The internal mammary artery gives off two terminal branches: the
superior epigastric and the musculophrenic. The musculophrenic artery
runs around the costal margin of the diaphragm and supplies the lower
intercostal spaces and the diaphragm. Thus, disconnection of the distal
end of the mammary artery for coronary grafting may impair the blood
supply to the diaphragm, intercostal muscles, and the phrenic nerve.
The relatively ischemic diaphragm thus may not contract in response to
stimulation of the phrenic nerve. A temporarily paralyzed diaphragm
becomes flaccid and raised just as in phrenic nerve injury, and
increases the chance of pulmonary collapse/consolidation. The ischemia
is obviously only temporary until the preformed collaterals open up.
Thus, the raised left hemidiaphragm after harvesting the left internal
mammary artery may not be secondary to overlying collapse/consolidation
or phrenic nerve palsy, but rather a primary event with
collapse/consolidation being a secondary event.