S dominal aortic aneurysms remains a challenging procedure. Improved Distal Circulatory Support for Repair of Descending Thoracic Aortic Aneurysms

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1 Improved Distal Circulatory Support for Repair of Descending Thoracic Aortic Aneurysms Ludwig K. von Segesser, MD, Igor Killer, Rolf Jenni, MD, Ulrich Lutz, PhD, and Marko I. Turina, MD Clinic for Cardiovascular Surgery and Departments of Medicine and Surgery, University Hospital, Zurich, Switzerland Bleeding is a well-known problem when cardiopulmonary bypass with full systemic heparinization is used for distal support during aortic cross-clamping. The recent advent of heparin-coated cardiopulmonary bypass equipment prompted our review of 91 consecutive patients who underwent repair of descending thoracic and thoracoabdominal aortic aneurysms. Two different surgical techniques were used: 42 of 91 patients had simple aortic cross-clamping and rapid reanastomosis, whereas 49 of 91 had distal support using all heparin-coated perfusion equipment with low systemic heparinization (1 IU/kg body weight; activated coagulation time >18 seconds). Baseline parameters, location (thoracoabdominal: 28/91; 31%), and type of aneurysm (ruptured 14/91; 15%) were similar in both groups. Cross-clamp time was 37 f 22 minutes for support versus 29 f 13 minutes for simple clamping (p <.5). There were fewer revisions due to bleeding for support (1/49 patients; 2%) versus simple (4/42; 1%; p <.5) and fewer patients with impaired renal function requiring temporary hemofiltration for support (4/49 patients; 8%) versus simple 3/42; 14%). Hospital mortality was lower for support (5/49; 1%) versus simple (8/42; 19%). Transfusion requirements during were 3,732 f 3,458 ml for simple versus 3,392 f 2,58 ml for support (not significant). Chest tube drainage totaled 982 f 1,12 ml for simple versus 72 f 618 ml for support (not significant). The total volume requirements were 8, ,753 ml for simple versus 7,495 f 3,342 ml for support (not significant) during and 4,416 f 2,422 ml for simple versus 3,38 f 1,432 ml for support (p <.25) during the 24 hours after. After declamping of the aorta the mean arterial ph dropped to 7.29 f.1 for simple clamping as compared with 7.37 f.8 for clamping supported with partial cardiopulmonary bypass (p <.5), partial arterial CO, pressures increased to 6.44 f 1.21 kpa for simple as compared with 5.22 f 1.9 kpa (p <.5), and mean negative base excess values increased to 4.9 f 5.8 for simple as compared with 2.6 f 3.9 (p <.5). No device failure occurred. We conclude that proximal unloading and support of distal circulation during resection of descending thoracic and thoracoabdominal aortic aneurysms prevents declamping shock and can now be realized without risk of bleeding complications. This approach provides superior hemodynamics and oxygenation. ( 1993;56:1373-8) urgical repair of descending thoracic and thoracoab- S dominal aortic aneurysms remains a challenging procedure. For earlier series, perioperative mortality rates of up to 92% have been reported [l]. This is in sharp contrast to the encouraging long-term results after successful repair of these lesions. In our hands, long-term follow-up of 74 patients surviving more than 3 days after repair of descending thoracic and thoracoabdominal aortic aneurysms showed a survival rate greater than 7% at 9 years [2]. Various preventive technical measures have been suggested in the past to improve the surgical results including support of the distal circulation during aortic cross-clamping. As a matter of fact, temporary diversion of the aortic blood flow during thoracic aortic occlusion was already recommended by Alexis Carrel in 191 [3]. However, control of the coagulation system was an unsolved problem at that time, as heparin was not yet Accepted for publication Feb 9, 1993 Address reprint requests to Dr von Segesser, Clinic for Cardiovascular Surgery, University Hospital, Ramistrasse 1, CH-891 Zurich, Switzerland. available [4]. After the introduction of cardiopulmonary bypass in clinical practice by John Gibbon (1952) the pump oxygenator was also used for distal perfusion during on the descending thoracic aorta [5]. Severe bleeding complications were the major drawback of this approach as long as full systemic heparinization was necessary during perfusion of these patients with very large surgical exposures. Bonding of heparin to synthetic surfaces to reduce the thrombogenicity was initiated in 1963 by V. Gott and associates [6], who also reported clinical application of heparinized shunts for thoracic aortic s without full systemic heparinization [7]. Despite the fact that improved heparin bonding techniques were developed thereafter, only very simple devices such as tubings were readily available for clinical application over many years. The recent advent of more complex heparin-bonded perfusion equipment [8] including centrifugal pumps [9], permitting superior shunt flows, as well as oxygenator/ heat exchanger structures, allowing for temperature control and additional gas exchange during single-lung ven by The Society of Thoracic Surgeons /93/$6.

2 1374 VON SEGESSER ET AL 1993;56:137MO Table 1. Baseline Characteristics Variable Simple Support Partial CPB n 42/91 (46%) 49/91 (54%) 38/91 (42%) Age (Y) Risk factors Diabetes mellitus 1(2%) 2(4%) 1(3%) Hyperlipidemia 3(7%) 3(6%) 2(5%) Hypertension 3 (71%) 35 (71%) 28 (74%) Nicotine 14 (33%) 17 (35%) 12 (32%) Coronary artery disease 7 (14%) 9 (18%) 8 (21%) Chronic obstructive 3 (7%) 8 (16%) 6 (16%) pulmonary disease Renal insufficiency 3 (7%) 7(14%) 7 (18%) Peripheral arterial 3 (7%) 4 (8%) 4 (11%) disease Mean number of risk * &.21 factors Type of lesion Aortic dissection 12 (29%) 2 (41%) 15 (39%) Thoracic aortic 12 (31%) 1 (2%) 7 (18%) aneurysm Thoracoabdominal 14 (33%) 14 (29%) 13 (34%) aneurysm Other 3 (7%) 5 (1%) 3 (8%) Operative variables Cross-clamp time (min) Transdiaphragmatic 16 (38%) 23 (47%) 21 (55%) repair Reimplantation of 12 (29%) 18 (37%) 15 (39%) intercostal, visceral, or renal vessels a p <.5 for simple versus support and for simple versus partial CPB. CPB = cardiopulmonary bypass. tilation [lo], prompted our review of patients undergoing repair of descending thoracic and thoracoabdominal aneurysms with and without these adjuncts. Patients and Methods From 1985 through 1991, 91 consecutive patients underwent repair of descending thoracic and thoracoabdominal aortic aneurysms at our institution. Patients with acute traumatic transsections of the aorta were not included in the study and were analyzed separately [ll]. Mean age of the 74 men and 17 women analyzed was 59 k 13 years (range, 2 to 78 years). Aortic dissections were diagnosed in 33/91 (36.2%), arteriosclerotic thoracic aortic aneurysms in 24/91 (26.4%), thoracoabdominal aortic aneurysms in 28/91 (3.8%), chronic posttraumatic aortic aneurysms in 5/91 (5.5%), and anastornotic aortic aneurysms in 1 patient. Emergency procedures were necessary in 15/91 patients (16.5%) and mainly due to ruptured lesions (14/91 patients; 15.4%). Complicated ruptures with aortoesophageal and aortobronchial fistulas were diagnosed in 1 patient each. Distal Protection Two basically different surgical techniques were used: 42/91 patients (46%), predominantly in the earlier part of the study, had normothermic simple aortic cross-clamping, rapid reanastomosis, and no distal circulatory support (simple group), whereas 49/91 patients (54%) had normothermic distal circulatory support using all heparinbonded perfusion equipment with low systemic heparinization (support group). The group with distal support can be divided into two main subgroups based on the emerging availability of the various heparin-coated components for clinical perfusion: 11/49 patients (22%) were supported by left heart bypass (left atrium-distal aorta) with heparin-bonded tubing sets (Carmeda Bioactive Surface; Carmeda, Stockholm, Sweden) using either an inletpressure servo-controlled roller pump (4/49; 8%) (modified roller pump [12]; Stockert Ltd, Munich, Germany) or a Carmeda Bioactive Surface heparin-coated centrifugal pump (7/49; 14%) (BioPump [9]; Biomedicus Inc, Minneapolis, MN); 38/49 patients (78%) were supported with partial cardiopulmonary bypass (partial CPB subgroup: left iliac vein to left iliac artery or pulmonary artery to left iliac artery) using a Duraflo I1 (Baxter Bentley, Irvine, CA) heparin-bonded tubing set and pump loop with a Duraflo I1 heparin-coated hollow fiber membrane oxygenator (BOS CM 5 Gold; Baxter Bentley [13]). Baseline characteristics for the three main groups analyzed (simple, support, and partial CPB) are given in Table 1. Systemic Heparinization, Perfusion, and Surgical Techniques All patients received low-dose systemic heparinization either before aortic cross-clamping (simple group) or cannulation (support group). The basic heparin doses, criteria for additional heparin doses, and protamine doses are given for each group in Table 2. If selected (available), distal circulatory support was started with a pump flow corresponding to 5% of cardiac output as determined by thermodilution and adapted after aortic cross-clamping to Table 2. Selected Heparin and Protarnine Doses Dose Simple Support Partial CPB Heparin priming dose Heparin loading dose Criterion for adding heparin Additional heparin dose Protamine equivalent Additional protamine titrated according to NA 5, IU Appearance of clots in surgical field 5, IU 5, IU ACT 1, IU/L 5, IU 1 IU/kg body weight ACT < 18 s ACT < 18 s 2, IU 2, IU 5, IU 1 IU/kg body weight ACT ACT ACT = activated coagulation time (measured with Hemochron; International Technidyne Inc, Edison, NJ); CPB = cardiopulmonary bypass; NA = not applicable.

3 ~ 1993: VON SEGESSER ET AL 1375 maintain adequate proximal and similar distal perfusion pressures. Staged segmental repair [14] of the descending thoracic and thoracoabdominal aorta was performed beginning proximally and replacing the aortic clamps distal to the completed anastomoses. Use of sealed prostheses and reimplantation of large intercostal arteries as well as reimplantation of visceral and renal arteries were routine. Shed blood was recovered in all groups by a red cell spinning device and pumped back to the patient (Autotrans; Dideco Spa, Mirandola, Italy). To avoid stagnant flow in the cardiopulmonary bypass system, recirculation through a shunt in the operating field was started immediately after weaning. Graft inclusion was used systematically. If necessary a piece of glutaraldehyde-preserved equine xenopericardium (Xenomedica, Division of Baxter- Edwards, Horw, Switzerland) was implanted for this purpose [15]. Transfusion Requirements Crystalloid fluids, noncrystalloid fluids, and homologous blood and blood products including packed red cells, fresh-frozen plasma, platelets, fibrinogen, and clotting factors required before, during, and after aneurysm repair were analyzed separately for each patient and compiled for the three main groups studied: simple, support, and partial CPB. Postoperative Renal Function Impairment The diagnosis of postoperatively impaired renal function was established for patients with normal preoperative renal function requiring temporary hemofiltration postoperatively due to inadequate urine output (oligouria or anuria: less than 4 ml/h of urine output over several hours) or increase in blood urea nitrogen level to more than 4 mmoyl (normal range, 2.5 to 7.5 mmoyl) despite maximal stimulation with volume load, mannitol, and furosemide. Data Analyses Continuous variables are presented as the mean? standard deviation. Comparison of continuous variables was made using Student's t test for paired or unpaired variables and analysis of variance (Solo; BMDP Statistical Software Inc, Los Angeles, CA) where applicable. Univariate analysis of descriptive data was performed using Fisher's exact test. Statistical significance was confirmed by a probability value less than.5. Results Hospital mortality accounted for 8/42 patients (19%) in the group undergoing with simple aortic crossclamping, 5/49 (1%) in the group operated on with circulatory support, and 2/38 (5.3%) in the subgroup supported with partial CPB. These improved results were achieved despite the fact that cross-clamp time was significantly longer in the group with circulatory support (37 f 22 minutes for support versus 29 f 13 minutes for simple; p <.5), there were more patients requiring transdiaphragmatic repair (23/49 patients [47%] for sup- Table 3. Transfusion Reguirements Simple Support Partial CPB Product (ml) (ml) (ml) Packed red cells During 2,593? 2,693 1,925 * 1,27 First 24 h after 53 f 1, f 633 Fresh-frozen plasma During 1,51 f 951 1,315 f 886 First 24 h after 474 f * 751 Platelets During 74 f First 24 h after f 11 Fibrinogen During 7 f 46 First 24 h after Other clotting factors During 7 f 46 First 24 h after 52 f 181 2,61 f 1, f 676 1,329 f f * 95 Total blood products During 3,732 f 3,458 3,392 2,58 3,566 f 2,41 During 24 h 1,14 f 1, f 1,278 1,35 f 1,373 after Total volume transfused including blood products, colloidal fluids, and noncolloidal fluids During 8,156 f 4,753 7,495 f 3,342 7,67 f 3,332 During 24 h 4,416 f 2,422 3,38 f 1,432" 3,426 f 1,465" after a p <.25 for simple versus support and for simple versus partial CPB. CPB = cardiopulmonary bypass. port versus 16/42 for simple [38%]), and there were more reimplantations of intercostal, visceral, or renal vessels (18/49 patients [37%] for support versus 12/42 [29%] for simple). The most complex procedures were performed in the subgroup supported with partial CPB as shown in Table 1. Although more complex procedures were performed in the group of patients operated on with distal support, there were fewer postoperative complications in these patients. In particular, there were fewer revisions for bleeding (1/49 patients [2%] for support versus 4/42 [lo%] for simple; p <.5) and fewer patients with impaired renal function requiring temporary hemofiltration (4/49 patients [8%] for support versus 6/42 [14%] for simple). Paraparesis or paraplegia was observed in 4/44 patients (9%) with support versus 3/34 (9%) with simple clamping (not significant []). One patient of each group

4 1376 VON SEGESSER ET AL 1993;56: Fig 1. Transfusion requirements of main homologous blood products (mean * standard deviation) during with simple aortic crossclamping (n = 42), distal circulatory support (n = 49), and partial cardiopulmonary bypass (CPB) (n = 38). There are no significant differences. ml 3 2 SIMPLE SUPPORT PARTIAL CPB 1 Packed red cell Fresh frozen plasma Platelet experienced significant recovery from the neurologic deficit. Transfusion requirements during as well as during the 24 hours after are tabulated in Table 3 for the simple, support, and partial CPB groups. There are no significant differences for packed red cells, fresh frozen plasma, platelet concentrates, fibrinogen and other clotting factors. During the procedures transfusion requirements totaled 3,732 * 3,458 ml in patients with simple aortic cross-clamping versus 3,392 k 2,58 ml in patients with distal support () versus 3,566 * 2,41 ml in patients with partial CPB (). The main blood products required during are broken down in Figure 1 as follows: packed red cells, fresh-frozen plasma, and platelet concentrates. There are no significant differences for these blood products. Chest tube drainage and transfusion requirements during the first 24 hours in the intensive care unit are depicted in Figure 2. Chest tube drainage totaled 982 * 1,12 ml in patients operated on with simple aortic cross-clamping versus 72 * 618 ml in patients with distal support () versus 732 k 658 ml in patients with distal support using partial CPB (). The total blood products transfused to compensate for these losses was 1,14 f 1,979 ml for the simple group versus 956 -C 1,278 ml for the support group () versus 1,35 f 1,373 ml for the partial CPB group (). The total volume requirements including blood products, colloidal fluids, and noncolloidal fluids were 8,156 * 4,753 ml for simple versus 7, ,342 for support () versus 7,67 -C 3,332 for partial CPB during () and 4,416 * 2,422 ml for simple versus 3,38 f 1,432 ml for support (simple versus support; p <.25) versus 3,426 * 1,465 ml for partial CPB (simple versus partial CPB; p <.25) during the 24 hours after (Fig 3). The results of the arterial blood gas analyses reflecting both adequacy of gas transfer and hemodynamics before anesthesia, before aortic cross-clamping during singlelung ventilation, after aortic cross-clamping (during repair of descending thoracic aorta), after release of the aortic cross-clamp, and at the end of the procedure are given in Table 4 for the group operated on with simple aortic cross-clamping and the group supported with partial CPB. The first set of data in Table 4 makes clear that there are no significant differences between the two groups before onset of anesthesia. Evolution of the arterial ph values during the procedures is shown in Figure 4. Mean arterial ph is significantly lower after aortic crossclamping for simple clamping (7.38 *.7) as compared with partial CPB (7.44 *.9; p <.5). After declamping of the aorta the mean arterial ph drops to 7.29 *.1 for the simple group as compared with 7.37 *.8 for the partial CPB group (p <.5). Normal ph values are finally found in both groups at the end of the procedures. Evolution of partial arterial CO, pressures is depicted in Figure 5. There are significantly higher arterial CO, partial pressures during aortic cross-clamping for the simple group ( kpa) as compared with partial CPB (4.5 * 1.2 kpa; p <.5). After declamping of the aorta the arterial CO, partial pressures increase to 6.44 * 1.21 kpa for simple clamping as compared with 5.22 k 1.9 kpa for partial CPB (p <.5). Partial arterial, pressures are shown in Figure 6. During aortic cross-clamping the mean arterial, partial pressures are at 13.2 * 7.9 kpa for simple clamping as compared with 3. f 16.2 kpa for -2 Transfusions ml 1-1 ml - 1 I SIMPLE SUPPORT PARTIAL CBP

5 1993; VON SEGESSER ET AL 1377 partial CPB (p <.5). Figure 7 depicts the arterial, saturation, which is 95.2% f 5.3% in the simple group as compared with 98.6% f 2.1% in the partial CPB group (p <.5) during aortic cross-clamping and 91.5% f 11.8% for simple clamping as compared with 95.9% f 4.4% for partial CPB (p <.5) after declamping of the aorta. Mean negative base excess values are depicted in Figure 8. After declamping of the aorta the negative base excess drops to 4.9 f 5.8 for simple clamping as compared with 2.6 f 3.9 for partial CPB (p <.5) and recovers by the end of the procedures. No device failure occurred during the procedures, and all oxygenators remained functional despite perfusion with low systemic heparinization. In the group with partial CPB, the mean activated coagulation time (ACT) was 12 & 23 seconds before heparinization and 27 * 79 seconds (p <.5) after low systemic heparinization with 1 IUkg body weight (before cannulation). Occasionally, ACT values up to 1, seconds were measured during perfusion, ie, after mixing with the crystalloid priming volume and the heparin loading dose. However, the mean of the lowest ACT measured per patient was 286 f 13 seconds. Excluding 5/38 patients with ACT values greater than 4 seconds, the mean of the lowest ACT value measured per patient was 25? 53 seconds. Seven patients had a lowest ACT value less than 2 seconds during the perfusion procedure: 197, 196, 188, 187, 183, 18, and 174 seconds. No modification of oxygenator performance was observed during these periods. No complications related to the use of cardiopulmonary bypass were observed and no bypassinduced distal embolization occurred. Comment Proximal unloading and support of distal circulation during resection of descending thoracic and thoracoabdominal aortic aneurysms prevents declamping shock and can now be realized without increased risk of bleeding com- ml t '*'OO 8 4 : I 1 Durlng surgery SIMPLE [IL3 Day I SUPPORT PARTIAL CPB Fig 3. Total volume transfused (mean f standard deviation) including homologous blood products, noncystalloid fluids, and crystalloid fluids during and 24 hours after (day 1) for simple aortic cross-clamping, distal circulatory support, and partial cardiopulmonary bypass (CPB). There was less volume required on day 1 for distal circulatory support and partial CPB as compared with simple aortic cross-clamping (p <.25). Table 4. Blood Gases Variable Simple Partial CPB p Value Baseline values before induction of anesthesia Arterial, Before aortic crossclamping Arterial, After aortic crossclamping Arterial, After declamping Arterial, End of Arterial, f f f f f f f f f f f f f.8 5.3? f f f f f f f f f f f f f f t_ f f t_ f f f f f f f f f f f f f 3.1 <.5 <.5 <.1 <.5 <.5 <.5 <.5 <.1 <.5 <.5 CPB = cardiopulmonary bypass; = not significant; PCO, = carbon dioxide tension; PO, = oxygen tension. plications. In the present study, we noted the best outcome in patients undergoing repair of the aorta during distal circulatory support with partial CPB. This group had the lowest morbidity and mortality rates despite the fact that they had more complex procedures with longer cross-clamp times and higher proportion of reimplanted intercostal, visceral, and renal vessels. The recent advent of relatively complex perfusion devices with increased thromboresistance due to heparin surface coating [&lo] allowed us to realize roller pump left heart bypass, centrifugal pump left heart bypass, and finally partial CPB with low systemic heparinization. The heparin doses used for perfusion with heparin-coated cardiopulmonary bypass equipment are similar to those used in vascular s and proved not to induce increased bleeding

6 1378 VON SEGESSER ET AL 1993;56: PH Partial CPB ---- Simple M Partial CPB ---- Simple T 7.1 Fig 4. (mean f standard deviation) before induction of anesthesia (baseline), before cross-clamping of the aorta (before, after aortic cross-clamping (after cx), after declamping of the aorta (declamped), and at the end of the (end). There is a significant acidosis for the group operated on with simple aortic crossclamping after declamping, whereas the ph in the group with partial cardiopulmona ry bypass (CPB) remains within the normal range (p <.5). even when partial CPB was realized under this regimen. The data shown in Table 3 demonstrate clearly that, with respect to blood loss and transfusion requirements, there are no significant differences between patients operated on with simple aortic cross-clamping versus patients I Fig 6. Arterial partial, pressure (pao,) (mean f standard deviation; 1 kpa = 7.5 mrn Hg) before induction of anesthesia (baseline), before cross-clamping of the aorta (before, after aortic crossclamping (after cx), after declamping of the aorta (declamped), and at the end of the (end). There are significantly lower values for the group operated on with simple aortic cross-clamping during single-lung ventilation and aortic cross-clamping (p <.5). (CPB = cardiopulmonary bypass.) undergoing aortic repair during distal circulatory support with low systemic heparinization and heparin-coated perfusion equipment versus patients undergoing aortic repair during distal circulatory support with partial CPB with low systemic heparinization and heparin-coated per- kpa Partial CPB ---- Simple % * Partial CPB ---- saturation Simple 6 T T Fig 5. Arterial partial CO, pressure (paco,) (mean? standard deviation; 1 kpa = 7.5 mm Hg) before induction of anesthesia (baseline), before cross-clamping of the aorta (before, after aortic cross-clamping (after cx), after declamping of the aorta (declamped), and at the end of the (end). There is significant hypercapnia for the group operated on with simple aortic cross-clamping after declamping, whereas the pac, in the group with partial cardiopulmonary bypass (CPB) remains within the normal range (p <.5) I 1 1 Fig 7. Arterial O2 saturation (mean f standard deviation) before induction of anesthesia (baseline), before cross-clamping of the aorta (before cx), after aortic cross-clamping (after cx), after declamping of the aorta (declamped), and at the end of the (end). There are significantly lower values in the group operated on with simple aortic cross-clamping during single-lung ventilation with clamped aorta and after declamping as compared with the group with partial cardiopulmonary bypass (CPB) (p <.5; p <.5, respectively).

7 19!33;56 137W3 VON SEGESSER ET AL 1379 Partial CPB Simple e--. I_ Fig 8. reflecting metabolic acidosis (mean 2 standard deviation) before induction of anesthesia (baseline), before cross-clamping of the aorta (before cx), after aortic cross-clamping (after, after declamping of the aorta (declamped), and at the end of the (end). There is a significantly higher acidosis for the group operated on with simple aortic cross-clamping after release of the aortic clamp as compared with the group with partial cardiopulmonary bypass (CPB) (p <.5). fusion equipment. This is in sharp contrast to our previous experience [16], where partial CPB using standard perfusion equipment with full systemic heparinization resulted in higher morbidity and mortality than simple aortic cross-clamping without further adjuncts. As partial CPB with standard equipment and full systemic heparinization failed to provide improved outcome at that time, DeBakey [17], Crawford [18], Livesay [19], Cooley [2], and others advocated simple aortic cross-clamping with expeditious graft replacement to avoid the risks of cannulation and anticoagulation for circulatory support. However, there are also various studies demonstrating improved organ perfusion if distal circulatory support is used during cross-clamping of the descending thoracic aorta. Kouchoukos and co-workers [21] studied patients undergoing thoracic aortic repair with Gott shunts and demonstrated substantial afterload reduction and significant left ventricular protection. The highest benefit of distal perfusion was found in patients with preoperatively impaired renal function [22]. Superior shunt flows were demonstrated for left heart bypass with centrifugal pumps [23], and improved distal protection as compared with a passive shunt was observed by Cartier and associates [24]. However, partial CPB using heparin-coated perfusion equipment with low systemic heparinization during repair of descending thoracic aortic aneurysms provides not only distal circulatory support during aortic cross-clamping but also a significant improvement in overall hemodynamics as well as overall gas transfer. Our data shown in Table 4 demonstrate clearly that superior arterial blood gases can be achieved during aortic crossclamping with partial CPB as compared with simple aortic cross-clamping. II Furthermore, the well-known "declamping shock" after release of the aortic cross-clamp including acidosis (see Fig 4), hypercarbia (see Fig 5), a decrease in arterial oxygen saturation (see Fig 7), and resulting negative base excess (see Fig 8) observed in simple aortic cross-clamping can be avoided if distal support with partial CPB is used. In comparison with left heart bypass with pumps only, distal circulatory support with partial CPB has the additional advantage of providing significant gas transfer. Hence the limited gas transfer that is due to single-lung ventilation (most desirable during extended thoracic aortic s) can be compensated for. This is in contrast to the usual decrease of blood flow below the aortic occlusion associated with simple aortic cross-clamping, which cannot be compensated for by the increased blood flow above the cross-clamp because of decreased oxygen uptake [25]. Outcome of s for descending thoracic and thoracoabdominal aortic aneurysms can be further improved in the near future by the possibility of use of cardiotomy reservoirs with improved thromboresistance in conjunction with heparin-coated perfusion equipment [26]. This will allow for recovery of aortic shed blood in quantities that are far superior to those that can be handled with red cell spinning devices. Furthermore, moderate systemic hypothermia, which is known to improve ischemic tolerance of the spinal cord [27, 281, can now also be used readily as heat exchangers are an integral part of the heparin-coated perfusion circuits used with low systemic heparinization. Finally, it has to be noted here that the antithrombotic activity of heparin bonded to blood-exposed surfaces is strictly flow dependent and requires adequate levels of antithrombin I11 in the perfusate. Hence, venting of cannulas into the venous line before onset of bypass, immediate recirculation through a shunt in the surgeon's field after weaning from bypass, and full systemic heparinization in case of unforeseen problems that might require temporary standstill of the pump are recommended. Nonresponding ACT after proper injection of heparin should be treated with a supplement of antithrombin 111 or fresh frozen plasma and not exclusively with additional heparin. This study was supported by grant from the Swiss National Foundation for the Development of Scientific Research. References 1. Chirillo F, Marchiori MC, Andriolo L, et al. Outcome of 29 patients with aortic dissection. A 12 year multicenter experience. Eur Heart J 199;11: Von Segesser LK, Burki H, Jenni R, et al. Spatresultate nach chirurgischer Sanierung von Aneurysmen der deszendierenden thorakalen Aorta. Schweiz Med Wochenschr 1991;121: Carrel A. On the experimental surgery of the thoracic aorta and the heart. Ann Surg 191;52: McLean J. The thromboplastic action of cephalin. Am J Physiol 1916;41: Gibbon JH. Application of mechanical heart and lung apparatus to cardiac surgery. Minn Med 1954;37171.

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