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Ann Thorac Surg 2000;70:1565-1570
© 2000 The Society of Thoracic Surgeons

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Original articles: cardiovascular

Thickened intima of the aortic arch is a risk factor for stroke with coronary artery bypass grafting

^a Department of Cardiovascular Surgery, Kameda Medical Center, Chiba, Japan
^b Department of Cardiovascular Surgery, Tokyo Takanawa Hospital, Tokyo, Japan

Address reprint requests to Dr Mizuno, Department of Cardiovascular Surgery, Kameda Medical Center, 929 Higashi-cho, Kamogawa City, Chiba 296-8602, Japan

	Abstract

Top Abstract Introduction Patients and methods Results Comment References

 
Background. Perioperative stroke is one of the most serious complications of cardiac surgery.

Methods. Using transesophageal echocardiography, we estimated the intimal thickness of the thoracic aorta as an index of the severity of aortic atherosclerosis to determine the risk of stroke in coronary artery bypass grafting (CABG) patients. The study population comprised 315 consecutive patients who underwent isolated CABG with cardiopulmonary bypass.

Results. Five patients (1.6%) had perioperative cerebral stroke or systemic emboli. We compared the mean intimal thicknesses of the ascending aorta, aortic arch, and descending aorta. Mean thicknesses in patients without stroke were 2.07 ± 0.76, 2.78 ± 1.15, and 2.32 ± 1.21 mm, respectively, and mean thicknesses in the stroke patients were 1.94 ± 0.55, 6.94 ± 3.79, and 3.39 ± 1.85 mm, respectively. The patients with an intima of more than 5 mm at the aortic arch had a significantly greater incidence of perioperative stroke (p = 0.007).

Conclusions. These results suggest that patients who have an aortic arch intima thickened to more than 5 mm are at a significantly high risk for perioperative stroke, and thus, the CABG procedure should be carefully evaluated to prevent such complications.

	Introduction

Top Abstract Introduction Patients and methods Results Comment References

 
In cardiac surgery, stroke is one of the most serious complications, significantly increasing morbidity and mortality [1]. Perioperative emboli have been considered to come from the severely atherosclerotic ascending aorta [2] or to form during arterial cannulation, cross-clamping, partial clamping, or declamping [3, 4]. To prevent emboli, many cardiac surgeons have assessed the ascending aorta by means of various examinations [5–8] and have modified surgical techniques in an endeavor to decrease the incidence of such strokes [9, 10], but perioperative emboli can not be eliminated completely.

There have been suggestions by some neurologists that protruding aortic arch atheroma is also a risk factor for cerebral stroke [11, 12]. Among cardiac surgeons, however, the aortic arch has not been investigated in detail as a potential source of emboli. From this standpoint, we used transesophageal echocardiography (TEE) to assess the thoracic aorta of patients who underwent isolated conventional coronary artery bypass grafting (CABG) with cardiopulmonary bypass (CPB) and examined the relevancy of the severity of thoracic aortic atherosclerosis as it pertains to stroke after CABG.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment References

 
About 360 patients underwent CABG in our hospitals between April 1995 and January 1999. Patients who had additional valvular or aneurysmal surgeries, those who had CABG on the beating heart without CPB, and those who could not received TEE for some reason were excluded from this study. We performed carotid arterial echography in patients older than 60 years, and those who showed stenosis of the carotid artery were also excluded because carotid arterial stenosis itself is an independent risk factor for perioperative cerebral complications. This study was conducted in the remaining consecutive 315 patients who underwent isolated conventional CABG with intraoperative TEE evaluation. The age of the 315 patients ranged from 33 to 86 years (mean 64.0 ± 9.9 years), and there were 252 men and 63 women. Ten patients (3.2%) had preoperative cerebral infarction. There was no patient with preoperative transient ischemic attack symptoms.

The mean number of distal anastomoses per patient in the total study was 2.90 ± 0.87, and the number of proximal anastomoses was 1.20 ± 0.61. Five-hundred thirty-one arterial conduits were employed: 280 left internal mammary arteries, 32 right internal mammary arteries, 72 right gastroepiploic arteries, and 147 radial arteries. Saphenous veins were used for the remaining grafting. Twelve patients (3.8%) had previous CABG. Femoral artery cannulation was used in 18 patients (Table 1).

Evaluation by TEE
After general anesthesia, TEE was performed using a biplane transducer (Echo system; 77025A Ultrasound system, transducer; 21,363 A, 5.0 MHz; Hewlett Packard, Palo Alto, CA). After examination of left ventricular and valvular function, the thoracic aorta was assessed by TEE. As an index of the severity of aortic atherosclerosis, the intimal thickness of the aorta was measured in the ascending aorta, in the aortic arch, and in the proximal descending aorta.

By rotating the probe counterclockwise from the short-axis transgastric view, the descending aorta can be visualized as a near-perfect circle. Until the probe is at the level of the aortic arch, the aorta can be seen as a perfect circle. When the probe is at the level of the aortic arch, the aorta can be imaged in an oblong or longitudinal view. All aortic arch vessels are difficult to image, but the left subclavian artery can be visualized. Using TEE, the proximal ascending aorta and the aorta distal to the middle portion of the aortic arch can be evaluated, but the distal ascending aorta and proximal aortic arch, which is shadowed by the air-filled trachea, can not be visualized.

In the short-axis view of the aorta, the intimal thickness could be measured accurately because the intima is visualized vertically. In the long-axis view, especially of the aortic arch, the intimal thickness was measured at the view where the aortic lumen was maximally visualized. In this view, the echo beam cut the aortic wall vertically and the measured intimal thickness was not mixed with angling artifacts. The ascending aorta was imaged just above the sino-tubular junction in the short-axis view (Fig 1A), and the proximal ascending aorta was imaged in the long-axis view (Fig 1B). The aortic arch was imaged in the transverse view (Fig 1C), and the proximal descending aorta in the short-axis view (Fig 1D). The intima was imaged as a low echogenic intraluminal layer. In this study, we paid attention only to the intimal thickness. The thickened intimal lesions consisted of various atheromatous materials like atheroma, ulceration, and calcification.

View larger version (83K): [in this window] [in a new window]   Fig 1. View of a normal aorta by transesophageal echocardiography. Normal ascending aorta in short-axis view (A) and in long-axis view (B); normal aortic arch (C) and normal descending aorta (D). Only the dista ascending aorta can not be imaged because it is behind the trachea and main bronchus.

Perioperative complication
Perioperative stroke was defined as any neurologic event consistent with a central source that required more than 24 hours for resolution, and brain computed tomography or magnetic resonance scanning was performed several days after the surgery for evaluation of the anatomy and pathology. Strokes from which the patient fully recovered within 24 hours were reported as transient, and all others were considered permanent.

Statistical analysis
All data are expressed as mean ± SD. Statistical analyses were performed using Fisher’s exact probability test or Mann-Whitney statistics. A p value of less than 0.05 was considered statistically significant.

	Results

Top Abstract Introduction Patients and methods Results Comment References

 
Early operative deaths (within 30 days after surgery) occured in eight cases (2.3%, one stroke, two arrhythmias, two mediastinitises, and three heart failures that resulted from preoperative cardiogenic shock from acute myocardial infarction). Five men among the 315 patients (1.6%) suffered perioperative cerebral stroke, and 1 of them suffered an additional massive systemic embolization and died. These stroke patients ranged in age from 51 to 84 (mean 71.4) years, and there was no statistical difference in age between the perioperative stroke and nonstroke patients (p = 0.092). The number of proximal and distal anastomoses in perioperative stroke patients (2.8 ± 1.1 and 1.3 ± 0.6, respectively) did not correlate statistically with the incidence of stroke (p = 0.992 and 0.72, respectively).

None of the 10 patients with previous cerebral infarction suffered perioperative stroke. One of the 12 patients who had redo CABG (patient 2) had perioperative cerebral stroke, but there was no statistically significant correlation between previous CABGs and perioperative strokes in this study (p = 0.164). Femoral artery cannulation was used in 18 patients, and 1 of them (patient 2) had perioperative stroke, but there was no statistically significant correlation between femoral artery cannulation and perioperative strokes (p = 0.264).

The TEE procedure was well tolerated in all patients and was without complications. We obtained clear aortic images throughout the thoracic aorta except at the upper ascending aorta, which was hidden behind the trachea and the bronchus. The lower ascending aorta, arch, and descending aorta could be imaged well (Fig 1). The descending aortas of all patients were imaged plainly, and only two ascending aortas and three aortic arches could not be imaged clearly enough in any view to measure the intimal thickness.

In all patients, the mean intimal thicknesses of the ascending aorta, aortic arch, and descending aorta were 2.07 ± 0.76, 2.84 ± 1.33, and 2.34 ± 1.22 mm, respectively. Intimal thickness was greater than 5 mm in two ascending aortas, 14 aortic arches, and 11 descending aortas, and was greater than 8 mm in five aortic arches and two descending aortas. In the patients without perioperative stroke, intimal thicknesses in the ascending aorta, aortic arch, descending aorta were 2.07 ± 0.76, 2.78 ± 1.15, and 2.32 ± 1.21 mm, respectively. In comparison, in the patients with perioperative stroke, the thicknesses were 1.94 ± 0.55, 6.94 ± 3.79, and 3.39 ± 1.85 mm, respectively (Table 2), and intimal thickness was greater than 5 mm in three aortic arches and one descending aorta, and greater than 8 mm in two aortic arches. Among patients with an intima thickened to more than 5 mm, the frequency of perioperative stroke was statistically high (22.2%, p = 0.0007). It was also high among patients with an intima thickened to more than 8 mm (40%, p = 0.020).

Characteristics of the 5 perioperative stroke patients are shown in Table 3. One patient (patients 1), who had mildly atherosclerotic aortic arches, suffered cerebral infarction several days after his surgeries. Two patients (patients 2 and 3) noticed right hemiplegia upon awakening from surgery. Patient 4 suffered a major systemic stroke including cerebral infarction and died. This patient had the thickest intima at the aortic arch of all patients. He awoke after surgery, but metabolic acidosis, which resulted from massive emboli of the lower body including the abdominal organs, progressed acutely, and he died 2 days after surgery. Patient 5, who had protruding aortic arch intima (Fig 2), complained of dizziness and disturbed vision after surgery, and he noticed weakness in his left leg during the postoperative rehabilitation period. He was diagnosed with cerebral infarction based on magnetic resonance imaging of the brain.

View larger version (76K): [in this window] [in a new window]   Fig 2. Thickened intima of the aortic arch in patient 5. The patient suffered a perioperative cerebral infarction.

One mobile atheroma was seen in an aortic arch, and it remained after CPB. The patient had no embolic complication after CABG.

	Comment

Top Abstract Introduction Patients and methods Results Comment References

 
Perioperative embolic complications of the central nervous system and the lower body have been one of the most important causes of morbidity and mortality since the early years of cardiac surgery using CPB. In previous reports, two kinds of emboli have been noted. One is the air embolus, which was thought to be produced by using CPB. Its incidence was reduced by introduction of the 40-mm arterial filter and membrane oxygenator [13]. The other kind of embolus is the atheromatous embolus. Patients with a severely atherosclerotic ascending aorta have shown a high frequency of perioperative cerebral embolic events, and thus, many groups have looked into assessment of the ascending aorta and surgical techniques to prevent stroke. Some investigators of neurology, however, have reported the diseased aortic arch to be the source of emboli and that TEE is useful for assessment of the aortic arch [11, 12]. They have demonstrated a good correlation between cerebral stroke and the severity of atherosclerosis in the aortic arch. Among cardiac surgery investigators, however, only a few have pursued the atherosclerotic aortic arch as a potential determinant for ischemic stroke [14–16].

In our study, we measured the intimal thickness of the aorta and used this measurement to index the severity of aortic atherosclerosis. It was demonstrated that patients with an aortic arch intima thickened to more than 5 mm had a significantly high frequency of perioperative stroke and that an even higher incidence (40%) occured among patients with an intima thickened to more than 8 mm. The 2 perioperative stroke patients with only mildly thickened aortic arch intimas (patients 1 and 2) showed a lag period between the surgery and the stroke. It is possible that these strokes could have been related to factors other than aortogenic factors. Marschall and associates [17] reported also that the arch intima greater than or equal to 5 mm or with a mobile component correlated significantly with perioperative stroke. Di Tullio and associates [11] suggested TEE-assessed large aortic atheromas (intimal thickness 0.5 cm) were an independent risk factor for stroke in nonoperative patients, and Amarenco and associates [18] found a high frequency of atheromas greater than or equal to 4 mm in thickness in nonoperative patients over 60 years of age. Katz and associates [15] reported mobile atheroma (but not protruding atheroma > 5 mm) to be correlated with perioperative stroke.

Many previous reports have suggested severe atherosclerosis of the ascending aorta is a main risk factor for stroke after cardiac surgery, and the emboli have been considered to be produced by direct manipulation of the diseased ascending aorta. The emboli of the aortic arch is, however, thought to be produced by a different process because the aortic arch is not manipulated directly. Groom and associates [19] observed that the peak flow velocity emanating from the arterial cannula tip ranged from 3 to 9.5 times the normal aortic flow velocity, suggesting a link between artificial flow and embolus formation. Katz and associates [15] suggested that even small, nonmobile atherosclerotic lesions in the aortic arch can result in debris being dislodged by aortic cannulation or by the high-pressure jet emanating from the cannula tip. The arterial jet flow might break down the fragile atheroumatous plaque of the aortic arch.

Noting the severity of the aortic atherosclerosis and understanding the relationship between the atherosclerotic grade and the frequency of stroke is very important for cardiac surgeons in determining whether to modify the surgical technique and strategy to avoid embolic complications in CABG patients. Ribakove and associates [16] proposed a grading scale of I to V for atheromatous disease of the aorta, but the grading scheme was not objective; the grades represent descriptive characteristics: minimal, extensive, sessile, protruding, and mobile. We measured only intimal thickness for quantitative analysis. The thickened intimal lesions included the other atherosclerotic forms such as calcified plaque, ulceration, and mobile thrombi in this study. Our findings suggest that whether or not the intima of the aortic arch is thickened to more than 5 mm is a good objective prognosticator of perioperative stroke. What kind of atherosclerotic formation most often becomes the source of emboli is not yet understood, but the intimal thickness of the aorta is considered to summarize the severity of aortic atherosclerosis.

Assessment of the proximal aorta is also very important. It is generally accepted that TEE is useful for assessment of the aorta. We could, in fact, clearly evaluate the proximal ascending aorta, aortic arch, and descending aorta. But because of the air-filled trachea and main bronchus, TEE was insensitive for assessing the distal ascending aorta, where the aortic cross-clamp and arterial cannula are placed [20]. Actually, in almost all patients, the tip of the arterial cannula could not be imaged by intraoperative TEE. This is a major drawback of TEE, but the atherosclerosis of this section can be estimated to some degree from TEE views of the aortic arch and proximal ascending aorta. With direct manual palpation of the ascending aorta, only large lesions will be felt, and small and soft atheromas will be missed in 50% to 75% of patients [5, 6]. Intraoperative epiaortic ultrasonography can detect atherosclerosis of the whole ascending aorta sensitively [5–8], but it can not image the aortic arch sufficiently. The most accurate intraoperative assessment of the ascending aorta and aortic arch might be achieved by an intraoperative combination of TEE and epiaortic ultrasonography.

In CABG, the formation of emboli from the aortic arch might differ in process from the formation of emboli from the ascending aorta. We think that in patients with protruding intima of the aortic arch, CPB itself could be harmful and should be avoided, and that the best approach to coronary revascularization in these patients is a combination of off-pump CABG using in-site arterial grafts and catheter intervention.

In conclusion, our findings suggest that severe atherosclerosis of the aortic arch is an independent risk factor for perioperative stroke in CABG patients undergoing CPB.

	References

Top Abstract Introduction Patients and methods Results Comment References

 

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