Which finding is most consistent with collateral flow into a limb?

• A. Monophasic waveform with broad systolic upstroke and diastolic forward flow
• B. Triphasic waveform with robust forward flow and diastolic reversal
• C. Biphasic waveform with absent diastolic flow
• D. Monophasic waveform with low amplitude and no diastolic flow

Answer: (A)

Collateral flow into a limb happens when alternative low-resistance channels supply the distal arterial bed because of upstream occlusion. This creates a Doppler pattern that is monophasic with a broad, rounded systolic upstroke and forward flow persisting through diastole. The continuous diastolic forward flow reflects the steady, low-resistance runoff provided by the collaterals, rather than a pulsatile, high-resistance native arterial interface.

In contrast, a triphasic waveform with a brisk systolic upstroke, forward diastolic component, and an early diastolic reversal points to normal high-resistance, native arterial inflow rather than collateral-dominated supply. A biphasic pattern with absent diastolic flow indicates increased distal resistance, not collateral-driven flow. A monophasic pattern with low amplitude and no diastolic flow suggests more severe downstream resistance or occlusion without collateral compensation. Thus, the described monophasic waveform with broad systolic upstroke and diastolic forward flow best fits collateral inflow.