Muscles of Ventilation: Mechanics and Pathologies

Intercostal Muscles

The intercostal muscles are located between the ribs and assist in stabilizing the chest wall and modulating thoracic volume.

1. External Intercostals: These muscles originate from the inferior border of the superior rib and insert onto the superior border of the lower rib, sloping downward and anteriorly. During inhalation, they contract to elevate the ribs, which increases both the transverse and A-P diameters of the chest. They also serve to stabilize the intercostal spaces to prevent bulging during pressure changes.
2. Internal Intercostals: These run at an oblique angle behind the external intercostals. Unlike the external muscles, the internal intercostals are primarily used during forced exhalation to pull the ribs together, thereby decreasing thoracic volume.

Exam Tip: Remember that in normal, quiet breathing, only the diaphragm and external intercostals are active. Exhalation is passive, relying on the elastic recoil of the lungs.

Accessory Muscles of Ventilation

When the demand for ventilation increases—due to exercise, voluntary effort, or respiratory pathology—the body recruits accessory muscles to assist the primary muscles.

Accessory Muscles of Inhalation (Deep/Forced)

These muscles work to further expand the thoracic cavity during deep or forced breaths:

• Sternocleidomastoid: Located in the neck, this muscle pulls upward on the sternum to increase the A-P diameter.
• Scalenes: These muscles attach to the 1st and 2nd ribs and elevate them during deep inhalation.
• Serratus Posterior-Superior: Located in the back, this muscle raises the 2nd through 5th ribs.
• Pectoralis Major and Minor: These chest muscles can pull the ribs upward when the arms are in a fixed position.

Accessory Muscles of Exhalation (Forced)

During forced exhalation (such as coughing or sneezing), the body utilizes the abdominal muscles. This group includes the rectus abdominis, external oblique, internal oblique, and transverse abdominis. When these muscles contract, they increase intra-abdominal pressure, which forces the diaphragm upward into the thoracic cavity, rapidly decreasing lung volume.

Pathological Implications

Disease states can significantly impair the mechanics of ventilation by affecting either the nerves, the muscles, or the physical space available for movement.

Spinal Cord Injuries

Because the diaphragm is controlled by the phrenic nerve (C3–C5), the level of a spinal injury is a critical predictor of ventilatory function:

• Injury at or above C3: This results in the paralysis of the diaphragm and other ventilatory muscles, necessitating artificial ventilatory support for survival.
• Injury at C5–C6: Because the innervation for the diaphragm originates higher up, a patient with an injury at this level will typically maintain functional diaphragmatic contraction.

Emphysema

In emphysema, the lungs lose their elastic recoil, leading to a hyperinflated state where air is trapped in the lungs. This has a devastating effect on the diaphragm:

1. The diaphragm remains abnormally flattened rather than dome-shaped.
2. Because the muscle fibers are now oriented horizontally rather than vertically, contraction may actually pull the lower ribs inward instead of outward, further decreasing thoracic diameter.
3. Consequently, patients must rely heavily on accessory muscles (like the scalenes and sternocleidomastoid) to breathe.

Ascites and Abdominal Pressure

Conditions like ascites (the accumulation of fluid in the peritoneal cavity) or large abdominal tumors create upward pressure against the diaphragm. This physical obstruction limits the diaphragm's ability to descend during inhalation, forcing the patient to use accessory muscles of inhalation to compensate for the restricted diaphragmatic movement.