The fan-shaped muscle of the diaphragm arises from the internal circumference of the thorax, with attachments to the sternum, the lower six or seven ribs, and the lumbar vertebral bodies. The muscle fibers also attach posteriorly to the aponeurotic arch of the ligamentum arcuatum externum, which overrides the psoas and quadratus lumborum muscles (Fig. 149-1). Laterally, the fibers of the diaphragm interdigitate with slips from the transversalis muscle of the abdomen to originate from the ribs.¹ The right crus is larger and longer than the left and arises from the bodies of the upper three or four lumbar vertebrae. The left crus arises from the upper two lumbar vertebral bodies.

There are three natural openings within the diaphragm (Fig. 149-2). The aortic opening is the most posterior of the three and is formed from fibers comprising the right and left diaphragmatic crura.¹ This tunnel is actually behind the diaphragm, not within it, and contains the aorta, azygos vein, and thoracic duct. The esophageal hiatus is slightly more ventral in relation to the aortic hiatus and consists of fibers passing between the aorta and the esophagus toward the right crus, as well as fibers converging on the pericardial tendon. The opening of the inferior vena cava lies within the confluence of the tendons of the right hemithorax and the tendon beneath the pericardium.

The muscular diaphragm acts as a boundary between the positive pressure abdominal cavity and the negative pressure thoracic cavity. Although diaphragmatic disease is infrequent, exposure to the diaphragm is commonplace, because it is visualized during every thoracic surgical procedure and most intraabdominal operations. Therefore the basic principles advocated by this chapter can be verified in the operating room. The diaphragm makes a good fence, but neighbors on both sides of that fence should know its anatomy, physiology, and surgical principles of resection and repair.

A number of incisions in the diaphragm are possible once the location of the nerve and vessel branches have been learned. These structures frequently lie within the muscle itself and are not seen on the cranial surface of the structure. Therefore, the concept of a neurovascular “manacle” around the junction of the central tendon to the muscle is a very helpful visual mnemonic.

The phrenic nerve originates from the C3, C4, and C5 nerve roots and then enters the chest anterior to the subclavian artery. On the left-hand side, the nerve lies medial to the internal mammary (thoracic) artery 64% of the time, and on the right-hand side, it lies medial to the internal mammary artery only 46% of the time (Fig. 149-3).² Thus the left nerve is more prone to injury during mobilization of the left internal mammary artery through a median sternotomy incision.

While the origin of the phrenic nerve and its proximal course through the mediastinum are well known, the distal extent of the nerve as it branches into the diaphragm proper is less well described. In 1956, Merendino et al.³ published the most descriptive study regarding this intradiaphragmatic portion of the phrenic nerve. Their anatomic findings and drawings are based on electrical stimulation studies and gross dissection in dogs as well as intraoperative dissection of approximately 40 human diaphragms.

The phrenic nerve usually divides at the level of the diaphragm or just above it. The right phrenic nerve enters the diaphragm just lateral to the inferior vena cava within the central tendon. The left phrenic nerve enters lateral to the left border of the heart just anterior to the central tendon within the muscle itself.

The intradiaphragmatic course of the phrenic nerve can be predicted, even when not directly seen, by knowing the distribution of the four main motor divisions (Fig. 149-4). The phrenic nerve first splits into an anterior and posterior trunk. The anterior trunk subsequently divides into a sternal and anterolateral branch near the anteromedial border of the central tendon. The posterior trunk likewise divides into a crural and posterolateral branch along the posteromedial border of the central tendon. The sternal and crural branches are short and continue to run in an anteromedial and posteromedial direction, respectively. The anterolateral and posterolateral branches are much longer and run close to the muscular fiber insertions into the central tendon. These two branches innervate the majority of the diaphragm. Their anatomic relation to one another is often described as a pair of handcuffs or manacles. Often these branches are within the muscle layers and are not readily visible.

The superior phrenic arteries are located on the thoracic surface of the diaphragm (Fig. 149-5). These represent small branches from the lower thoracic aorta and traverse the posterior diaphragm over the top portion of each crus close to the mediastinum.¹ They terminate in small anastomoses with the musculophrenic and pericardiophrenic arteries, which are both branches from the internal mammary artery. These latter two arteries also supply blood to the phrenic nerve and the pericardial fat pad.⁴

The inferior phrenic arteries lie on the undersurface of the crus and the dome of the diaphragm (Fig. 149-6). These are small, paired vessels with frequent anatomic variations. They can originate separately from the aorta above the celiac artery. Alternatively, a common trunk arising from either the aorta or the celiac artery gives rise to these two arteries. Occasionally, one vessel will originate from the aorta, whereas the other emerges from one of the renal arteries. The inferior phrenic arteries then course obliquely superior and lateral along the inferior surface of the diaphragm. The left phrenic artery passes posterior to the esophagus and then runs anteriorly along the lateral side of the esophageal hiatus. The right inferior phrenic artery passes behind the inferior vena cava.¹