Introduced by Cornelis Drost during the last quarter of the last century, transit-time ultrasound technology has become recognized as the gold standard for accurate intraoperative volume flow measurements. Measurements are quick, simple and repeatable. However, in order to achieve accurate flow measurements, good technique matters.

As with any skill, surgical team members must first familiarize themselves and be comfortable with the correct procedure for measuring coronary bypass graft flow. A flowprobe sized so that the vessel fills 75 percent of the flowprobe’s sensing window should be used with adequate ultrasound couplant to produce a good ultrasonic signal. Motion artifacts should be avoided by holding the flowprobe still on the graft during the measurement. Occlude the native coronary temporarily to assess the presence of competitive flow and to know the full flow capacity of the coronary artery and graft to the distal myocardial territory fed by that respective graft.

Flow Measurement Protocol

A step-by-step protocol for measuring flow follows:

1. If using an internal mammary artery graft, skeletonize a 1.5 cm segment of the artery before performing the anastomosis. Vein grafts require no additional preparation.
2. Select a flowprobe sized so that the graft will fill at least 75 percent of the sensing window of the flowprobe. Take care not to undersize the probe for the graft.
3. Apply a water-soluble ultrasound couplant (such as Surgilube or Aquasonic 100) to the window of the flowprobe.
4. Turn on FlowSound. A low-pitch zero flow sound (“hum”) indicates that the probe is properly connected to the flowmeter, and that there is adequate ultrasound signal coupling for a measurement.
5. Place the flowprobe on the graft, bending its flexible neck as needed. Avoid kinking the graft or placing the flowprobe over surgical clips or sutures. The ultrasound signal quality is indicated on the AureFlo monitor or the flowmeter’s front panel display.
6. Observe the contraction of the heart while listening to FlowSound. Listen for a strong diastolic flow component.
7. Note, after 10 seconds, the average (mean) flow displayed on the AureFlo screen or the front panel of the flowmeter.
8. Occlude the native coronary artery and note any changes in the pitch and pattern of FlowSound. An increase in FlowSound pitch (i.e., mean flow) indicates the presence of competitive flow. If no competitive flow is observed, the occlusion may be released.
9. When flow has stabilized in 10 to 15 seconds, press PRINT on the flowmeter, or tap SNAPSHOT or RECORD on AureFlo to document the measurement. Hold the probe steady on the graft until the printer stops or for 8 seconds if taking a recording.