Double arterial vs. single axillary cannulation in acute type A aortic dissections: a meta-analysis.

Aim: To evaluate the effects of double (axillary and femoral) vs. single (axillary) cannulation on early outcomes of acute type A aortic dissection (ATAAD). Materials & methods: Meta-analysis using PubMed/MEDLINE, Scopus, and Cochrane databases through August 23, 2023. Focused on operative mortality, postoperative stroke, re-exploration for bleeding, spinal cord injury, and renal replacement therapy. Results: Among 5 propensity score-matched studies with 2127 patients, double cannulation showed comparable mortality and higher rates of postoperative stroke (pooled odds ratio: 1.69, 95% confidence interval: 1.19-2.39) and need for renal replacement therapy (pooled odds ratio: 1.35, 95% confidence interval: 1.13-1.60) compared with single cannulation. Conclusion: Double arterial cannulation in ATAAD surgery is associated with increased postoperative stroke and renal replacement therapy than single cannulation.

What is this summary about? We studied the optimal way to maintain blood flow during surgery for acute aortic dissection. We focused on comparing the use of one tube placement site in the axillary artery with two sites, both in the axillary and femoral arteries, in five previous studies.What were the results? Using two sites was associated with a higher risk of stroke and need for dialysis after surgery than using only one site.What do the results mean? Adding a tube in the femoral artery for blood flow may increase the risk of complications. It appears that placing the tube only in the axillary artery may be a safer choice for appropriately selected patients having this surgery.

Clinical Presentation and Management of the Cervical Aortic Arch in the Adult Population: A Review of Case Reports.

The cervical aortic arch (CAA) is an uncommon congenital anomaly in aortic development, characterized by an elongated aortic arch extending at or above the medial ends of the clavicles. Our objective was to examine the clinical and surgical characteristics of this infrequent condition in the adult population. PubMed, ScienceDirect, SciELO, DOAJ, and Cochrane Library databases were searched until December 2023 for case reports describing the presence of a cervical aortic arch in patients aged ≥18 years. Case reports and series were included if the following criteria were met: (1) description of the cervical aortic arch, (2) age ≥18 years, and (3) English language. The literature search identified 2,325 potentially eligible articles, 61 of whom met our inclusion criteria and included a combined number of 71 patients. Mean age was 38.6 ± 15.4 years, with a female prevalence of 67.1% (47/70). Two-thirds of the CAA were left-sided (48/71, 67.6%), and 62.0% (44/71) of patients presented a concomitant arch aneurysm. Asymptomatic patients were 45.7% (32/70), while of those that were symptomatic, 60.5% (23/38) had symptoms related to vascular-induced compression of trachea and esophagus. Surgery was performed in 42 patients (62.7%) among 67 cases that reported the patient's treatment, and 5 patients (11.9%) among those surgically treated underwent the procedure through an endovascular approach. CAA is an uncommon congenital abnormality that presents challenges in diagnosis and treatment due to its high anatomical variability, diverse clinical manifestations, and presence of concomitant diseases. Surgery seems to be a safe and effective option for the resolution of symptoms.

Risk Score for Long-Term Survival and Major Adverse Cardiovascular and Cerebrovascular Events After Coronary Artery Bypass Grafting Surgery.

To develop risk scoring models predicting long-term survival and major adverse cardiovascular and cerebrovascular events (MACCEs), including myocardial infarction and stroke after coronary artery bypass grafting (CABG). All 4,821 consecutive patients who underwent isolated CABG at Lankenau between January 2005 and July 2021 were included. MACCE was defined as all-cause mortality + myocardial infarction + stroke. Variable selection for both outcomes was obtained using a double-selection logit least absolute shrinkage and selection operator with adaptive selection. Model performance was internally evaluated by calibration and accuracy using bootstrap cross-validation. Mortality and MACCEs were compared in patients split into 3 groups based on the predicted risk scores for all-cause mortality and MACCEs. An external validation of our database was performed with 665 patients from the University of Brescia, Italy. Preoperative risk predictors were found to be predictors for all-cause mortality and MACCEs. In addition, being of African-American ethnicity is a significant predictor for MACCEs after isolated CABG. The areas under the curve (AUCs), which measures the discrimination of the models, were 80.4%, 79.1%, 81.3%, and 79.2% for mortality at 1, 2, 3, and 5 years follow-up. The AUCs for MACCEs were 75%, 72.5%, 73.8%, and 72.7% at 1, 2, 3, and 5 years follow-up. For external validation, the AUCs for all-cause mortality and MACCEs at 1, 2, 3, and 5 years were 73.7%, 70.8%, 68.7%, and 72.2% and 72.3%, 68.2%, 65.6%, and 69.6%, respectively. The Advanced (AD) Coronary Risk Score for All-Cause Mortality and MACCE provide good discrimination of long-term mortality and MACCEs after isolated CABG. External validation observed a more AUCs greater than 70%.

Hybrid robotic-assisted coronary revascularization and transcatheter aortic valve replacement: a single-center experience.

OBJECTIVES: The efficacy of hybrid robotic-assisted coronary artery bypass grafting (CABG) and transcatheter aortic valve replacement (TAVR) for coronary and aortic valve disease is poorly reported. Herein, we report our experience with this hybrid approach. METHODS: Between January 2018 and June 2022, 10 (7 male, 3 female) patients with a mean age of 81 years underwent the hybrid procedure. Coronary revascularization was performed prior to TAVR with robotic-assisted left internal mammary artery-to-left anterior descending (LAD) bypass grafting for left main or proximal LAD lesions with or without multivessel disease with or without hybrid percutaneous coronary intervention (PCI). RESULTS: Five patients had left main disease, and 5 had proximal LAD disease with or without multivessel disease. All patients tolerated the robotic-assisted CABG procedure well; 9 patients were extubated in the operating room and all patients were ambulatory on postoperative day 1. Five patients underwent hybrid PCI for non-LAD lesions. TAVR was subsequently performed at intervals ranging from 3 days to 5 months after CABG. One patient with end-stage renal disease on hemodialysis required hospitalization for heart failure during the interval period. The 1-year mortality rate was 0%, and 3 patients died during late follow-up (24-43 months). CONCLUSIONS: This innovative, less invasive approach demonstrates the potential for early recovery in appropriately selected patients with complex coronary and aortic valve disease with promising mid-term outcomes.

Direct aortic cannulation versus femoral arterial cannulation for early outcomes in acute type A aortic dissection: A study-level meta-analysis.

PURPOSE: To investigate the impact of direct aortic cannulation (DAC) versus femoral arterial cannulation (FAC) on clinical outcomes of surgery for acute type A aortic dissection. METHODS: PubMed/MEDLINE, Scopus, and Cochrane Central Register of Controlled Trials were searched until August 25, 2023, to conduct a meta-analysis. Primary endpoints of the study were operative mortality and postoperative stroke. Secondary endpoints were cardiopulmonary bypass time, myocardial ischemic time, hypothermic circulatory arrest time, temporary neurological dysfunction (TND), combined stroke and TND, re-exploration for bleeding, and need for renal replacement therapy. A random-effect model was used to estimate the pooled effect size, and a leave-one-out method was used for the primary endpoints for sensitivity analysis. RESULTS: 15 studies met our eligibility criteria, including a total of 7941 samples. Operative mortality was significantly lower in the DAC group with a pooled odds ratio (OR) of 0.72 [95% confidence interval (CI): 0.61-0.85)]. Incidence of postoperative stroke was also lower in the DAC group with a pooled OR of 0.79 (95% CI: 0.66-0.94). However, after excluding one study with the greatest weight, the difference became nonsignificant. DAC was also associated with a lower incidence of postoperative TND, and re-exploration for bleeding with a pooled OR of 0.52 (95% CI: 0.37-0.73), and 0.60 (95% CI: 0.47-0.77), respectively. CONCLUSIONS: This meta-analysis showed that patients who underwent ATAAD repair with DAC had a lower incidence of operative mortality, postoperative stroke, TND, and re-exploration for bleeding compared to those who underwent FAC.

Aortic versus axillary cannulation in acute type A aortic dissection repair: A meta-analysis.

BACKGROUND: Research comparing the effectiveness of central aortic cannulation to axillary artery cannulation in repairing acute type A aortic dissection is limited and controversial. This meta-analysis aimed to compare early outcomes of central aortic cannulation versus axillary artery cannulation for surgery for acute aortic dissection type A. METHODS: A comprehensive systematic search was conducted across PubMed/MEDLINE, Scopus, and the Cochrane Central Register of Controlled Trials up to September 1, 2023. The primary endpoints were operative mortality and incidence of postoperative stroke. Secondary endpoints encompassed cardiopulmonary bypass time, myocardial ischemic time, hypothermic circulatory arrest time, postoperative temporary neurological dysfunction, combination of stroke and temporary neurological dysfunction, as well as the need for reexploration for bleeding, renal replacement therapy, and tracheotomy. A random-effect model was utilized to calculate the pooled effect size. RESULTS: Eleven studies met our eligibility criteria, enrolling a total of 7204 patients (2760 underwent aortic cannulation and 4444 underwent axillary cannulation). The operative mortality and incidence of postoperative stroke did not show statistical differences between the two groups, with a pooled odds ratio of 1.07 (95% confidence interval: 0.73-1.55) and 1.17 (0.95-1.42), respectively. Similarly, none of the secondary endpoints exhibited significant statistical differences between the two groups. CONCLUSIONS: Aortic cannulation can be a viable alternative to axillary artery cannulation for repair of acute aortic dissection type A, as both approaches present similar early clinical outcomes.

[Stanford Type A Acute Aortic Dissection with Myonephropathic Mrtabolic Syndrome].

A 57-year-old man was transferred with sudden onset chest pain and evolving paralysis and numbness in the left leg. Contrast computed tomography (CT) revealed Stanford type A acute aortic dissection from the ascending aorta to bilateral internal and external iliac arteries with blood flow obstruction to the left kidney and left lower limb. Surgery was initiated 10 hours after onset of ischemic symptoms in the leg. Femoro-femoral bypass was carried out first, and we ensured sufficient phlebotomy from the ischemic limb during reperfusion and continuous hemodiafiltration to prevent myonephropathic metabolic syndrome. Total aortic arch replacement was then performed. Our treatment strategy was effective in this case of Stanford type A aortic dissection with prolonged lower limb ischemia. Although left hip disarticulation was subsequently required due to intractable infection, the patient became able to walk with an artificial limb after post-rehabilitation.

Surgical repair of the adult form of scimitar syndrome.

Scimitar syndrome has various clinical presentations and anatomic features, and some cases are diagnosed in adulthood. Reconstruction using autologous tissue to reroute the right pulmonary venous blood flow to the left atrium is ideal. However, if the scimitar vein drains to the caudal segment of the inferior vena cava, reconstruction using prosthetic material may be necessary due to the distance between the left atrium and the scimitar vein. We describe the case of a 16-year-old boy with scimitar syndrome. We anastomosed the scimitar vein to the right atrium using an artificial graft and created an atrial septal defect for rerouting the right pulmonary venous blood to the left atrium. It has been 9 years since this procedure, and the patient has not experienced graft stenosis or thromboembolic events.

Postoperative Maximal Aortic Diameter is a Significant Predictor of Dilation of the Residual Dissected Aorta after Aortic Replacement for Acute Debakey Type I Aortic Dissection.

BACKGROUND: This study investigated the impact of aortic diameter on late aortic dilation of the residual dissected aorta after tear-oriented aortic replacement for acute DeBakey type I aortic dissection. METHODS: Of 133 patients who underwent aortic replacement for acute DeBakey type I/II aortic dissection between 2008 and 2019, 45 patients with a residual dissected aorta after surgery for acute DeBakey type I aortic dissection and who underwent computed tomography at predischarge and after 1 year were retrospectively assessed. The aortic diameter and false lumen area were measured at 3 levels: the maximal aortic site, seventh thoracic vertebra, and celiac axis. Multivariable Cox regression analysis was employed to identify the predictors of late aortic dilation, defined as an aortic growth rate of ≥5 mm/year or a maximal aortic diameter of ≥55 mm. RESULTS: During a median follow-up of 75 [range: 13-152] months, 6 patients (5 men; mean age: 57 ± 14 years) experienced aortic dilation. All 6 patients had the maximal aortic diameter between the distal aortic arch and seventh thoracic vertebra level at the last computed tomography. Multivariable Cox regression analysis showed that the predischarge maximal aortic diameter was an independent determinant of late aortic dilation (hazard ratio: 2.28/mm, 95% confidence interval: 1.10-5.86). CONCLUSIONS: Predischarge maximal aortic diameter is a significant predictor of late aortic dilation in patients with a residual dissected aorta after tear-oriented surgical repair of acute DeBakey type I aortic dissection.

Coronary artery patch augmentation for congenital left coronary ostial stenosis in Williams syndrome.

Left coronary ostial stenosis, which is associated with sudden death, occasionally occurs in individuals with Williams syndrome. However, surgical methods that provide reliable long-term revascularization remain unknown among infants and young children with coronary ostial stenosis. We describe the case of an 18-month-old boy with Williams syndrome who presented with cardiogenic shock due to left coronary ostial stenosis. We performed patch augmentation of the left coronary ostium using glutaraldehyde-treated autologous pericardium. At the last follow-up, the patient was well without any adverse events or myocardial ischemia.

Unilateral Embolization of the Internal Iliac Artery for Endovascular Aortic Repair Does Not Induce Gluteal Muscle Atrophy.

BACKGROUND: To investigate the effect of unilateral internal iliac artery (IIA) embolization for endovascular aortic repair (EVAR) on gluteal muscle size. METHODS: We assessed the gluteal muscle size in 111 consecutive patients who underwent elective EVAR with unilateral IIA embolization (n = 31) or without IIA embolization (n = 80) for abdominal aortic and/or iliac artery aneurysm. The cross-sectional area (CSA) of the gluteus maximus (Gmax) and gluteus medius/minimus (Gmed/min) was measured on computed tomography preoperatively, 6 months postoperatively, and final follow-up. Mean changes in the Gmax and Gmed/min CSA were evaluated using a mixed model analysis of variance. RESULTS: In the patients with embolization, both the Gmax and Gmed/min CSA significantly decreased over time on the embolization and nonembolization sides (P < 0.001); however, embolization did not affect the changes in the Gmax CSA (P = 0.64) and Gmed/min CSA (P = 0.99). In the patients with embolization and those without embolization, both the Gmax and Gmed/min CSA significantly decreased over time (P < 0.001); however, embolization did not affect the changes in the Gmax CSA (P = 0.76) and Gmed/min CSA (P = 0.11). CONCLUSIONS: Unilateral IIA embolization was not associated with gluteal muscle atrophy after EVAR. Pre-emptive unilateral IIA embolization for EVAR seems to be an acceptable procedure in terms of maintenance of gluteal muscle size.

Determinants of Changes in Arterial Stiffness after Thoracic Endovascular Aortic Repair.

BACKGROUND: Aortic stent grafting can cause aortic stiffening and increase pulse wave velocity (PWV), which can potentially affect long-term cardiovascular outcomes. The aim of this study was to clarify the factors contributing to increases in PWV after thoracic endovascular aortic repair (TEVAR). METHODS: We included 64 patients with thoracic aortic pathology (51 men; mean age, 73 years) who underwent elective TEVAR, in this study. TEVAR was performed for degenerative aortic aneurysm (n = 43) or aortic dissection (n = 21), and the treatment length was 175 ± 52 mm. Brachial-ankle PWV (baPWV) was obtained before and 1 week after TEVAR. Univariable and multivariable logistic regression analyses were used to determine the predictors of increases in baPWV of ≥100 cm/sec after TEVAR. RESULTS: baPWV increased from 1,851 ± 392 cm/sec to 2,047 ± 479 cm/sec, and the change in baPWV (ΔbaPWV) was 195 ± 339 cm/sec (95% confidence interval, 111-280). Thirty-seven patients (58%) had ΔbaPWV ≥100 cm/sec after TEVAR. In the multivariable analysis, in addition to Δheart rate and Δsystolic blood pressure, age (odds ratio, 1.21/year; 95% confidence interval, 1.05-1.40) and coronary artery disease (odds ratio, 12.0; 95% confidence interval, 1.20-121) were independent determinants of ΔbaPWV ≥100 cm/sec after TEVAR, whereas ΔbaPWV ≥100 cm/sec was not associated with treatment length or device type. CONCLUSIONS: TEVAR was associated with PWV progression, especially in older patients with coronary artery disease, whereas treatment length or device type was not a predictor of PWV progression after TEVAR.

Preoperative distal aortic diameter is a significant predictor of late aorta-related events after endovascular repair for chronic type B aortic dissection.

OBJECTIVES: Long-term therapeutic effects of thoracic endovascular aortic repair (TEVAR) for chronic type B aortic dissection remain controversial. This study aimed to evaluate the possible predictors of late aortic rupture and re-interventions after TEVAR. METHODS: We retrospectively reviewed the operative outcomes of 40 patients who underwent TEVAR for chronic type B aortic dissection at Kyushu University Hospital. During a mean follow-up period of 39.2 months, we assessed aortic morphology via computed tomography and then employed a multivariable Cox regression analysis in an attempt to identify the predictors of late aorta-related events. RESULTS: The early success rate of TEVAR was 100%. During the follow-up, however, three patients died from aortic rupture. Eight patients required aortic re-intervention, including thoraco-abdominal aortic graft replacement, repeated TEVAR, total arch replacement and EVAR. Multivariable Cox regression analysis revealed that the preoperative maximum distal aortic diameter was a significant predictor of late aorta-related events. The cutoff value of the distal aortic diameter was 40 mm. Freedom from aorta-related events was 94.6% at 1 year and 78.3% at 3 years. The survival rate was not significantly different despite the re-intervention. CONCLUSIONS: TEVAR is an effective treatment for chronic type B dissection, with acceptable mid-term results. The preoperative distal aortic diameter is a significant risk factor for late aorta-related events. When the maximum distal aortic diameter is ≥ 40 mm, a therapeutic strategy should be developed taking into consideration the possible need for aortic re-intervention.

Comparison of cardiac energetics after transcatheter and surgical aortic valve replacements.

OBJECTIVES: The effect of transcatheter aortic valve replacement (TAVR) on cardiac energetics has not been described. We compared changes in cardiac energetics after TAVR with those after surgical aortic valve replacement (SAVR). METHODS: We retrospectively estimated end-systolic elastance (Ees) and effective arterial elastance (Ea) using blood pressure and left ventricular (LV) volume obtained from echocardiography. LV efficiency [ventriculoarterial coupling (Ea/Ees) and the stroke work to pressure-volume area ratio (SW/PVA)] was calculated. Measurements were taken before, 1 week after and 1 year after bioprosthetic aortic valve replacement (TAVR, n = 56; SAVR, n = 61) in patients with severe aortic stenosis and preserved ejection fraction. RESULTS: Patients with TAVR had a lower aortic valve pressure gradient and larger stroke volume 1 week after the procedure than those with SAVR. Ea was more markedly decreased, and LV efficiency was significantly improved 1 week after TAVR (SW/PVA 68.1% ± 8.4% to 72.0% ± 8.5%, P < 0.001), but LV efficiency was unchanged 1 week after SAVR (SW/PVA 70.1% ± 7.4% to 69.1% ± 8.0%). LV efficiency was improved 1 year after both procedures (SW/PVA 75.5% ± 6.1% in TAVR; 74.7% ± 6.4% in SAVR). CONCLUSIONS: TAVR decreases the transvalvular pressure gradient further without deteriorating stroke volume in the early postoperative period, which is accompanied by early improvement in afterload and LV efficiency compared with SAVR. Improvement in LV efficiency at mid-term follow-up is satisfactory after both procedures.

Thoracic endografting increases cardiac afterload and leads to left ventricular hypertrophy in dogs.

OBJECTIVES: Aortic endografting can cause aortic stiffening. We aimed to determine the chronic effect of thoracic endografting on cardiac afterload, function and remodelling. METHODS: Eleven dogs were included, and all except 1 was successfully assessed [endograft, n = 5; sham operation (control), n = 5]. We deployed a stent graft in the descending aorta. The ascending aortic pressure and flow were measured, and aortic input impedance was obtained by frequency analysis to determine characteristic impedance and arterial compliance. Left ventricular pressure-volume relations were measured with an admittance catheter. Measurements were performed before, 10 min after and 3 months after endografting. Following euthanasia, we weighed the left ventricle of each dog and measured the cardiomyocyte cell size. RESULTS: Arterial compliance decreased from 0.47 ± 0.07 to 0.36 ± 0.06 and to 0.31 ± 0.05 ml/mmHg (both P < 0.01 versus baseline), and characteristic impedance increased from 0.11 ± 0.04 to 0.19 ± 0.05 and to 0.21 ± 0.04 mmHg/ml/s (both P < 0.01 versus baseline) 10 min and 3 months after endografting, respectively. Pressure-volume relation analysis showed that arterial elastance increased from 5.3 ± 1.0 to 6.7 ± 1.6 (at 10 min) and to 6.8 ± 1.0 mmHg/ml (at 3 months) (both P < 0.05 versus baseline), but end-systolic elastance and ventriculo-arterial coupling remained unchanged. Left ventricular weight to body weight ratio and left ventricular cardiomyocyte cell width in the endograft group were larger compared with the control's results (5.06 ± 0.27 g/kg vs 4.20 ± 0.49 g/kg, P = 0.009, 15.1 ± 1.7 µm vs 13.9 ± 1.5 µm, P = 0.02, respectively). CONCLUSIONS: The mid-term effect of the descending aortic endografting on left ventricular contractility and efficiency in canine normal hearts was minimal. However, endografting resulted in increased cardiac afterload and left ventricular hypertrophy.

Axillo-iliac artery bypass for recurrent aortic coarctation to reduce cardiac afterload.

A 13-year-old girl, who had undergone interrupted aortic arch repair with an 8-mm graft as a neonate and Fontan completion in childhood, developed ventricular fibrillation due to long-QT syndrome. Cardioverter defibrillator implantation was planned. Preoperative catheterization showed a 45-mmHg aortic pressure gradient and ventricular end-diastolic pressure of 11 mmHg. This indicated that recurrent coarctation had adversely affected ventricular function. After consideration of the patient's age, symptoms and anatomical/surgical complexities, axillo-iliac bypass with cardioverter defibrillator implantation was performed. Postoperative ventricular end-diastolic pressure was 6 mmHg. Axillo-iliac bypass is a surgical option for coarctation that can reduce cardiac afterload.

Concealed Left Ventricle to Right Atrium Fistula Complicating Infective Endocarditis.

We report a case of aortic prosthetic valve endocarditis presenting with subaortic stenosis without perivalvular leakage and vegetations in the left ventricular outflow and right atrium, the latter being attached to the atrioventricular septum. Intraoperatively, an abscess that had formed on the aortic annulus and perforated to the right atrium was unexpectedly found, the fistula being occluded by vegetations. Even when no left-to-right shunts are detected by imaging, vegetations adjacent to the atrioventricular septum may conceal a left ventricle-right atrium fistula, resulting in prosthetic valve endocarditis presenting clinically as subaortic stenosis without perivalvular leakage.

Successful resection of a huge metastatic liposarcoma in the pericardium resulting in improvement of diastolic heart failure: a case report.

Although liposarcoma often metastasizes to various organs, cardiac metastasis, including to the pericardium, is rare. We present a case of a third recurrence of pericardial metastasis from the thigh, which required surgical resection because of cardiac failure. Surgery was effective for improving cardiac function and reintroducing chemotherapy. This is the first reported case of metastatic pericardial liposarcoma, which was successfully resected three times. We believe that aggressive surgical treatment, when it can resolve cardiac impairment, potentially leads to a more favorable prognosis.

Endovascular Surgery for Traumatic Thoracic Aortic Injury: Our Experience with Five Cases, Two of Whom were Young Patients.

OBJECTIVES: We present our experience of endovascular surgery for traumatic aortic injury and the results of our procedures. MATERIALS AND METHODS: From January 2009 to December 2013, we performed endovascular repairs of traumatic thoracic aortic injury on 5 male patients 16-75 years old (mean, 50.8), two of whom were young. Three of the patients had multiple organ injuries. The mean interval time to the operation is 22.0 hours (range, 10-36). All patients underwent endovascular repair with heparinization. The isthmus regions were seen in three cases and all of them were needed left subclavian artery (LSA) coverage. In the two young patients, the deployed stent graft was 22 mm (22.2% oversizing for diameter of aorta) and 26 mm (36.8% oversizing), respectively. RESULTS: The procedures were successful in all patients, with no early mortality, paraplegia or stroke. During 3-63 months (mean, 30.8) follow-up period, no one experienced stent graft-related complications. One patient with LSA coverage experienced arm ischemia but the symptom improved with time. CONCLUSION: Endovascular surgery for traumatic thoracic aortic injury can be performed safely with low mortality or morbidity even in young small aorta. Accumulation of clinical experience and evaluation of long-term outcomes are necessary.

Precursor mediated cycloaddition reaction of ethylene to the Si(100)c(4 x 2) surface.

We report the direct observation of a precursor state for the cycloaddition reaction (the di-sigma bond formation) of ethylene on Si(100)c(4 x 2) using high-resolution electron energy loss spectroscopy at low temperature, and the meta-stable precursor state is identified as a weakly bonded pi-complex type. The activation energy from the pi-complex precursor to the di-sigma bonded species is experimentally estimated to be 0.2 eV. First-principles calculations support the pi-complex precursor mediated cycloaddition reaction of ethylene on Si(100)c(4 x 2).