Coronary collaterals.

Coronary collaterals are probably enlargements of pre-existing channels which respond to local vasodilators and which function whenever pressure differences exist across them. Thus, in human coronary atherosclerosis collaterals are only seen when there is a severe intervening arterial obstruction (in excess of 75%). Coronary collaterals follow epicardial and intramycardial pathways, and the intermediary connections may be at vessels of highly varying caliber. The flow potential of most collateral pathways in man is possibly adequate for segmental myocardial function at lower than normal demands but clearly is inadequate for most, if not all, stressful interventions. In the last analysis, coronary collaterals in man are more an indication of severe regional ischemia (present or potential) than a sign of biological "compensation'' for a perfusion deficit.