[0081] FIG. 2B illustrate outer catheter 112 and inner catheter 114 after engagement of thrombus 126. When inner catheter distal opening 122 is blocked by thrombus (or partially blocked by thrombus 126 such that adequate flow of fluid is not maintained through inner catheter distal opening 122), aspiration pump 6 may deliver aspiration fluid through outer catheter lumen 116 between an inner surface of outer catheter 112 and an outer surface of inner catheter 114. However, the aspiration fluid may be delivered through sidewall openings 124 into inner catheter lumen 120, even though inner catheter distal opening 122 is blocked by thrombus 126. For example, aspiration fluid may be delivered from outer catheter inner lumen 116 through a sidewall opening 124 within outer catheter distal opening 116 into inner catheter lumen 120 and/or delivered through outer catheter distal opening 118 into a volume outside outer catheter distal opening 118, such as near thrombus 126, and through a sidewall opening 124 outside outer catheter distal opening 118. In some examples, the aspiration fluid may be delivered through sidewall openings 124 in response to a differential pressure (i.e., difference between positive pressure and negative pressure) exceeding a differential pressure threshold (i.e., minimum differential pressure) of the sidewall openings 124. The aspiration fluid (or combination of fluids) may flow near a surface of the engaged thrombus 126 within inner catheter lumen 120, such that pieces of thrombus 126 may continue to be loosened and removed from thrombus 126. As such, the fluid flow shown in FIG. 2B may characterize a second flow path of fluid from outer catheter lumen 116 through outer catheter distal opening 118 and at least a first sidewall opening of the plurality of sidewall openings 124 to inner catheter lumen 120, and a third flow path of fluid from outer catheter lumen 116 directly through a second sidewall opening of the plurality of sidewall openings 124 to inner catheter lumen 120. [0082] Aspiration pump 6 may draw the pieces of thrombus 126, along with fluid, into inner catheter lumen 120 far removal of the pieces of thrombus 126. In this way, aspiration catheter system 110 may maintain both suction and -partial flow through inner catheter lumen 120 on engaged thrombus 126 to remove thrombus 126, even when distal opening 122 is partially or fully blocked by thrombus 126. [0083] In some examples, the sidewall openings discussed herein may include one‹ way valves having a variety of opening mechanisms. FIGS. 3A-3C are schematic diagrams illustrating various valve configurations for sidewall openings. FIG. 3A is a schematic diagram illustrating an example duckbill valve 200. Duckbill valve 200 may include two or more flaps 204 extending from an inner catheter 202. Duckbill valve 200