CD34 and angina pectoris: Support for this mechanism comes from two groups who have reported independently that patients treated with EECP experienced increases in CD34+ endothelial progenitor cell (EPC) stem cell subpopulations in their peripheral blood [15,16] while animal and human studies [9,10,11,17,18,19,20,21,22] and direct evidence in human patients treated with EECP for stable angina have demonstrated an increased collateral flow index [23].