M. Arisan Ergin

Cerebral metabolic suppression during hypothermic circulatory arrest in humans.

BACKGROUND Hypothermic circulatory arrest (HCA) is used in surgery for aortic and congenital cardiac diseases. Although studies of the safety of HCA in animals have been carried out, the degree to which metabolism is suppressed in patients during hypothermia has been difficult to determine because of problems with serial measurements of cerebral blood flow in the clinical setting. METHODS To quantify the degree of metabolic suppression achieved by hypothermia, we studied 37 adults undergoing operations employing HCA. Cerebral blood flow was estimated using an ultrasonic flow probe on the left common carotid artery, and cerebral arteriovenous oxygen content differences were calculated from jugular venous bulb and arterial oxygen saturations. Cerebral metabolic rates while cooling were then ascertained. The temperature coefficient, Q10, which is the ratio of metabolic rates at temperatures 10 degrees C apart, was determined. RESULTS The human cerebral Q10 was found to be 2.3. The cerebral metabolic rate is still 17% of baseline at 15 degrees C. If one assumes that cerebral blood flow can safely be interrupted for 5 min at 37 degrees C, and that cerebral metabolic suppression accounts for the protective effects of hypothermia, the predicted safe duration of HCA at 15 degrees C is only 29 min. CONCLUSIONS The safe intervals calculated from measured cerebral oxygen consumption suggest that shorter intervals and lower temperatures than those currently used may be necessary to assure adequate cerebral protection during hypothermic circulatory arrest.

Prospective Study of the Natural History of Thoracic Aortic Aneurysms

BACKGROUND The decision whether or not to recommend resection of moderately large descending thoracic and thoracoabdominal aneurysms requires weighing the relatively high mortality and significant risk of paraplegia associated with operation against the likelihood that the aneurysm will rupture spontaneously, with an almost invariably fatal outcome. To better define the risk of aneurysm rupture, we undertook a prospective study of patients who had not had operation on their moderately large descending thoracic and thoracoabdominal aneurysms. METHODS Patients were enrolled at the time of their second computed tomographic scans: three-dimensional computer-generated reconstructions allowed determination of several dimensional parameters for each study, including diameters and cross-sectional areas at the site of maximal dilatation in the descending aorta and in the abdomen as well as total thoracoabdominal surface area. Comparisons of serial studies permitted calculation of yearly rates of change in these dimensions. RESULTS Of 114 patients, 8 died of causes unrelated to the aneurysm, 26 died of rupture, 20 met previously determined criteria for operation, and 60 survived without operation or rupture. Multivariate regression analysis identified maximal diameter in the descending and in the abdominal aorta as independent risk factors for rupture, as well as older age, the presence of even uncharacteristic pain, and a history of chronic obstructive pulmonary disease. A piecewise exponential model enabled construction of an equation allowing calculation of rate of rupture in patients in whom the values of the risk factors are known, and also of the probability of rupture in a given individual over a specified time interval. CONCLUSIONS Because using this equation--based on easily determined risk factors (age, pain, chronic obstructive pulmonary disease, maximal thoracic and maximal abdominal aortic diameter)--allows the risk of aneurysm rupture within a given interval to be estimated fairly accurately for each individual patient, it is our current practice to recommend operation when the calculated risk of rupture within 1 year exceeds the anticipated mortality of elective operation, rather than relying on general operative guidelines based almost exclusively on aneurysm size.

Surgical treatment of the dilated ascending aorta: when and how?

BACKGROUND The aorta is considered pathologically dilated if the diameters of the ascending aorta and the aortic root exceed the norms for a given age and body size. A 50% increase over the normal diameter is considered aneurysmal dilatation. Such dilatation of the ascending aorta frequently leads to significant aortic valvular insufficiency, even in the presence of an otherwise normal valve. The dilated or aneurysmal ascending aorta is at risk for spontaneous rupture or dissection. The magnitude of this risk is closely related to the size of the aorta and the underlying pathology of the aortic wall. The occurrence of rupture or dissection adversely alters natural history and survival even after successful emergency surgical treatment. METHODS In recommending elective surgery for the dilated ascending aorta, the patients age, the relative size of the aorta, the structure and function of the aortic valve, and the pathology of the aortic wall have to be considered. The indications for replacement of the ascending aorta in patients with Marfans syndrome, acute dissection, intramural hematoma, and endocarditis with annular destruction are supported by solid clinical information. Surgical guidelines for intervening in degenerative dilatation of the ascending aorta, however, especially when its discovery is incidental to other cardiac operations, remain mostly empiric because of lack of natural history studies. The association of a bicuspid aortic valve with ascending aortic dilatation requires special attention. RESULTS There are a number of current techniques for surgical restoration of the functional and anatomical integrity of the aortic root. The choice of procedure is influenced by careful consideration of multiple factors, such as the patients age and anticipated survival time; underlying aortic pathology; anatomical considerations related to the aortic valve leaflets, annulus, sinuses, and the sino-tubular ridge; the condition of the distal aorta; the likelihood of future distal operation; the risk of anticoagulation; and, of course, the surgeons experience with the technique. Currently, elective root replacement with an appropriately chosen technique should not carry an operative risk much higher than that of routine aortic valve replacement. Composite replacement of the aortic valve and the ascending aorta, as originally described by Bentall, DeBono and Edwards (classic Bentall), or modified by Kouchoukos (button Bentall), remains the most versatile and widely applied method. Since 1989, the button modification of the Bentall procedure has been used in 250 patients at Mount Sinai Medical Center, with a hospital mortality of 4% and excellent long-term survival. In this group, age was the only predictor of operative risk (age > 60 years, mortality 7.3% [9/124] compared with age < 60, mortality 0.8% [1/126], p = 0.02). CONCLUSIONS This modification of the Bentall procedure has set a standard for evaluating the more recently introduced methods of aortic root repair.

Risk factors for rupture of chronic type B dissections

OBJECTIVE This study was an attempt to determine risk factors for rupture and to improve management of patients with type B aortic dissection who survive the acute phase without operation. METHODS We studied 50 patients by means of serial computer-generated 3-dimensional computed tomographic scans. All patients who did not undergo operative treatment before the completion of at least 2 computed tomographic scans a minimum of 3 months apart after an acute type B dissection were included in the study. The median duration of follow-up was 40 months (range 0.9-112 months). Only 1 patient died of causes unrelated to the aneurysm during follow-up. Nine patients had fatal rupture (18%); 10 patients underwent elective aneurysm resection because of rapid expansion or development of symptoms, and 31 patients remained alive without operation or rupture. Possible risk factors for rupture in patients in the rupture, operative, and event-free groups were compared, as were dimensional data from first follow-up and last computed tomographic scans. RESULTS Older age, chronic obstructive pulmonary disease, and elevated mean blood pressures were unequivocally associated with rupture (rupture versus event-free survival, P <.05), and pain was marginally significantly associated. Analysis of dimensional factors contributing to rupture was complicated by the fact that patients who underwent elective operation had significantly larger aneurysms and faster expansion rates than did either of the other groups, leaving comparisons of aneurysmal diameter between groups with and without rupture showing only marginal statistical significance. The last median descending aortic diameter before rupture in the rupture group was 5.4 cm (range 3.2-6. 7 cm). CONCLUSIONS In an environment in which patients with large and rapidly expanding aneurysms are usually referred for surgical treatment, older patients with chronic type B dissections, especially if they have uncontrolled hypertension and a history of chronic obstructive pulmonary disease, are significantly more likely to have rupture than are younger, normotensive patients without lung disease. Neither the presence of a persistently patent false lumen nor a large abdominal aortic diameter appears to increase the risk of rupture. Overall, our nondimensional data strikingly resemble the natural history of patients with nondissecting aneurysms, suggesting that calculations derived from data on chronic descending thoracic and thoracoabdominal aneurysms would provide an overly conservative individual estimate of rupture risk for patients with chronic type B dissection, who tend toward earlier rupture of smaller aneurysms. A more aggressive surgical approach toward treatment of patients with chronic type B dissection seems warranted.

Significance of distal false lumen after type A dissection repair

Fifty-eight patients underwent repair of acute type A dissection between 1986 and 1992. Follow-up aortogram, computed tomographic scan with contrast, magnetic resonance imaging scan, or a combination of these tests was available in 38 patients with preoperatively patent distal false lumens. All distal anastomoses were constructed with the open technique during a period of circulatory arrest. There were 25 suture and 13 intraluminal graft anastomoses. Patency of the distal false lumen was found in 47.3%. Use of the intraluminal graft for the distal anastomosis decreased patency, although not significantly (4/13, 30% versus 14/25, 56%; p = 0.14). The direction of flow into the false lumen was antegrade in 11 of 24 (45.8%) of sutured anastomoses and 0 of 9 intraluminal graft anastomoses (p < 0.01). Actuarial survival at 5 years for patients with closed distal false lumen was 95% +/- 4.8% versus 76% +/- 15% for patients with patency of the distal false lumen (p = not significant). Event-free survival at 5 years for both groups was 84% +/- 8.3% (closed false lumen) and 63% +/- 13.5% (patency of distal false lumen; p = not significant). This experience indicates that in the treatment of acute type A dissections, operative strategy and anastomotic technique play a role in reducing the incidence of patency and related complications of the distal false lumen.

Effect of Aneurysm on the Tensile Strength and Biomechanical Behavior of the Ascending Thoracic Aorta

BACKGROUND Rupture of an ascending thoracic aortic aneurysm (ATAA), which is associated with significant mortality, occurs when the mechanical forces acting on the aneurysm exceed the strength of the degenerated aortic wall. The purpose of this study was to evaluate changes in biomechanical properties of the aortic wall related to ATAA formation. METHODS Ascending thoracic aortic aneurysm tissue was obtained from surgery; control (nonaneurysmal) aorta was obtained from autopsy. Tissue strips with longitudinal (LONG) or circumferential (CIRC) orientation were stretched to failure. Maximum tissue stiffness and tensile strength were determined from plots of stress (normalized force) versus strain (normalized deformation). Students t test was used for all comparisons. RESULTS Tensile strength of LONG (nATAA = 17, n(control) = 7) and CIRC (nATAA = 23, n(control) = 7) ATAA specimens were 29% and 34% less than that of control tissue, respectively (p < 0.05). Maximum tissue stiffness was 72% stiffer for LONG ATAA (p < 0.05) and 44% stiffer for CIRC ATAA (p = 0.06) than for control tissue, respectively. CONCLUSIONS The data suggest that ATAA formation is associated with stiffening and weakening of the aortic wall, which may potentiate aneurysm rupture.

Natural history of descending thoracic and thoracoabdominal aneurysms

BACKGROUND A review of 165 patients with chronic dissecting and degenerative aneurysms of the descending thoracic and thoracoabdominal aorta initially managed nonoperatively was carried out to ascertain factors associated with a high risk of rupture. METHODS Changes in the aneurysms were followed with three-dimensional reconstructions of computed tomograph scans. Risk factors were compared in patients with dissecting and nondissecting aneurysms who experienced rupture, in whom operation was recommended during the course of follow-up, and in those without rupture or operation. RESULTS Nondimensional variables associated with an enhanced risk of rupture include age, the presence of chronic obstructive pulmonary disease, and even uncharacteristic continued pain. Patients with rupture of dissections had significantly higher blood pressures than survivors, and significantly smaller maximal descending thoracic aortic diameters (median 5.4 cm) than patients with rupture of degenerative aneurysms (median 5.8 cm). The extent of the aneurysm, as reflected by the maximal abdominal aortic diameter, was a significant risk factor for rupture only in nondissecting aneurysms. Mortality from rupture was significantly higher in patients with chronic dissections than in patients with nondissecting aneurysms: 9/10 vs 26/34 (p = 0.004). CONCLUSIONS Almost 20% of patients followed nonoperatively succumbed to rupture, suggesting that a more aggressive surgical approach toward patients with chronic aneurysms of the descending thoracic and thoracoabdominal aorta is warranted. An individualized risk of rupture within 1 year can now be calculated, and patients whose operative risk is lower than their calculated risk should be offered elective surgery.

Hypothermic Circulatory Arrest and Other Methods of Cerebral Protection During Operations on the Thoracic Aorta

Current surgical techniques in operations on the thoracic aorta frequently require exclusion of the cerebral circulation for varying periods. During these periods, hypothermic circulatory arrest (HCA), selective cerebral perfusion (SCP), and retrograde cerebral perfusion (RCP) can be used for cerebral protection. Hypothermia is the principle component of these methods of protection. The main protective effect of hypothermia is based on reduction of cerebral energy expenditures and largely depends on adequate suppression of cerebral function. It is most effective at deep hypothermic levels (13°C to 15°C). Measures that preserve autoregulation of cerebral blood flow help increase the margin of safety with all methods of protection. There is solid experimental and clinical data indicating the safe limits and outcome following HCA. Current applications of SCP and RCP are fairly recent developments and do not have comparable supporting data. SCP can be used without deep hypothermia and allows prolonged periods of cerebral protection, but is complex in application. RCP is simpler, but always requires deep hypothermia. Present clinical data do not allow separation of its protective effect from that of HCA alone. Recent modifications in the application of HCA include monitoring of cerebral O2 extraction, and selective use of supplemental SCP to limit arrest times to less than 50 minutes, or RCP to prevent embolic strokes, as indicated. These changes appear to have reduced the overall mortality, the severity of embolic strokes, and stroke‐related mortality. (J Card Surg 1994;9:525–537)

Selective management of acute type B aortic dissection: Long-term follow-up

BACKGROUND Since 1985, we have selectively treated acute type B aortic dissections. Initial treatment lowered blood pressure and heart rate. Transesophageal echocardiography and computed tomographic scans were used to diagnose and follow up the patients. Patients were operated on for organ ischemia, pain, hypertension, or increasing subpleural fluid on computed tomographic scan. METHODS We retrospectively reviewed consecutive patients admitted over a 10-year period to the Mt. Sinai Hospital. RESULTS From August 1985 to May 1995, 68 patients were seen. Three died soon after admission during initial diagnostic evaluation. Seventeen patients underwent operation without mortality or paraplegia (group 1). Forty-seven of 48 patients treated nonoperatively were discharged; 1 patient died of rupture on day 7 (group 2). Actuarial survival for all 68 patients at 1 and 5 years was 92% +/- 4% and 82% +/- 8%. Group 1 survival was 93% +/- 4% and 68% +/- 5%, and group 2 survival was 90% +/- 6% and 87% +/-14%. There were no differences between groups. Late intervention was required in 2 group 1 patients (12%) and in 12 of 48 group 2 patients (25%), again without mortality or paraplegia. CONCLUSIONS This experience suggests that selective management of acute type B aortic dissection results in acceptable short-term and long-term survival. Avoiding early operation did not compromise late results.

Subtypes of acute aortic dissection

BACKGROUND This series consists of a 12-year experience with a policy of identifying and replacing the aortic segment containing the primary intimal tear for repair of acute aortic dissection. METHODS Patients with type A dissection underwent urgent surgery. Patients with type B dissection were referred for surgery based on selective criteria, including aortic dilatation greater than 5 cm. A classification system for acute dissection is described that specifies the site of intimal tear while retaining the clinical relevance of the Stanford system. RESULTS Of 168 acute dissections, 139 were type A and 29 were type B. The site of intimal tear was as follows: ascending aorta, 83 cases; arch, 32 cases; descending aorta, 29 cases; multiple tears, 11 cases (10 included arch tears); no tear (intramural hematoma), 6 cases; not noted, 7 cases. Only 60% of acute type A dissections arose from solitary intimal tears in the ascending aorta, whereas 30% had arch tears. Hospital mortality for type A dissection was 13.7% (18.8% for arch tears, NS) and 0% for type B. False lumen patency was 57.1% for type A dissection and 18.8% for type B dissection (p = 0.002), yet survival was similar for these groups. Ten-year survival for type A dissection with arch tear (0.51 +/- 0.12) was lower than 10-year survival for type A dissection with ascending tear (0.74 +/- 0.05; p = 0.77), and significantly lower than for type A dissection with descending tear (0.88 +/- 0.12; p = 0.029). CONCLUSIONS Systematic resection of the primary tear yielded similar hospital mortality, 5-year survival, and aorta-related event-free survival rates for subtypes of acute type A dissection. Excellent results were obtained with a selective approach to type B dissection.