Absence of atherosclerosis in human intramyocardial coronary arteries: a neglected phenomenon.

Atherosclerosis is absent in human intramyocardial (buried) coronary arteries but atherosclerosis may be severe in superficial segments of the same vessels. The development of atherosclerosis has three phases: a plasma phase, a transfer phase and an intramural phase. The transfer phase involves the transfer of low-density lipoproteins and macrophages from the plasma into the arterial wall. Efficiency of transfer is low where plasma flow near the wall is rapid. Eddy currents caused by arterial branches produce low flow near the arterial wall. Plasma recalculates and moves slowly in these eddy currents and thus prolongs contact of LDL and macrophages with the wall, increasing the occurrence of atherosclerosis. Absence of atherosclerosis in buried vessels appears due to the effects of myocardial contraction on the transfer phase. Contraction of the myocardium surrounding buried arterial vessel compresses these vessels and moves the plasma, LDL and macrophages away from the wall. This will decrease transfer into the wall and act to prevent the development of atherosclerosis. Similar but less striking effects occur where bridges of myocardium cross arterial vessels. Possible applications to human disease are discussed briefly.