The intercostal spaces (ICS) are the 11 anatomical gaps found between the 12 pairs of ribs in the human chest wall. These spaces house muscles, nerves, and blood vessels that enable the expansion and contraction of the rib cage during breathing. Accurate identification and counting of these spaces are a foundational skill in clinical assessment. This skill informs where a practitioner should listen to lung sounds, place an electrocardiogram (ECG) electrode, or perform procedures like a thoracentesis. Each intercostal space is numbered according to the rib that forms its superior boundary; for example, the first intercostal space lies just below the first rib. Counting them reliably begins with identifying a specific, easily felt bony landmark on the chest.
Locating the Sternal Angle
Accurate counting of the intercostal spaces relies on first locating the Sternal Angle, also known as the Angle of Louis. This structure is a slight, palpable horizontal ridge on the sternum where the upper section (manubrium) connects to the main body. To find this landmark, place a finger in the jugular notch, the depression felt at the top of the sternum between the collarbones.
Slide the finger downward along the sternum until it encounters the distinct ridge of the Sternal Angle. This angle consistently aligns with the second costal cartilage, the attachment point of the second rib. Moving the finger outward (laterally) from the Sternal Angle leads immediately to the bony structure of the second rib.
The space immediately below this second rib is the second intercostal space (2nd ICS). The first rib is difficult to palpate as it is partially hidden by the clavicle. Therefore, the 2nd ICS is the most practical starting point for counting.
Step-by-Step Anterior Counting Method
Once the 2nd intercostal space has been located adjacent to the Sternal Angle, counting proceeds sequentially down the chest. The technique involves walking the fingers down the chest wall, using a light but firm touch. This touch helps distinguish between the hard bone of the ribs and the softer depression of the space. The numbering sequence alternates between a rib and a space.
To maintain accuracy, the palpating fingers should be walked along the chest wall in a slightly inferior and lateral direction. This movement is necessary because the ribs angle downward from the spine toward the sternum. The spaces also become wider as they move away from the midline.
The anterior count typically terminates around the lower limit of the sternum, defined by the attachment of the 7th costal cartilage. Below this point, ribs 8, 9, and 10 connect indirectly to the sternum, forming the costal margin. This structure makes individual rib and space identification near the midline much harder.
Anatomical Exceptions
The anterior method limits the ability to locate spaces across the entire thorax, particularly laterally and posteriorly. When counting laterally, the ribs must be followed around the curved side of the chest, usually in the mid-axillary line. This area often presents a challenge due to increased muscle mass and soft tissue, which can obscure the distinction between the bony rib and the space below it.
For posterior counting, a different set of landmarks establishes the starting point. The inferior angle of the scapula typically aligns with the level of the 7th rib in a patient standing in the anatomical position. Counting upward from the lower margin of the rib cage can also be effective, using the most inferior rib, the 12th rib, as an anchor.
Further complexities arise from the 11th and 12th ribs, known as floating ribs, which do not attach to the sternum or the costal margin. The spaces below these ribs cannot be counted using the anterior method. Palpation is also challenging in individuals with variations in body composition, such as obesity or large breasts, which obscure subtle bony landmarks.
In challenging cases, alternative bony reference points may serve as secondary guides for estimation. These include the clavicle or the prominent spinous process of the C7 vertebra in the neck.