Midclavicular Line / Mid-Axillary Line
Vertical anatomic reference lines on the chest wall, used to identify specific locations for needle decompression, chest tube placement, and other procedures.
In the Field
The midclavicular and mid-axillary lines are how trauma providers describe exactly where on the chest a procedure should be performed. Saying "second intercostal space, midclavicular line" tells another provider precisely where you placed a needle, even if they cannot see the patient. The lines are not visible markings. They are imaginary vertical reference lines based on anatomic landmarks. Knowing them is part of being able to communicate effectively in trauma operations and to follow protocol-specified procedures correctly.
Common Mistake
Performing needle decompression at incorrect locations because the midclavicular or mid-axillary line was misidentified, resulting in procedural failure or injury.
Technical Detail
The midclavicular line and mid-axillary line are two of several vertical anatomic reference lines used to describe locations on the thoracic and abdominal walls. These imaginary lines provide precise descriptive language for anatomy, examination findings, and procedure locations.
Midclavicular line (MCL). A vertical line running through the midpoint of the clavicle, extending downward across the chest and abdomen. The midpoint of the clavicle is identified by:
Locating the medial end of the clavicle at the sternoclavicular joint.
Locating the lateral end of the clavicle at the acromioclavicular joint.
Identifying the point halfway between.
Common procedural reference at the midclavicular line:
Second intercostal space, midclavicular line. The traditional site for needle decompression of tension pneumothorax. Located on the upper anterior chest, approximately one to two finger widths below the clavicle, at the level just below the second rib.
Mid-axillary line (MAL). A vertical line running through the midpoint of the axilla (armpit), extending downward across the lateral chest and abdomen. The mid-axillary line is identified by:
Locating the anterior axillary fold (formed by the pectoralis major muscle).
Locating the posterior axillary fold (formed by the latissimus dorsi muscle).
The mid-axillary line passes through the midpoint between these two folds.
Common procedural references at or near the mid-axillary line:
Anterior axillary line. Slightly anterior to the mid-axillary line, at the anterior axillary fold.
Fifth intercostal space, anterior axillary line. The current CoTCCC-preferred site for needle decompression of tension pneumothorax. Located on the lateral chest, approximately at the level of the nipple in a male patient, slightly forward of the armpit.
Posterior axillary line. Slightly posterior to the mid-axillary line, at the posterior axillary fold.
Other thoracic reference lines. Several other vertical reference lines are commonly used in clinical and procedural anatomy:
Midsternal line. Through the center of the sternum.
Sternal lines (right and left). At the lateral edges of the sternum.
Parasternal lines (right and left). Just lateral to the sternum.
Scapular line. Through the inferior angle of the scapula, on the back.
Vertebral (midspinal) line. Through the spinous processes of the vertebrae, on the back.
Practical use in tactical trauma. The midclavicular and mid-axillary lines (and the related anterior axillary line) are the most operationally important reference lines in tactical trauma because they define needle decompression sites:
Traditional (anterior) site: 2nd intercostal space, midclavicular line.
Current preferred site: 5th intercostal space, anterior axillary line.
The anterior axillary line site became the CoTCCC preferred location based on chest wall anatomy research showing higher procedure success rates and lower rates of needle insertion failure due to chest wall thickness. The traditional midclavicular line site remains acceptable but has higher failure rates in larger patients.
Site identification in field conditions. Identifying these reference lines reliably under stress requires practice:
Patient positioning matters. The mid-axillary line shifts with patient position, and identification is more reliable with the patient supine and arms at the side.
Body habitus affects landmark visibility. Larger patients have less prominent landmarks; thinner patients have more visible bony anatomy.
Tactical conditions (low light, gloves, body armor partially obscuring the patient) make landmark identification more difficult.
Practice and familiarity are required to perform reliable site identification under operational conditions.
Provider scope. Use of these reference lines for needle decompression is a provider-level skill, taught in TCCC at the Combat Lifesaver level and above and in TECC at provider levels. Aid bag specifications include needle decompression catheters meeting CoTCCC length and gauge requirements when these procedures are within the carrier's scope of practice.
Midclavicular line (MCL). A vertical line running through the midpoint of the clavicle, extending downward across the chest and abdomen. The midpoint of the clavicle is identified by:
Locating the medial end of the clavicle at the sternoclavicular joint.
Locating the lateral end of the clavicle at the acromioclavicular joint.
Identifying the point halfway between.
Common procedural reference at the midclavicular line:
Second intercostal space, midclavicular line. The traditional site for needle decompression of tension pneumothorax. Located on the upper anterior chest, approximately one to two finger widths below the clavicle, at the level just below the second rib.
Mid-axillary line (MAL). A vertical line running through the midpoint of the axilla (armpit), extending downward across the lateral chest and abdomen. The mid-axillary line is identified by:
Locating the anterior axillary fold (formed by the pectoralis major muscle).
Locating the posterior axillary fold (formed by the latissimus dorsi muscle).
The mid-axillary line passes through the midpoint between these two folds.
Common procedural references at or near the mid-axillary line:
Anterior axillary line. Slightly anterior to the mid-axillary line, at the anterior axillary fold.
Fifth intercostal space, anterior axillary line. The current CoTCCC-preferred site for needle decompression of tension pneumothorax. Located on the lateral chest, approximately at the level of the nipple in a male patient, slightly forward of the armpit.
Posterior axillary line. Slightly posterior to the mid-axillary line, at the posterior axillary fold.
Other thoracic reference lines. Several other vertical reference lines are commonly used in clinical and procedural anatomy:
Midsternal line. Through the center of the sternum.
Sternal lines (right and left). At the lateral edges of the sternum.
Parasternal lines (right and left). Just lateral to the sternum.
Scapular line. Through the inferior angle of the scapula, on the back.
Vertebral (midspinal) line. Through the spinous processes of the vertebrae, on the back.
Practical use in tactical trauma. The midclavicular and mid-axillary lines (and the related anterior axillary line) are the most operationally important reference lines in tactical trauma because they define needle decompression sites:
Traditional (anterior) site: 2nd intercostal space, midclavicular line.
Current preferred site: 5th intercostal space, anterior axillary line.
The anterior axillary line site became the CoTCCC preferred location based on chest wall anatomy research showing higher procedure success rates and lower rates of needle insertion failure due to chest wall thickness. The traditional midclavicular line site remains acceptable but has higher failure rates in larger patients.
Site identification in field conditions. Identifying these reference lines reliably under stress requires practice:
Patient positioning matters. The mid-axillary line shifts with patient position, and identification is more reliable with the patient supine and arms at the side.
Body habitus affects landmark visibility. Larger patients have less prominent landmarks; thinner patients have more visible bony anatomy.
Tactical conditions (low light, gloves, body armor partially obscuring the patient) make landmark identification more difficult.
Practice and familiarity are required to perform reliable site identification under operational conditions.
Provider scope. Use of these reference lines for needle decompression is a provider-level skill, taught in TCCC at the Combat Lifesaver level and above and in TECC at provider levels. Aid bag specifications include needle decompression catheters meeting CoTCCC length and gauge requirements when these procedures are within the carrier's scope of practice.