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Ann Thorac Surg 2000;70:1558-1564
© 2000 The Society of Thoracic Surgeons
a Escorts Heart Institute and Research Centre, New Delhi, India
Address reprint requests to Dr Manisha Mishra, Escorts Heart Institute and Research Centre, Okhla Rd, New Delhi—110025, India
e-mail: manishamishra{at}yahoo.com
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Abstract |
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Methods. Between January 1993 and July 1997, 3,660 patients scheduled for CABG underwent intraoperative transesophageal echocardiography to identify aortic atheromatous disease. The disease was graded as follows: grade I, plaques extending less than 5 mm into the aortic lumen; grade II, plaques extending more than 5 mm into the aortic lumen; and grade III, plaques with a mobile element. Only patients with grade III atheromas were included in the study. Various surgical modifications were done depending on the location of the lesion, eg, aortic arch atherectomy, CABG combined with transmyocardial laser revascularization, off-pump CABG by median sternotomy, and minimally invasive direct coronary artery bypass. Measured outcomes were death, stroke, and other vascular events, both early (within 1 week) and late (1 to 5 years) after operation.
Results. Of the 3,660 patients, 104 (2.84%) had mobile atheromas. The perioperative stroke rate was 0.96%, and the incidence of other vascular events was 1.92% at 1 week. There was no embolic event in the group of 88 patients who underwent off-pump CABG. Of the study group, 98.07% are in regular follow-up. At 5 years, 1 patient had had a nonfatal stroke, and 2 patients had died of causes unrelated to atheromatous disease.
Conclusions. The stroke rate was very low in patients with mobile aortic atheromas who underwent CABG after modification in surgical technique, especially off-pump CABG. A follow-up of 5 years showed that patients with mobile atheromas have a very low incidence of spontaneous embolization.
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Introduction |
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The importance of the aorta as a source of emboli has become apparent only since the advent of transesophageal echocardiography (TEE) [9]. This technique has made possible high-resolution imaging of the atherosclerotic aortic wall in great detail.
The severity of aortic atheromas increases sharply with age, from 20% among patients in their forties at postmortem examination to 80% in patients more than 75 years old [4], and the stroke rate increases from 1% in patients 51 to 60 years old to 7% or more in those older than 75 years [4, 8]. Among patients dying after coronary bypass procedures, evidence of atheroembolism was present in only 4.5% in 1982 and in as many as 48% in 1989 [4]. Although the proportion of strokes caused by aortic atheroemboli rather than concomitant cerebral atherosclerotic disease has not been clearly defined, it has been demonstrated in small numbers of patients with severe aortic atherosclerosis that a decrease in perioperative stroke incidence can be affected by modifying cross-clamping, cannulation, and graft anastomosis techniques that specifically respect the embolic potential of aortic plaques [7].
The atherosclerotic process is one of a slowly progressive inflammatory disease of the arterial vasculature, with plaque formation taking place over several decades [10, 11]. The slow but inexorable progression is interspersed with episodes of plaque rupture that can lead to changes in lesion morphology and associated clinical events [12]. This prospective study was undertaken to identify mobile aortic atheromas in patients undergoing CABG and to determine the incidence of immediate perioperative embolic events after suitable surgical modifications. The late clinical events attributable to the natural history of the atheroma were also studied.
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Material and methods |
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Of the 3,660 patients studied, 104 patients had grade III atheroma in the ascending aorta or aortic arch and also completed at least 1 year of follow-up. They comprised the study group. The study was approved by the ethics committee of the institute.
Transesophageal echocardiography
Following a protocol in use at our institution since 1993, all 3,660 patients were studied. After induction of general anesthesia, a multiplane phased-array dual-frequency 3.7/5-MHz transesophageal transducer was placed in position to view the thoracic aorta. Hewlett-Packard SONOS 1500 and 2500 systems were used. All images were recorded on 
-inch (12.5-mm) videotape for subsequent playback and evaluation. The descending aorta was visualized by rotating the transducer at the level of the left atrium. The probe was then introduced into the stomach and gradually withdrawn up to the aortic arch while recording all images from 0 degrees to 180 degrees. The ascending aorta was imaged from the aortic root. Five to seven centimeters of the supravalvular ascending aorta could be visualized.
Definition of aortic disease
Particular attention was paid to obtaining good echocardiographic images of the thoracic aorta systematically. Instrument settings, gain, and time-gain compression were established for each patient to optimize arterial wall brightness and definition. An attempt was made not to alter these settings throughout the study. Three methods were used to evaluate aortic arch disease:
Atheromatous lesions of the aorta on TEE were graded as follows:
A cerebrovascular event was classified as a transient ischemic attack or a stroke. A transient ischemic attack was defined as focal cerebral dysfunction of presumed vascular origin that resolved completely within 24 hours. A stroke was defined as a focal or global cerebral dysfunction of presumed vascular origin lasting more than 24 hours. For the statistical analysis of neurological events, transient ischemic attack and stroke were considered a single group. A cerebrovascular event was considered embolic only if strong clinical and computed tomographic evidence was available. Computed tomography was done for every patient who had clinical signs of a neurological event.
A peripheral vascular event was classified as embolic if there were supporting clinical, angiographic, or surgical findings.
Measured outcomes were death, stroke, and other vascular events, both early (within 1 week) and late (1 to 5 years) after operation.
Duplex carotid Doppler echocardiography was also performed in all patients. Lesions were described as obstructive plaques when stenosis of greater than 50% was present with increased Doppler velocities and as nonobstructive when stenosis of less than 50% was present.
Surgical strategy
Surgical modifications were done only in patients with grade III atheroma located in the ascending aorta and aortic arch. Most surgical techniques concentrate on minimizing the direct handling of the diseased aorta. In the initial period of the study, the atheromas were managed very aggressively, leading to a major surgical exercise.
Changes in technique to less invasive measures were brought about by our interest in off-pump CABG. This permits multivessel coronary artery grafting because of better anesthesia backup in the form of monitoring and drugs to reduce heart rate and the availability of a variety of mechanical devices to stabilize the heart from companies such as Cardiothoracic Systems, Inc, Medtronic (Octopus tissue stabilization system), and Genzyme Surgical Products.
After a median sternotomy, the pericardium was opened and the ascending aorta gently palpated for calcification or atherosclerosis. In the meantime, TEE was performed and the aorta imaged. Depending on the location and the extent of the atheromatous lesion in the thoracic aorta, the need to modify surgical technique was evaluated.
Ascending aorta
In the presence of grade III atheromatous disease, if the disease was in the proximal portion, high aortic cannulation was done along with relocation of the vein graft and cardioplegia needle to another site. Greater use was made of arterial conduits, and a side-biting clamp was avoided. The proximal anastomoses were done with the cross-clamp in place (after the distal anastomosis) during the rewarming period. Warm potassium-free perfusate was injected through the retrograde coronary sinus cannula using the Shiley blood cardioplegia delivery system. If fewer proximal anastomoses were desirable on the ascending aorta, the conduits were taken piggyback on internal mammary artery or vein grafts.
Patients with disease in the distal portion of the ascending aorta precluding aortic cannulation had femoral artery cannulation and fibrillatory arrest without cross-clamping. Arterial grafts were used. In patients who had extensive disease of the ascending aorta and suitable coronary anatomy, grafting was done on a beating heart without cardiopulmonary bypass.
Our standard technique of cardiopulmonary bypass we use a membrane oxygenator, hypothermia to 30°C, hemodilution to a hematocrit of 25% to 30%, and pulsatile perfusion. Mean arterial pressure is maintained at 50 to 70 mm Hg. After cross-clamping, both antegrade and retrograde cardioplegia are used for myocardial protection. However, this technique was modified in several instances (depending on the aortic atherosclerosis) as will be detailed.
Aortic arch
All grade III atheromas in the proximal two thirds of the arch required modification in technique consisting of either femoral artery cannulation with fibrillatory arrest or a long cannula beyond the atheroma. In 6 patients, aortic arch debridement was done. After right atrium—femoral artery bypass was established, the patient was cooled to a rectal temperature of 15°C, and on low flows (up to 500 mL/min), the aortic arch was opened by making a longitudinal incision of 3 to 4 cm. The atheromatous material was found bulging into the lumen of the aorta. These atheromatous areas correlated with those seen on TEE. Rongeurs were used to remove all the loose friable atheromatous material, after which vigorous irrigation was performed. During the period of low flow, the brain was perfused retrogradely through a cannula in the superior vena cava at 200 mL/min to prevent air and debris from entering cerebral vessels. The aorta was then closed using 4-0 Prolene (Ethicon). The average period of low flow was about 12 minutes (range, 10 to 15 minutes). After air was evacuated from the arch, it was closed and CABG then performed.
Minimally invasive technique
In the presence of extensive atherosclerosis of the ascending aorta or the arch, the technique of coronary artery bypass was individualized to avoid atheroembolism. Minimally invasive direct coronary artery bypass (MIDCAB) and off-pump CABG were used. In the MIDCAB technique, the internal thoracic artery was harvested through a small anterior thoracotomy (4 to 6 cm) and anastomosed to the left anterior descending coronary artery without cardiopulmonary bypass.
With the off-pump method, multivessel CABG was performed through a median sternotomy using stabilization devices such as the Medtronic Octopus system and Genzyme Elite instruments. If the ascending aorta was normal, the proximal anastomosis was constructed on the ascending aorta. If the ascending aorta was diffusely diseased, the internal mammary arteries, gastroepiploic artery, radial artery, or even vein grafts were used in the form of T grafts and sequential anastomoses to obviate the need of proximal anastomose on the ascending aorta. Occasionally the lifting of the heart was not well tolerated hemodynamically by the patient. In such instances, after the left anterior descending coronary artery–right coronary artery anastomosis on a beating heart, either transmyocardial laser revascularization of the left circumflex territory was performed or percutaneous transluminal coronary angioplasty and stenting (hybrid procedure) of the left circumflex artery was done.
Follow-up
All patients underwent clinical examination at our institution at 1 month, 6 months, and then every year after operation. The follow-up comprised clinical examinations, questionnaires; and telephone interviews. A clinical examination was done by the cardiologist and then by a neurologist at every visit. All patients underwent a stress test 6 months postoperatively and every year thereafter. If there was clinical suspicion of residual myocardial ischemia or angina recurrence, an angiogram was made. Detailed questionnaires about any symptoms pertaining to an adverse event, in particular, a neurological or cardiac event, were sent to patients who could not have regular follow-up because of long travel distances. Telephone interviews were also held for this group of patients who could not be seen at the time of the routine follow-up.
Statistical analysis
The values are expressed as the mean ± the standard deviation. A 
2 analysis was done for the categorical data. A p value of less than 0.05 was considered significant.
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Results |
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Atherosclerosis was seen most frequently in the aortic arch, including grade III atheroma with large, mobile filamentous protrusions (Fig 1). There were 98 patients with grade III atheroma in the arch. Of these 98 patients, 6 underwent aortic arch debridement. Early in the study, in 1 patient with a large, nonmobile proximal arch atheroma, we inserted a long aortic cannula beyond the left subclavian artery to prevent the sandblast effect. Subsequently we began to use femoral artery cannulation for such patients. In 1 patient with a proximal arch atheroma, the aorta was opened for atherectomy. However, contrary to expectation, it was not ulcerated but hard and calcified. Atherectomy was abandoned and CABG performed.
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Neurological and vascular complications
Of the 3,660 patients evaluated by TEE, 104 patients had mobile atheroma. In the remaining 3,556 patients, the perioperative stroke rate was 0.4% (Table 5). In the study group, 1 patient sustained a stroke. The atheroma was in the ascending aorta, and femoral artery cannulation was done with hypothermic fibrillation without aortic cross-clamping. Right-sided hemiplegia developed, and a computed tomographic scan revealed an infarct in the territory of the left middle cerebral artery. Another patient with atheroma in the distal aortic arch experienced acute abdominal symptoms 72 hours after operation. Laparotomy revealed massive gangrene of the gut suggestive of superior mesenteric artery embolism. The condition of this patient deteriorated rapidly, and he died after 2 days. One patient had a minor episode of peripheral embolism, which resolved on its own. There was only one neurological event (0.96%) in the entire study group of 104 patients. There was no vascular event in the group of 88 patients in whom off-pump CABG was done. The operative mortality resulting from neurological events was nil.
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Comment |
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Although palpation has been the standard method of identifying aortic disease, it has been found to be relatively insensitive [5, 15]. Wareing and colleagues [4] reported that palpation identified atheroma in only 38% of 68 patients who had ultrasonographic evidence of major atheromatous disease of the ascending aorta. Palpation also underestimated the severity of disease.
The only literature available on the distribution of atherosclerotic lesions in the thoracic aorta in the Indian population is our previous study [9]. We found a very high incidence of atherosclerosis in the transverse aortic arch (31.07%) compared with the ascending aorta (2.53%) and the descending thoracic aorta (16.92%).
Barbut and coauthors [16] demonstrated in their pathological study that the ascending aortic segment is the least atheromatous portion of the thoracic aorta. In their series, severe atheromatosis of the ascending segment was detected in only 0.5% of patients as opposed to 6% in the aortic arch. These findings are consistent with those of Konstadt and associates [17], who found moderate to severe atheroma in the ascending segment in only 4.9% of patients and in the arch, in 12.3%. Although the prevalence of ulcerated plaque in the arch is twice that in the ascending segment in autopsy specimens, TEE–determined segmental differences appear to be largely artifactual. The current method of choice is epicardial echocardiography, and with this technique, moderate to severe disease has been detected in 17% of patients in all three segments [17].
Several studies underline the diagnostic impact of TEE in the evaluation of patients with suspected aortic atherosclerosis [18, 19]. An unequivocal association between TEE–detected aortic plaque and stroke was established by Amarenco and associates in the early 1990s. They [6] performed a prospective, case-control study of the frequency and thickness of atherosclerotic plaques in the ascending aorta and proximal arch in 250 patients with stroke and in 250 controls. They found protruding (4 mm) plaques in 14.4% of patients with stroke but in only 2% of controls (odds ratio for stroke, 9.1). Plaques of all thickness were associated with stroke, but the association was strongest for plaques more than 4 mm thick.
Aortic manipulations involving cross-clamping, side-clamping, clamp removal, aortic cannulation, and aortic angiography with advancement of an arterial catheter are all procedures that can disrupt an atherosclerotic plaque or a thrombus. In patients with intraaortic debris, the risk of embolization with angiography is estimated at 17% [26].
The cause of stroke is attributed to embolization of atherosclerotic material after aggressive manipulation of the aorta in an attempt to locate a cannulation site.
In this study, we found that spontaneous atheroembolic events from the thoracic aorta were a relatively uncommon cause of stroke and peripheral embolization. Other groups have reported similar results. Using TEE, Montgomery and coworkers [20] followed up a cohort of 30 patients with severe disease. The overall atherosclerotic grade did not change in the majority of patients (61%) throughout the 1-year follow-up, a finding reflecting relative stability in the disease process. In contrast, the morphology of individual severe atherosclerotic lesions represents a dynamic process, with the formation and the resolution of mobile components of the plaque occurring frequently.
Tunick and Kronzon [21] reported the case of a relatively young patient with an ulcerated plaque and a superimposed thrombus in the aortic arch who had multiple embolic episodes over a 12-year period. Intraaortic atherosclerotic debris that is pedunculated and highly mobile is more likely to embolize than is layered immobile debris (73% versus 12%). Karalis and colleagues [2] noted atherosclerotic debris in 38 of 556 patients referred for TEE. A history of an embolic event was documented in 8 (73%) of 11 patients with mobile plaque versus 3 (12%) of 25 patients with no mobile plaque.
We and others [5, 22–24] have shown that alteration in surgical techniques reduces the incidence of stroke. Hosoda and coauthors [25] identified moderate to severe atherosclerosis of the ascending aorta by ultrasonographic scanning in 36 of 100 patients studied. No surgical modification was done. Three (8.3%) of the 36 patients had a stroke compared with none of 64 patients without atherosclerosis. Using a hand-held ultrasonic probe to evaluate the ascending aorta, Wareing and associates [26] screened 1,200 patients who were older than 50 years and were to undergo a cardiac surgical procedure. The stroke rate in the series was 1.6%. We [9] found the overall incidence of neurological events in our previous report to be 0.76% (6 of 792 patients). None of the 6 were in the group in whom surgical modification was done.
Several management decisions should be made when atherosclerotic disease of the aorta is diagnosed in a patient scheduled for a cardiac operation. These decisions regard the following issues: What thickness and what extent of aortic atherosclerosis require a change in surgical technique? What changes in technique of bypass grafting are available when severe disease is present? In cases of severe disease, should the surgical procedure be modified to the point that extremely aggressive changes in management are advocated, such as aortic atherectomy or resection of diseased segments with graft replacement? Will changes in management result in improved neurologic outcome for the patient? This last is the most difficult to answer.
Most authorities agree that discrete areas of thickening less than 3 mm in depth and away from areas that are normally manipulated during a cardiac surgical procedure do not necessitate a change in technique. Focal areas of moderate to severe disease greater than 3 mm thick and located in the surgical field are often managed by using techniques that avoid aortic manipulation, such as changing the site of aortic cannulation or changing the cardioplegia technique or cross-clamp placement or both using procedures described. More aggressive management for severe, extensive disease, especially protruding plaque in areas of aortic manipulation, is the most controversial because the changes in technique often involve deep hypothermic circulatory arrest and atherectomy or aortic replacement.
There are reasons to believe that manipulation of the aorta carries an inherent risk of dislodging atheromatous debris, if present. In some reports [1, 2], invasive procedures on the aorta were deferred (eg, insertion of an intraaortic balloon pump) or modified (brachial instead of femoral route for cardiac catheterization) after aortic atheroma was seen on TEE.
In summary, the stroke rate was very low in patients with mobile aortic atheromas who underwent CABG after modification in surgical technique. Of the surgical modifications done, we prefer the technique of off-pump CABG in this very high risk group of patients. Follow-up of 5 years showed that patients with mobile atheromas have a very low incidence of spontaneous embolization of debris from the atheroma.
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