Lecture on Ventilation by Dr. Alexandra Kapelovic

Introduction

• Ventilation: Mechanical process of air inhalation and exhalation.
  • Occurs typically 12 to 20 times per minute at rest.
  • Divided into:
    • Quiet Ventilation: During sedentary activities, low metabolic demand.
    • Forced Ventilation: During strenuous activities, requires rapid and voluminous air exchange.

Forces Driving Ventilation

• Driven by active and passive forces.
• Intrathoracic Volume changes drive air pressure changes.

Boyle's Law

• At a fixed temperature and mass, inverse relationship between volume and pressure of gas.
• Increase in intrathoracic volume leads to a decrease in gas pressure.

Inspiration Mechanics

• Increase in Intrathoracic Volume:
  • Contraction of muscles attached to ribs and sternum.
  • Expansion of thorax reduces pressure in intrapleural space, creating suction that expands lungs.
  • Results in reduced alveolar pressure below atmospheric pressure, drawing air into lungs.

Expiration Mechanics

• Quiet Expiration: Passive process, no muscular activity.
  • Decrease in intrathoracic volume increases intrapleural pressure.
  • Lungs recoil, increasing alveolar pressure, causing air to flow from alveoli to atmosphere.

Rib Motion and Axis

• Upper Ribs:
  • Axis of rotation closer to frontal plane, allowing motion mainly in sagittal plane (Pump Handle movement).
• Lower Ribs:
  • Axis of rotation towards sagittal plane, allowing motion in frontal plane (Bucket Handle movement).

Biomechanics of Inspiration

• Ribs:
  • Move through elevation (upper ribs - pump handle; lower ribs - bucket handle).
  • Increase intrathoracic volume and diameters (anterior-posterior, medial-lateral).
• Costovertebral Joint:
  • Moves through posterior rotation during inspiration.

Sternum and Thoracic Spine Movement

• Sternum: Moves anteriorly and superiorly with inspiration.
• Thoracic Spine: Moves into extension with forced inspiration.

Muscles Responsible for Inspiration

1. Diaphragm:

   • Primary muscle, performs 60-80% of ventilatory work during quiet inspiration.
   • Dome drops 1.5 cm in quiet inspiration, 6-10 cm in forced inspiration.
   • Increases intrathoracic volume in all three diameters.

2. Scalenes:

   • Three muscles: anterior, middle, posterior scalene.
   • Attach to transverse processes C3-C6 and first two ribs.
   • Can affect brachial plexus if hypertrophied or spasmed.

3. Intercostal Muscles:

   • External Intercostals: Primary muscles of inspiration.
   • Internal Intercostals:
     • Parasternal fibers assist in inspiration.
     • Interosseous fibers responsible for forced expiration.