Respiratory mechanics and MV

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Inspiratory Movements

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Thoracic Cavity• Diaphragm:• Sheets of striated muscle divides anterior body cavity into 2

parts.

• Above diaphragm: thoracic cavity:• Contains heart, large blood vessels, trachea, esophagus,

thymus, and lungs.

• Below diaphragm: abdomino-pelvic cavity• -important contributor to respiratory pathology and insufficiency!

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Mechanics of breathing• Gas: the more volume, the less pressure (Boyle’s law) • Inspiration:

• lung volume increases ->

• decrease in intrapulmonary pressure, to just below atmospheric pressure ->

• air goes in!• Expiration: vice versa (normally passive)

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Mechanics of breathing

• Intrapleural space:

• “Space” between visceral & parietal pleurae. • Visceral and parietal pleurae (membranes) are flush against each

other.• Lungs normally remain in contact with the chest wall. • Lungs expand and contract along with the thoracic cavity.

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Mechanics of breathing

• Compliance: • This the ability of the lungs to stretch during inspiration • lungs can stretch when under tension.

• Elasticity: • It is the ability of the lungs to recoil to their original collapsed

shape during expiration• Elastin in the lungs helps recoil

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Inspiration

• Inspiration – Active process• Diaphragm contracts -> increased thoracic volume vertically.• Intercostals contract, expanding rib cage -> increased thoracic

volume laterally.• More volume -> lowered pressure -> air in.• Negative pressure breathing

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Expiration

• Expiration – Passive

• Due to recoil of elastic lungs.• Less volume -> pressure within alveoli is just above

atmospheric pressure -> air leaves lungs.• Note: Residual volume of air is always left behind,

so alveoli do not collapse.

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Mechanics of breathing

• During Quiet breath:• +/- 3 mmHg intrapulmonary pressure.

• During Forced breath:

• Extra muscles, including abdominals• +/- 20-30 mm Hg intrapulmonary pressure

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Dynamics of Respiration

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Elastic properties of the lung• Pressure-volume curve• Hysteresis• Airway closure• Compliance• Surface tension• Regional differences in

ventilation• Elastic properties of chest wall• Airway resistance• Causes of uneven ventilation• Tissue resistance• WOB

Sequence of events (SV)

Pressure-volume curve• Non-linear, with lung stiffer at higher volumes• Hysteresis between inflation and deflation• Compliance is the slope ΔV/ΔP• Behaviour depends on both structural proteins (collagen,

elastin) and surface tension

Compliance• Volume change per unit pressure change• 200 mL/cm H2O• Compliance decreased at higher expanding pressures and at

lower lung volumes, with increased fibrous tissue and with alveolar oedema

• Compliance increased with emphysema

Surface tension• Inherent tendency of a fluid-filled chamber to attain the

smallest surface area possible (Pressure = 4T/R)• Pulmonary surfactant contributes greatly to lowering surface

tension in the alveolus• Increases compliance• Increases stability of laveolus• Keeps alevoli dry

• Loss of surfactanr decreases complinace, promotes atelectasis and a tendency to pulm oedema

Regional differences in ventilation• Lung weight reduces intrapleural pressures at the base• Bases more compressed than apices• Therefore, for same distending pressure (change in

intrapleural pressure), bases expand more than apices!!!• Basal portions are more compliant than upper portions

Airway closure• Lungs (esp. basal regions) never empty completely• Closure of small airways with increasing intrapleural pressure• Loss of elastin• Closing volume increases with age and with chronic lung

disease

Elastic properties of chest wall

Mechanical ventilation-physiologic interactions• Effects on respiratory drive• Effects on airway• Effects on oxygenation and ventilation• Effects on lung volumes and pulmonary pressures• Haemodynamic effects (Heart-lung interactions)• Effects on GI and renal systems• Fluid retention

Effects on respiratory drive• Improved oxygenation and CO2 elimination can depress

respiratory drive• Tendency to over-ventilate and cause respiratory alkalosis :

depresses drive

Effects on airway• Positive pressure

splints airway open• Treat or prevent

airway collapse

Oxygenation, ventilation and WOB• PEEP or CPAP increases airway and alveolar pressures• Prevents collapse at end of expiration• Thus improves V/Q matching• PSV lends inspiratory support and helps with CO2 elimination• Decreases the WOB

Lung volumes and pressures• FRC and alveolar volumes increased• Contribute to increased dead space if alv over-distended• Raised Pulmonary vascular pressures (alveolar vessels) and

resistance• Increases R heart load and decreases cardiac output