What is transit-time flow measurement (TTFM), and what intraoperative values suggest good graft patency?

• A. TTFM is a noninvasive ultrasound assessment of graft flow; mean graft flow >15 mL/min and PI <2 indicate satisfactory flow; abnormal values prompt revision.
• B. TTFM is an invasive angiographic test to measure graft patency.
• C. TTFM uses MRI to measure flow.
• D. TTFM monitors flow quality but not overall patency.

Answer: (A)

Transit-time flow measurement uses a perivascular ultrasonic flow probe placed around the graft to quantify blood flow in real time during the operation. It provides two key numbers: the mean graft flow in milliliters per minute and the pulsatility index (PI), which reflects resistance to flow.

A healthy graft typically shows a mean flow above about 15 mL/min and a PI below about 2. This combination suggests adequate blood supply with low resistance, consistent with good patency. If the values are low or the PI is high, it raises concern for a technical problem at the anastomosis, graft kinking, twisting, or competitive native flow, prompting intraoperative revision or further assessment.

This method is not an imaging study like MRI or conventional angiography; it’s an intraoperative flow measurement tool that helps confirm patency and function of the graft during the operation.