Flow dynamics of angiographically no-flow patent internal mammary artery grafts

Changes in Internal Thoracic Artery Blood Flow According to the Degree of Stenosis of the Anterior Descending Branch of the Left Coronary Artery

PURPOSE

Computational fluid dynamics has enabled the evaluation of coronary flow reserve. The purpose of this study was to clarify the hemodynamic variation and reserve potential of the left internal thoracic artery (LITA).

METHODS

Four patients were selected on the basis of various native coronary stenosis patterns and graft design. The wall shear stress and oscillatory shear index were measured, and one patient was selected. Next, we created three hypothetical lesions with 75%, 90%, and 99% stenosis in front of the graft anastomosis, and compared the changes in LITA blood flow and coronary flow distribution.

RESULTS

In the 75% to 90% stenosis model, blood flow was significantly higher in the native coronary flow proximal to the coronary artery bypass anastomosis regardless of time phase. In the 99% stenosis model, blood flow from the LITA was significantly dominant compared to native coronary flow at the proximal site of anastomosis. The range of LITA flow variability was the largest at 99% stenosis, with a difference of 70 ml/min.

CONCLUSION

The 99% stenosis model showed the highest LITA flow. The range of LITA flow variability is large, suggesting that it may vary according to the rate of native coronary stenosis.

HEMODYNAMIC INVESTIGATION OF SUBCLAVIAN-CORONARY STEAL SYNDROME IN DIALYSIS PATIENTS WITH CORONARY ARTERY OCCLUSION AND DIFFERENT STENOSIS PERCENTAGES IN SUBCLAVIAN ARTERY

Purpose: Diseases of the coronary arteries represent critical cardiovascular bleakness and mortality around the world. The main focus of this study is to simulate hemodialysis patients undergoing Coronary Artery Bypass Graft (CABG) surgery by the left internal mammary artery. The survey is to investigate whether subclavian-coronary steal syndrome will occur in patients with stenosis in their left subclavian.

Methods: A three-dimensional model of Brachial—Radial—Ulnar tree arterial system and dialysis graft are constructed and then simulated by the implementation of fluid–structure interaction (FSI) and non-FSI models. After consent was obtained, data attributed to dialysis patients who had undergone coronary bypass surgery at the Tehran Heart Center (THD) was collected.

Results: Stenosis in the subclavian artery causes the left internal mammary artery (LIMA) graft to lack flow that, in CABG cases, is supplying the heart muscle. With the increase of stenosis from zero to 54%, the flow results show a negative flow indicating reversed flow in the artery. Meanwhile, the comparison between flow quantity of a normal Left Anterior Descending (LAD) and LIMA in case of 38% stenosis in subclavian shows that the amount of flow reaching the heart is less than standard. The threshold stenosis in the subclavian artery for which subclavian-coronary steal phenomena occurs is simulated to be around 54%. Furthermore, investigation of the effect of flow quantity in cases of hemodialysis versus nonhemodialysis shows that the flow is lessened in case of hemodialysis.

Conclusions: In hemodialysis patients with more than 38% stenosis in left subclavian, it is well advised that LIMA bypass graft is not the preferred graft choice, and at 54% of subclavian stenosis, the steal phenomenon occurs.

Physiology of in-situ arterial revascularization in coronary artery bypass grafting: Preoperative, intraoperative and postoperative factors and influences

Surgical revascularization with coronary artery bypass grafting (CABG) has become established as the most effective interventional therapy for patients with moderately severe and severe stable ischemic heart disease (SIHD). This recommendation is based on traditional 5-year outcomes of mortality and avoidance of myocardial infarction leading to reintervention and/or cardiac death. However, these results are confounded in that they challenge the traditional CABG surgical tenets of completeness of anatomic revascularization, the impact of arterial revascularization on late survival, and the lesser impact of secondary prevention following CABG on late outcomes. Moreover, the emergence of physiologic-based revascularization with percutaneous cardiovascular intervention as an alternative strategy for revascularization in SIHD raises the question of whether there are similar physiologic effects in CABG. Finally, the ongoing ISCHEMIA trial is specifically addressing the importance of the physiology of moderate or severe ischemia in optimizing therapeutic interventions in SIHD. So it is time to address the role that physiology plays in surgical revascularization. The long-standing anatomic framework for surgical revascularization is no longer sufficient to explain the mechanisms for short-term and long-term outcomes in CABG. Novel intraoperative imaging technologies have generated important new data on the physiologic blood flow and myocardial perfusion responses to revascularization on an individual graft and global basis. Long-standing assumptions about technical issues such as competitive flow are brought into question by real-time visualization of the physiology of revascularization. Our underestimation of the impact of Guideline Directed Medical Therapy, or Optimal Medical Therapy, on the physiology of preoperative SIHD, and the full impact of secondary prevention on post-intervention SIHD, must be better understood. In this review, these issues are addressed through the perspective of multi-arterial revascularization in CABG, which is emerging (after 30 years) as the “standard of care” for CABG. In fact, it is the physiology of these arterial grafts that is the mechanism for their impact on long-term outcomes in CABG. Moreover, a better understanding of all of these preoperative, intraoperative and postoperative components of the physiology of revascularization that will generate the next, more granular body of knowledge about CABG, and enable surgeons to design and execute a better surgical revascularization procedure for patients in the future.

Computational modeling of blood flow steal phenomena caused by subclavian stenoses

The study of steal mechanisms caused by vessel obstructions is of the utmost importance to gain understanding about their pathophysiology, as well as to improve diagnosis and management procedures. The goal of this work is to perform a computational study to gain insight into the hemodynamic forces that drive blood flow steal mechanisms caused by subclavian artery stenosis. Such condition triggers a flow disorder known as subclavian steal. When this occurs in patients with internal thoracic artery anastomosed to the coronary vessels, the phenomenon includes a coronary-subclavian steal. True steal can exist in cases of increased arm blood flow, potentially resulting in neurological complications and, in the case of coronary-subclavian steal, graft function failure. In this context, the anatomically detailed arterial network (ADAN) model is employed to simulate subclavian steal and coronary-subclavian steal phenomena. Model results are verified by comparison with published data. It is concluded that this kind of model allows us to effectively address complex hemomdynamic phenomena occurring in clinical practice. More specifically, in the studied conditions it is observed that a regional brain steal occurs, primarily affecting the posterior circulation, not fully compensated by the anterior circulation. In the case of patients with coronary revascularization, it is concluded that there is a large variability in graft hemodynamic environments, which physically explain both the success of the procedure in cases of severe occlusive disease, and the reason for graft dysfunction in mildly stenosed left anterior descending coronary artery, due to alternating graft flow waveform signatures.

Competitive flow in coronary bypass surgery: is it a problem?

PURPOSE OF REVIEW

Competition flow is a common finding in arterial grafting. This phenomenon can lead to graft occlusion.

RECENT FINDINGS

Internal thoracic arteries are the best equipped arterial conduit to withstand the competition flow thanks to their endothelial function. Radial as well as right gastroepiploic arteries support much less flow competition because of their different anatomy, histology and endothelial function than that of internal thoracic artery, leading to spasm and occlusion. Therefore, these two arterial conduits should be used only in case of critical lesion to avoid graft occlusion. Saphenous vein graft is the only conduit that is not significantly affected by flow competition, mainly because of its nonresistivity and common reimplantation in the aorta. Graft configuration is the second important factor influencing the equation between graft flow and native coronary flow. Therefore, composite grafting should be reserved for the case of severely stenotic coronary target, especially if multiple arterial sequential anastomoses have to be performed on the lateral-inferior wall of the heart. Finally, an accurate tool such as the fractional flow reserve to evaluate the stenosis severity should be the milestone of coronary surgery in order to decrease the rate of flow competition and improve arterial grafting functionality.

CONCLUSION

Competition flow plays a crucial role in arterial grafting functionality. Grafting strategy should address this by appropriate graft choice and configuration in order to avoid graft attrition.

Different flow patterns between left and right internal thoracic artery grafts influence the evaluation of severe graft stenosis by transthoracic Doppler echocardiography

OBJECTIVE

An increase in the diastolic to systolic flow velocity ratio (D/S) in the proximal left internal thoracic artery (ITA) after coronary artery bypass grafting (CABG) enables noninvasive assessment of graft patency by transthoracic Doppler echocardiography (TTDE). The increase in the D/S can be less pronounced at a site distant from the anastomosis. We postulated that proximal ITA flow patterns differ between the left and right ITAs and that the increase in D/S is less pronounced in the right than in the left proximal ITA.

METHODS

Proximal ITA flow was examined by TTDE in 129 consecutive patients after CABG of the left (75) or right (69) ITA to the left coronary artery. The mean D/S of the ITAs was compared with coronary angiography.

RESULTS

The D/S was lower in the group with a patent right ITA than in the group with a patent left ITA (P < .05). The D/S of both the left and right ITAs negatively correlated with angiographic stenosis (r = 0.56 or 0.67, P < .001, respectively). The regression line was significantly shifted downward in the right ITA compared with the left ITA, according to analysis of covariance (P = .01). Graft stenosis was predicted by a D/S of <0.57 and <0.28 with an accuracy of 91% and 97% in the left and right ITAs, respectively.

CONCLUSION

The patency of both left and right ITA grafts to the left coronary artery can be assessed using TTDE, but different cutoff values of D/S are required to diagnose severe ITA stenosis.

Improvement of flow capacity of the left internal thoracic artery graft assessed by using a pressure wire

OBJECTIVE

We sought to evaluate improvement of flow capacity in a left internal thoracic artery graft by means of pressure measurement.

METHODS

Eighteen patients who received a left internal thoracic artery graft to the left anterior descending coronary artery were studied. Angiography and pressure measurement at the proximal and distal portions of the left internal thoracic artery graft during maximal hyperemia with a pressure guide wire were performed at 1 month (early study) and 6 months (late study) after surgical intervention.

RESULTS

There are no significant differences between the early and late studies in resting mean aortic pressure, left ventricular end-diastolic pressure, left ventricular ejection fraction, and percentage diameter stenosis of the recipient left anterior descending coronary artery. There was no stenosis in the anastomosis site of the left internal thoracic artery graft and the distal left anterior descending coronary artery, as determined by means of angiography, in the early and late studies. The mean diameter of the distal left internal thoracic artery graft was significantly increased in the late study (1.6 +/- 0.2 vs. 1.8 +/- 0.2 mm, P = .011). There was a significant difference between the early and late studies in the pressure gradient through the graft (15 +/- 4 vs 13 +/- 3 mm Hg, P = .036). The ratio of distal to proximal pressure within the left internal thoracic artery graft in the late study was significantly increased from that in the early study (0.80 +/- 0.04 to 0.84 +/- 0.03, P = .0003).

CONCLUSIONS

The pressure ratio within the left internal thoracic artery graft became higher as the left internal thoracic artery graft adapted itself to the myocardial circulation. This finding might relate to decreasing the resistance of the left internal thoracic artery graft.

Can Late Supraclavicular Echo Doppler Reliably Predict Angiographical String Sign of Lima to Lad Area Grafts?

OBJECTIVE

To investigate whether supraclavicular ultrasonography of left internal mammary artery (LIMA) to left anterior descending (LAD) area grafts can reliably predict (distal) string sign grafts on arteriography.

METHODS

Fifty-five patients (42 M, 61 +/- 7 years) with the LIMA to LAD area grafting were prospectively studied. Control arteriography was performed at 1.4 +/- 0.8 years postoperatively. Angiography demonstrated in 46 patients (group I) functional grafts, in 4 patients (group II) sequential distal string sign grafts and in 5 patients (group III) total string sign grafts. Ultrasonography was performed at 1.8 +/- 0.8 year postoperatively and compared with control angiography. Data were tested by unpaired t- and ANOVA tests. The diagnostic accuracy was assessed by the area under the curve of the Receiver Operator Characteristic. A formula was developed to predict the probability of (distal) string sign phenomena of sequential as well as single LIMA grafts.

RESULTS

Between the groups all duplex parameters showed a highly significant linear relation (p < or = 0.004) and all parameters between group I and III are significantly different with high Area Under Curve values. The model for the probability of (distal) string sign grafts fitted best with diastolic and systolic peak velocities as the most discriminative factors for (distal) string sign grafts.

CONCLUSIONS

Postoperative supraclavicular duplex as a method to assess the patency of LIMA to LAD area grafts allows discriminating functional grafts from (distal) string sign grafts.

Flow capacity of gastroepiploic artery versus vein grafts for intermediate coronary artery stenosis

BACKGROUND

Native flow competition is a significant factor affecting bypass graft patency. The objective of this study was to compare the effect of competitive flow on conduit flow dynamics in the gastroepiploic artery (GEA) and the saphenous vein graft (SVG).

METHODS

In 51 patents, 23 GEAs (in-situ grafts) and 28 SVGs (aortocoronary grafts) were examined using a Doppler-tipped guidewire during coronary angiography after coronary artery bypass. Graft flow volume at rest and maximum graft flow volume during hyperemia were calculated from graft diameter and average peak velocity at rest and maximum average peak velocity induced by papaverine hydrochloride injection. Grafts were classified according to the grade of native coronary artery stenosis; group S (14 GEAs and 16 SVGs) displayed over 75% stenosis and group M (9 GEAs and 12 SVGs) exhibited over 50% up to 75% stenosis.

RESULTS

In group S, no difference in flow volume was apparent between the GEA and the SVG at rest (36+/- 17 vs 42 +/- 16) and during hyperemia (78 +/- 30 vs 88 +/- 28). In group M, flow volume of the GEA was significantly lower than that of the SVG at rest (17 +/- 11 vs 38 +/- 12; p = 0.029) and during hyperemia (32 +/- 19 vs 94 +/- 46; p = 0.001).

CONCLUSIONS

These data suggest that in intermediate coronary stenosis, GEA flow is compromised by native flow competition, whereas the SVG flow dynamics is maintained. However, the GEA can provide comparable flow capacity to the SVG and will achieve good surgical results when target coronary artery selection is appropriate.

Evaluation of flow in the left anterior descending coronary artery but not in the left internal mammary artery graft predicts significant stenosis of the arterial conduit

OBJECTIVES

The purpose of this study was to evaluate which Doppler-derived flow index best predicts new distal left anterior descending coronary artery (LAD) stenosis in patients with left internal mammary artery (LIMA) graft.

BACKGROUND

The LIMA flow measurement has been proposed to assess graft function, but it may be misleading in case of new distal LAD stenosis and/or competitive flow from native LAD. Distal LAD coronary flow reserve (CFR: hyperemic/baseline peak flow velocity ratio) may be more appropriate.

METHODS

The LIMA and distal LAD flow was measured by transthoracic Doppler echocardiography in 96 patients undergoing diagnostic/therapeutic coronary angiography, 7 +/- 4 years after cardiac bypass surgery. The LIMA flow indexes (systolic-to-diastolic peak velocity ratio [SDPVr] >1, diastolic time velocity integral fraction [DTVIf] <0.5, and CFR <2) and LAD CFR <2 were used to predict > or =70% new LAD stenosis.

RESULTS

The LAD CFR <2 predicted new LAD stenosis, found in 21 of 77 patients without competitive flow from native LAD, with significantly higher diagnostic accuracy (98%) than LIMA flow indexes (SDPVr >1 = 61%, DTVIf <0.5 = 69%, and CFR <2 = 72%). The LIMA flow indexes were abnormal in 17 of 19 patients with competitive graft flow, but only 5 had graft restriction, and none had significant LAD stenosis. In a multivariate model of new distal LAD stenosis prediction, competitive flow from native LAD reduced the predictive role of LIMA but not of LAD CFR.

CONCLUSIONS

In patients without competitive flow from native LAD, LAD CFR is more accurate for the detection of LAD stenosis than LIMA CFR. In patients with competitive graft flow, abnormal LIMA flow patterns and blunted LIMA CFR do not reflect downstream LAD flow as LAD CFR does.

Noninvasive assessment of flow velocity and flow velocity reserve in the right gastroepiploic artery graft by transcutaneous Doppler echocardiography: comparison with an invasive technique

BACKGROUND

The measurement of flow velocity (FV) in coronary artery bypass grafts using a Doppler guidewire has provided useful clinical and physiologic information. The recently developed transcutaneous Doppler echocardiography is a noninvasive technique to measure FV and FV reserve (FVR) in the right gastroepiploic artery (GEA) graft. The purpose of this study was to evaluate whether transcutaneous Doppler echocardiography accurately measures FV and FVR in the right GEA graft in a clinical setting.

METHODS

In 33 patients who underwent graft angiography for the assessment of the right GEA graft, FV in the right GEA graft was measured by transcutaneous Doppler echocardiography under the guidance of color flow Doppler imaging at the time of examination using a Doppler guidewire. FV in the midportion of the right GEA graft was measured at baseline and during hyperemic conditions using both transcutaneous Doppler echocardiography and a Doppler guidewire.

RESULTS

There were excellent correlations between the value of FV obtained by transcutaneous Doppler echocardiography and those obtained with the Doppler guidewire (averaged peak velocity: y = 0.95 x + 1.46, r = 0.98, standard error of the estimate [SEE] = 2.94 cm/s; averaged systolic peak velocity: y = 0.94 x + 1.18, r = 0.97, SEE = 3.15 cm/s; diastolic peak velocity: y = 0.97 x + 1.62, r = 0.98, SEE = 4.40 cm/s; averaged diastolic peak velocity: y = 0.95 x + 1.75, r = 0.98, SEE = 3.60 cm/s). The FVR as determined by transcutaneous Doppler echocardiography showed a good correlation with that determined using the Doppler guidewire method (y = 0.90 x + 0.21, r = 0.92, SEE = 0.31).

CONCLUSIONS

Transcutaneous Doppler echocardiography proved to be an accurate noninvasive method to measure FV and FVR in the right GEA graft.

Coronary revascularization in the 21st century. Emphasis on contributions by Japanese surgeons

The first three decades of coronary artery surgery have provided the foundation for the next century of this evolution. It is apparent that a multitude of events including the development of cardioplegia, improving surgical instrumentation, technological advances including endoscopic approaches and computer assisted robotics and biologic discoveries such as the role of the endothelium have provided the underpinnings for improved surgical outcomes. However, the single most important determinant of late results is the type of bypass conduit used for grafting. Thus, use of the left internal thoracic artery (ITA) grafted to the left anterior descending coronary is a more important determinant of survival than is any other factor (progression of coronary artery disease, increased age, poor left ventricular function, diabetes, female gender and off-pump operations). Use of two ITAs provides further benefit and it is likely that three or more arterial conduits will be shown to be advantageous in this regard in due time. Japanese cardiothoracic surgeons have made significant contributions to the continuing evolution of coronary bypass surgery and particularly to the advance of arterial conduits. This report will address those contributions to this evolution.

Severity of coronary artery stenosis at preoperative angiography and midterm mammary graft status

BACKGROUND

The purpose of this study was to evaluate the correlation between the midterm angiographic results of mammary artery grafts and the preoperative stenosis of the target vessel.

METHODS

We analyzed preoperative and postoperative angiograms of 93 patients who underwent postoperative midterm (> or = 3 years) angiograms of an internal mammary artery (IMA) to left anterior descending artery graft for clinical or study purposes. Patients were divided into three groups on the basis of the percentage of the coronary artery stenosis at preoperative angiography: < 70%, 70% to 90%, and > 90% stenosis.

RESULTS

Preoperative characteristics were similar in the three groups. The overall incidence of IMA occlusion was 19% in the entire population, without significant differences between groups (19% versus 29% versus 14%). The mean mammary artery diameter significantly increased in direct proportion to the severity of the coronary stenosis (2.0 +/- 0.2 mm in the < 70% versus 2.5 +/- 0.3 mm in the 70% to 90% and 2.7 +/- 0.4 mm in the > 90% series; p < 0.05).

CONCLUSIONS

Chronic native competitive flow does not significantly affect midterm graft status but does influence mammary graft diameter.

Remodeling of arterial conduits in coronary grafting

In the initial decade of coronary surgery, serial angiography of internal thoracic artery grafts revealed increased caliber in some, decreased caliber in others, and "string sign" in a few, which was occasionally documented to be reversible. Although we speculated on possible causes of these changes, it was not until discovery of the endothelial role in modulating arterial diameter to maintain shear stress in a narrow range that we began to gain insight into the mechanisms responsible for remodeling of the arterial wall. This review provides a glimpse of the physiology and biology of arterial remodeling and summarizes observations on the various arterial conduits when subjected to flow alterations.

Flow wire measurements after complete arterial coronary revascularization with T-grafts

BACKGROUND

The T-graft procedure achieves complete arterial revascularization in coronary three-vessel disease. In this technique, all bypass anastomoses are supplied by the left internal mammary artery (IMA). This prospective study explores the question of whether the quantitative flow in such grafts is influenced by the pathology in the native coronary arteries.

METHODS

Eighty-two patients with coronary three-vessel disease were studied after complete arterial coronary revascularization with T-grafts. Quantitative flow and coronary flow reserve were measured in the proximal IMA with a Doppler guide wire. Three groups were compared: group 1, all native coronary arteries were stenosed but patent (n = 31); group 2, one occluded native coronary vessel (n = 33); group 3, two or more occluded native coronary arteries (n = 18).

RESULTS

Quantitative flow was significantly higher in group 3 than in group 2 at 1 week (93.9 +/- 39.5 vs 75.8 +/- 27.3 mL/min, p < 0.05) and 6 months postoperatively (86.0 +/- 40.1 vs. 69.1 +/- 35.5 mL/min, p < 0.05). Flow in group 2 was significantly (p < 0.05) higher than in group 1 (1 week: 58.0 +/- 28.4 mL/min, 6 months: 55.2 +/- 29.2 mL/min) in both examinations. There were no significant differences in coronary flow reserve between the three groups (1: 2.88 +/- 0.97, 2: 2.84 +/- 0.96, 3: 2.74 +/- 0.94).

CONCLUSIONS

After complete arterial revascularization with T-grafts, the quantitative flow in the IMA is influenced by the status of the native coronary arteries. As a result of competitive flow phenomena, blood flow in the bypasses is significantly lower when the coronary arteries are affected only by stenosis.

Effect of flow competition on internal thoracic artery graft: postoperative velocimetric and angiographic study

OBJECTIVES

To assess the effects of competitive blood flow on internal thoracic artery grafts, we investigated postoperative flow velocity characteristics and angiographic findings of the grafts with various grades of native coronary artery stenosis.

METHODS

Fifty patients who had an internal thoracic artery graft to the left anterior descending artery underwent intravascular Doppler graft velocimetry during postoperative angiography. Patients were divided into 3 groups according to the grade of native coronary stenosis: group H (28 patients), 80% stenosis or greater; group M (16 patients), 60% to 79% stenosis; and group L (6 patients), 40% to 59% stenosis. Phasic flow velocity of the grafts was measured with an intravascular Doppler ultrasound-tipped guide wire during angiography. Graft flow volume was calculated from the diameter and the average peak velocity.

RESULTS

Average peak velocity (group H, 27.1 +/- 8.6 cm/s; group M, 16.9 +/- 3.9 cm/s; group L, 7.2 +/- 3.7 cm/s), distal graft diameter (group H, 2.27 +/- 0.23 mm; group M, 2. 00 +/- 0.28 mm; group L, 1.07 +/- 0.27 mm), and calculated graft flow volume (group H, 33.1 +/- 12.0 mL/min; group M, 16.2 +/- 5.8 mL/min; group L, 2.3 +/- 2.0 mL/min) significantly differed among the 3 groups. Graft flow in diastole and systole also differed among the 3 groups.

CONCLUSIONS

Competitive blood flow reduces internal thoracic artery graft flow and diameter according to the grade of the native coronary artery stenosis. These data suggest that grafting the internal thoracic artery to the coronary artery with stenosis of a low grade can cause graft atrophy and failure.