How do I check the pulsatility index of a coronary bypass graft intraoperatively?

How to Check Pulsatility Index Intraoperatively During CABG

The pulsatility index (PI) is measured using transit-time flow measurement (TTFM), which is the most widely used intraoperative method for assessing coronary bypass graft function. 1

Technique for Measuring Pulsatility Index

Equipment and Method

• TTFM uses an ultrasonic flow probe placed directly around the bypass graft after completion of the anastomosis to measure real-time blood flow patterns. 2
• The device automatically calculates and displays the PI along with other parameters including mean graft flow (MGF) and insufficiency ratio (IR). 2, 3
• Measurements should be obtained twice: first after completing the anastomosis, and again after protamine administration before chest closure. 4

What the Pulsatility Index Represents

• PI reflects the resistance to flow in the graft and distal coronary bed—higher values indicate greater resistance or flow impairment. 1
• The calculation is derived from the flow waveform pattern captured by the transit-time probe. 2

Interpreting Pulsatility Index Values

Critical Thresholds

• PI > 5 is strongly associated with graft failure and predicts major adverse cardiac events, including operative death. 1
• For left coronary artery grafts, PI > 3 suggests potential graft dysfunction (sensitivity 72%, specificity 70%). 5, 3
• For right coronary artery grafts, the threshold is higher: PI > 5 indicates abnormal flow due to the different flow characteristics in right coronary territories. 3
• Combining PI with mean graft flow improves diagnostic accuracy: MGF < 15 mL/min plus PI > 3 (left) or PI > 5 (right) has 96% sensitivity and 77% specificity for detecting graft abnormalities. 2, 3

Flow Pattern Assessment

• In left coronary territories, normal grafts show diastolic-dominant flow patterns; systolic-dominant or balanced patterns suggest dysfunction. 3
• In right coronary territories, systolic-dominant patterns indicate abnormal flow. 3
• None of the abnormal grafts in validation studies showed diastolic-dominant flow patterns. 3

Important Limitations and Pitfalls

False Negatives Are Common

• TTFM has a critical weakness: it can miss poor grafts with low PI (false negatives), meaning a normal PI does not guarantee graft patency. 1
• Studies show that 63-70% of grafts flagged as abnormal by TTFM criteria (PI > 3 or combined low flow/high PI) were actually patent on follow-up angiography. 5
• TTFM failed to detect occluded grafts in controlled studies when relying on PI alone. 6

False Positives Lead to Unnecessary Revisions

• High PI values (false positives) rarely occur in truly good grafts, but intermediate elevations (PI 3-5) frequently lead to unnecessary graft revisions. 1, 5
• The literature-suggested cutoffs result in unnecessary revisions in the majority of flagged cases. 5

Context-Dependent Interpretation

• Right coronary artery grafts normally have higher PI values than left-sided grafts due to different flow dynamics, so use territory-specific thresholds. 3
• Competitive flow from non-critically stenosed native vessels can elevate PI even in technically perfect grafts. 3

Recommended Approach to Intraoperative Graft Assessment

Combine TTFM with Additional Modalities

• Because TTFM assesses function but not anatomy, combine it with intraoperative fluorescence imaging (IFI) or high-resolution epicardial ultrasound to evaluate both graft function and anastomotic integrity. 1, 2
• Epicardial ultrasound provides 2D imaging and color-flow mapping to directly visualize the graft body and anastomosis. 2

When to Revise Based on PI

• Revise immediately if PI > 5 in any graft, as this threshold has the strongest association with graft failure and adverse outcomes. 1
• For PI 3-5, do not revise based on PI alone—obtain additional assessment with epicardial ultrasound or intraoperative angiography to confirm anatomic defects before revision. 1, 2
• If MGF < 15 mL/min AND PI > 3 (left) or > 5 (right), strongly consider revision after confirming with imaging. 2, 3

Gold Standard Remains Angiography

• Intraoperative coronary angiography is the most reliable method for assessing graft patency and anastomotic quality, but it requires hybrid operating room infrastructure that is rarely available. 1
• When available, angiography should be used to confirm TTFM findings before graft revision. 4, 6

Practical Algorithm

1. Measure TTFM parameters (PI, MGF, IR) after anastomosis completion and after protamine
2. If PI ≤ 3 (left) or ≤ 5 (right) with MGF ≥ 15 mL/min → graft likely adequate
3. If PI > 5 → high risk for failure, obtain confirmatory imaging or revise
4. If PI 3-5 with low flow → add epicardial ultrasound or IFI before deciding on revision
5. Document flow patterns: diastolic-dominant (left) or non-systolic-dominant (right) patterns are reassuring 3

The clinical value of TTFM is established—it predicts graft failure at 3,6, and 12 months post-CABG—but its limitations require cautious interpretation and integration with other assessment methods. 1, 5