The Doppler waveform from a collateral channel shows high resistance compared with contralateral normal artery. The increased resistance may be attributed to:

The Doppler waveform from a collateral channel shows high resistance compared with contralateral normal artery. The increased resistance may be attributed to:

• A. Decreased tube diameter
• B. Increased vessel length
• C. Decreased resistance in the vascular bed
• D. Both Decreased tube diameter and Increased vessel length

Answer: (D)

High-resistance flow shows when the path for blood is more impedance to movement, which happens when a vessel is longer and narrower. Poiseuille’s law links resistance to both vessel length and radius: R ∝ L / r^4. That means longer vessels increase resistance, and even more dramatically, smaller radii (diameter) increase resistance because of the radius to the fourth power. Collateral channels are typically longer and have a smaller diameter than a normal artery, so both factors raise resistance and produce the high-resistance Doppler waveform. If the vascular bed’s resistance were decreased, overall impedance would drop, not rise, so that doesn’t explain the high-resistance pattern. The combination of reduced diameter and increased length best accounts for the observation.

High-resistance flow shows when the path for blood is more impedance to movement, which happens when a vessel is longer and narrower. Poiseuille’s law links resistance to both vessel length and radius: R ∝ L / r^4. That means longer vessels increase resistance, and even more dramatically, smaller radii (diameter) increase resistance because of the radius to the fourth power. Collateral channels are typically longer and have a smaller diameter than a normal artery, so both factors raise resistance and produce the high-resistance Doppler waveform.

If the vascular bed’s resistance were decreased, overall impedance would drop, not rise, so that doesn’t explain the high-resistance pattern. The combination of reduced diameter and increased length best accounts for the observation.