1. A system for determining cardiovascular information for a patient, the system comprising: at least one computer system configured to: obtain a first geometric model representing at least a location of interest in a lumen of a patient's anatomy using received patient-specific data; determine a first blood flow rate within the patient's anatomy at the location of interest in the lumen of the first geometric model; receive a change in the lumen geometry of the lumen of the first geometric model; modify the first geometric model by incorporating the change in the lumen geometry of the first geometric model; determine a second blood flow rate at a location in the modified model equivalent to the location of interest of the first geometric model; and determine a blood flow characteristic of the patient by calculating a ratio of the second blood flow rate calculated from the modified model to the first blood flow rate calculated from the first geometric model.