The thoracic cage, or rib cage, is an anatomical structure composed of 12 pairs of ribs that protects the heart and lungs and facilitates respiration. The ribs curve around the chest and attach to the spine in the back. The mobility of this bony structure allows the chest to expand and contract during breathing, a movement made possible by the muscles and tissues that occupy the spaces between the ribs. These distinct, organized anatomical regions are known as the intercostal spaces.
The Anatomical Count
There are precisely 11 intercostal spaces on each side of the human body, totaling 22 spaces between the ribs. This count is determined by the 12 pairs of ribs, as each space is defined by the ribs above and below it. The spaces are numbered sequentially according to the rib forming their superior boundary; for example, the space between the first and second ribs is the first intercostal space.
The final pair of ribs, the 12th ribs, are often “floating” and do not connect to any rib below them. Consequently, the region directly below the 12th rib is referred to as the subcostal space, which limits the official count of true intercostal spaces to 11 pairs.
Defining the Intercostal Space
An intercostal space is a three-dimensional region bordered by the bony framework of the ribs and vertebrae. Superiorly, the space is limited by the lower margin of one rib, and inferiorly, by the upper margin of the next rib. The space extends anteriorly to the sternum and posteriorly to the thoracic vertebrae.
The space is composed of three distinct layers of muscle that span between the ribs. The outermost layer is the external intercostal muscle, which aids in inhalation by elevating the ribs. Deep to this is the internal intercostal muscle, which assists mainly with forced exhalation. The deepest layer is the innermost intercostal muscle, which is often incomplete. These three muscle layers provide structural integrity to the chest wall and facilitate breathing.
Essential Contents of the Space
Nestled between the internal and innermost muscle layers is the neurovascular bundle. This bundle consists of the intercostal vein, intercostal artery, and intercostal nerve, which supply the chest wall. These three components follow a strict vertical arrangement, remembered by the mnemonic VAN: Vein, Artery, Nerve, listed from superior to inferior.
The neurovascular bundle travels along the costal groove, a protective channel on the undersurface of the rib above. The vein is the most superior structure, followed by the artery, and the nerve is the most inferior. This placement shields the vessels and nerve from external injury. The intercostal nerve is a branch of the thoracic spinal nerve, providing both sensory and motor function.
The intercostal artery supplies oxygenated blood to the tissues, while the vein returns deoxygenated blood. Each space receives blood supply from a large posterior intercostal artery, which branches off the aorta for the lower spaces, and two smaller anterior intercostal arteries.
Clinical Significance and Applications
Knowledge of the intercostal spaces is fundamental for medical procedures involving the chest. One common procedure is thoracentesis, which involves inserting a needle into the space to remove fluid or air from the pleural cavity. This is used for diagnosis and therapeutic relief of conditions like pleural effusion or pneumothorax.
To avoid damaging the neurovascular bundle, instruments for procedures like thoracentesis are intentionally inserted along the superior border of the rib forming the lower boundary of the space. By staying high on the rib below, a practitioner ensures the needle passes well clear of the protective costal groove of the rib above, minimizing the risk of hitting the vein, artery, or nerve.
Conversely, an intercostal nerve block requires injecting an anesthetic agent near the nerve for pain management. The needle is directed toward the inferior margin of the rib above to target the VAN bundle. These blocks are effective for managing pain from conditions like rib fractures or post-operative discomfort.
Accurately locating and accessing a specific intercostal space is a life-saving skill. Examples include the second space for needle decompression of a tension pneumothorax or the fifth space for chest tube insertion.