The intercostal space (ICS) is the anatomical region located between two adjacent ribs. These spaces contain muscles, nerves, and blood vessels, playing a large role in the expansion and compression of the rib cage during breathing. Locating a specific intercostal space is a foundational skill in health assessment and clinical procedures. This guide provides a practical method for accurately locating the 5th intercostal space, a key anatomical landmark.
Understanding Ribs and Intercostal Spaces
The human rib cage generally consists of twelve pairs of ribs, forming eleven intercostal spaces on each side. These spaces are numbered according to the rib located immediately above them; for example, the 5th intercostal space is the soft tissue area found directly below the 5th rib.
The first seven pairs of ribs, known as true ribs, connect directly to the sternum via their own costal cartilage. The 8th, 9th, and 10th pairs (false ribs) connect to the cartilage of the rib above them, while the 11th and 12th pairs are floating ribs with no anterior connection. When counting on the anterior chest, the focus is on the upper ribs. The first rib is often difficult to palpate as it sits just below the clavicle, making it an unreliable starting point.
Identifying the Critical Starting Landmark
Accurately finding any intercostal space requires first identifying a single, reliable anatomical landmark: the Sternal Angle. This angle, also known as the Angle of Louis, is a distinct horizontal ridge where the upper part of the sternum (the manubrium) joins the main body of the sternum. This ridge is nearly always palpable, even in individuals with significant body mass.
To locate the Sternal Angle, begin by finding the suprasternal notch, the indentation at the top center of the breastbone between the collarbones. Place a finger in this notch and slowly slide it down the sternum a few centimeters. The first distinct, horizontal bony prominence encountered is the Sternal Angle. The costal cartilage of the second rib articulates precisely at this joint on either side of the sternum.
Since the second rib attaches at this point, the soft space immediately below the Sternal Angle is the 2nd intercostal space. Counting must always begin here to ensure accuracy in subsequent identification.
Step-by-Step Palpation and Counting Technique
Once the Sternal Angle is located, finding the 5th intercostal space involves a sequential, walking motion using the fingertips. Begin by confirming the location of the 2nd intercostal space by sliding a finger laterally from the Sternal Angle, feeling the soft depression below the second rib. It is essential to use the finger pads and apply gentle pressure, as pressing too hard can compress the soft space and make the rib margins difficult to distinguish.
From the 2nd intercostal space, the technique involves moving one space at a time, inferiorly and slightly laterally along the sternal border. The sequence involves finding the soft space (the ICS) and then the firm bone (the rib) below it. Slide the finger down from the 2nd ICS to the rib below it, which is the 3rd rib, and then the space below that, the 3rd ICS. Continue this “rib-space” sequence: below the 3rd ICS is the 4th rib, and below that is the 4th ICS. Finally, the soft area immediately below the 5th rib is the target: the 5th intercostal space.
The 5th intercostal space is often most relevant when followed laterally away from the sternum. The point of maximal impulse (PMI), also called the apex beat, is typically located here near the left midclavicular line. The midclavicular line is an imaginary vertical line that drops straight down from the midpoint of the clavicle. For procedures like placing ECG electrodes, the 5th intercostal space may be identified closer to the sternum or further laterally toward the anterior axillary line.
Clinical Relevance and Common Palpation Errors
The 5th intercostal space is a significant anatomical landmark due to its consistent relationship with the heart. On the left side of the chest, this space, particularly along the midclavicular line, is the standard location for auscultating the mitral valve and for palpating the apex beat, which represents the lowest and outermost point of the heart’s contraction. Furthermore, the 5th intercostal space is the standardized location for placing the V4, V5, and V6 electrodes during a 12-lead electrocardiogram (ECG).
Accurate palpation is important, but several common errors can lead to misidentification. One frequent mistake is confusing the firm costal cartilage with the actual rib bone, leading to a miscount. Another error is counting too quickly and skipping a rib or an intercostal space, especially as the spaces become narrower further down the sternum.
A common pitfall involves losing the count when moving laterally away from the sternum. To maintain accuracy, a healthcare professional typically keeps one finger anchored in the last confirmed space (like the 4th ICS) while the other hand searches for the next landmark. Relying solely on the nipple line to estimate the 5th intercostal space is unreliable, especially in women, due to anatomical variability.