Surgical Treatment for Aneurysms of the Descending Aorta Using Temporary Perfusion by a Centrifugal Pump: Clinical Analysis of 33 Cases

Abstract: Since 1987, 33 patients have undergone surgery at Kobe University Hospital for aneurysm of the descending aorta using left heart bypass with a heparin-coated centrifugal pump and heparin-coated tubes. Sixteen patients had true aneurysms of the descending thoracic aorta, 7 had thoracoabdominal aneurysms, and 10 had aortic dissection (DeBakey's Type III). Heat exchangers and oxygenators were not included in the bypass circuit in any of the cases. Perfusion time was from 42 to 205 min (average 90 min). Left heart bypass was established with 1 mg/kg of systemic heparinization in 5 cases, 0.5 mg/kg in 5 cases, and 0 mg/kg in 23 cases. There were no complications such as perioperative embolism, acidosis, or hypothermia. During aortic cross-clamping, the arterial pressure of the lower extremity was maintained above 70 mm Hg, but there was no relationship between the distal perfusion pressure and bypass flow. The urine output during left heart bypass was related to the distal perfusion flow by centrifugal pump. Of 23 patients who underwent bypass with less than 40 ml/kg/min of distal perfusion flow, 7 showed transient renal dysfunction post-operatively, and 1 developed postoperative renal failure. The other patients who were bypassed with over 40 ml/kg/min of pump flow stayed in the normal range of renal function. Postoperative paresis occurred in 2 patients, who were also perfused with less than 40 ml&kg/min of bypass flow. It could be concluded that left heart bypass by centrifugal pump is safe and acceptable as a circulatory support in the surgical treatment of aneurysm of the descending aorta. In addition, during such assistance, more than 40 ml/kg/min of perfusion flow is recommended to maintain the distal circulation and to prevent ischemic renal damage and spinal cord injury.     

Beneficial Circulatory Support by Left Heart Bypass with a Centrifugal (BioMedicus) Pump for Aneurysms of the Descending Thoracic Aorta

Abstract: A comparative study between left heart bypass with a centrifugal (BioMedicus) pump and with a temporary external shunt was performed to assess the efficacy of distal organ perfusion in the surgical treatment of 31 patients with aneurysm of the descending thoracic aorta. Eighteen patients were supported with a centrifugal pump, and the remaining 13 were supported by temporary shunt with either a Gott shunt or a Dacron graft. Heparinless bypass with a centrifugal pump provided a significant decrease of intraoperative blood loss and blood transfusion by the combined application of Cell-Saver. The pressure difference between upper and lower extremities decreased (p < 0.05) in the centrifugal pump group even with aortic cross-clamping, and the urine output increased during operation. Among 13 patients supported with the temporary shunt, 3 had postoperative renal failure, and 2 died of it. All patients with a centrifugal pump survived without any complications. It could be concluded that the left heart bypass with a centrifugal (BioMedicus) pump was safe and was favorable for support of the distal circulation during aortic cross-clamping and to prevent ischemic complications such as renal failure and spinal cord injury.     

Early and late results following repair of dissections of the descending thoracic aorta

Management of dissections of the descending thoracic aorta remains controversial, especially with regard to timing and method of repair. To clarify these and other issues we have reviewed our total experience with repair of descending aortic dissections between 1962 and 1983. The 44 men and 20 women had a mean (+/- SEM) age of 59 +/- 2 years (range, 19 to 83 years), and in all patients the dissection originated in and was limited to the aorta distal to the left carotid artery (Stanford type B, DeBakey types IIIa and IIIb). Twenty-nine patients underwent operation within 2 weeks of the onset of symptoms (acute), and the remainder had later repair (chronic). During repair, circulation distal to the aortic cross-clamp was supported with cardiopulmonary bypass or shunt in two thirds of patients. Overall, 18 deaths occurred less than or equal to 30 days postoperatively (operative risk 28%), and risk was higher in acute (45%) than in chronic (14%) dissections. Operative risk was not significantly related to protection of the distal circulation. The most serious postoperative complication was spinal cord ischemia manifested by paraplegia in five patients (8%) and transient or permanent paraparesis in six patients (9%). Risk of spinal cord ischemia was significantly lower in patients who had protection of the distal circulation during operative repair (8% vs. 44%, p = 0.003). Late survival, including hospital deaths, was 49% +/- 7% at 5 years after operation; 22 of the 46 patients who survived repair were found to have aneurysms involving the thoracic and/or abdominal segments of the aorta. Our results indicate that repair of chronic dissection of the thoracic aorta has a lower operative risk than repair of acute dissections, and initial medical management of acute dissection may be indicated if no early complications occur. Risk of spinal cord ischemia is significantly reduced by cardiopulmonary bypass or shunt and is preferred over aortic cross-clamping alone. Finally, these patients require careful long-term follow-up because of the high incidence of residual or recurrent aortic aneurysms.     

Descending thoracic aortic aneurysms: surgical treatment with the Gott shunt.

Over the past 16 years, 267 consecutive patients underwent surgery for a descending thoracic aortic aneurysm. To provide optimal protection of surrounding organs during aortic occlusion, a 9-mm Gott shunt was used for distal perfusion in all cases. The shunt was placed preferentially between the ascending aorta and the descending aorta; however, alternative sites of proximal and distal cannulation were chosen according to the location and the extent of the aneurysmal disease and the presence of a concomitant aneurysm along the aortic conduit. In one-third of the patients, a flowmeter on the shunt recorded shunt flows, which varied from 1100 mL/min to 4900 mL/min (mean 2526 mL/min). Because the highest shunt flows were obtained with proximal systolic pressures lower than 140 mm Hg, nitroglycerin and nitroprussate were used routinely to improve distal perfusion by arterial vasodilation and release of proximal organs from a circulatory overload. The mean aortic cross-clamp time was 33 minutes for the entire series but was reduced to 25 minutes for the last 140 patients. The hospital death rate was 14.6% overall (12.2% if ruptured aneurysms were excluded). Of the 267 patients, 260 survived the operation and underwent clinical neurologic assessment. No paraplegia or other spinal-cord ischemic deficit occurred.     

Surgical treatment for cervical aortic arch with aneurysm formation

Cervical aortic arch is an unusual malformation. Cervical aortic arch with aneurysm formation is very rare. We report a case of cervical aortic arch associated with a saccular aneurysm in a 59-year-old Japanese man. The aneurysm protruded caudally and was located between the left common carotid and left subclavian arteries. Cardiopulmonary bypass and deep hypothermic circulatory arrest was applied as adjunct methods. A Dacron graft was sutured just distal to the left common carotid artery, with the patient in the Trendelenburg position. The proximal site was left open while oxygen-saturated venous blood was supplied in a retrograde manner to perfuse the lower body during occlusion of the descending aorta. Distal anastomosis to the descending aorta was performed during rewarming. The left subclavian artery was reconstructed by using a branch of the graft. This procedure is simple and useful for distal arch operations, especially in patients with Haughton D type aneurysms.     

Surgical treatment for cervical aortic arch with aneurysm formation

Cervical aortic arch is an unusual malformation. Cervical aortic arch with aneurysm formation is very rare. We report a case of cervical aortic arch associated with a saccular aneurysm in a 59-year-old Japanese man. The aneurysm protruded caudally and was located between the left common carotid and left subclavian arteries. Cardiopulmonary bypass and deep hypothermic circulatory arrest was applied as adjunct methods. A Dacron graft was sutured just distal to the left common carotid artery, with the patient in the Trendelenburg position. The proximal site was left open while oxygen-saturated venous blood was supplied in a retrograde manner to perfuse the lower body during occlusion of the descending aorta. Distal anastomosis to the descending aorta was performed during rewarming. The left subclavian artery was reconstructed by using a branch of the graft. This procedure is simple and useful for distal arch operations, especially in patients with Haughton D type aneurysms.     

Surgery of the descending thoracic aorta: spinal cord protection with the Gott shunt

From July, 1974, to July, 1987, surgical treatment of descending thoracic aortic aneurysms was performed in 173 patients at l'H?pital du Sacré-Coeur de Montréal. The cause of the aneurysms was arteriosclerosis or medial degeneration in 83 patients, trauma in 50, dissection in 34, and a congenital malformation in 6. A single method of external shunting provided distal perfusion in all patients in the series. A 9-mm Gott aneurysm shunt was placed preferentially between the ascending aorta (67%) and the descending aorta (60%). Alternative sites of proximal cannulation (aortic arch, 9%; proximal descending aorta, 22%; left ventricle, 2%) and distal cannulation (abdominal aorta, 3%; left femoral artery, 37%) were chosen based on the location and the extent of the aortic aneurysm. No systemic heparinization was used. In the last 40 patients, a flowmeter adapted for use with the shunt allowed the recording of shunt flow (mean, 2,475 ml/min; range, 1,100 to 4,000 ml/min). Hospital mortality, including patients with ruptured aneurysms, was 15% (26/173). The mean aortic cross-clamp time was 37 minutes (range, 8 to 105 minutes). Of the 173 patients, 168 survived long enough to allow accurate clinical evaluation of the function of the spinal cord: no paraplegia or other spinal cord ischemic injury occurred. To date, our clinical experience has demonstrated the effectiveness of the 9-mm Gott shunt in preserving the functional integrity of the spinal cord during cross-clamping of the thoracic aorta.     

Surgery for thoracic aortic aneurysms involving the distal arch through a left thoracotomy

Surgical outcome for thoracic aortic aneurysms involving the distal arch via a left thoracotomy using retrograde cerebral perfusion combined with profound hypothermic circulatory arrest was reviewed. Twelve patients with a atherosclerotic aortic aneurysm between 1994 and 1997 were involved. A proximal aortic anastomosis was made by means of an open aortic technique. For the first four patients, oxygenated arterial blood from cardiopulmonary bypass was perfused retrogradely through a venous cannula positioned into the right atrium. In the last eight cases, venous blood provided by a low-flow perfusion of the lower half body via the femoral artery, which was still oxygen-saturated, was circulated passively in the brain in a retrograde fashion with the descending aorta clamped. Prosthetic replacement was done between the distal arch and the proximal descending aorta in 6 patients and from the distal arch to the entire descending thoracic aorta in 6 patients. The median duration of hypothermic circulatory arrest and continuous retrograde cerebral perfusion was 36 minutes and 33 minutes respectively. The overall outcome was satisfactory without early mortality--all patients survived, although an octogenarian died of respiratory failure 1 year postoperatively. Another octogenarian with a ruptured aneurysm developed delay of meaningful consciousness, and other two patients with a severely atherosclerotic aneurysm suffered permanent neurological dysfunction (stroke) presumably due to an embolic episode. The safe and simple combination of profound hypothermic circulatory arrest, retrograde cerebral perfusion, and open aortic anastomosis protects the brain adequately and produces satisfactory results in surgery for aortic aneurysms involving the distal arch through a left thoracotomy.     

Fate of aneurysms of the distal arch and proximal descending thoracic aorta after transaortic endovascular Stent-Grafting

BACKGROUND: The purpose of this study was to evaluate the midterm results of transaortic stent-grafting for distal aortic arch aneurysms or proximal descending aortic aneurysms and the feasibility of this method for thoracic aortic aneurysm repair. METHODS: Twenty-three patients with true distal aortic arch aneurysms or proximal descending thoracic aortic aneurysms were repaired with the stent-graft introduced through the incision on the proximal arch aorta. Follow-up computed tomography was performed every 6 months in 21 surviving patients. The maximum dimension of the excluded aneurysmal space and the maximum aneurysmal diameter were measured and evaluated to determine whether the aneurysmal space decreased or disappeared after this alternative procedure. RESULTS: There was 1 hospital death (4.3%) due to cerebral embolism. Another patient died of pneumonia 1 year after surgery. Twenty-one patients (91%) survived during the follow-up period, but 1 patient (4.3%) suffered from paraplegia. The follow-up period ranged from 12 to 62 months (average, 34.3 +/- 15.2). There were no instances of aneurysmal rupture during the follow-up period. Postoperative serial computed tomography scans showed disappearance or significant shrinkage of the excluded aneurysmal space in 20 of 21 patients (95%), except for the one patient with endoluminal leakage. CONCLUSIONS: Transaortic endovascular stent-grafting was an effective alternative approach to treating distal aortic arch aneurysms or proximal descending aortic aneurysms. The excluded aneurysm disappeared or shrunk after successful placement of the stent-graft.     

Surgical repair of dissection of the upper descending thoracic aorta and discrete ascending aortic aneurysm.

This report describes successful staged surgical repair in 2 patients with dissection of the upper descending thoracic aorta (DeBakey type III) with coexisting discrete Marfan's aneurysms of the ascending aorta. Initial repair of the descending aortic dissection was done through a left thoracotomy using a transverse aorta--femoral artery shunt in 1 patient and a left ventricular apex--femoral artery shunt without systemic heparinization in the other. Emphasis is placed on the need for pharmacological reduction of blood pressure during aortic cross-clamping as well as the use of a shunt to prevent dissection of the ascending aortic aneurysm. In both patients, subsequent repair of the ascending aortic aneurysm was accomplished using composite graft replacement of the aortic valve and ascending aorta. This operation is advised for such patients even in the absence of notable aortic valve incompetence.     

Progress in the treatment of aneurysms of the distal aortic arch: approach through median sternotomy

AIM: A review of past and current operative procedures for the treatment of aneurysms of the distal aortic arch is presented in conjunction with a series of 43 patients. In this study, distal aortic arch aneurysm refers to an aneurysm involving at least the origin of the left subclavian artery, but not extending beyond the left common carotid artery. We excluded dissection aneurysm and extended aneurysm to the descending thoracic aorta from this study. METHODS: Between January, 1985, and March, 2000, 43 consecutive patients (37 males, 6 females; mean age 67.5 years) underwent repair of aneurysms of the distal aortic arch. The approach to the aneurysm was through a left thoracotomy in 4 patients and a median sternotomy in 39 patients, including an additional left thoracotomy continued to a median sternotomy in 2 patients. The supportive methods during surgery were left heart bypass using a centrifugal pump in 4 patients (LHB group), cardiopulmonary bypass with selective cerebral perfusion in 11 patients (SCP group), and cardiopulmonary bypass with continuous retrograde cerebral perfusion in 28 patients (RCP group). In the RCP group, the "aortic no-touch technique" was applied in 21 patients. The operative methods were patch closure in 4 patients, graft replacement of the distal arch using the inclusion technique in 14 patients, and total arch replacement using the exclusion technique in 25 patients. RESULTS: There were 5 hospital deaths: 1 patient in the LHB group, intractable bleeding; 1 patient in the SCP group, rupture of the distal anastomosis; 3 patients in the RCP group, stroke, rupture of the dissection arising from the distal anastomosis, and perioperative myocardial infarction. Stroke occurred in 1 patient (25%) with LHB, 3 patients (27.2%) with SCP, and 1 patient (3.6%) with RCP. Among the postoperative survivors, a new onset of left recurrent nerve palsy occurred in 2 patients (66.7%) with LHB, 1 patient (10%) with SCP, and in 1 patient (4%) with RCP. No neurological injury or left recurrent nerve palsy occurred in the patients who underwent the "aortic no-touch technique". CONCLUSION: Total arch replacement with the graft exclusion technique under profound hypothermic circulatory arrest using RCP through the median sternotomy is a promising surgical treatment for atherosclerotic distal aortic arch aneurysm. The "aortic no-touch technique" further improved the surgical results of the distal aortic arch aneurysm.     

The frozen elephant trunk technique for the treatment of extensive thoracic aortic aneurysms: operative results and follow-up.

OBJECTIVE: The 'frozen' elephant trunk technique allows for single-stage repair of combined aortic arch and descending aortic aneurysms using a 'hybridprosthesis' with a stented and a non-stented end. This report summarizes the operative- and follow-up data (mean follow-up 14 months) with this new treatment. METHODS: Between 09/01 and 4/04, 22 patients (62+/-9 years; 9 female) with different aortic pathologies (15 aortic dissections, 7 aneurysms) were operated on after approval from the local institutional review board. The stented end of the hybridprosthesis was deployed in the descending aorta through the opened aortic arch during hypothermic circulatory arrest and selective antegrade cerebral perfusion. RESULTS: All patients survived the procedure but one patient died of acute hemorrhage due to rupture of the false lumen in the descending aorta on the second postoperative day. Two patients required reexploration of the chest for bleeding complications. In 2 of 4 patients who developed neurological dysfunction, symptoms resolved completely. In one of them, the descending aorta was perforated intraoperatively due to misplacement of the stented end of the hybridprosthesis. In all follow-up CT-scans thrombus formation in the descending aortic aneurysm excluded by the stented end of the hybridprosthesis has been observed. CONCLUSIONS: This procedure is performed through median sternotomy and combines the concepts of the elephant trunk operation and endovascular stenting of descending aortic aneurysms. Favourable intraoperative and postoperative results during follow-up with regard to thrombus formation around the stented descending aortic segment encourage us to evaluate all patients with thoracic aneurysms extending to proximal and distal of the left subclavian artery for this treatment.     

Spinal cord perfusion and protection during descending thoracic and thoracoabdominal aortic surgery: the collateral network concept

In the last two decades, as an increasing number of patients with descending thoracic and thoracoabdominal aneurysms are being diagnosed and treated, a more sophisticated understanding of spinal cord perfusion has become important in the attempt to minimize the frequency of spinal cord injury. The synthesis of information from laboratory studies and clinical experience has led to the collateral network concept, a framework for understanding spinal cord perfusion and thereby improving spinal cord protection during treatment of aneurysmal disease of the aorta distal to the left subclavian artery. Application of principles based on the collateral network concept has resulted in falling rates of spinal cord injury, which now approach 1% in descending thoracic aneurysm resection and less than 10% in extensive thoracoabdominal resections. These accomplishments suggest that, with further investigation, routine sacrifice of segmental aortic branches can be carried out in a way that will allow surgical and endovascular therapy of extensive distal aortic aneurysms without neurologic injury.     

Endovascular stent-grafting through the aortic arch: an alternative approach for distal arch aortic aneurysm

BACKGROUND: Endovascular stent-grafting is an innovative procedure; we have developed a novel approach to treat distal arch aortic aneurysm through a small incision in the aortic arch. METHODS: Eight patients with thoracic aortic aneurysms were treated with an endovascular stent-graft that was introduced into the thoracic aorta through a small incision in the aortic arch. Of these patients, 7 had distal arch aortic aneurysms, and 1 had chronic aortic dissection of Stanford type B. Four of these patients had received concomitant coronary artery bypass grafting, and 1 patient had undergone tricuspid valvular annuloplasty. The stent-graft was introduced into the distal arch aorta and descending aorta through a small incision in the aortic arch, under selective cerebral perfusion and hypothermic circulatory arrest. RESULTS: The selective cerebral perfusion time ranged from 52 to 86 minutes (mean, 68 minutes) and the operating time from 289 to 422 minutes (mean, 318 minutes). There was no endoluminal leakage into the aneurysm. Seven patients survived and were discharged, but 1 patient suffered a cerebral infarction and died during the follow-up period. CONCLUSIONS: Placing an endovascular stent-graft through the aortic arch is an acceptable alternative treatment for distal arch aortic aneurysms.     

Preventive effect of Fluosol-DA for paraplegia encountered after surgical treatment of the thoracic aorta. Preliminary results in a dog model

The effectiveness of Fluosol-DA (Green Cross Corporation, Osaka, Japan) on circulatory dynamics and neurologic outcome in dogs with ischemic spinal cord injury produced by aortic crossclamping was tested. The control group (receiving saline solution) had an elevated mean aortic proximal pressure (112.9 +/- 30.2 mm Hg versus 175.3 +/- 20.5 mm Hg, p greater than 0.05) and a drastic drop in mean distal aortic pressure (112.9 +/- 30.2 mm Hg versus 29.8 +/- 11.2 mm Hg, p less than 0.05). Although the same trend occurred in dogs treated prophylactically with Fluosol-DA, these changes were not statistically significant. However, there was a significant difference in mean distal aortic pressure during the ischemic phase between the two groups (58.9 +/- 16.0 mm Hg versus 29.8 +/- 11.2 mm Hg, p less than 0.05). Postoperatively all animals had mean arterial pressures within the normal range. All dogs in the control group were paraplegic (partial or complete); the treatment group had one dog with partial paraplegia. The difference between the mean neurologic scores of the two groups was of high statistical significance (3.7 +/- 0.5 versus 1.6 +/- 1.0, p less than 0.05). Our preliminary results show that prophylactic use of Fluosol-DA has favorable effects on hemodynamics and neurologic outcome in dogs with spinal cord ischemia produced by aortic crossclamping. The high propensity of the drug to carry oxygen and carbon dioxide and to provide nutritional support to the ischemic area with resultant improvement in local microcirculation and blood rheology are some speculative mechanisms advocated for these changes.     

Partial cardiopulmonary bypass, hypothermic circulatory arrest, and posterolateral exposure for thoracic aortic aneurysm operation

Partial cardiopulmonary bypass, profound hypothermia, and circulatory arrest were used in the treatment of 25 patients with thoracic aortic aneurysms exposed through left posterolateral incisions. Indications included aortic clamp laceration (4 patients), pulmonary artery tear (1), treatment of ruptured aneurysm without clamping (5), right-sided arch (2), exposure of proximal aorta in a patient with a large aneurysm (1), inability to expose the proximal aorta for clamping (3), to permit removal of both arch and distal aorta (4), and to avoid distal arch clamping because of atheromatous disease (5). Aortic segments ranging from the upper descending thoracic aorta to most of the aorta were replaced, with early survival in 21 patients. Cerebral protection was satisfactory.     

A report of 4 cases of aortic operation using open proximal anastomosis under the hypothermic circulatory arrest and left lateral exposure]

Four cases of aortic operations using open proximal anastomosis (OPA) under the hypothermic circulatory arrest (HCA) and left lateral approach (LLA) are reported. Three of 4 cases had extensive aortic disease from distal arch to descending thoracic aorta (Stanford type B chronic aortic dissection, double false aneurysms, and double true aneurysms). Another one case had ruptured aneurysm of thoracic aorta. LLA should have been selected in all cases, however, aortic proximal cross-clamp was impossible in them, because of giant pseudolumen, diseased lesion of aortic arch, hemothorax followed rupture of aneurysm. Therefore OPA under the HCA was performed. There were no complication associated with HCA, bleeding, neurological deficiency and respiratory dysfunction. We conclude that, although the HCA may have some problems, if there is the proper indication, OPA under the HCA is useful method at aortic operation for difficult aortic disease.     

Complications associated with clamping the aorta between the left common carotid artery and left subclavian artery

BACKGROUND: Hypothermic circulatory arrest using a left thoracotomy has recently been recommended for repair of distal arch lesions to prevent the atheroembolism that often results from clamp injury. The recommendation holds even for cases in which aortic cross-clamping between the left common carotid artery and left subclavian artery is possible. METHODS: Over the last 16 years, 69 patients underwent repair of the distal arch or descending thoracic aorta using distal perfusion with the proximal aortic clamp placed between the left common carotid and left subclavian artery. The average age of the patients was 61+/-12 years; 18 of them (26%) were older than 70 years. Forty-four patients (64%) had atherosclerotic true aneurysms. RESULTS: The surgical procedures used included patch closure of saccular aneurysms in 20 patients (29%) and graft replacement in 47 (71%). The left subclavian artery was reattached in 7 patients (10%). Although there were 3 hospital deaths (4%), no cerebral complications occurred aside from temporary neurologic dysfunction in 4 patients (6%). CONCLUSIONS: An acceptably low incidence of cerebral complications is associated with cross-clamping the aorta between the left common carotid artery and left subclavian artery.     

Distal Aortic Arch Replacement for Aneurysmal Disease: The Value of Preparatory Carotid Subclavian Reconstruction

Between November 2000 and January 2002, two patients with aneurysms that involved the distal part of the aortic arch including the left subclavian artery were treated at our institution. Patient 1 had an aneurysm of 5.8 cm extending to the proximal descending aorta. Patient 2 had a 6.8 cm type II thoracoabdominal aneurysm extending proximal to the aortic bifurcation. Both patients had left subclavian-to-carotid transposition in preparation for distal aortic arch replacement. Complete replacement of the descending thoracic and abdominal aorta was carried out in patient 2. Both cases were done with distal aortic perfusion, spinal catheter drainage, and dual lumen endotracheal anesthesia. There was no mortality. There were no cerebrovascular complications in spite of the fact that patient 1 required aortic cross-clamping between the innominate and left carotid artery. There was no paraplegia, renal failure, or mesenteric or lower extremity complications. Patient 1 had postoperative vocal cord palsy, eventually requiring medialization procedure. He recovered normal voice. Both patients remain alive and well at the time of last follow-up (7 to 20 months). Carotid subclavian reconstruction in preparation for distal aortic arch replacement facilitates the performance of the proximal anastomosis and attempts to maintain flow through the left vertebral system during aortic cross-clamping. This may reduce the risk of stroke during distal aortic arch replacement.