What best explains the finding of a new systolic bruit on the left after losing the bruit on the right in a patient with a prior 70% stenosis of the right ICA?

The presence of a new systolic bruit on the left side, following the loss of a bruit on the right side where there was a previously documented 70% stenosis of the right internal carotid artery (ICA), is indicative of a progression of vascular disease.

In this scenario, the bruit on the right side may have previously signified turbulent blood flow due to significant stenosis. The loss of this bruit suggests either a decrease in the degree of stenosis, perhaps due to treatment or changes in the hemodynamics, or a complete occlusion of the right ICA. However, the development of a new bruit on the left side points towards an increase in flow velocity through the left ICA, which might occur as a compensatory response to the condition of the right ICA.

In cases where one arterial pathway becomes compromised, the opposite side often experiences increased blood flow to maintain cerebral perfusion. This can lead to the formation of a new bruit indicating significant flow disturbances due to either new disease formation on the left or compensatory mechanisms initiated by the worsening status of the right ICA. Given that the patient has a history of stenosis, it is plausible that the right ICA has worsened, thereby contributing to increased flow or turbulence in the left ICA