Department of Cardiothoracic Surgery
Harvesting the internal mammary artery is a standard part of the
majority of coronary artery bypass operations, but it is associated
with an increased incidence of postoperative pulmonary
complications.1 The use of bilateral mammary arteries
increases this risk appreciably. Leaving the ipsilateral pleura
unopened does not prevent collapse/consolidation development. The
explanations vary, none of which truly explain the excessively high
risk of collapse/consolidation after harvesting the internal mammary
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.
The importance of the internal mammary artery was demonstrated in a
swine model that is being developed for other experiments; it was
utilized to demonstrate the extensive blood supply to the diaphragm.
Crystal violet dye was injected into the left atrium, after the left
internal mammary artery had been harvested and disconnected distally.
The left hemidiaphragm demonstrated virtually no uptake of the dye,
compared with the right side. Direct injection into the internal
mammary artery resulted in the whole ipsilateral hemidiaphragm taking
up the crystal violet dye virtually immediately (Fig 1).
Obviously the data are preliminary and could have been bolstered by
phrenic nerve tests, local tissue oxygen saturation, nicotinamide
adenine dinucleotide/nicotinamide adenine dinucleotide (reduced forms),
ratios and videofluoroscopy of the diaphragms postoperatively. The aim
of this small study was not to be exhaustive and comprehensive, but
merely to draw the cardiac surgeon’s attention to another possible
reason that pulmonary complications after harvesting the internal
mammary artery may be due to diaphragmatic ischemia.
Now that the internal mammary artery is being used in pulmonary
transplantation for bronchial artery revascularization, diaphragmatic
complications could become more important in an already critical field.
Correspondence to: Michael Poullis, BSc, Department of
Cardiothoracic Surgery, Hammersmith Hospital, DuCane Road, East Acton,
London, W12 ONN; e-mail: firstname.lastname@example.org
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