Functional evaluation of internal mammary artery bypass grafts in the early and late postoperative periods

Diagnostic radiology methods for assessing coronary artery bypass graft viability

The review describes available modern radiological methods which are currently applied for a detailed and comprehensive anatomical and functional assessment of the viability of various coronary artery bypass grafts. In addition, it presents some aspects of the implementation of these methods and clinical interpretation of the results.

Predictors and prevention of flow insufficiency due to limited flow demand

Background

We investigated the impacts of flow demand and native coronary stenosis on graft flow and patency.

Methods

We reviewed the angiograms of 549 bypass grafts in 301 patients who underwent off-pump coronary artery bypass grafting since 2007. Grafts consisted of 237 internal thoracic artery to left anterior descending artery; 97 internal thoracic artery and 52 saphenous vein grafts to left circumflex artery; and 109 gastroepiploic artery and 54 saphenous vein grafts to right coronary artery. We selected only individual bypass grafts created as the sole bypass graft to the coronary vascular region. Flow insufficiency was defined as ≤ 20 ml/min measured intraoperatively. When a significant difference in the incidence of flow insufficiency or “not functional” occurred between higher and lower values rather than the particular minimal luminal diameter value, the highest value was defined as the cut-off minimal luminal diameter. Distal lesions were defined as stenosis at segment #4, 7, 8, 12, 13, 14, or 15.

Results

Flow insufficiency was found in 112/549 (20.4%) bypass grafts. For internal thoracic artery to left circumflex artery grafts, the cut-off minimal luminal diameter for proximal and distal lesions was 1.25 mm and 0.75 mm, respectively. For gastroepiploic artery to right coronary artery grafts, the cut-off minimal luminal diameter was 0.82 mm for proximal lesions (p = 0.005), while 10 (71%) of 14 gastroepiploic artery grafts for distal lesions presented with flow insufficiency. Univariate and multivariate analysis identified a distal lesion (odds ratio (OR): 3.12, p < 0.0001); minimal luminal diameter greater than the cut-off value (OR: 3.64, p < 0.0001); right coronary artery (OR: 18.2, p = 0.0002) and left circumflex artery (OR; 2.29, p = 0.009) grafting; and a history of myocardial infarction in the grafted region (OR: 2.21, p = 0.02) as significant predictors of flow insufficiency.

Conclusions

Both competitive flow and insufficient flow demand cause graft failure. For distal lesions, more severe stenosis is necessary to avoid graft failure, compared with proximal lesions. A revascularization strategy for distal lesions should be discussed separately from that for proximal lesions.

Electronic supplementary material

The online version of this article (doi:10.1186/s13019-014-0188-3) contains supplementary material, which is available to authorized users.

Effects of skeletonized versus pedicled radial artery on postoperative graft patency and flow

Background

Radial artery (RA) was the second arterial graft introduced in clinical practice for myocardial revascularization. The skeletonization technique of the left internal thoracic artery (LITA) may actually change the graft's flow capacity with potential advantages. This leads to the assumption that the behavior of the RA, as a coronary graft, is similar to that of the LITA, when skeletonized.

Objective

This study evaluated 'free' aortic-coronary radial artery (RA) grafts, whether skeletonized or with adjacent tissues.

Methods

A prospective randomized study comparing 40 patients distributed into two groups was conducted. In group I, we used skeletonized radial arteries (20 patients), and in group II, we used radial arteries with adjacent tissues (20 patients). After the surgical procedure, patients underwent flow velocity measurements.

Results

The main surgical variables were: RA internal diameter, RA length, and free blood flow in the radial artery. The mean RA graft diameters as calculated using quantitative angiography in the immediate postoperative period were similar, as well as the flow velocity measurement variables. On the other hand, coronary cineangiography showed the presence of occlusion in one RA graft and stenosis in five RA grafts in GII, while GI presented stenosis in only one RA graft (p = 0.045).

Conclusion

These results show that the morphological and pathological features, as well as the hemodynamic performance of the free radial artery grafts, whether prepared in a skeletonized manner or with adjacent tissues, are similar. However, a larger number of non-obstructive lesions may be observed when RA is prepared with adjacent tissues.

Angiographic evaluation of flow distribution in sequential and composite arterial grafts for three vessel disease

OBJECTIVES

We sought to delineate the effects of the severity of target lesions and their combinations on the occurrence of competitive flow, especially in the composite Y-graft and to establish an optimal strategy for graft arrangement and patient selection.

METHODS

We reviewed early and late angiograms of 2514 bypass grafts in 601 patients, who underwent off-pump coronary revascularization to three-vessel vascular regions using the internal thoracic artery (ITA) and radial artery (RA) without aortic manipulation. As a standard technique, the left anterior descending artery (LAD) was bypassed with the in situ ITA, and the left circumflex and right coronary arteries (RCA) were bypassed with the composite RA. Bypass flow was graded as antegrade, competitive or no flow.

RESULTS

The early patency rate was 98.1% (2466/2514), while competitive flow was detected in 6.4% (162/2514). For the LAD, the individual and sequential in situ ITA provided lower incidence of competitive flow than the composite graft (0.3% (1/298) versus 7.6% (23/303), P < 0.0001). Regarding the RA to non-LAD bypass, 86.3% (113/131) of competitive flow occurred at the distal end of the I- or Y-graft, and the cumulative patency rate was significantly lower than that of sequential proximal anastomosis (80.1 versus 56.6% at 5 years, P < 0.0001). The number of sequential anastomoses did not affect the cumulative patency rate (P = 0.09). For the composite Y-graft to three-vessel regions, the rate of antegrade flow in patients with 76-100% stenosis in both the LAD and the RCA was 95.7% (178/186), which was significantly higher than that of 78.1% (100/128) in patients with 76-100% stenosis in the LAD and 51-75% stenosis in RCA (P < 0.0001).

CONCLUSIONS

Sequential and composite grafting was considered reliable, exclusively in appropriately selected situations. To secure entire patency of the Y-graft to three-vessel regions, balanced bypass flow toward LAD and RCA would be crucial.

Physiological and metabolic effects of grafts in coronary artery bypass surgery

The internal thoracic artery (ITA) has become the gold standard graft material for modern coronary artery bypass grafting (CABG) because of its excellent long-term patency. The use of ITA grafts has also prolonged the postoperative survival of patients when applied to the left anterior descending artery or used bilaterally as 2 grafts for the left coronary system. Moreover, recent large-scale randomized clinical trials comparing the survival rates between CABG and percutaneous coronary intervention (PCI) with stents have shown that CABG is more effective for improving the survival of patients with severe coronary artery disease and/or in those with diabetes mellitus. The fundamental principle underlying these clinical benefits of CABG is the excellent endothelial function of the ITA, which provides physiological and metabolic effects that are beneficial not only for the graft itself, but also for the recipient coronary system. The production of nitric oxide and prostanoids by the ITA endothelium and their beneficial effects on the downstream coronary artery should therefore be taken into consideration when debating the merits of CABG vs. PCI.

Evaluation of early and late postoperative flow capacity of internal thoracic artery bypass by means of stress Doppler echocardiography

OBJECTIVES

Arterial revascularization with the internal thoracic artery (ITA) has shown exceptional long-term results, even though early hypoperfusion can occur and can cause ischemia and contractile dysfunction. Therefore, it is still controversial as to whether the bypass vessel can guarantee the required demand for blood flow during the early postoperative (EPO) phase or whether this is only possible a long time after the operation. This question is important particular in the early postoperative phase to manage afterload and reduce left ventricular oxygen demand. Therefore, stress Doppler echocardiography was performed to assess EPO and late postoperative (LPO) flow capacity after minimal-invasive coronary artery bypass grafting (MIDCAB) in subjects with single vessel disease.

METHODS

Doppler echocardiography was performed after MIDCAB in 15 patients (mean age 65 years+/-12 standard deviation) in the EPO (15 days) and LPO (266 days) at rest and under stress.

RESULTS

The mean diastolic velocity (MDV) as a measure for the coronary perfusion in the bypass-graft was comparable (30.9+/-14.5 EPO to 30.8+/-13.9 cm/s LPO, P>0.05) for both settings (early vs. late). In both tests, the stress reactions by means of handgrip maneuver lead to comparable increases in blood flow: MDV 65.0+/-54.4% (EPO) to 62.5+/-53.7% n.s. (LPO). The flow increase in the bypass graft was just as similar [48.2+/-46.2% (EPO) to 51.1+/-41.6% n.s. (LPO)].

CONCLUSION

The stress echocardiography revealed for the first time that the ITA-graft after MIDCAB-operation is able to produce the same flow reserve through the EPO phase as well as the LPO phase. Restrictions to a maximum early flow adaptation are not justified. Thus, particularly a patient with severe comorbidities or higher age should be mobilized in the EPO phase to minimize the postoperative complications without risk of myocardial ischemia caused by impaired early blood flow through recent implanted ITA. This method for assessment of ITA blood flow allows for long-time observations and can detect disturbances in perfusion at an early stage.

Progressive improvement of myocardial perfusion after off-pump revascularization with bilateral internal thoracic arteries: Comparison of early versus 1-year postoperative myocardial single photon emission computed tomography

OBJECTIVE

We evaluated the time course of myocardial perfusion improvement after off-pump revascularization.

METHODS

Seventy-six patients who underwent off-pump coronary bypass with bilateral internal thoracic arteries for revascularization of the left coronary territory were studied. Myocardial single photon emission computed tomography was performed preoperatively and at 3 months and 1 year postoperatively. Myocardial perfusion was quantified with automatic software, and the left coronary territory was divided into 16 segments. As an indicator of the ischemic myocardium, the reversibility score was defined as a measure of rest minus stress perfusion values. A total of 403 segments that showed a reversibility score of 7 or greater preoperatively were included.

RESULTS

The reversibility score was improved significantly at 3 months postoperatively (P < .001) and further improved 1 year after the operation (P < .001). When the patients were divided into 2 groups based on the type of bilateral internal thoracic arteries used (group Y: Y-composite graft, n = 39; group I: bilateral in situ grafts, n = 37), the reversibility score was improved significantly 3 months after the operation (P < .001) and further improved 1 year after the operation (P < .001) in both groups. There were no significant differences in the reversibility scores between groups Y and I at postoperative month 3 (P = .463). The scores approached zero in both groups at 1 year postoperatively (group Y, -1.7% +/- 7.9%; group I, 1.3% +/- 7.2%).

CONCLUSIONS

The myocardial reversibility score improved greatly during the first 3 months after the operation and further improved until 1 year postoperatively. Perfusion improvement was similar between the Y-composite and bilateral in situ internal thoracic artery grafts in terms of the reversibility score.

Resistance to flow of arterial Y-grafts 6 months after coronary artery bypass surgery

BACKGROUND

The use of both internal thoracic arteries (ITAs) as a Y-graft configuration has been proposed as a technique allowing complete arterial revascularization. Controversy remains, however, about the capacity of this Y-graft configuration to provide sufficient blood flow to the whole left coronary system and about possible steal phenomenon occurring during periods of maximal myocardial blood flow demand.

METHODS AND RESULTS

To evaluate graft conductance 6 months after Y-graft revascularization of the left coronary system with both ITAs, 11 consecutive patients were studied during cardiac catheterization. In all of the cases, the left ITA had been connected to the left anterior descending coronary artery (LAD) territory (mean, 1.3 anastomoses), and the free right ITA was anastomosed proximally to the left ITA and distally to the left circumflex (LCX) territory (mean, 1.9 anastomoses). Pressure and fractional flow reserve (FFR) were recorded using a 0.014-inch pressure wire advanced distally in the left ITA main stem close to the proximal anastomosis of the free right ITA (ITA-stem) and in the distal part of each ITA branch at the site of their implantation to the LAD (ITA-LAD) or LCX (ITA-LCX) system. At each of these sites, the pressure gradient between aorta and the graft was measured in basal condition and during maximal hyperemia induced by intragraft bolus injection of 40 microg of adenosine. In basal conditions, the pressure gradient was minimal between the aorta and the ITA-stem (2+/-2 mm Hg), the ITA-LAD (3+/-3 mm Hg), and the ITA-LCX (3+/-2 mm Hg; P value was not significant versus ITA-LAD). During maximal hyperemia, the pressure gradient increased to 7+/-2 mm Hg in the ITA-stem, to 9+/-5 mm Hg in the ITA-LAD, and to 9+/-3 in the ITA-LCX (P value not significant versus ITA-LAD). The fractional flow reserve was 0.93+/-0.03 in the ITA-stem, 0.91+/-0.04 in the ITA-LAD, and 0.91+/-0.03 in the ITA-LCX.

CONCLUSIONS

A Y-graft configuration with a free right ITA attached to a pedicled left ITA allows an adequate revascularization of the whole left coronary system with an even distribution of perfusion pressure in both distal branches and minimal resistance to maximal blood flow.

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.

Persistent but reversible coronary microvascular dysfunction after bypass grafting

The effect of coronary artery bypass grafting (CABG) on absolute myocardial blood flow (MBF) has not been investigated previously. MBF (ml. min(-1). g(-1)) was measured at rest and during hyperemia (0.56 mg/kg iv dipyridamole) using H(2)(15)O and positron emission tomography in eight patients with three-vessel disease before surgery and 1 and 6 mo after full revascularization. Baseline MBF was 0.87 +/- 0.12 preoperatively and 1.04 +/- 0.14 and 0.95 +/- 0.13 at 1 and 6 mo after CABG, respectively (P < 0.05, 6 mo vs. preoperatively). Hyperemic MBF was 1.36 +/- 0.28 preoperatively and increased to 1.98 +/- 0.50 and 2.45 +/- 0.64 at 1 and 6 mo after CABG, respectively (P < 0.01, 6 mo vs. preoperatively). Coronary vasodilator reserve (hyperemic/baseline MBF) increased from 1.59 +/- 0.40 preoperatively to 1.93 +/- 0.13 and 2.57 +/- 0.49 at 1 and 6 mo, respectively (P < 0.05, 6 mo vs. preoperatively). Minimal (dipyridamole) coronary resistance (mmHg. min. g(-1). ml(-1)) fell progressively from 59.37 +/- 14.56 before surgery to a nadir of 35. 76 +/- 10.12 at 6 mo after CABG (P < 0.01 vs. preoperatively). The results of the present study confirm that CABG improves coronary vasodilator reserve progressively as a result of reduction in minimal coronary resistance. These data suggest persistent microvascular dysfunction that recovers slowly after surgery.

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.

Functional integrity and aging of the left internal thoracic artery after coronary artery bypass surgery

OBJECTIVE

To study the endothelial function in the left internal thoracic artery after coronary artery bypass surgery and to identify predictors of early dysfunction, we performed a provocative test with acetylcholine in 23 male patients who underwent routine postoperative coronary angiography.

METHODS

The change in mean diameter of the proximal thoracic artery was assessed by quantitative angiography after selective injections of acetylcholine and nitroglycerin.

RESULTS

The thoracic artery showed a 6.8% (P <. 001) and 9.0% (P <.001) increase in mean diameter after acetylcholine and nitroglycerin administration, respectively. Vasodilative responses to acetylcholine and nitroglycerin administration were strongly correlated (R: 0.88; P <.001). Among the common risk factors, only age was associated with an impairment in the vasodilative response of the arterial graft (P =.001), and acetylcholine-induced vasodilation was inversely correlated to the age of the patient (R: 0.69; P <.001).

CONCLUSIONS

Endothelium-dependent vasodilative response to acetylcholine administration seems well preserved in the left internal thoracic artery after surgery. Common risk factors, except for age, do not affect the functional integrity of the arterial graft. The vasodilative properties of the graft depend on the age of the patient and do not deteriorate over time after operation.

Systolic and diastolic changes in human coronary blood flow during Valsalva manoeuvre

Valsalva manoeuvre is reported to be sometimes successful for the relief of angina pectoris. The present study investigated how haemodynamic changes produced by Valsalva manoeuvre can interact to improve the relationship between cardiac work and coronary blood flow. Ten male subjects aged 53 +/- 12 years (SD) were considered. Blood velocity in the internal mammary artery, previously anastomosed to the left descending coronary artery, was studied with Doppler technique. The subjects performed Valsalva manoeuvres by expiring into a tube connected to a mercury manometer, to develop a pressure of 40 mmHg. The arterial blood pressure curve was continuously monitored with a Finapres device from a finger of the left hand. During expiratory effort, an increase in heart rate and a decrease in arterial pulse pressure were followed by a more delayed and progressive increase in mean and diastolic pressures. Systolic blood velocity markedly decreased along with the reduction in pulse pressure and increase in heart rate. By contrast, diastolic and mean coronary blood velocities did not show any significant change. Since it is known that the Valsalva manoeuvre strongly reduces stroke volume and cardiac output, it is likely that a reduction in cardiac work also takes place. Since in diastole, i.e. when the myocardial wall is better perfused, coronary blood velocity did not show any significant reduction, it is likely that unchanged perfusion in the presence of reduced cardiac work is responsible for the relief from angina sometimes observed during Valsalva manoeuvre. It is also likely that the increase in heart rate prevents the diastolic and mean blood coronary velocity from decreasing during the expiratory strain, when an increased sympathetic discharge could cause vasoconstriction through the stimulation of the coronary alpha-receptors.

Clinical results of coronary artery bypass grafting with use of the internal thoracic artery under low free flow conditions

OBJECTIVE

Even when left internal thoracic artery flow is very low, we have used the artery for grafting without any further maneuvers. In this study, we investigated the clinical results of coronary bypass surgery using the left internal thoracic artery with low free flow.

METHODS

A total of 163 patients were divided into 2 groups: group L (n = 43) had free flow of 20 mL/min or less and group H (n = 120) had free flow of more than 20 mL/min. We performed a comparative study on the basis of coronary angiography and pulsed Doppler echocardiography. Furthermore, 12 months' postoperative graft angiography was carried out in 11 patients from group L.

RESULTS

No patient had low output syndrome or perioperative myocardial infarction. One month after the operation, 3 cases of graft occlusion and 9 cases of the "string sign" were identified in group H. However, group L had no graft occlusion and only 1 case of the "string sign." The 1-month postoperative Doppler echocardiographic study showed no significant differences in the diastolic fraction of velocity time integrals and the diastolic/systolic peak velocity ratio of the grafts. In the 11 patients undergoing angiography after 1 year, graft patency was excellent. Moreover, the graft diameter was significantly larger than it was 1 month after the operation.

CONCLUSION

These results suggest that the left internal thoracic artery can be used for coronary artery bypass grafting even when the flow is less than 20 mL/min.

T grafts with the right internal thoracic artery to left internal thoracic artery versus the left internal thoracic artery and radial artery: flow dynamics in the internal thoracic artery main stem

OBJECTIVE

Complete arterial coronary artery bypass grafting with 2 grafts can be achieved even in triple vessel disease by use of a T configuration. There is still uncertainty whether the coronary flow reserve in the main stem of the left internal thoracic artery is sufficient to supply more than 1 anastomosed coronary vessel.

METHODS

Between March 1996 and February 1999, 251 patients with multivessel coronary artery disease underwent complete arterial revascularization with T grafts, using either the left internal thoracic artery with the free right internal thoracic artery graft (n = 73, group I) or the left internal thoracic artery and radial artery (n = 178, group II). A mean of 4.0 (group I) versus 4.3 (group II) coronary vessels were anastomosed per patient. One week (n = 92) and 6 months (n = 28) after the operation, flow was measured in the proximal left internal thoracic artery with a Doppler guide wire. Maximum flow was determined after injection of adenosine (30 microg).

RESULTS

The in-hospital mortality was 2.7% (group I) versus 2.3% (group II). At angiography (n = 142, 56.6%) the patency rate was 96.3% (group I) versus 98.2% (group II). There was no significant difference between baseline flow, maximum flow, and coronary flow reserve between the 2 groups. Coronary flow reserve increased in both groups within the first 6 postoperative months (group I, 1.85 +/- 0.31 vs 2.77 +/- 0.77, P =.0002; group II, 1.82 +/- 0.4 vs 2.53 +/- 0.73, P =.009).

CONCLUSION

Both variants of T grafts allow for complete arterial revascularization with good perioperative results. The flow reserve of the proximal internal thoracic artery is adequate for multiple coronary anastomoses irrespective of the choice of the second arterial graft.

Multiple arterial grafts and survival

Limitations in the long-term patency of saphenous veins for bypass grafts have encouraged interest in the use of arterial conduits. The positive effect of an internal thoracic artery graft on survival has been accepted for more than a decade, but it has proven difficult to show additional benefit from additional arterial conduits; this is probably due to multiple factors, including inappropriate choice of target vessels, short follow-up, and inadequate numbers of patients. Recently, however, the positive effect of a second arterial graft was confirmed. It will probably be difficult to show a survival benefit from a third or fourth arterial graft, but we believe that complete arterial revascularization will result in improved long-term freedom from reintervention. Interest in arterial conduits for coronary artery bypass was primarily limited to the left internal thoracic artery until the mid-1980s, when enthusiasm for the use of bilateral internal thoracic arteries grew. More recently, the gastroepiploic artery, the inferior epigastric artery, and especially the radial artery have all found advocates. However, the original conduit--and the standard against which all others are compared--is the greater saphenous vein.

Noninvasive assessment of left and right internal mammary artery graft patency with high-frequency transthoracic echocardiography

OBJECTIVES

The aim of this study was (1) to visualize internal mammary artery grafts (IMAG) on coronary artery by transthoracic echocardiography and (2) to assess the patency of the grafts.

METHODS

Twenty-three patients (21 men, 56 +/- 6 years) with previous coronary artery bypass grafting were studied at baseline and after they underwent low-dose dipyridamole infusion. The parameters obtained were systolic (SPV) and diastolic (DPV) peak velocities and their ratio (DPV/SPV); the dipyridamole infusion to baseline ratio of DPV was an index of IMAG blood flow reserve (FR). Two groups of patients were selected at baseline: group A, (n = 12) with a DPV/SPV >1, and group B (n = 11), with a DPV/SPV <1.

RESULTS

The IMAG was identified in all patients. Intraluminal flow signals obtained with pulsed wave Doppler showed a biphasic pattern (1 systolic and 1 diastolic wave). After dipyridamole infusion was administered, flow velocities increased in 11 of 12 patients in group A and in 5 of 11 patients in group B. In group A the DPV/SPV increased from 1.79 +/- 0.47 to 1.8 +/- 0.43 (P = not significant), and the FR was 1.8 +/- 0.4. In group B the DPV/SPV increased from 0. 46 +/- 0.05 to 0.5 +/- 0.09 (P = not significant), and the FR was 1. 3 +/- 0.41. Coronary angiography showed the graft patency in all patients in group A and in 5 patients in group B with increased flow velocity after dipyridamole infusion. In the identification of graft stenosis at baseline, DPV/SPV showed 100% sensibility and 58% specificity, and FR showed 92% sensibility and 84% specificity.

CONCLUSION

Doppler echocardiographic evaluation of the IMAG is a simple noninvasive method to assess the functional impairment of the vessel.

Adaptive mechanisms of arterial and venous coronary bypass grafts to an increase in flow demand

OBJECTIVE

To compare the mechanisms by which arterial and venous grafts increase their flow during pacing induced tachycardia, early and later after coronary bypass surgery.

DESIGN

43 grafts (13 epigastric artery, 15 mammary artery, 15 saphenous vein) evaluated early (9 (3) days (mean (SD)) after bypass surgery were compared with 41 other grafts (15 epigastric, 11 mammary, 15 saphenous vein) evaluated later after surgery (mean 23 months, range 6 to 168 months) by quantitative angiography and intravascular Doppler velocity analysis during atrial pacing. Controls were 17 normal coronary arteries.

RESULTS

Baseline graft flow tended to be lower later after surgery than early (41 (16) v 45 (21) ml/min, NS). Blood flow increased during pacing by 30 (16)% early after surgery, less than later after surgery (+46 (18)%, p < 0.001) and less than in normal coronary arteries (+54 (27)%, p < 0.001 v early grafts; NS v late grafts). There was no difference between venous and arterial grafts. No significant vasodilatation was observed during pacing early after surgery in arterial and venous grafts. Later after surgery, significant vasodilatation was observed only in arterial grafts (mammary and epigastric grafts), from 2.41 (0.37) to 2.53 (0. 41) mm (+5.1% v basal, p < 0.001). Early after surgery and in venous grafts later after surgery, the increase in flow was entirely due to an increase in velocity. In later arterial grafts, the relative contribution of the increase in velocity to the increase in flow during pacing was lower in arterial grafts (70 (22)%) than in venous grafts (102 (11)%, p < 0.001) and similar to normal coronary arteries (68 (28)%).

CONCLUSIONS

Early and later after surgery, arterial grafts and venous grafts both increase their flow similarly during pacing. Early arterial grafts and venous grafts increase their flow only through an increase in velocity. Later after surgery, arterial grafts act as more physiological conduits and increase their flow in the same way as normal coronary arteries, through an increase in velocity and calibre mediated by the endothelium.

Effect of distal stenosis of internal thoracic artery bypass grafts on longitudinal phasic blood flow velocity characteristics

BACKGROUND

Left internal thoracic artery (LITA) bypass conduits show gradual longitudinal transition in their phasic flow velocity patterns from the proximal to distal segments, but little is known about the influence of distal stenosis, particularly early after surgery, on that characteristic. The purpose of this study was to evaluate the influence of distal stenosis on these flow velocity patterns.

METHODS

We examined 24 LITAs within 1 month (7 to 30 days) after surgery with a Doppler-tipped guide wire at the proximal, mid, and distal segments. Maximum peak velocities (MPV), time averaged peak velocities (APV), and velocity-time integrals (VTI) were measured.

RESULTS

In LITAs without stenosis (n = 14, group A), the APV, MPV, and VTI values at the diastole were significantly greater than those for distal stenosis (minimal lumen diameter >75%, n = 10, group B). The values of the 3 indexes at the systole in each segment did not differ significantly between the 2 groups. Both groups showed gradual increases in the diastolic/systolic ratios of the 3 indexes from the proximal to distal portions, the ratios in group A being significantly larger than that in group B (APV, P <. 001; MPV, P <.01; TVI, P <.01, respectively). For these indexes, sensitivity and specificity for predicting stenosis of LITA was higher in the proximal and mid portion than in the distal.

CONCLUSIONS

Anastomotic stenosis decreases the diastolic flow component but not the systolic one. By using diastolic/systolic ratios of the 3 indexes, it is possible to predict distal stenosis of LITA from the resting phasic flow velocity pattern.

The continuing evolution of arterial conduits

Vineberg used the internal thoracic artery (ITA) to achieve indirect myocardial revascularization in 1950, and Green reported direct coronary anastomosis with an operating microscope in 1968. It was not until the early 1980s that superior 10-year patency compared with saphenous vein was established for the ITA. In 1986, Loop proved better patient survival at 10 years when the left ITA was grafted to the left anterior descending artery rather than vein in patients with triple-vessel disease having complete revascularization. Only in 1998 has Lytle shown enhanced survival with use of two ITAs over one in triple-vessel disease. This report came 4 months after a report showing no additional benefit from the second ITA. Increasingly, complex use of arterial conduits allows complete revascularization with two arterial conduits (both ITAs or one ITA and one radial artery) in most patients (with 10-year data soon to be available in the former). The T-graft configuration (ITA T-graft or radial T-graft) is highly complex and utilizes single-source inflow to the entire heart (in addition to native coronary flow), which may not provide adequate inflow and remains controversial. Although patency for the ITA is well established, this cannot be said for the radial artery (one report of 55 conduits to 5.5 years), the gastroepiploic artery (one report of 44 conduits to 7 years), or the inferior epigastric artery (one report of 154 conduits to 43 months). Long-term follow-up of each conduit and each configuration is required to prove its durability and, therefore, value in the operative management of coronary disease.

Usefulness of flow reserve in the left internal mammary artery to determine graft patency to the left anterior descending coronary artery

Two-dimensional Doppler echocardiography (DE) and intravascular Doppler-tipped guidewire (flowire) have been used to measure flow in aortocoronary conduits at rest and during hyperemia, but they have not been compared. We investigated which flow velocity parameters obtained with these 2 different techniques can predict left internal mammary artery (LIMA) graft patency. Twenty-nine patients with previous coronary artery bypass grafting referred for evaluation of symptoms of coronary artery disease were studied after cardiac catheterization using the flowire and DE. Proximal LIMA graft flow velocity was measured at rest and during hyperemia produced by 140 microg/kg/min of intravenous adenosine infusion over 6 minutes with both methods. Normal LIMA grafts and left anterior descending artery (LAD) distal to the anastomosis were present in 16 patients, whereas 13 had >70% graft or native vessel stenosis. The coronary flow velocity reserve (r = 0.79) and the diastolic-to-systolic velocity ratio during hyperemia (r = 0.73) correlated very well between the 2 techniques. Among the variables obtained with the 2 techniques, the intragraft coronary flow velocity reserve measured by both methods was the only independent predictor of graft/recipient LAD patency. This variable had a sensitivity and specificity of 86% at a cutoff point of 2.07 with the flowire method and 83% at a cutoff point of 1.54 with DE. The areas below the receiver-operating characteristic curves were 0.91 and 0.93, respectively. Coronary flow velocity reserve measurements obtained with DE appears a reliable noninvasive method for assessing LIMA graft and/or LAD distal to the anastomosis patency in patients after bypass surgery and correlate very well with those directly obtained by intravascular Doppler.

[The Doppler color detection of internal mammary artery anastomoses and a functional assessment of their patency]

BACKGROUND

Left internal mammary artery coronary graft patency is usually evaluated invasively by cardiac catheterization. The objective of this study was to assess the ability of a high-frequency transthoracic Doppler echocardiographic system in the evaluation of patency and flow velocities changes in left internal mammary artery.

METHODS

Twenty-two patients (63 +/- 9 years, 17 male) who had previously undergone left internal mammary artery to left anterior descending artery were evaluated by transthoracic echocardiography, with an Acuson Sequoia C256 equipment (Acuson, Mountain view, USA), placing the transducer (5 MHz) in the second-third left intercostal space. Left internal mammary artery flow was considered to be found in the presence of one systolic/diastolic biphasic blood flow. Dipiridamol 0.5 mg/kg was administered in absence of contraindications in order to evaluate the flow reserve.

RESULTS

In 16 patients (73%), a biphasic systolic-diastolic flow was visualized. Peak velocity of systolic and diastolic waves was 38 +/- 13 and 37 +/- 15 cm/s, respectively. Eleven patients received i.v. dipiridamol 0.5 mg/kg. After dipiridamol, systolic and diastolic velocities rose from 32 +/- 8 to 43 +/- 14 cm/s (p = 0.0429) and from 25 +/- 8 to 50 +/- 17 cm/s (p = 0.0002), respectively (an increasing of 33 +/- 22% and 103 +/- 46% over the baseline, respectively).

CONCLUSION

This descriptive study shows that a non-invasive functional evaluation of left internal mammary artery grafts by a transthoracic approach with a high-frequency echocardiographic equipment is possible.

Assessing flow during minimally invasive coronary artery bypass: an Allen's test equivalent

The use of a provocative test to elicit selective regional myocardial dysfunction (detected with intraoperative TEE) as a method to infer adequacy of regional myocardial perfusion following MIDCAB is described. We liken the similarity of this technique to the originally described "Allens test" for determination of collateral blood flow adequacy.

Functional assessment of internal mammary artery bypass grafts: case reports and review of the literature

The utility of intra-arterial Doppler flow in assessing internal mammary artery (IMA) bypass grafts has infrequently been reported. Two patients are described in which Doppler evaluation of the IMA graft provided valuable diagnostic information prior to intervention. Also provided is a review of the available literature concerning evaluation of pathology in IMA grafts.

How to standardize vasomotor tone in serial studies based on quantitation of coronary dimensions?

In patients with coronary artery disease including those after coronary bypass graft operation and heart transplantation intervention studies based on serial quantitative coronary angiography, in part combined with intravascular ultrasound, are of increasing relevance. Since vasomotor tone of epicardial coronary arteries is influenced by a variety of factors, angiographic follow-up studies require standardization of coronary tone by induction of maximal dilation. We reviewed the effects of the most potent coronary vasodilatory drug groups, calcium antagonists and nitrocompounds, on coronary diameters. Intravenous or intracoronary injections of verapamil, diltiazem, nifedipine, nicardipine, and nisoldipine can cause profound coronary dilation which has been shown to be maximal with verapamil and nisoldipine. Shortcomings of calcium antagonists include short or unknown duration of action after bolus administration, severe drop in blood pressure, and lack of commercial availability of solutions for injection of many substances. Isosorbide dinitrate induces profound coronary dilation; however, after sublingual administration marked blood pressure decrease can occur, and the duration of action and ideal dose of intracoronary isosorbide dinitrate has not been investigated yet. Injections of molsidomine and its active metabolite, SIN-1, cause longlasting, reproducible, maximal coronary dilation, although only after a waiting period of at least 10 minutes; unfortunately, SIN-1 is only commercially available in France. Nitroglycerin induces reproducible maximal coronary dilation and is easy to administer sublingually or as intracoronary bolus injection with rapid onset of action and no major side effects. The short duration of action may require repeated administrations. To date, repeated intracoronary bolus injections of 0.1 mg nitroglycerin every 10 minutes seem to be the optimal known regimen for standardization of coronary vasomotor tone in serial angiographic studies. Further investigations in this field with old and new vasodilatory drugs are recommendable.

Noninvasive functional assessment of left internal mammary artery grafts by transcutaneous Doppler echocardiography

A noninvasive method to assess left internal mammary artery (LIMA) patency and function would be useful because this vessel is frequently used for revascularization of the left anterior descending coronary artery. The purpose of this study was to assess the feasibility of measuring changes in LIMA velocities by transcutaneous Doppler during dipyridamole-induced vasodilation. Twenty-five patients with a LIMA graft anastomosed to the left anterior descending coronary artery were studied at least 1 month after surgery by the use of a 5 MHz transducer placed in the left supraclavicular fossa. Doppler velocity parameters were measured at baseline and after intravenous administration of dipyridamole. Dipyridamole increased mean velocity by 127% +/- 54% (p < 0.001), systolodiastolic velocity time integral by 89% +/- 31% (p < 0.001), and diastolic-to-systolic peak velocity ratio from 0.7 +/- 0.3 to 1.2 +/- 0.4 (p < 0.001). The dipyridamole-to-baseline mean velocity ratio was 2.3 +/- 0.5. We conclude that it is possible to measure dipyridamole-induced changes in LIMA flow velocities and thus obtain an index of LIMA blood velocity reserve by transcutaneous Doppler echocardiography.

Endothelium-dependent and endothelium-independent flow reserve in vascular regions supplied by the internal mammary artery before and after bypass grafting

OBJECTIVE

It was the goal of this study to compare endothelium-dependent and endothelium-independent flow reserve in vascular regions supplied by the left internal mammary artery before and after bypass graft surgery.

METHODS

The native internal mammary artery in situ was investigated in 13 patients (age 61.8 +/- 8.0 years) with angiographically proven coronary artery disease. The internal mammary artery after bypass grafting was investigated in ten patients (age 60.8 +/- 7.3 years) 3.5 +/- 2.8 years after the operation. Flow reserve was evaluated endothelium-dependent with acetylcholine (ACh 25 and 50 microg i.c.) and endothelium-independent with nitroglycerin (NTG 0.3 mg i.c.) followed by papaverin (10 mg i.c.). Flow indices were calculated from intraluminal Doppler blood flow velocity measurements and the vascular cross-sectional area as determined by quantitative angiography. An index for vascular resistance was defined as the ratio of pressure gradient and resting or peak flow.

RESULTS

After endothelium-dependent stimulation with acetylcholine 25 microg (50 microg), flow in the internal mammary increased by 352.3 +/- 152% (412 +/- 145%) before surgery, whereas it increased only by 213 +/- 134% (193 +/- 120%) after surgery (P < 0.05). Endothelium independent stimulation with papaverin resulted in a flow increase of 391 +/- 234% before surgery vs. 315 +/- 135% after surgery (n.s.). The resistance index decreased after endothelium-dependent stimulation with acetylcholine 25 microg(50 microg) to 35 +/- 16.8% (28 +/- 8.9%) before surgery, whereas it decreased only to 59 +/- 26% (72 +/- 43%) after surgery (P < 0.05). Endothelium independent stimulation with papaverin resulted in a decrease of the vascular resistance index to 31 +/- 14% before surgery vs. 32 +/- 14% after surgery (n.s.).

CONCLUSION

Vascular regions supplied by the internal mammary artery as a graft demonstrate a significantly reduced endothelium-dependent flow reserve but a preserved endothelium-independent flow reserve as compared to vascular regions supplied by the native internal mammary artery. The selective decrease in endothelium-dependent flow reserve may be due to microvascular changes in the myocardial region supplied by the internal mammary artery after bypass grafting.

Evaluation in vivo of the endothelial function of the native gastroepiploic artery

The endothelial function of a coronary bypass graft is an important aspect, contributing not only to its patency but to its functional performance. To evaluate this aspect in vivo, we studied 16 patients who underwent selective catheterization of the native gastroepiploic artery (GEA). Quantitative angiography of the GEA was performed at baseline, after 2 minutes' infusion of acetylcholine in three ascending doses, and after 2 mg isosorbide dinitrate injection directly into the GEA. Mean GEA diameter was 2.02 +/- 0.38 mm at baseline. We observed dose-dependent vasodilation during acetylcholine infusion: The mean diameter increased slightly to 2.11 +/- 0.32 mm (+6%, not significant) with the second dosage and, more significantly, with the highest dosage, to 2.32 +/- 0.33 mm (+18%, p < 0.001). More important vasodilation was observed after administration of nitrates (+36%, p < 0.001). We found no difference between patients with and without coronary artery disease and no relationship with risk factors for atherosclerosis. A positive correlation was seen between the vasodilation observed after nitrate administration and the highest dose of acetylcholine (r = 0.728, p = 0.002). In conclusion, the GEA demonstrates a notable vasodilatory response to nitrates (non-endothelium-dependent) and a dose-related dilator response to acetylcholine, reflecting preserved endothelial function. This sensitivity should affect favorably the hemodynamic performance of grafts performed with GEA, as well as these grafts' long-term patency rate.

Intraoperative monitoring of IMA flow: what does it mean?

BACKGROUND

This study examines whether the measurement of internal thoracic artery (ITA) graft flow can determine the adequacy of the ITA-left anterior descending coronary artery (LAD) anastomosis.

METHODS

To study a wide range of clinical problems, we used a computer simulation of the cardiovascular system. The model included a time-varying elastance model of the heart, a systemic circulation represented by a multielement nonlinear model of the aorta and its major branches, a nonlinear model of the LAD circulation, and a model of the ITA bypass graft.

RESULTS

With a mild LAD stenosis, ITA graft flow was low and flow reversal occurred. As the percent stenosis increased, ITA flow and the percentage of ITA-to-total LAD flow increased. The ITA graft helped to maintain resting LAD blood flow. A partial obstruction (40%) at the ITA-LAD anastomosis reduced ITA graft flow at similar levels of LAD stenosis. However, overlap in flow values comparing a normal with a partially obstructed anastomosis occurred.

CONCLUSIONS

Flow patterns in the ITA are highly dependent on the degree of stenosis of the LAD as well as the integrity of the anastomosis. The predictive power of ITA flow measurement increases with severe stenosis or total occlusion of the proximal LAD and with high coronary blood flow demands.

Does the internal thoracic artery graft delay the recovery of myocardial metabolism?

BACKGROUND

The left internal thoracic artery (LITA) bypass graft to the left anterior descending artery has greater long-term patency than a saphenous vein graft. However, surgeons may be reluctant to use the LITA graft in some patients because they are unable to deliver cardioplegia to the left anterior descending artery territory.

METHODS

We compared the myocardial levels of high-energy phosphates and their metabolites in patients who received an LITA graft with those in patients who received a saphenous vein graft to the left anterior descending artery territory during elective coronary artery bypass grafting. Right and left ventricular biopsy specimens were obtained at three times: before aortic cross-clamping, after cross-clamp removal, and after 10 minutes of reperfusion.

RESULTS

No differences were found between the LITA graft group and the saphenous vein graft group in any right ventricular metabolites. There was an improvement in myocardial protection over time and a higher proportion of LITA graft patients in the late time period (early group, 63% versus late group, 80%; p < 0.01). Within each time period, there were no differences between the LITA and saphenous vein graft groups. Among patients receiving cold antegrade cardioplegia, the myocardial levels of high-energy phosphates were better preserved in those receiving an LITA graft.

CONCLUSIONS

Advances in myocardial protection have led to improved preservation of high-energy phosphate levels after cardioplegic arrest. In patients undergoing elective coronary artery bypass grafting, the use of an LITA graft does not adversely affect myocardial metabolism. Further investigations are required to determine the effects of the use of the LITA during urgent or emergent procedures.