. Sivapalan

2. Mechanics of Ventilation
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
Mechanics of Ventilation

3. Mechanics of Ventilation
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
Mechanics of Ventilation

4. Mechanics of Ventilation
Respiratory Muscles
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
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Mechanics of Ventilation

5. Recording Chest Movements
Stethograph is used to record chest movements.
2013
Mechanics of Ventilation

6. 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

7. 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 out
2013
Mechanics of Ventilation

8. 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

9. Mechanics of Ventilation
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 chest
2013
Mechanics of Ventilation

10. Mechanics of Ventilation
Lung Volumes
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.
2013
Mechanics of Ventilation

11. Mechanics of Ventilation
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 L/m
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Mechanics of Ventilation

12. Mechanics of Ventilation
Compliance
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.
2013
Mechanics of Ventilation

13. Mechanics of Ventilation
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.
2013
Mechanics of Ventilation

14. Mechanics of Ventilation
Dead Space
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
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Mechanics of Ventilation

15. Mechanics of Ventilation
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+Gray
2013
Mechanics of Ventilation

16. Mechanics of 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 = [ ]x12 = 4200 ml/min
Effects of increasing and decreasing dead space
Ventilation to different alveoli depends on gravity and expansion
2013
Mechanics of Ventilation

17. Mechanics of Ventilation
Perfusion
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
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Mechanics of Ventilation

18. 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.
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Mechanics of Ventilation

19. Ventilation Perfusion Ratio
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.
2013
Mechanics of Ventilation