Evaluation of coronary bypass flow with color-Doppler and magnetic resonance imaging techniques: comparison with intraoperative flow measurements

Assessment of the Graft Quality and Patency during and after Coronary Artery Bypass Grafting

Coronary artery bypass grafting (CABG) is the gold standard procedure for multi vessels and left main coronary artery disease. The prognosis and survival outcomes of CABG surgery are highly dependent on the patency of the bypass graft. Early graft failure which can occur during or soon after CABG remains a significant issue, with reported incidences of 3-10%. Graft failure can lead to refractory angina, myocardial ischemia, arrhythmias, low cardiac output, and fatal cardiac failure, emphasizing the importance of ensuring graft patency during and after surgery to prevent such complications. Technical errors during anastomosis are among the leading causes of early graft failure. To address this issue, various modalities and techniques have been developed to evaluate graft patency during and after CABG surgery. These modalities aim to assess the quality and integrity of the graft, thus enabling surgeons to identify and address any issues before they lead to significant complications. In this review article, we aim to discuss the strengths and limitations of all available techniques and modalities, with the goal to identify the best modality for evaluating graft patency during and after CABG surgery.

Localizing intramyocardially embedded left anterior descending artery during coronary bypass surgery: literature review

Proper detection of the deeply embedded left anterior descending artery remains a challenge. Many authors proposed different methods for artery identification, such as ultrasound Doppler, cineangiography, retrograde dissection overlying tissues, and exposure over the probe. Choice of the technique often depends on the surgeon's acquaintance and experience. The article compares and summarizes different procedures for the detection of intramyocardially located left anterior descending artery.

Comparison of arterial and venous coronary artery bypass flow measurements using 3-T magnetic resonance phase contrast imaging

OBJECTIVE

Comparison of arterial and venous coronary artery bypass flow measurements using 3-T magnetic resonance (MR) phase contrast in correlation with intraoperative Doppler flow measurements.

METHODS

Fifty-six coronary bypasses (right coronary artery n=18, left internal mammary artery to left anterior descending artery n=16, marginal artery n=7, circumflex artery n=7, diagonal artery n=6, left anterior descending artery n=1, and right internal mammary artery to right coronary artery n=1) were studied in 27 asymptomatic patients. In this prospective study, each bypass was studied intra-operatively using Doppler flow measurement. Within one week post surgery, patients were studied using a 3-T MR scanner (Magnetom Verio, Siemens, Erlangen, Germany) using velocity encoded phase-contrast flow measurements.

RESULTS

Intraoperative Doppler flow measurements demonstrated regular flow patterns in all vascular territories supplied. All bypasses were patent on MRI and flow measurement results were as follows: median flow 60ml/min (interquartile range (IQR): 37.5-78.5ml/min). For comparison, the corresponding median intraoperative flow was 58ml/min (IQR: 41-80ml/min) (p<0.001; R=0.44). Linear regression analysis demonstrated a significant correlation for venous bypasses (p=0.0002; R=0.48), but not for arterial bypasses (p=0.09; R=0.24).

CONCLUSION

This study demonstrated that MR flow measurements of venous bypass grafts agreed more with Doppler than arterial bypass grafts. However, bypass patency was confirmed for all patients. In the future, this technique may be used for non invasive coronary bypass graft follow-up.

Quantifying coronary sinus flow and global LV perfusion at 3T

Background

Despite the large availability of 3T MR scanners and the potential of high field imaging, this technical platform has yet to prove its usefulness in the cardiac MR setting, where 1.5T remains the established standard. Global perfusion of the left ventricle, as well as the coronary flow reserve (CFR), can provide relevant diagnostic information, and MR measurements of these parameters may benefit from increased field strength. Quantitative flow measurements in the coronary sinus (CS) provide one method to investigate these parameters. However, the ability of newly developed faster MR sequences to measure coronary flow during a breath-hold at 3T has not been evaluated.

Methods

The aim of this work was to measure CS flow using segmented phase contrast MR (PC MR) on a clinical 3T MR scanner. Parallel imaging was employed to reduce the total acquisition time. Global LV perfusion was calculated by dividing CS flow with left ventricular (LV) mass. The repeatability of the method was investigated by measuring the flow three times in each of the twelve volunteers. Phantom experiments were performed to investigate potential error sources.

Results

The average CS flow was determined to 88 ± 33 ml/min and the deduced LV perfusion was 0.60 ± 0.22 ml/min·g, in agreement with published values. The repeatability (1-error) of the three repeated measurements in each subject was on average 84%.

Conclusion

This work demonstrates that the combination of high field strength (3T), parallel imaging and segmented gradient echo sequences allow for quantification of the CS flow and global perfusion within a breath-hold.

PIV-validated numerical modeling of pulsatile flows in distal coronary end-to-side anastomoses

This study employed particle image velocimetry (PIV) to validate a numerical model in a complementary approach to quantify hemodynamic factors in distal coronary anastomoses and to gain more insights on their relationship with anastomotic geometry. Instantaneous flow fields and wall shear stresses (WSS) were obtained from PIV measurement in a modified life-size silastic anastomosis model adapted from a conventional geometry by incorporating a smooth graft-artery transition. The results were compared with those predicted by a concurrent numerical model. The numerical method was then used to calculate cycle-averaged WSS (WSS(cyc)) and spatial wall shear stress gradient (SWSSG), two critical hemodynamic factors in the pathogenesis of intimal thickening (IT), to compare the conventional and modified geometries. Excellent qualitative agreement and satisfactory quantitative agreement with averaged normalized error in WSS between 0.8% and 8.9% were achieved between the PIV experiment and numerical model. Compared to the conventional geometry, the modified geometry produces a more uniform WSS(cyc) distribution eliminating both high and low WSS(cyc) around the toe, critical in avoiding IT. Peak SWSSG on the artery floor of the modified model is less than one-half that in the conventional case, and high SWSSG at the toe is eliminated. The validated numerical model is useful for modeling unsteady coronary anastomotic flows and elucidating the significance of geometry regulated hemodynamics. The results suggest the clinical relevance of constructing smooth graft-artery transition in distal coronary anastomoses to improve their hemodynamic performance.

Intraoperative Transit Time Flow Measurement: Off-Pump Versus On-Pump Coronary Artery Bypass

BACKGROUND

Off-pump coronary artery bypass grafting (OPCAB) has attracted increasing attention. Performing the anastomosis off-pump is technically more demanding. The objective of the study is to assess the quality of anastomosis in OPCAB in comparison with conventional on-pump coronary artery bypass grafting using the transit time flow measurement.

METHODS

Four hundred forty-five patients operated on using OPCAB technique were included in the study. For each patient in this group a similar patient from the on-pump coronary artery bypass grafting population was selected according to the number of grafts, bypass material, and target coronary arteries. The mean flow and the pulsatile index were measured in every bypass graft in both groups.

RESULTS

The average pulsatile index in OPCAB was 2.09 +/- 1.03 (mean flow, 39 +/- 22.63 mL/min), whereas with on-pump coronary artery bypass grafting it was 1.9 +/- 0.98 (mean flow, 44.19 +/- 23.58 mL/min); p = 0.005. Subgroup analysis showed significantly lower mean flows and higher pulsatile index with OPCAB in grafts to the obtuse marginal, diagonal, and right coronary artery, but not to the left anterior descending territory.

CONCLUSIONS

The quality of the anastomosis performed using the OPCAB technique might be jeopardized by less accessibility as in the case of lateral and posterior wall coronary arteries. Techniques to optimize the accessibility of the coronary artery like combining sling support with cup stabilizers, together with systematic training, should be strongly considered in OPCAB. Whenever there is good accessibility of the coronary artery as in the case of the left anterior descending, the anastomosis performed under OPCAB has a quality as good as that performed using the conventional technique.

Perioperative monitoring of flow and patency in native and grafted internal mammary arteries using a combined MR protocol

The objective of this study was to evaluate graft flow (f) and patency (p) in patients with internal mammary artery (f,p) and venous (p only) grafts using a combined MR protocol with phase-contrast technique and MR angiography. 42 patients with 42 left internal mammary artery (LIMA) and 63 venous grafts were examined pre and 6 months post coronary artery bypass graft (CABG) surgery. Phase-contrast flow measurements were applied to the IMA. Post-operatively, a contrast enhanced MR angiogram was performed to assess bypass patency. LIMA/venous grafts were occluded in 3/42 and 13/63, respectively. Flow in LIMA decreased from 19.4+/-10.4 ml min(-1) m(-2) pre-operatively to 13.4+/-9.7 ml min(-1) m(-2) post-operatively (p<0.002). In contrast, flow in the native right IMA increased from 17.6+/-8.7 ml min(-1) m(-2) pre-operatively to 24.8+/-9.0 ml min(-1) m(-2) post-operatively (p<0.001). MRI allows a combined assessment of bypass patency and flow. This study protocol may be applicable to perioperative follow-up studies in patients after CABG surgery.

Evaluation of in-stent stenosis by magnetic resonance phase-velocity mapping in nickel-titanium stents

PURPOSE

To evaluate different grades of in-stent stenosis in a nickel-titanium stent with MRI.

MATERIALS AND METHODS

Magnetic resonance phase velocity mapping (MR-PVM) was used to measure flow velocity through a 9-mm NiTi stent with three different degrees of stenosis in a phantom study. The tested stenotic geometries were 1) axisymmetric 75%, 2) axisymmetric 90%, and 3) asymmetric 50%. The MR-PVM data were subsequently compared with the velocities from computational fluid dynamic (CFD) simulations of identical conditions.

RESULTS

Good quantitative agreement in velocity distribution for the 50% and 75% stenoses was observed. The agreement was poor for the 90% stenosis, most likely due to turbulence and the high-velocity gradients found in the small luminal area relative to the pixel resolution in our imaging settings.

CONCLUSION

The accuracy of the MRI velocities inside the stented area renders MRI a modality that may be used to assess moderate to severe in-stent restenosis (ISR) in medium-sized vascular stents in peripheral vessels, such as the iliac, carotid, and femoral arteries. Advances in MR instrumentation may provide sufficient resolution to obtain adequate velocity information from smaller vessels, such as the coronary arteries, and allow MRI to substitute for invasive and expensive catheterization procedures currently in clinical use.

Visualization of intramuscular left anterior descending coronary arteries during off-pump bypass surgery

In off-pump coronary artery bypass surgery, an appropriate method for intraoperative evaluation of grafts and vessels has been awaited. We report the usefulness of a 15-MHz linear transducer for this purpose. A 15-MHz linear transducer with a SONOS 5500 (Philips Medical Systems, Best, Netherlands) was applied epicardially in off-pump coronary artery bypass surgery patients. Vascular anatomy was easily discerned when the transducer was applied in an appropriate way. In 6 patients, intramuscular coronary arteries were easily detected, and in all of these patients, anastomoses were successful. The shapes of the anastomoses were very clearly shown, and the flow and its phase in the bypass graft or coronary artery were measured with synchronization of electrocardiograms in all patients. The total left internal thoracic artery (LITA) flow (28.4 +/- 6.8 mL/s) and the pattern of the flow was dominantly diastolic in all patients. The 15-MHz linear transducer system (Philips) is very useful for detecting intramuscular left anterior descending coronary arteries and may become one of the standard tools for intraoperative evaluation in off-pump coronary artery bypass surgery.

Intraoperative flow measurement of native coronary artery can help decision making before CABG

We report the case of a patient with three-vessel coronary artery disease whose right coronary artery had been stented at the time of the diagnostic procedure. He had recurrent angina 12 days later and was transferred for urgent coronary artery bypass grafting. No repeat coronary angiography was performed. In the operating room, the flow on the native right coronary artery was determined with an ultrasonic flow probe.

Differences in LIMA Doppler characteristics for different LAD perfusion areas

OBJECTIVES

To correlate supraclavicular left internal mammary artery (LIMA) to left anterior descending artery (LAD) area Doppler characteristics with angiographically perfused area.

METHODS

Sixty patients (50 male, mean age 62+/-7.3 years) with LIMA to LAD area grafting were prospectively entered in a follow up study. Supraclavicular echo Doppler of the LIMA was studied at the LIMA origin preoperatively, and at 4.8+/-3.8 months and 1.8+/-0.9 years postoperatively. The potential area to be revascularized judged from preoperative angiography was called the 'target' area. Control angiography (native and LIMA) was done at 1.5+/-0.9 years. The perfused area % was classified into group I < or =17.0% (n=16), group II >17.0% and <22.50% (n=17), and group III > or =22.50% (n=18) and related to LIMA Doppler characteristics. Multivariate linear regression analyses (MLRA) were performed to assess the relations between Doppler variables and the perfused area, target area and ratio of perfused/target area.

RESULTS

At MLRA perfused area was significantly related to the natural logarithm of diastolic peak velocity (DPV) (P=0.013) and diastolic mean velocity (P=0.048) and the ratio only to the degree of LAD stenosis (P=0.004). In hyperaemic response maximal DPV (DPV max) showed significant correlation to the perfused area (P=0.005) as well as to the ratio (P=0.017). When analyzing the additive power of both investigations, only DPV max (P=0.005) correlated significantly to the perfused area and for the ratio only the degree of stenosis of the LAD emerged as significant (P=0.004).

CONCLUSIONS

At MLRA the diastolic flow pattern at rest and the maximal DPV in hyperaemic response correlated significantly with the LIMA run-off area whereas the last variable is the strongest predictor of the LIMA run-off area.

Mr flow measurement in the internal mammary artery-to-coronary artery bypass graft: comparison with graft stenosis at radiographic angiography

PURPOSE

To evaluate the sensitivity and specificity of breath-hold magnetic resonance (MR) flow measurement for detection of significant stenosis in internal mammary artery bypass grafts.

MATERIALS AND METHODS

Twenty-six consecutive patients who had undergone coronary artery bypass surgery were examined. Breath-hold velocity-encoded cine MR images were obtained at the midpoint of the internal mammary artery between its origin from the subclavian artery and the distal anastomosis to the left anterior descending artery.

RESULTS

MR images were obtained successfully in 24 patients. At conventional angiography, no significant stenosis was observed in 17 patients (group A), and significant stenosis (diameter > 70%) was observed in seven patients (group B). The mean diastolic-to-systolic peak velocity ratio in group B (0.61 +/- 0.44 [SD]) was significantly lower than that in group A (1.88 +/- 0.96; P <.01). Evaluation of graft stenosis with the diastolic-to-systolic peak velocity ratio revealed a sensitivity of 86% and a specificity of 88%. The mean blood flow rate at baseline in group B (16.9 mL/min +/- 5.5) was significantly lower than that in group A (79.8 mL/min +/- 38.2; P <.01). The sensitivity and specificity of MR blood flow measurement in predicting significant stenosis were 86% and 94%, respectively. The mean pharmacologic flow reserve ratios were 2.00 +/- 1.43 in group A and 1.39 +/- 1.46 in group B (P >.05).

CONCLUSION

Fast MR blood flow measurement at baseline is highly useful for predicting significant stenosis in internal mammary arterial grafts.

Accuracy of segmented MR velocity mapping to measure small vessel pulsatile flow in a phantom simulating cardiac motion

The purpose of this study was to investigate the accuracy of conventional, segmented, and echo-shared MR velocity mapping sequences to measure pulsatile flow in small moving vessels using a phantom with simulated cardiac motion. The phantom moved either cyclically in-plane, through-plane, in- and through-plane, or was stationary. The mean error in average flow was -2% +/- 3% (mean +/- SD) for all sequences under all conditions, with or without background correction, as long as the region of interest (ROI) size was equal to the vessel cross-sectional size. Overestimation of flow as a result of an oversized ROI was less than 20%, and independent of field of view (FOV) and matrix, as long as the offset in angle between the imaging plane and flow direction was less than 10 degrees. Segmented velocity mapping sequences are surprisingly accurate in measuring average flow and render flow profiles in small moving vessels despite the blurring in the images due to vessel motion. J. Magn. Reson. Imaging 2001;13:722-728.

Graft patency verification in coronary artery bypass grafting: principles and clinical applications of transit time flow measurement

The increasing popularity of beating-heart coronary surgery has raised concerns and doubts about the quality of the coronary anastomoses performed. Intraoperative graft patency verification methods are not commonly used after coronary surgery and, most of the cardiac surgeons, rely on the simple clinical signs (electrocardiogram tracings and hemodynamic stability) to make a diagnosis of coronary graft occlusion. New transit time ultrasound based methods for graft-patency verification have been adopted in many centers during beating-heart and traditional bypass grafting. Although the results are very encouraging, correct interpretation of the flow findings may prove difficult if specific rules are not properly followed. Flow curves, pulsatility index, and flow values should always be considered simultaneously before revising a coronary graft. Measurements should also be always performed with and without a proximal coronary snare. This article summarizes the main features of flowmetry and provides some technical pitfalls and suggestions to achieve an adequate intraoperative flow measurement adopting the transit time method.

Graft revision after transit time flow measurement in off-pump coronary artery bypass grafting

OBJECTIVE

To determine whether coronary graft patency can be predicted by transit time flow measurement (TTFM).

METHODS

From May 1 1997 to December 31 1998, TTFM was prospectively evaluated in 409 patients undergoing coronary artery bypass grafting (CABG) without cardiopulmonary bypass (CPB). All grafts (1145) were tested with TTFM.

RESULTS

Thirty-seven out of 1145 grafts (3.2%) were revised in 33 patients (7.6%). In six cases (18.1%) use of CPB was necessary during revision due to hemodynamic instability. The remaining patients underwent revision off-pump. Thirty-four grafts (91.9%) were revised for both low flow and abnormal flow curve patterns. Findings at revision included: thrombosis of the anastomosis (n=6), stenosis at the toe or heel of the anastomosis (n=8), coronary flap or dissection (n=5), dissection of the internal mammary artery (n=5), graft kinking (n=4), flap at proximal anastomosis (n=1), coronary stenosis distal to the graft (n=3), and no findings (n=2). After revision all flow values and flow patterns improved. Although three additional grafts (8.1%) were revised for low flow (<7 ml/min) despite normal flow patterns, there were no findings at revision and flow values and curves remained unchanged after revision. Postoperatively, one patient developed a stroke (3%), one had an acute myocardial infarction (MI) (3%), one had a sternal wound infection (3%), and one required prolonged ventilatory support (3%).

CONCLUSION

Evaluation of TTFM is valuable in determining the status of a coronary graft after CABG. Correct interpretation of flow patterns allows for correction of abnormalities prior to chest closure.

Difficulties in the interpretation of coronary angiogram early after coronary artery bypass surgery on the beating heart

OBJECTIVE

The major objective of this study was to evaluate the findings in early postoperative coronary angiography in patients who underwent coronary revascularization on the beating heart without cardiopulmonary bypass.

METHODS

Eighty-four consecutive patients receiving 113 grafts were studied. A coronary angiography was performed 0 to 5 days postoperatively. All the grafts were reviewed and classified in the following way: grade A (unimpaired run-off); grade B1 (<50 stenosis); grade B2 (>50% stenosis); grade O (occlusion). A second coronary angiography was performed in patients with a stenosis grade B2, 4 to 30 months postoperatively. An exercise test was performed by patients with B1 stenosis.

RESULTS

Overall graft patency was 96% in the 113 grafts. None of the 14 patients with B1 stenosis in the early coronary angiography had any clinical signs of ischemia. Eight of the 12 patients who exhibited B2 stenosis either at the anastomotic site, in the graft or in the distal coronary artery at the first coronary angiography had a normal angiogram at the re-angiography.

CONCLUSION

A majority of stenoses visualized at the early coronary angiography could not be seen at a later coronary angiography, which makes the interpretation of the angiogram unreliable as a tool for the decision as to redo-procedure in the early postoperative period.