Mechanics of Ventilation

Mechanics of Ventilation

1. VENTILATION

   • Caused by pressure-Volume relationship
   • Boyle's Law
     • P~1/V
     • Pressure of gas in inversely proportional to volume causing inspiration
     • Decreased in lung volume raises intrapulmonary pressure causing expiration
   • Air moved from higher pressure to low pressure

2. INTRAPULMONARY & INTRAPLEURAL PRESSURE

   • At Rest
     • Atmospheric pressure
       • 760 mmHg
     • Alveolar Pressure
       • 760mmHg
   • Inspiration
     • Atmospheric Pressure
       • 760
     • Alveolar Pressure
       • 757
     • Atmospheric pressure is greater than alveolar pressure therefore air will go IN
   • Expiration
     • Atmospheric pressure
       • 760
     • Alveolar Pressure
       • 763
     • Atmospheric pressure is less than alveolar pressure therefore air will go OUT

3. ALVEOLAR PRESSURE DROPS

   • Diaphragm contracts
     • Which Increases VOLUME
       • Decreases Pressure
         • Air comes

4. PNEUMOTHORAX

   • Collapsed Lung
     • Airs enters into the intrapleural space through comm. from chest wall
       • Occurs because alveolar pressure is normally greater than the pleural cavity pressure
         • As air escapes from the lung, it recoils and will begin to collapse
         • Because air comes inside pressure increases

5. PROPERTIES OF LUNGS

   • Ventilation of the lung requires:
     • Compliance
       • Ability to strecth under pressure (inspiration)
         • Lung volume: transmural pressure V/P
           • Reduced by factors that cause resistance to distension
       • High Compliance
         • Stretches Easily
           • Facillitates lung expansion
       • Low Compliance
         • Requires more force
           • Restrictive lung disease
             • Fibrotic lung disease
               • Inadequate surfactant production
     • Easticity
       • Returning to initial size after stretch (expiration)
         • Elastic recoil after inspiration
           • Tension increases during inspiration & is reduced recoil during expiration
     • Surface Tension
       • Promotes alveolar collapse & resist distension
         • Lung secrete & absorb ions and water
       • Absorption
         • Osmosis driven by Na+ active transport INWARD across alveolar endothelium
       • Secretion
         • Osmosis driven by active transport of Cl- OUTWARD
       • Surface Tension
         • H2O molecules are attached to other H20 molecules with increase alveolar pressure

6. SURFACTANT

   • Production begins in fetal life
     • Phospholipid DPPC proteins
       • Type II alveolar cells of lung into single alveolus
         • More concentrated in small alveolus
         • Increases pulmonary Compliance
           • Without surfactant, small alveoli collapse into larger alveoli (reducing compliance
   • Respiratory distress syndrome
     • <34 WEEKS
       • Have trouble inflating lungs
   • Acute Respiratory Distress Syndrome
     • Sepsis bacterial infection
     • Decreased compliance & surface secretion
       • impaired gas exchange
         • Death due to muti-organ failure
   • is also impaired with Emphysema & Pulmonary fibrosis