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Ann Thorac Surg 2005;79:1217-1224
© 2005 The Society of Thoracic Surgeons
a Department of Cardiac Surgery, Villa Torri Hospital, Bologna, Italy
b Department of Nuclear Medicine, Bologna General Hospital, Bologna, Italy
Accepted for publication September 24, 2004.
* Address reprint requests to Dr Gatti, via Pignolini 5, Peschiera d/G, 37019 Verona, Italy (E-mail: giusep.gatti{at}tiscali.it).
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Abstract |
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METHODS: In 53 of 60 consecutive patients who underwent complete myocardial revascularization using only this composite arterial graft, good transthoracic echocardiographic images and pulsed Doppler signals of the Y-graft main stem were obtained at rest and early after standard exercise. Stress/rest 99mTc-sestamibi myocardial perfusion single-photon emission computed tomography (SPECT) was the gold standard for residual myocardial ischemia. The patients with negative SPECT were divided into groups according to the number of coronary artery systems grafted, and history of preoperative myocardial infarction.
RESULTS: Diastolic peak velocity, diastolic velocity-time integral, the diastolic-to-systolic ratio of the peak velocities and velocity-time integrals, and the stress-to-rest ratio of the diastolic peak velocities and diastolic velocity-time integrals in the negative-SPECT patients were significantly greater than in the 6 positive-SPECT patients. Sensitivity and specificity for ischemia of the stress-to-rest ratio of the diastolic peak velocities less than 1.5 were 100%. The stress-to-rest ratio of the diastolic velocity-time integrals in the patients with three coronary systems grafted, and in those without preoperative myocardial infarction, were respectively greater than in the patients with two systems grafted (p < 0.0001), and in those with preoperative myocardial infarction (p = 0.0048).
CONCLUSIONS: Noninvasive dynamic assessment with transthoracic echocardiography of a composite arterial graft, including in situ left internal thoracic artery, is feasible and correlates with myocardial perfusion SPECT. The Y-graft used was able to regulate its flow capacity to myocardial demand.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsCommentReferences |
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So far, in situ LITA has been reliably evaluated with transthoracic echocardiography (TTE) both at baseline [10–24] and after physiologic [21] or nonphysiologic stress tests [11, 15–20], either at its origin from the left subclavian artery [11–24] or at the coronary anastomotic site [10], only after coronary artery bypass grafting of the left anterior descending coronary artery with or without cardiopulmonary bypass [12, 18]. These evaluations, performed generally after 2 to 6 months from operation in symptomatic or asymptomatic patients, were compared with intraoperative ultrasound transit-time flowmeter results [21], or postoperatively tested with arteriography [10–13, 15, 17, 18, 20], exercise test [14], and myocardial scintigraphy [20].
The aim of this prospective study was to perform a reliable and reproducible noninvasive dynamic assessment of the main stem of the composite RA and in situ LITA Y-graft using TTE. Complete myocardial revascularization was obtained with this composite arterial Y-graft only.
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Patients and Methods |
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TopAbstractIntroductionPatients and MethodsResultsCommentReferences |
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Age, preoperative clinical status and cardiac function (Table 1), coronary arteries grafted (Table 2), and postoperative outcome were prospectively recorded and analyzed. Only 1 patient had a major perioperative complication (myocardial infarction).
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The Institutional Review Committee approved the study. Each patient entering the study gave informed consent specific to the type of surgery and postoperative evaluation. The procedures followed were in accordance with institutional guidelines.
Operative Procedure
After standard median sternotomy incision, the in situ LITA was dissected as skeletonized conduit. The RA was simultaneously harvested as pedicled conduit from the nondominant arm. A warm diluted papaverine hydrochloride solution (200 mg papaverine in 100 mL saline) was applied to the LITA and RA throughout the dissection, and gently injected into the two conduits early before the distal end clip-occlusion. Cautery was used only to cut the endothoracic fascia and to obtain hemostasis on the chest wall. The RA was maintained in situ until the end-to-side Y-anastomosis (using 8-0 polypropylene running suture) with the in situ LITA (at the third intercostal side branch) had to be performed.
Cardiopulmonary bypass was then instituted after standard cannulation. Myocardial protection was achieved with intermittent anterograde and retrograde cold crystalloid cardioplegia. Mild systemic hypothermia was employed.
Every anastomosis between the Y-graft and coronary artery was performed, end-to-side or side-to-side, with 8-0 polypropylene running suture. The left anterior descending coronary artery system was bypassed with the LITA branch of the Y-graft, and the circumflex coronary artery and right coronary artery systems with the RA branch (Table 2).
Drug Management
The use of a continuous intravenous infusion of nitroglycerin (0.5 to 5 mg/h), starting at the stop of the aortic cross-clamping time, was the sole specific systemic antispasmic drug strategy (combined with the topical application and injection of diluted papaverine solution) we adopted during operation to prevent the immediate RA graft failure. This infusion was followed by transdermal nitroglycerin (10 or 15 mg/d) until intensive care unit discharge, and by oral isosorbide-5-monohydrate (20 mg/d) throughout 1 year of follow-up. In case of acute myocardial ischemia, the infusion of nitroglycerin (0.5 to 10 mg/h) should be restored until complete episode resolution. A calcium-channel antagonist (diltiazem) was used only orally (180 mg/d), and only when headache and chronic obstructive pulmonary disease or peripheral vascular disease limited, respectively, the use of nitrates and ß-adrenergic blockers.
Antiplatelet therapy included a single intravenous dose of 125 mg lysine acetylsalicylate 6 hours after surgical procedure, and every postoperative day until extubation; aspirin (150 mg/d) or ticlopidine (500 mg/d) was then administered, and continued for the rest of patient's life.
Atorvastatin was the lipid-lowering agent we used for the duration of 1 year from operation, and indefinitely continued in case of hyperlipidemia.
The ß-adrenergic blockers were stopped 24 hours before performing the postoperative evaluation.
Noninvasive Dynamic Assessment With Transthoracic Echocardiography of the Y-Graft
At a mean of 7.9 ± 0.8 months (range, 6 to 9) after cardiac surgery, TTE evaluation of the main stem of the Y-graft was attempted in all the 60 patients. Color Doppler TTE was performed using computed instrument (Vivid 3 Expert; GE Medical Systems, Solingen, Germany) equipped with a 5.0 and 7.5 MHz linear array transducer, both at rest and early after a treadmill exercise test for 15 minutes on a flat surface at 5 km/h. The double product was calculated before and after exercise. The scanner head was placed in the left supraclavear space with the patient in the supine position. The main stem of the Y-graft was localized by means of low nondirectional color flow mapping, and the diameter was measured on B-mode. The intraluminal flow signals were obtained using the pulsed Doppler method, with the ultrasound beam maintained as parallel as possible to the long axis of the main stem. The diastolic (DPV) and systolic peak velocities (SPV), the diastolic (DVTI) and systolic velocity-time integrals (SVTI), the diastolic-to-systolic ratio of the peak velocities (DPV/SPV) and velocity-time integrals (DVTI/SVTI), both at rest and after stress, as well as the stress-to-rest ratio of the diastolic peak velocities (DPVs/DPVr) and diastolic velocity-time integrals (DVTIs/DVTIr) were the evaluated echocardiographic variables. The DPV, SPV, DVTI, and SVTI were determined on the basis of the shape of the Doppler signal curve; the peak velocity being the highest point of the Doppler wave, and the velocity-time integral the area between the line traced on the Doppler wave and the baseline.
Both at rest and after stress, satisfactory ultrasonic visualizations of the main stem as a tubular structure characterized by the color flow directed from the left subclavian artery to the heart and by the typical biphasic Doppler waveform (with a larger diastolic component and a smaller systolic one) were obtained in 53 patients (88%). In 2 patients with a muscular neck it was impossible to individuate the main stem already at rest, and in another patient it was impossible to revisualize the main stem after stress. In 3 patients, the pulsed Doppler signals were unreliable because the angle between the ultrasound beam and the long axis of the main stem greater than 30 degrees caused underestimation of the true velocity; in these 6 patients, there were no signs or symptoms of myocardial ischemia. Finally, in 1 patient with perioperative myocardial infarction and the Doppler waveform with a very small diastolic peak followed by a larger systolic component, the stress test was prudently not attempted at all; arteriography confirmed stenosis at the Y-anastomosis.
Each TTE examination was recorded on VHS videotape for separate offline analysis by two experienced observers unaware of the myocardial scintigraphy results.
Myocardial Perfusion Single-Photon Emission Computed Tomography Evaluation
In the 53 patients with satisfactory dynamic assessment with TTE of the Y-graft main stem, the 99mTc-sestamibi myocardial perfusion scintigraphy with single-photon emission computed tomography (SPECT) approach was chosen to show any residual myocardial ischemia; the 2-day stress/rest protocol was used [25]. The SPECT scan was always performed within 7 days of the echocardiographic evaluation in each patient, and was considered positive for myocardial ischemia if any reversible filling defect was visible between the stress and resting images. The Bruce exercise was used as stressing test. The 90% of the predicted maximal heart rate was the considered peak exercise.
Statistical Methods
Values of variables were expressed as mean ± standard deviation, or as percentage. The Mann-Whitney test was used for comparing continuous variables. Statistical significance was assumed for a p value less than 0.05. The sensitivity, specificity, predictive value, and diagnostic accuracy for ischemia of DPV/SPV and DVTI/SVTI <0.6—both at rest and after stress—as well as of DPVs/DPVr and DVTIs/DVTIr less than 1.5 were calculated using standard formulas, with myocardial perfusion SPECT as the gold standard. We plotted the individual data points of the main transthoracic echocardiographic variables according to the SPECT results, the number of coronary artery systems grafted, and history of preoperative myocardial infarction. The threshold values we adopted—0.6 and 1.5—were obtained by choosing them in the sample of data points.
The highest interobserver and intraobserver variability allowed for the echocardiographic measurements was 3%.
Statistical analysis was performed using MINITAB release 13 statistical software (MINITAB, State College, PA).
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Results |
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Of all 53 patients with satisfactory dynamic assessment with TTE of the Y-graft, 6 patients (11%) had a positive myocardial perfusion SPECT. Five positive-SPECT patients of the 6 were the only patients of this study with the Bruce exercise test positive (Table 3). At rest, while there were no statistically significant differences as the evaluated systolic echocardiographic variables—SPV and SVTI—between these positive-SPECT patients and the 47 patients with negative SPECT, the diastolic echocardiographic variables—DPV and DVTI—were significantly lower in the positive-SPECT patients (p < 0.0001 and = 0.0009, respectively), so even the DPV/SPV and DVTI/SVTI ratios were significantly inferior (p < 0.0001 and = 0.0079, respectively). After stress, these differences between the two groups of patients both as the diastolic echocardiographic variables and the diastolic-to-systolic ratios were still more evident (Fig 1). The DPVs/DPVr and DVTIs/DVTIr were severely impaired in the positive-SPECT patients, and significantly lower with respect to the negative-SPECT patients (p < 0.0001 and = 0.0002, respectively; Fig 2). Stress increased the specificity, predictive value, and diagnostic accuracy for residual myocardial ischemia of DPV/SPV and DVTI/SVTI less than 0.6. The DPVs/DPVr less than 1.5 had 100% of sensitivity, specificity, predictive value, and diagnostic accuracy for ischemia (Table 4).
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Comment |
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TopAbstractIntroductionPatients and MethodsResultsCommentReferences |
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Both coronary arteriography and flow wire determinations are invasive and expensive. Moreover,as selective catheterization of the target vessel is required, these examinations may induce spasm of the catheterized artery, so causing a catastrophic event if the target artery is the main stem of a composite arterial graft achieving complete myocardial revascularization. Recent experiences have shown that noninvasive assessment with TTE of the in situ LITA graft blood flow, either at the origin from the left subclavian artery or at the coronary anastomotic site with the left anterior descending coronary artery, is feasible and reliable [10–24]. Thanks to Doppler TTE, it is possible to perform a complete (morphologic and functional) evaluation of explored coronary graft. Morphologic informations concern visualization of the graft, and measurements of diameter and cross-sectional area throughout the graft. Functional information concerns the changes of diameter and cross-sectional area of the graft after pharmacologic or nonpharmacologic stress tests, the Doppler waveform morphology (the diastolic-to-systolic ratio), and the changes of Doppler waveform morphology throughout the graft and after stress tests.
This study aimed at proving that it is possible to perform a reliable and reproducible noninvasive dynamic assessment with TTE of the main stem of a composite arterial graft including in situ LITA.
Our choice of exclusion criteria comes from the need for a standard population of patients with coronary artery disease, and for good arterial conduits to make good composite Y-grafts; in case of impaired blood flow throughout the Y-graft, it was mandatory to rule out a possible extrinsic cause such as compression by an hyperinflated left lung. Our average patient was younger than the typical patient undergoing coronary surgery; there were no other differences. We chose 99mTc-sestamibi myocardial perfusion SPECT as a gold standard for residual myocardial ischemia because it is effective in predicting subsequent cardiac events even in the asymptomatic patients after coronary artery bypass grafting, and in determining need for coronary arteriography [26]. All the patients were assessed after 6 to 9 months from operation. They underwent the same surgical management and drug treatment. Nuclear investigation was always performed early after TTE to rule out any clinical change of the patient between the two evaluations. The average number of coronary anastomoses per patient and the number of RA anastomoses to coronary arteries were high.
Our rate (88%) of satisfactory ultrasonic visualizations and evaluations of the main stem Y-grafts agree with the literature data [10–24]. Because TTE assessments after exercise were performed in polypneic and stressed patients, we think this is a very good result. It may be ameliorated by introducing complex computed elaborations to overcome underestimation of the true velocity in the 3 patients with the angle between the ultrasound beam and the long axis of the main stem greater than 30 degrees [11]; however, the following need for special software might make this study nonreproducible. Cause of failure in the patient whose main stem was not visible after stress was probably linked to exuberant activation of accessory respiratory muscles, eg, the (left) anterior scalenus muscle. In 2 of the 6 positive-SPECT patients, coronary arteriography showed a stenotic RA anastomosis. A 34-year-old woman suffering from combined familial hypercholesterolemia and insulin-dependent diabetes mellitus had extensively positive SPECT and confirmed diffuse and peripheral coronary artery disease at postoperative coronary arteriography, but negative Bruce exercise test. Both the patients with two coronary systems (left anterior descending and right coronary artery) grafted, and positive SPECT and Bruce exercise test for inferior or inferolateral myocardial ischemia but angiographic patency of the Y-graft, had only one RA anastomosis and neither occlusion nor subocclusive stenosis of the right coronary artery proximally. Perhaps, in the presence of a small coronary flow reserve and in the absence of both good coronary run-off and proximal protective stenosis, stress caused a crisis in the RA branch of the Y-graft of these patients (a kind of steal phenomenon to the LITA branch). Finally, failure of the main stem to perfuse suitably the three stenotic coronary systems was a plausible explanation in the patient with diffuse myocardial ischemia but Y-graft angiographic patency.
Diastolic echocardiographic variables values and the diastolic-to-systolic ratios were lower in the positive-SPECT patients than in the negative-SPECT ones. Stress increased diagnostic accuracy of DPV/SPV and DVTI/SVTI less than 0.6 for myocardial ischemia, and optimized specificity and predictive value. The DPVs/DPVr less than 1.5 had the same diagnostic capacity for myocardial ischemia as the 2-day stress/rest 99mTc-sestamibi SPECT. There was a good correlation between DVTIs/DVTIr and expected myocardial demand.
This simple prospective study is the first evaluation with TTE of the main stem of a composite RA and in situ LITA Y-graft. This graft achieved complete myocardial revascularization, so any influences due to the presence of other coronary grafts were eliminated. All the patients were evaluated at midterm follow-up to remove any time-linked variability as the growth potential of the graft [7, 21, 24, 28]. Results were compared with a high diagnostic tool such as the 2-day stress/rest 99mTc-sestamibi myocardial perfusion SPECT. There are some limitations, however.
Neither intravascular Doppler guidewire nor quantitative arteriography was used to validate our TTE variables values. Postoperative coronary arteriography was performed only in the positive-SPECT patients and in the patient with perioperative myocardial infarction. Our echocardiographic measurements were obtained neither after pharmacologically induced maximal hyperemic blood flow (with adenosine, dipyridamole, or dobutamine) nor during atrial pacing [9], but after peak muscular exercise. The vasodilatation induced by physiologic stimuli does not appear to be as complete as that pharmacologically induced [27]. Moreover, we did not assess simple graft bypassing one coronary artery but composite graft, sustaining even three coronary artery systems. However, this is seemingly a limitation of the study: we were interested in the relative values of echocardiographic variables depending on the results of SPECT, number of coronary systems bypassed with the Y-graft, and presence of preoperative myocardial infarction, but not in the absolute values; differences in the values after maximal hyperemia might be even more evident. We did not consider the impact of the coronary stenosis severity on coronary blood flow reserve. The determinants of myocardial demand we adopted are arbitrary and debatable, but simple and intuitive [6, 7, 21–23].
Last but not least, the 100% sensitivity and specificity for ischemia of DPVs/DPVr less than 1.5 was obtained by carefully choosing the threshold in the sample of data points. We are well aware that our results would have been even more convincing if the threshold was derived a priori from the literature. However, since this study is the first evaluation with TTE of the main stem of a composite Y-graft achieving complete myocardial revascularization, we think it is incorrect to adopt a priori any threshold used for a simple graft supporting a very limited number of coronary anastomoses, such as the in situ LITA to the left anterior descending coronary artery.
Composite arterial Y-graft including in situ LITA should be able to sustain the whole stenotic coronary artery tree, even during peak muscular exercise stress. It is also possible to perform a complete dynamic assessment of the main stem with TTE. Demanding techniques are necessary, however, and proximal protective stenosis and peripheral coronary run-off play an important role in the Y-graft physiology and pathophysiology. In this time of minimally invasive cardiac operations, one should tend to obtain minimally invasive or noninvasive cardiac diagnosis. The Y-grafts including an in situ internal thoracic artery and TTE may be two good presuppositions for a noninvasive evaluation of the health of the coronary grafts.
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References |
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