Flow measurement to assess graft patency should be routinely performed for multiarterial grafting and off-pump coronary bypass, as well as in other situations when doubts about the integrity of a graft arise. Likewise, the use of flow measurement findings to inform optimal intraoperative decision-making requires a good understanding of techniques for graft patency assessment.
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These are leading takeaways from an invited expert opinion by two Cleveland Clinic cardiothoracic surgeons recently published in JTCVS Techniques summarizing common problems affecting bypass grafts, techniques used to verify graft function and tips for interpreting flow measurements.
“Suboptimal flow measurements warrant investigation into possible sources of flow compromise but should not trigger an automatic graft revision,” says Faisal Bakaeen, MD, co-author of the piece along with his colleague Rami Akhrass, MD. “Surgeon judgment remains a key component of determining how to react in such situations.”
CABG is evolving
Multiarterial grafting is becoming a favored approach in coronary artery bypass graft surgery (CABG), with evidence mounting that graft patency and clinical outcomes are improved over saphenous vein or single internal thoracic artery grafts. At Cleveland Clinic, Dr. Bakaeen notes, multiarterial grafting is the default procedure unless contraindicated; it is used in about 30% of all CABG operations and for most patients undergoing elective CABG.
This trend has triggered the need for real-time assessment of graft patency. Properties of arteries make routine flow verification especially important: arterial grafts are more susceptible to dissections and hematomas that may compromise flow. They also have a more dynamic nature, with flow influenced by multiple factors, such as spasm, hemodynamic parameters and severity of target vessel stenosis. In addition, manual injection to test distal anastomotic flow is not possible in an in situ arterial graft.
“Intraoperative validation of graft patency provides the opportunity to correct any technical issues before closing the patient’s chest,” Dr. Bakaeen adds. “Having this extra information is important for both safety and peace of mind.”
According to the expert opinion piece, off-pump CABG — which is physiologically less invasive than traditional CABG and can benefit selected high-risk patients — also merits routine intraoperative flow assessment.
The hows and whys of flow measurement
Methods of intraoperative graft patency assessment are described in the article. Coronary angiography, although often considered the gold standard for flow assessment, is impractical and not available in most operating rooms. Use of a transit time ultrasonic flow meter is covered in some detail, as is interpretation of its results. Epicardial ultrasonography, which is typically performed in conjunction with transit time flow measurement, is also discussed.
The authors note that flow assessment can be particularly useful in identifying a number of flow-limiting scenarios, including the following:
- Technical errors. Mistakes in vessel harvesting or anastomosis suturing are not always visually apparent, and neither is misjudgment of the lay of a graft. All of these can be detected by graft flow measurements or imaging.
- Competitive native flow. Flow through a graft can be compromised when the target vessel is not severely stenotic, leading to reduced graft patency. Anticipating such situations by measuring flow after completing each bypass is key.
- Coronary-coronary or coronary-subclavian steal syndromes. These can result in retrograde flow.
- Even subtle tension can cause spasm, as can air embolism, disrupting flow. Once spasm is detected, systemically or directly injected milrinone or nitroglycerine may help.
Key takeaways for flow assessment
The article, which includes a short embedded video, summarizes leading observations and recommendations from the Cleveland Clinic experience. Dr. Bakaeen emphasizes the following messages:
- Routinely measure flow for multiarterial grafts and off-pump CABG. Flow measurement is especially important with multiarterial grafts when bypasses are based off only one inflow source.
- If in doubt, measure flow in other situations. Competitive or imbalanced flow can also complicate venous and single-artery CABG.
- Measure flow under optimal hemodynamic conditions. This advice stems from the fact that low systemic pressures are associated with low graft flow and suboptimal flow meter readings. Surgeons should also be aware that high-dose pressors can cause spasm-related diminished flow.
- Don’t rush to graft revision. Several measurements should be taken before embarking on a graft revision unless a cause is identified. The authors observe that the Achilles’ heel of transit time flow measurement is typically regarded as its potential for false negativity, with apparent good flow in the face of a bad graft because of retrograde flow. But they point out that a more common issue is false positivity — e., poor measured flow with a good graft — which is more often related to patient risk factors and anatomic causes than to technical errors. However, persistent zero or nearly no flow is a red flag for real graft compromise.
“Flow measurements are an important supplement to a surgeon’s clinical judgment, and it is critical that he or she know how to employ and interpret them,” Dr. Bakaeen emphasizes. “Judgment remains the key component in determining how to react to a poor graft flow measurement, and it is what ultimately determines whether intraoperative flow assessment proves to be a friend or foe.”
Image at top: Photo showing assessment of an ITA graft with a transit time flow meter. Inset shows a resulting transit time flow measurement. Images are from Akhrass and Bakaeen. JCTVS Tech. 2021 Jan 5 [Epub]. © The Authors.