proximal of distal opening 18). For example, the clinician may desire that a higher flow rate of aspiration fluid passes through sidewall openings 24 that are distal of outer catheter distal opening 18; in such a case, the clinician may select a predetermined position corresponding to an increased number of sidewall openings proximal of catheter distal opening 18 (e.g., within lumen 16). [0097] As pieces of the thrombus are removed, the thrombus may block inner catheter distal opening 22 to a lesser extent, such that flow of fluid into inner catheter distal opening 22 may increase. In response, the clinician may reposition aspiration catheter system 10, such as by advancing aspiration catheter system in a distal direction, to continue to remove the thrombus. [0098] Catheter 12 may be removed from the vasculature once the procedure is complete. [0099] In some examples, inner catheter 14 may be used during a medical procedure with all of sidewall openings 24 in a blood vessel of a patient distal to or otherwise not within outer catheter 12 or another outer catheter. For example, inner catheter 14 may be used on its own without outer catheter 12, although a guide catheter may be used to deliver inner catheter 14 to a treatment site within the vasculature of a patient. In some of these examples, inner catheter 14 may be used as an aspiration catheter. For example, aspiration pump 6 may create a negative pressure on inner catheter 14 to draw fluid into inner catheter lumen 20, e.g. to aspirate a thrombus from a blood vessel. In some of these examples, sidewall openings 24 may be configured to enable flow of fluid from the blood vessel into inner catheter lumen 20 through sidewall openings 24, such as in response to a positive pressure in the blood vessel being greater than a negative pressure in inner catheter lumen 20. In some examples, the plurality of sidewall openings 24 may be configured to open in response to a differential pressure in the blood vessel and in inner catheter lumen 20 (i.e., a difference between a positive pressure in the blood vessel and a negative pressure in inner catheter lumen 20) being greater than or equal to a predetermined differential pressure threshold value. This may enable fluid to flow into inner catheter lumen 20 when inner catheter distal opening 22 is blocked, such as by the thrombus. In this way, the clinician may maintain both suction on an engaged thrombus and continuous flow of fluid on the engaged thrombus, which may result in more effective removal of the thrombus from the vasculature of the patient. [0100] Various aspects of the disclosure have been described. These and other aspects are within the scope of the following claims. While the dimensions of catheters described