Diaphragm Location by Intercostal Space
The diaphragm typically sits at or below the 5th intercostal space (ICS) in most individuals, but its position varies significantly with respiration, body habitus, and underlying pathology—making the 5th ICS a potentially unsafe landmark for procedures without ultrasound guidance.
Anatomical Position During Respiration
The diaphragm's position is dynamic and varies considerably:
- During tidal breathing: The diaphragm crosses into or above the 5th ICS in approximately 10-13% of cases 1, 2
- During maximal inspiration: The diaphragm rises to cross the 5th ICS in 27% of pediatric patients and up to 20% of adults 1, 2
- Right vs. left asymmetry: The right hemidiaphragm crosses the 5th ICS more frequently than the left, with rates reaching 45% during maximal respiration 2
Clinical Context: Surgical Landmarks
Surgical guidelines provide practical reference points for diaphragm location:
- Minimally invasive cardiac surgery: The working incision at the 4th or 5th ICS at the anterior axillary line "usually falls just at the dome of the diaphragm" 3
- Diaphragm plication procedures: Access is typically performed via lateral thoracotomy at the 8th intercostal space, well below the diaphragm's dome 4
- Radiation therapy planning: Treatment volumes extend inferiorly to the diaphragmatic insertion point, usually near the bottom of the L2 vertebral body 3
Body Habitus Effects
Critical caveat: Diaphragm position is significantly influenced by patient characteristics:
- Obesity: Each 1 kg/m² increase in BMI increases the odds of the diaphragm crossing the 5th ICS by >10% during both tidal and maximal respiration 2
- Obstructive pulmonary disease: Low diaphragm position can cause V3 and V4 ECG leads to be located above ventricular boundaries when placed in standard positions 3
- Subcutaneous fat: Variable filtering effects occur with surface electrode recordings over the diaphragm 3
Electrophysiologic Landmarks
For diaphragm EMG recordings, anatomical references include:
- Crural diaphragm motor innervation zone: Located 1-3 cm cephalad to the gastroesophageal junction, with the left side approximately 1 cm higher than the right 3
- Optimal surface electrode placement: 5 cm superior to the xiphoid process (G1) with reference electrode 16 cm away at the 6th intercostal space costochondral junction 5
Safety Implications for Procedures
The 5th ICS landmark is unreliable and potentially dangerous:
- Traditional landmark-based chest tube placement at the 5th ICS results in 81% of sites being below the diaphragm, but 19-20% are at or above the diaphragm, risking subdiaphragmatic insertion or diaphragmatic injury 1
- Ultrasound guidance improves accurate site identification from 48% (palpation) to 91% 6
- Without imaging, the average placement error is 0.88 rib spaces using traditional technique versus 0.09 rib spaces with ultrasound 6
Bottom line: While the diaphragm generally sits at or below the 5th ICS, significant individual variation—particularly with respiration, obesity, and lung disease—makes this an unreliable anatomical landmark. Ultrasound verification is strongly recommended before any procedure targeting this region 1, 6, 2.