The sixth intercostal space is the gap between the sixth and seventh ribs, sitting roughly at the level of the xiphoid process (the small cartilaginous tip at the bottom of the sternum). It serves as a key anatomical landmark for procedures ranging from chest drain insertion to cardiac access, and its clinical importance comes from what lies just beneath and around it: the lower lung fields, the diaphragm’s dome, the pericardium, and a set of blood vessels and nerves that vary in position more than textbooks suggest.
How to Find It on Your Own Body
Counting ribs is trickier than it sounds, because the first rib hides almost entirely behind the collarbone. The reliable starting point is the sternal angle, the horizontal ridge you can feel where the manubrium meets the body of the sternum. The second rib attaches right there. From that landmark, you count downward: the space just below the second rib is the second intercostal space, the space below the third rib is the third, and so on. The sixth intercostal space sits just below the sixth rib. Along the midclavicular line (an imaginary vertical line dropped from the middle of the collarbone), it falls near the lower border of the breast in most adults. Along the side of the chest, at the midaxillary line, it sits a few finger-widths below the armpit.
This counting method works well in lean individuals but becomes unreliable in people with higher body mass, large breasts, or chest wall deformities. Research using ultrasound to verify rib counting has shown that clinicians frequently misidentify intercostal levels by one or even two spaces, especially in women. One study found that in close to half of female subjects, landmark-based guidelines placed the target at the sixth intercostal space or below rather than the intended fifth.1PubMed. Finding the fifth intercostal space for chest drain insertion: guidelines and ultrasound That matters because going one space too low can bring a needle or tube dangerously close to the diaphragm or abdominal organs.
What Runs Through It
Each intercostal space contains a neurovascular bundle tucked along the underside of the rib above. In the sixth space, the bundle typically includes an intercostal vein, an intercostal artery, and an intercostal nerve, running in that order from top to bottom.2The FASEB Journal. A Cadaveric Study of Intercostal Neurovascular Bundle Variation in Intercostal Spaces: Chest Tube Insertion Approaches The traditional teaching is that these structures nestle safely in a groove along the lower border of the upper rib, so inserting a needle or tube just above the top of the lower rib should avoid them.
Cadaveric dissections have complicated that picture. The neurovascular bundle does not always sit neatly in the subcostal groove. In the fourth through sixth intercostal spaces at the midaxillary line, vessels and nerves wander to varying positions within the space. An additional collateral artery often runs along the upper border of the lower rib, meaning that both the top and bottom edges of the space carry some vascular risk. One cadaver study concluded that the safest needle path through these spaces is roughly halfway to two-thirds of the way down the interspace, a narrower window than many clinicians realize.3PubMed. Neurovascular anatomy and variation in the fourth, fifth, and sixth intercostal spaces in the mid-axillary line: a cadaveric study in respect of chest drain insertion This finding has practical weight: hugging the top of the lower rib is not as safe as older guidelines implied, because that collateral vessel can sit right there.
The Safe Triangle and Chest Drain Insertion
When air or fluid collects between the lung and the chest wall, a chest drain (also called a chest tube or thoracostomy tube) needs to go in. The standard teaching defines a “safe triangle” on the lateral chest wall, bounded by the anterior border of the latissimus dorsi muscle, the lateral border of the pectoralis major, and a horizontal line at the level of the nipple. Within that triangle, the fourth or fifth intercostal space is the classic target. The sixth intercostal space sits at or just below the lower boundary of this triangle, depending on the patient’s body habitus.
Going too low carries real consequences. The diaphragm rises higher than many people expect, especially during expiration or in patients lying flat. A tube placed into the sixth or seventh intercostal space can accidentally puncture the diaphragm and enter the abdominal cavity, injuring the liver on the right side or the spleen on the left. The study that found landmark-based guidelines placed the insertion point at the sixth space or lower in close to half of women underscores why many emergency physicians now advocate for ultrasound confirmation before making the incision.4PubMed. Finding the fifth intercostal space for chest drain insertion: guidelines and ultrasound Ultrasound takes only seconds and can verify both the rib level and the presence of lung sliding (which confirms you are above the diaphragm and looking at pleural space, not solid organ).
Cardiac Access Through the Sixth Intercostal Space
The heart sits behind the sternum and slightly to the left, and the pericardium (the sac surrounding it) can be reached through left-sided intercostal spaces. When fluid accumulates around the heart and needs to be drained, a procedure called pericardiocentesis is performed. The classic approach uses a subxiphoid route, inserting a needle just below and to the left of the xiphoid process and angling toward the left shoulder. But in many patients, especially older or larger children, a direct intercostal approach through the fifth or sixth intercostal space on the left side works as well or better.
A study of children undergoing pericardiocentesis found that the fifth and sixth intercostal spaces were the most commonly chosen entry points when a non-subxiphoid approach was used. That approach was associated with shorter procedure times compared to the subxiphoid route, with no major complications in either group.5PubMed. Anatomic Approach and Outcomes in Children Undergoing Percutaneous Pericardiocentesis The sixth intercostal space, in particular, provides a window that avoids the thickest portion of the left lung while allowing a relatively direct path to the pericardial sac. Echocardiographic guidance is standard practice for this procedure, both to confirm the fluid’s location and to track the needle in real time.
Intercostal Nerve Blocks for Pain Relief
The intercostal nerves that pass through each space carry sensory signals from the chest wall and upper abdomen. Blocking those nerves with local anesthetic can provide substantial pain relief after surgery or rib fractures. Because the sixth intercostal space corresponds roughly to the dermatome covering the upper abdomen and lower chest, it falls within the range targeted for pain control after abdominal procedures.
A clinical study of intercostal nerve blocks performed before gallbladder removal (cholecystectomy) through a subcostal incision found that injecting a small volume of long-acting local anesthetic into each of the intercostal spaces from T5 through T11 produced excellent pain relief lasting an average of about 12 hours, with no complications in the series.6PubMed. Posterior intercostal nerve block for pain relief after cholecystectomy. Anatomical basis and efficacy The sixth space sits in the middle of that range and corresponds to the area of maximal incisional pain for many upper abdominal surgeries. Modern practice often performs these blocks under ultrasound guidance, which helps visualize the pleura and the neurovascular bundle before injecting.
The main risk of intercostal nerve blocks is pneumothorax, where the needle punctures the pleura and allows air into the chest cavity. This risk is small with experienced hands and ultrasound guidance, but it is the reason clinicians take care to stay superficial enough to deposit the anesthetic around the nerve without advancing into the pleural space.
How the Sixth Space Differs in Children
Pediatric anatomy does not simply scale down from adult anatomy. The ribcage in infants and young children is more horizontal, the thorax is more cylindrical, and the diaphragm inserts at different relative levels. A study reappraising pediatric thoracic surface anatomy found that the lower border of the diaphragm sat at the level of the sixth or seventh rib along the midclavicular line in children.7PubMed. A reappraisal of pediatric thoracic surface anatomy Along the midaxillary line, it was at the seventh intercostal space and eighth rib, and posteriorly at roughly the eleventh thoracic vertebra.
The practical takeaway is that the sixth intercostal space in a child is closer to the diaphragm than it is in most adults. A chest tube or needle inserted at the sixth space in a small child carries a higher risk of hitting the diaphragm, which is why pediatric guidelines tend to favor the fourth or fifth intercostal space at the midaxillary line for thoracostomy. The relatively higher diaphragm in children also means the liver and spleen sit higher in the abdomen, making subdiaphragmatic injury a concern at intercostal levels that would be considered safe in adults.
How Wide Is the Space, and Why It Varies
The physical width of an intercostal space matters whenever a procedure or therapeutic device needs to pass through it. Sonographic measurements of intercostal spaces have shown that widths vary considerably depending on the individual and the location along the chest wall. In one study measuring multiple intercostal regions, average widths ranged from roughly 14 to 20 millimeters, with individual measurements spanning as low as 4 millimeters to as high as 33 millimeters depending on the region.8American Journal of Roentgenology (AJR). Sonographic analysis of the intercostal spaces for the application of high-intensity focused ultrasound therapy to the liver That wide range means a space that is comfortably accessible in one patient may be barely passable in another.
Several factors influence width. The spaces tend to be wider toward the front and side of the chest and narrower toward the back. They also narrow with age, as rib cartilage calcifies and the chest wall stiffens. Body position affects them too: raising the arm on the same side widens the lateral intercostal spaces, which is why patients having a chest tube placed are typically positioned with their arm above their head. Deep inspiration also widens the spaces slightly, which can help during ultrasound-guided procedures.
The variability in width is especially relevant for focused ultrasound therapies targeting abdominal organs like the liver. These devices deliver energy beams that must pass between the ribs without being blocked or scattered by bone. If the sixth intercostal space is too narrow in a given patient, the treatment window may need to shift to an adjacent space, or the approach angle may need to be adjusted.
Trauma and Lung Herniation
Rib fractures involving the fifth and sixth ribs can disrupt the integrity of the intercostal space and, in rare cases, allow lung tissue to herniate outward through the gap. One case report described a patient who developed a lung herniation through an enlarged fifth intercostal space after fractures of the fifth and sixth ribs following thoracic surgery.9International Journal of Surgery Case Reports. A rare case of an intercostal lung herniation with fractures of the fifth and sixth ribs after thoracic surgery The herniation was visible on imaging and confirmed during a subsequent operation, where the lung tissue was reduced back into the chest and the chest wall defect was repaired.
Lung herniation through an intercostal space is uncommon, but it illustrates how much the structural integrity of the ribs and intercostal muscles matters. The fifth and sixth ribs are among the most frequently fractured ribs in blunt chest trauma (along with the seventh through ninth), in part because they sit at the widest part of the thorax and absorb impact directly. When fractures occur at adjacent ribs, the intercostal space between them can widen dramatically, losing the muscular and fascial support that normally keeps the lung contained. Surgical repair typically involves plating the fractured ribs and closing the defect with mesh or sutures.
Common Misconceptions About Intercostal Anatomy
One persistent myth is that the neurovascular bundle is always safely tucked in the groove under the upper rib, so inserting a needle right above the lower rib guarantees you will miss it. Cadaver studies have shown this is not reliably true, especially in the fourth through sixth intercostal spaces. Vessels and nerves can wander into the middle of the space or sit along the upper border of the lower rib, and a collateral branch of the intercostal artery often runs near that lower border.10PubMed. Neurovascular anatomy and variation in the fourth, fifth, and sixth intercostal spaces in the mid-axillary line: a cadaveric study in respect of chest drain insertion The safest approach is to aim for the middle portion of the space and use imaging guidance when available.
Another misconception is that the diaphragm sits well below the sixth rib and is not a concern at this level. In reality, the diaphragm’s dome reaches as high as the fourth intercostal space on the right side during full expiration, and routinely sits at or near the sixth rib along the midclavicular line. In children, as noted earlier, it can be even higher.11PubMed. A reappraisal of pediatric thoracic surface anatomy Clinicians who assume the sixth intercostal space is safely above the diaphragm in every patient risk a subdiaphragmatic insertion, which is one of the more dangerous complications of chest tube placement.
A third area of confusion involves counting itself. Many people assume that if the nipple is at the fourth intercostal space, the sixth space is simply two spaces below. But nipple position varies enormously with sex, age, and body habitus. In some individuals, particularly women with ptotic breasts, the nipple can sit at the sixth or seventh rib. Using the nipple as your only reference point for any intercostal level is unreliable. The sternal angle remains the most dependable palpable landmark for starting a rib count, and ultrasound remains the most dependable tool for confirming one.
Ultrasound-Guided Procedures at This Level
Ultrasound has changed how clinicians interact with the sixth intercostal space. For chest tube insertion, a quick scan confirms the rib level, identifies the diaphragm, and verifies that there is fluid or air to drain. For nerve blocks, ultrasound visualizes the pleura, the intercostal muscles, and sometimes the neurovascular bundle itself, allowing the clinician to deposit anesthetic precisely. For pericardiocentesis, echocardiography identifies the largest pocket of fluid and the safest trajectory through the intercostal window.
Beyond procedural guidance, ultrasound at the sixth intercostal space is used diagnostically. A probe placed at the right sixth intercostal space in the midaxillary line is one of the standard positions in the eFAST exam (extended Focused Assessment with Sonography for Trauma), used to look for fluid around the liver and above the diaphragm. This view can detect both a hemothorax (blood in the chest cavity) and free fluid in the hepatorenal recess. The left sixth intercostal space gives a comparable view of the spleen and left hemidiaphragm. These windows rely on the sixth intercostal space being wide enough and positioned correctly to give acoustic access, which loops back to the variability in space width described above. In patients where the intercostal window is too narrow, the probe can be angled or repositioned to an adjacent space.