Brachial artery shows triphasic flow but with blunting of the dialostic component and suggests collateral distal flow. what does this mean?

Perhaps a fistula: Arterial blood flow has three phases, rapid forward flow systolic flow, transient reversal of flow during early diastole, and slow forward flow during late diastole. In your case, there appears to be some change in the diastolic flow, suggesting that there may be some connection that has formed over time or resulted from some trauma. The significance depends on the clinical history and findings.