Chest Wall and Diaphragm

Describe the structure of the chest wall and diaphragm and to relate these to respiratory mechanics.

The chest wall is formed by the ribs and intercostal muscles:

• Ribs
  Slope antero-inferiorly, and are connected by the external, internal, and innermost intercostal muscles.
• Intercostal muscles
  • External intercostals slope antero-inferiorly
  • Internal and innermost intercostals slope infero-posteriorly
• Diaphragm
  Complex dome-shaped membranous structure, consisting of a central tendon and peripheral muscles
  • Performs the majority of inspiratory work of breathing
  • Able to dramatically increase intraabdominal pressure, so is essential in:
    • Coughing
    • Vomiting
    • Sneezing
  • Role in maintaining lower oesophageal sphincter tone
  • It has three perforations:
    • T8 for the vena cava (eight letters)
    • T10 for the oesophagus (ten letters)
    • T12 for the aorta, thoracic duct, and azygos vein

Inspiration

• During inspiration, the diaphragm and external intercostal muscles contract
  • Diaphragm pushes the intraabdominal contents down, increasing thoracic volume and generating a negative intrathoracic pressure
    • Diaphragm is supplied by the phrenic nerves from C3/4/5.
  • External intercostals pull the ribs antero-superiorly, which increases the cross-sectional area of the chest, further increasing thoracic volume (and negative pressure)
    • Intercostal muscles are supplied by intercostal nerves from the same spinal level
    • Paralysis of the external intercostals does not have a dramatic effect on inspiratory function provided the diaphragm is intact

• Accessory muscles include sternocleidomastoid and the scalene, which elevate the sternum and first two ribs respectively. They are active in hyperventilation.

Expiration

• Expiration is passive during quiet breathing as elastic recoil of the lung will return them to FRC
• When minute ventilation is high, expiration becomes an an active process:
  • Abdominal wall muscles (rectus abdominis, internal oblique, external oblique, transversus abdominis) contract, raising intraabdominal pressure and forcing the diaphragm up
  • Internal and innermost intercostals contract, pulling the ribs downwards and inwards, further decreasing thoracic volume

Respiratory Mechanics in Spinal Injury

• Paralysis of the abdominal wall muscles (e.g. spinal injury) has significant affect on respiratory mechanics:
  • In the initial phases of injury, spinal shock results in a flaccid paralysis of the abdominal wall
    • Intraabdominal pressure is low, and so the diaphragm moves inferiorly
      This results in a higher FRC but limits tidal volumes, as contraction of the diaphragm only increase thoracic volume by a small fraction.
    • Nursing in a supine position causes the abdominal contents to push the diaphragm superiorly, causing:
      • Lower FRC
      • Greater proportional expansion with respiration, improving tidal volumes
  • Once spastic paralysis ensues, the abdominal wall is rigid and the patient can be sat up

References

1. West J. Respiratory Physiology: The Essentials. 9th Edition. Lippincott Williams and Wilkins. 2011.