Pressures in Ventilation

Pressures Important in Ventilation

• Air flows from an area of high pressure to an area of low pressure due to a pressure gradient
• Atmospheric pressure is the pressure of the atmosphere on the earth's surface, with a value of 760 mmHg at sea level
• Atmospheric pressure decreases with increasing altitude above sea level and increases below sea level
• It is used as a reference point (zero) when relating other pressures to it

Intrapulmonary Pressure (Intra-Alveolar Pressure)

• Intrapulmonary pressure is the pressure within the alveoli
• It changes during inspiration (-1 or 759 mmHg) and expiration (+1 or 761 mmHg)
• It is equal to atmospheric pressure (0 or 760 mmHg) at the end of both inspiration and expiration
• Because alveoli communicate with the atmosphere through conducting airways, air quickly flows down its pressure gradient until the two pressures equilibrate (become equal)

Intrapleural Pressure

• Intrapleural pressure is the pressure within the pleural sac
• It is the pressure exerted outside the lungs within the thoracic cavity
• It is not connected to atmospheric or alveolar pressure; thus, it does not equilibrate with them
• It is sub-atmospheric at rest, with a value of -4 mmHg (756 mmHg)
• It can be measured indirectly by an intra-oesophygeal balloon connected to a manometer
• Causes of negativity of intrapleural pressure include:
  • Normal lung at rest
  • Elastic recoil of the chest wall and lung creates an inward pull
  • The chest wall tries to pull outward, while the lung tries to pull inward, resulting in a negative pressure