Pulmonology: Lung Pressures

Types of Lung Pressures

1. Intrapulmonary Pressure: Pressure in the lung or alveoli. Can be positive or negative.
2. Intrapleural Pressure: Intrathoracic pressure between two layers of the pleura. Always negative in normal conditions.
3. Transmural (Transpulmonary) Pressure: Pressure that inflates the lung. Difference between intrapulmonary and intrapleural pressures. Always positive.

Intrapulmonary Pressure

• Pressure inside the lung/alveoli.
• Follows Boyle's Law: Pressure and volume are inversely related.
  • Inspiration: Increase in lung volume → Decrease in pressure → Negative pressure (
    • Sucks air in.
  • Expiration: Decrease in lung volume → Increase in pressure → Positive pressure (
    • Pushes air out.
• At the end of both inspiration and expiration, pressure is atmospheric.

Intrapleural Pressure

• Always negative due to the opposing forces of the chest wall (wants to expand) and the lungs (want to recoil).
• Example: Two glass slides with a drop of water create a negative pressure that keeps them from separating.
• During inspiration: Chest wall expands, moving away from lung → Pressure becomes more negative.
• During expiration: Lung recoils, chest wall contracts → Pressure becomes less negative.
• Typical values: Negative 5 cm H2O (end expiration) to negative 7 cm H2O (end inspiration).

Transmural (Transpulmonary) Pressure

• Difference between intrapulmonary and intrapleural pressures.
• Determines lung inflation.
• Example values: 7 cm H2O (end inspiration), 5 cm H2O (end expiration).

Pathological Conditions

• Pneumothorax: Air in the pleural space neutralizes negative pressure.
  • Tension Pneumothorax: Valve mechanism allows air in but not out → Positive pressure → Mediastinal shift.
• COPD: Positive intrapleural pressure → Reduced venous return → Ankle edema.

Practical Measurement

• Intrathoracic pressure can be measured indirectly using intraesophageal pressure because they are identical.
  • Intraesophageal balloon method.

Importance of Negative Intrapleural Pressure

• Sucks air into lungs during inspiration.
• Facilitates venous return to the heart.
• Valsalva Maneuver: Can dangerously decrease venous return.

Boyle's Law in Lung Function

• Inspiration: Increased lung volume → Decreased intrapulmonary pressure → Air in.
• Expiration: Decreased lung volume → Increased intrapulmonary pressure → Air out.

Transmural Pressure Summary

• Positive pressure that inflates the lungs.
• Pressure difference between intrapulmonary and intrapleural pressures.
• Values: 7 cm H2O (end inspiration), 5 cm H2O (end expiration).

Additional Notes

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