Why is the 5th intercostal space (ICs) at the mid-axillary line the preferred site for tube thoracostomy?

Why the 5th Intercostal Space at the Mid-Axillary Line is Preferred for Tube Thoracostomy

The 5th intercostal space (ICS) at the mid-axillary line is the preferred site for tube thoracostomy because it provides the thinnest chest wall thickness (mean 3.5 cm ± 0.9 cm), achieves 100% successful pleural cavity penetration, and minimizes risk of injury to vital structures including the diaphragm, heart, and major vessels. 1, 2

Anatomical Advantages of the 5th ICS Mid-Axillary Line

Superior Success Rates

• The 5th ICS at the mid-axillary line achieves 100% successful pleural cavity penetration compared to only 57.5% success at the traditional 2nd ICS midclavicular line. 2
• The chest wall is significantly thinner at the 5th ICS (3.5 cm ± 0.9 cm) compared to the 2nd ICS (4.5 cm ± 1.1 cm), making tube insertion more reliable. 2, 3
• This 1 cm difference in chest wall thickness translates to substantially improved procedural success, particularly in obese patients or those with increased body mass index. 2, 3

Safety Profile

• The anterior to mid-axillary line between the 4th or 5th intercostal space contains no vital organs or organ structures except lung parenchyma when proper technique is used. 4
• The v. thoracoepigastrica is the only significant vessel at the lateral thoracic wall, and it can be avoided by inserting above the superior rib margin. 4
• For right-sided procedures, both the 5th ICS mid-axillary line and 2nd ICS midclavicular line are acceptable options. 5, 6
• For left-sided procedures, the 2nd ICS midclavicular line is safer due to lower risk of cardiac injury, though the 5th ICS can be used on the right side. 5, 6

Critical Technical Considerations

Proper Insertion Technique

• Insert the tube at the superior border of the rib to avoid the neurovascular bundle that runs along the inferior rib margin. 6
• The insertion should be perpendicular to the chest wall at the 4th or 5th intercostal space in the mid- or anterior-axillary line. 1
• Use blunt dissection rather than a steel trocar, as trocar insertion significantly increases complications including hemothorax, lung lacerations, and injury to thoracic or abdominal organs. 1

Ultrasound Guidance Strongly Recommended

• Ultrasound should be used to identify the diaphragm position before insertion, as 20% of right hemidiaphragms and 18% of left hemidiaphragms are above or crossing the 5th ICS during the respiratory cycle. 7
• Blind landmark-based approaches at the 5th ICS result in a significant percentage of subdiaphragmatic insertions or diaphragmatic injuries. 7
• Ultrasound can identify safer insertion sites and reduce thoracostomy complications substantially. 7

Common Pitfalls and How to Avoid Them

Diaphragm Injury Risk

• The most critical pitfall is inserting too low and penetrating the diaphragm or entering the abdominal cavity. 7
• Always use ultrasound to visualize the diaphragm through an entire respiratory cycle before marking the insertion site. 7
• If ultrasound is unavailable, err on the side of inserting one intercostal space higher (4th ICS) rather than risk subdiaphragmatic placement. 1

Patient-Specific Factors

• Women have significantly thicker chest walls than men at both the 2nd ICS (52 mm vs 38 mm) and 5th ICS (38 mm vs 33 mm). 3
• Patients with trauma, subcutaneous emphysema, multiple rib fractures, lung contusion, or sternum fractures have thicker chest walls at the 5th ICS. 3
• Despite these variations, the 5th ICS remains thinner than the 2nd ICS in all patient subgroups. 3

Tube Size Considerations

• Standard 5 cm catheters are inadequate for approximately 33% of women and 13% of men at the 5th ICS. 3
• Use larger bore tubes (28-32 French) with adequate length to ensure pleural cavity access. 1

Distinction from Needle Thoracostomy

It is important to distinguish tube thoracostomy from needle thoracostomy (emergency decompression):

• For needle decompression of tension pneumothorax, the American College of Surgeons recommends the 5th ICS at the anterior axillary line with a 7-8 cm needle. 8
• Tube thoracostomy is the definitive treatment following needle decompression. 5, 8
• The 5th ICS mid-axillary line serves as the optimal site for both procedures due to consistent anatomical advantages. 8, 1, 2