MMCTS
(June 19, 2007). doi:10.1510/mmcts.2006.002535
Copyright © 2007 European Association for Cardio-thoracic Surgery
Procedure
Perfusion techniques during surgery of the thoracic and thoraco-abdominal aorta: the veno-arterial bypass
Ludwig K. von Segesser*
Department of Cardio-Vascular Surgery, Centre Hospitalier Universtaire Vaudois, CCV BH 10-275, Rue du Bugnon 46, CH-1011 Lausanne, Switzerland
* Corresponding author: * Tel.: +41-21-314 2279; fax: +41-21-314 2879 ludwig.von-segesser{at}chuv.ch; Web: www.cardiovasc.net
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Summary
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Open repair of the descending thoracic aorta and the thoraco-abdominal aorta remains a challenging procedure. On the proximal side, pressure overload in the supra-aortic territories and pump failure are just two examples for undesirable effects of descending thoracic aortic cross-clamping which may end with a disaster. On the distal side, potential problems include among others, paraplegia, renal failure, visceral and peripheral ischemia. Finally, there are per-procedural issues like hypoxia due to single lung ventilation, uncontrollable inflow or backflow, excessive blood loss, unrealistic time constraints for systematic revascularization of intercostals arteries etc. Partial (femoro-femoral) cardiopulmonary bypass provides answers to many of the issues raised above. In addition, perfusion with heparin coated equipment allowing for significant reduction of anticoagulation has been shown to be useful for maintaining the hemostatic potential of the patient and limiting blood loss. Our current approach to perfusion for descending thoracic and thoraco-abdominal aneurysm repair has evolved into a versatile cardiopulmonary bypass strategy allowing for rapid conversion from partial cardiopulmonary bypass with peripheral cannulation to full, and if necessary, central, cardiopulmonary bypass, and all sorts of combinations including deep hypothermic circulatory arrest.
Key Words: Aortic aneurysm • Cannulation • Cardiopulmonary bypass • Thoracic aortic aneurysm • Thoraco-abdominal aortic aneurysm • Perfusion • Renal perfusion • Spinal chord protection • Visceral perfusion
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Introduction
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Repair of the descending thoracic aorta, and even more so the thoraco-abdominal aorta, is in general a major undertaking. This is due to the pivotal position of this principal pipeline of the body. As a result, shutting it down, even for a short period of time, interferes not only with the downstream end organ perfusion, but has also major consequences upstream. Pressure overload in the supra-aortic territories and pump failure are just two examples for undesirable effects of descending thoracic aortic cross-clamping which may end with a disaster. On the distal side, potential problems include among others, paraplegia, renal failure, visceral and peripheral ischemia. Finally, there are per-procedural issues like hypoxia due to single lung ventilation, uncontrollable inflow or backflow, excessive blood loss, unrealistic time constraints for systematic revascularization of intercostal arteries, etc.
In addition to the traditional ‘clamp and go’ approach, as well as the single clamp technique [1] with or without spinal fluid drainage, numerous techniques and adjuncts have been advocated for descending thoracic and thoraco-abdominal aneurysm repair:
- Surface induced hypothermia [2]
- Full cardiopulmonary bypass [2, 3]
- Deep hypothermia with circulatory arrest [4, 5]
- Apico-aortic shunting with heparin coated tubing (Gott shunt: [6,7,8])
- Left heart bypass with heparin coated centrifugal pump and tubing [9,10,11,12]
- Partial cardiopulmonary bypass with heparin coated perfusion equipment [13,14,15,16].
Over the years, we have worked with all of these techniques. Our current perfusion strategy for descending thoracic and thoraco-abdominal aneurysm repair is based on partial cardiopulmonary bypass with heparin coated perfusion equipment as previously reported [13,14,15,16] which has evolved since into a versatile cardiopulmonary bypass circuit which allows for rapid conversion from partial cardiopulmonary bypass with peripheral cannulation to full, if necessary central, cardiopulmonary bypass, and all sorts of combinations.
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Methods
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Partial cardiopulmonary bypass with low systemic heparinization
Following our work on heparin surface coated perfusion equipment allowing for low systemic heparinization we introduced for descending thoracic aortic aneurysm repair in successive fashion:
- The inlet pressure controlled roller pump left heart bypass [11]
- The centrifugal pump left heart bypass [13]
- The centrifugal pump left heart bypass with an oxygenator [14], and
- Partial cardiopulmonary bypass [16]
Tip-to-tip heparin coated perfusion equipment including the venous cannula, the venous line, the soft venous reservoir, the pump loop, the heat-exchanger/oxygenator structure (Photo 1), the arterial filter, the arterial line and the arterial cannula as well as a heparin coated cardiotomy reservoir are used – MMCTSLink 115, MMCTSLink 145, MMCTSLink 150.
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Photo 1 Heparin coated oxygenator/heat exchanger with a heparin coated soft venous reservoir for perfusion with low systemic heparinization. This set-up was used for many years [17, 18]. No oxygenator thrombosis was observed during perfusion with low systemic heparinization (ACT >180 s) and immediate recirculation after weaning from CPB. Various other oxygenator/heat-exchanger structures with improved thrombo resistance have been introduced since (MMCTSLink 115, MMCTSLink 145, MMCTSLink 150).
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As previously reported [13, 14], the perfusion circuit is primed with Ringer's lactate and 1000 IU of heparin per litre of priming volume are added (priming dose). Heparin loading dose for the patient is 100 IU of heparin per kg bodyweight and the target ACT is >180 s. During perfusion ACT is monitored continuously and small additional heparin doses (1000–2000 IU) are added when necessary. A shunt between the arterial and the venous line in the surgeon's field (Schematic 1) allows for recirculation before and after cardiopulmonary bypass. If non-circulating blood volume has to be stored within the cardiotomy reservoir, target ACT is increased >300 s in order to avoid cardiotomy reservoir thrombosis. Target pump flow during perfusion is 50% of cardiac output. Special care is taken to maintain the proximal (cardiac output dependant) and distal (pump flow dependant) mean perfusion pressures above 60 mmHg. For further protection of the spinal chord during aortic cross-clamping, we reduce the blood temperature to 28 °C [17].
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Schematic 1 Table set with arterio-venous shunt carrying a side-arm suitable for recirculation, selective organ perfusion (clamp in position Y) or additional drainage (clamp in position X).
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Femoral arterial cannulation for partial cardiopulmonary bypass
The left groin is the preferred cannulation site for both the arterial and the venous side. In accordance to the surgeon's preference, the common femoral vessels are cannulated in percutaneous, semi-open, or open fashion [19]. The traditional landmark for this purpose is the inguinal ligament which spans between the anterior superior iliac spine and the pubic tuberculum. Alternatively, to the common femoral vessels, the external iliac vessels are also suitable candidates for remote cannulation. The main advantages of the latter include their larger diameter and the absence of major lymph nodes. However, cannulation of the iliac vessels usually requires open access. In contrast, the common femoral arteries can be cannulated in percutaneous, semi-open or open fashion. If a semi-open or open approach is selected we prefer a cranio-caudal incision which may be slightly curved and lateral to the access vessels but still in the same direction as the main vessels in order to allow for secondary extension if a more complex revision is required.
Schematic 2 is a topographic reminder of the anatomy of the groin. For arterial cannulation, care is taken to respect the deep femoral artery in order to allow for collateral perfusion during the pump run (Schematic 3).
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Schematic 2 View of the left inguinal ligament which spans between the anterior superior iliac spine and the pubic tuberculum. The inguinal ligament is undercrossed by the femoral artery.
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Schematic 3 The distal landmark of the surgical field is the deep femoral artery, which should not be clamped or compromised during perfusion in order to allow for collateral perfusion.
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Schematic 4 shows the anterior circumflex iliac artery which has a common origin with the superficial epigastric artery.
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Schematic 4 Care is taken not to tear the iliac circumflex artery which has a common origin with the superficial epigastric artery.
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Care is taken to avoid these collaterals which are major bleeders if torn or cut. The common femoral artery is controlled with a vessel loop (Schematic 5) which can later be passed through a snare. For open cannulation, the femoral artery is clamped close to the inguinal ligament and proximal to the deep femoral artery. For healthy vessels we prefer a transverse incision, whereas a longitudinal arteriotomy is recommended for diseased vessels (requires patch repair after decannulation).
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Schematic 5 A vessel loop is passed around the common femoral artery between the inguinal ligament and the origin of the deep femoral artery.
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Typical heparin coated cannula for open or percutaneous insertion (over a guide wire) is shown in Photo 2.
Once a suitable cannula is selected and positioned within the arteriotomy, the femoral artery is controlled with the vessel loop, the proximal clamp is removed and the cannula is gently advanced within the vessel lumen. Insertion of a few centimetres is usually enough and avoids conflicts between the cannula tip and tortuous arteries. Under the latter circumstances, positioning of a guide wire within the vessel lumen (position can be checked with TEE) prior to cannula insertion helps to stay out of trouble. Once in the correct position and checked for adequate back flow, the cannula can be secured in various fashions: a suture to the skin, a snare and a tie, and/or a tape.
Femoral venous cannulation for partial cardiopulmonary bypass
As outlined above, the left groin is also the preferred cannulation site for venous cannulation. In accordance to the surgeon's preference, the common femoral vein can be cannulated in percutaneous, semi-open, or open fashion. Like for the arterial side, the external iliac vein is also a suitable candidate for remote cannulation. Again the main advantages of the latter include its larger diameter and the absence of major lymph nodes. We usually use the same skin incision for both the arterial and the venous cannulation.
Schematic 6 is a topographic reminder of the anatomy of the groin. For venous cannulation, the greater saphenous vein and its branches are the distal landmark (Schematic 7).
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Schematic 6 View of the left inguinal ligament which spans between the anterior superior iliac spine and the pubic tuberculum. The inguinal ligament is undercrossed by the femoral vein.
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Schematic 7 The distal landmark of the surgical field is the greater saphenous vein and its collaterals, which should be avoided during preparation of the cannulation site.
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Care is taken to respect the deep femoral vein in order to allow for collateral perfusion during the pump run (Schematic 8). The common femoral vein is controlled with a vessel loop (Schematic 9) which can later be passed through a snare.
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Schematic 8 The deep femoral vein should not be clamped or compromised during perfusion in order to allow for collateral drainage.
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Schematic 9 A vessel loop is passed around the common femoral vein between the inguinal ligament and the origin of the deep femoral artery.
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For open cannulation, the femoral vein is clamped close to the inguinal ligament and proximal to the deep femoral vein. A transverse incision is made. Typical heparin coated cannula for open or percutaneous insertion (over a guide wire) is shown in Photo 3.
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Photo 3 Heparin coated venous cannula for remote cannulation and perfusion with low systemic heparinization with its mandrel ready for insertion over a guide wire (MMCTSLink 148).
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Once a suitable cannula is selected and positioned within the phlebotomy, the femoral vein is controlled with the vessel loop, the proximal clamp is removed and the cannula is gently advanced within the vessel lumen. Deep insertion up to the vena cava is necessary to reach acceptable flows (e.g. 2 l/min for effective proximal unloading and distal perfusion in an adult). If a resistance is felt and the cannula cannot be advanced sufficiently, it has to be removed. A guide wire is positioned within the vena cava (can be checked with TEE) prior to re-insertion of a guide wire compatible venous cannula. Once the venous cannula is in the correct position and checked for adequate back flow, it can be secured in various fashions: a suture to the skin, a snare and a tie, and/or a tape.
After connection to the venous line and verification of the arterial connection, both sides are vented into the pump loop with open (uncompressed) venous reservoir (typical recirculation flow of the perfusion system is 1 l/min). The a-v shunt is clamped and partial CPB is initiated. For additional protection of the spinal chord, we set the blood temperature at 28 °C. Staged aortic cross-clamping, systematic re-implantation of intercostal, visceral and renal arteries (island technique and/or separate grafts) as well as spinal fluid drainage are routine. Rewarming is started after completion of all vascular anastomoses. At a core temperature of 35 °C, the patient is weaned from CPB and recirculation through the a-v shunt in the surgeon's field is started immediately (typically 1 l/min; Schematic 1). After hemodynamic stabilization and completion of hemostasis at the level of the anastomoses, the patient is decannulated and the access vessels are repaired. Neutralization of low dose systemic heparin with protamine (1 to 1) usually results in immediate hemostasis.
Conversion of partial cardiopulmonary bypass with remote femoral cannulation to full cardiopulmonary bypass
The versatile perfusion circuit which we now use in routine fashion for rapid conversion from partial peripheral cardiopulmonary bypass to full central cardiopulmonary bypass and its combination (simultaneous proximal and distal perfusion) is an extension of our previous experience. As a matter of fact, this technique has evolved from the Partial cardiopulmonary bypass with remote cannulation and low systemic heparinization strategy described above and remains the corner stone of our current strategy. Likewise, there is in addition to proximal unloading and distal perfusion also oxygenation as well as shed blood recovery. Furthermore, our circuit allows for simultaneous selective organ perfusion as the necessary side arm built into arterio-venous shunt of the table set (Schematic 1) can be extended. As a matter of fact, it is this side arm which can be used for rapid conversion of peripheral partial cardiopulmonary bypass to central full cardiopulmonary bypass or its combinations.
In addition to remote femoral (or external iliac) arterial cannulation for distal perfusion (A) there are, however, two conditions to be met for combined central and peripheral perfusion with clamped descending thoracic aorta, namely, (B) adequate venous blood drainage for full flow with gravity drainage alone, and (C) additional blood return to the aortic arch or its branches.
A) Remote femoral cannulation for distal perfusion allowing for rapid conversion to full cardiopulmonary bypass
Basically, femoral cannulation with full cardiopulmonary bypass in mind is similar to the technique outlined above for partial cardiopulmonary bypass except for the selection of larger cannulas allowing for higher flows (typically 2.4 l/m2 translates into 4–5 l/min in adults). Usually we use for this purpose thin wall percutaneous cannulas (Photo 4) which can be used in accordance to the surgeon's preference in a percutaneous, semi-open, or open fashion. If a semi-open or open approach is selected we prefer a cranio-caudal incision which may be slightly curved and lateral to the access vessels but still in the same direction in order to allow for secondary extension if a more complex revision is required. The common femoral artery is punctured with a hollow needle and a flat angle in the middle between the inguinal ligament and the origin of the deep femoral artery (Schematic 10).
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Photo 4 Thin wall percutaneous arterial cannula (top) allowing for full flow. Typical size for this purpose is 21F which allows for flows of 4 l/min and more (MMCTSLink 120). A venous smart canula® is visible behind the retractor (MMCTSLink 149).
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Schematic 10 The common femoral artery is punctured in the middle between the inguinal ligament and the deep femoral artery, provided the vessel wall is soft and free from major atheromatous lesions.
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High pressure backflow of red blood (pulsatile jet: Schematic 11) has to be witnessed prior to insertion of a guide wire (Schematic 12).
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Schematic 11 High pressure, pulsatile backflow of arterial blood (red) has to be witnessed prior to insertion of a guide wire.
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Schematic 12 A J-type guide wire (typical radius 3.5 mm) is inserted through the hollow needle and advanced into the iliac vessels.
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The hollow needle is removed and a small incision (in adults typically 8 mm) is made in contact with the guide wire. Serial dilators are used up to 20F for preparation of a suitable access orifice prior to percutaneous cannula insertion (Schematic 13).
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Schematic 13 A percutaneous cannula stiffened by its corresponding mandrel is inserted over the guide wire and positioned with all its orifices within the artery.
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A 21F thin wall percutaneous arterial cannula is usually sufficient for flows up to 4 l/min. Once in position, the cannula can be secured in various fashions: a suture to the skin, a snare and a tie, or a tape.
B) Remote smart cannulation for adequate venous blood drainage and full cardiopulmonary bypass
Adequate venous blood drainage is one of the main challenges for conversion of partial cardiopulmonary bypass into full cardiopulmonary bypass. In the past we relied for this purpose on percutaneous venous cannulas which were positioned in trans-femoral fashion into the right atrium. At best, approximatively 90% of target flow can be achieved with the best traditional percutaneous cannulas provided venous return is augmented with a centrifugal pump, vacuum or other suitable means [18]. However, with the advent of self-expanding venous cannulas based on the ‘collapsed insertion and expansion in situ’ principle (MMCTSLink 146) our strategy has changed. As a matter of fact, with smart cannulation, full flow can usually be achieved with gravity drainage alone.
For open repair of the descending thoracic and thoraco-abdominal aortic aneurysm, the left groin is prepped and the common femoral vessels are cannulated in percutaneous, semi-open, or open fashion. Either the common femoral vessels or the external iliac vessels are suitable for smart cannulation. Schematic 6 is a topographic reminder of the anatomy of the groin. For smart venous cannulation, care is taken to respect the deep femoral veins in order to allow for collateral drainage during perfusion (Schematic 8). The common femoral vein is punctured with a relatively flat angle between the vein and the needle (Schematic 14).
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Schematic 14 For percutaneous smart venous cannulation, the common femoral vein is punctured in a relatively flat angle with reference to the vessel and in such a fashion that its wall is penetrated roughly in the middle, between the inguinal ligament and the deep femoral vein.
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Low pressure back flow of dark blood (droplets only) has to be witnessed prior to insertion of a guide wire with a relatively large J (Schematic 15).
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Schematic 15 A J-type guide wire, with a large radius (typically 7.5 mm=a J diameter of 15 mm) is used for soft exploration of the ilio-caval axis.
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Typically a J with 7.5 mm radius (15 mm diameter) has a low risk of being trapped by collaterals during insertion. The tip of the guide wire is brought up into the superior vena cava (Video 1) and its position is checked with TEE (Photo 5). The hollow needle is removed and a small incision (in adults typically 10 mm) is made in contact with the guide wire. Serial dilators are used up to 24F for preparation of a suitable access orifice prior to percutaneous insertion of a self-expanding smart canula® (Schematic 16: collapsed insertion diameter <18F (Photo 6), target expansion within the access orifice 24F for flows up to 6 l/min, expansion within the vena cava and the right atrium up to 36F as shown in Schematic 17 and Photo 7). This set-up together with A) Remote femoral cannulation for distal perfusion is adequate for cooling and rewarming if deep hypothermia and circulatory arrest are mandatory for the planned procedure.
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Video 1 Trans-femoral semi-open venous smart cannulation: a flexible guide wire with a large J-tip is inserted through a hollow needle up to the level of the superior vena cava (check position with TEE). The needle is removed and the vascular access aperture is enlarged with a 20F dilator. Digital control of the orifice is recommended during insertion of the stretched smart canula® over the wire. The guide wire has to be withdrawn prior to the mandrel and the cannula expands.
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Schematic 16 A collapsed smart canula® (typical length for adults is 53 cm or 63 cm) is inserted over the guide wire and advanced through the inferior vena cava and the right atrium up into the superior vena cava as checked by TEE.
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Schematic 17 Once the tip of a long smart canula® is positioned in the superior vena cava, the guide wire is removed first and the mandrel second. This order prevents cannula tip dislocation. The smart canula® expands and is fixed with a suture around its reinforced body, a snare, and/or a tape. After connection to the venous line, full flow can usually be achieved with gravity drainage alone.
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C) Remote arterial cannulation of the aortic arch for rapid conversion to full cardiopulmonary bypass with central perfusion
If the strategy has to be changed with clamped descending thoracic aorta (e.g. cardiac arrest under normothermia) rapid conversion to full cardiopulmonary bypass requires also perfusion of the supra-aortic vessels. In theory, there are various possibilities for cannulation of the aortic arch through a left thoracotomy or a thoraco-phreno-laparotomy, the standard incisions for descending thoracic and thoraco-abdominal aortic aneurysm repair. However, there are situations where the standard routes for aortic arch perfusion cannot be used (e.g. the left subclavian artery emerging from the aneurysm). We prefer under such circumstances the trans-apical cannulation of the ascending aorta. Dual purse string sutures with snares are prepared for this purpose at the left ventricular apex in a coronary artery free zone. The apex is punctured with a hollow needle (cf. Remote femoral cannulation for distal perfusion) and a J-type guide wire is inserted. Usually the ascending aorta is entered directly as witnessed by TEE. The hollow needle is removed and a percutaneous arterial cannula is inserted over the guide wire into the ascending aorta. Care is taken that all orifices of the percutaneous cannula are positioned within the ascending aorta. The trans-apical arterial cannula is tied to the two snares and connected to the additional limb emerging from the arterio-venous shunt in the surgical field (Schematic 18).
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Schematic 18 A closed clamp in position X and brief opening of the clamp in position Y allows for debubbling of the trans-apical cannula into the venous line.
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Potential air is vented into the venous line (while a clamp is in position X, the clamp in position Y is briefly opened). Once the trans-apical cannula and its line is vented, the clamp in position Y is closed and the clamp in position X is opened. This maneuver results in dual arterial perfusion (Schematic 19) in trans-apical (central) and trans-femoral (peripheral) fashion.
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Schematic 19 A clamp in position Y transforms the additional line emerging from the arterial line in a second arterial line.
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At the end of the aneurysm repair, after rewarming and weaning from cardiopulmonary bypass, all arterial and venous cannulas are clamped and recirculation is started through the arterio-venous shunt in the surgical field (Schematic 20) until the patient is hemodynamically stable and decannulated.
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Schematic 20 All arterial and venous cannulas are clamped and recirculation is started through the a-v shunt in the surgeon's field.
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Results
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We have previously reported our results with active cooling with partial cardiopulmonary bypass and low systemic heparinization during open repair of thoraco-abdominal aortic aneurysms [17]. In a prospective analysis of 100 consecutive patients normothermic (36 °C) or hypothermic (29 °C) perfusion was selected in accordance to the surgeon's preference. In the hypothermic group, aortic cross clamp was applied when the target temperature of the venous blood was achieved and rewarming was started after declamping. 52/100 patients (62.2±10.9 years) received normothermic and 48/100 patients hypothermic perfusion (63.8±10.6 years: NS). Emergent procedures accounted for 18/52 (35%) with normothermia vs. 21/48 (44%: NS) with hypothermia. The number of aortic segments (eight=maximum including aortic arch, 3 thoracic, 3 abdominal, and the aortic bifurcation) replaced was 3.9±1.5 with normothermia vs. 4.1±1.5 with hypothermia (NS); Crawford type II aneurysms accounted for 21/52 patients (40%) for normothermia vs. 20/48 (42%: NS) for hypothermia. Total clamp time was 38±21 min with normothermia vs. 47±28 min with hypothermia (P=0.05). Pump time was 55±28 min with normothermia vs. 84±34 min with hypothermia (P=0.001). Mortality at 30 days was 8/52 patients (15%) with normothermia vs. 2/48 (4%) with hypothermia (P=0.06; odds ratio 4.1). Parapareses/plegias occurred in 4/52 patients (8%) with normothermia vs. 4/48 (8%) with hypothermia (NS). Revisions for bleeding were required in 4/52 patients (8%) with normothermia vs. 2/48 patients (4%) with hypothermia (P=0.38). Revisions for distal vascular problems were necessary in 5/52 patients (10%) with normothermia vs. 2/48 (4%) with hypothermia (P=0.25). Freedom from death, paraplegia, and surgical revision was 89.9% with normothermia vs. 94.8% with hypothermia (P=0.04; odds ratio 2.0). Hence, we concluded that cooling during repair of thoraco-abdominal aortic aneurysms allows for longer cross-clamp times, more complex repairs and improves outcome.
With regard to full cardiopulmonary bypass relying on optimized trans-femoral cannulation of the right atrium and the superior vena cava, we have recently reported our experience with redo aortic root replacement [20]. Smart canula® (MMCTSLink 147) performance was assessed in a small series of patients (76±17 kg) undergoing redo procedures. The calculated target pump flow (2.4 l/min/m2) was 4.42±61l/min. Mean pump flow achieved during cardiopulmonary bypass was 4.84±87 l/min or 110% of the target. Reduced atrial chatter, kink resistance in situ, and improved blood drainage despite smaller access orifice size, are the most striking advantages of smart cannulation.
Femoro-femoral cardiopulmonary bypass with the set-up described above which allows for rapid conversion to full cardiopulmonary bypass with femoral smart venous cannulation and both peripheral as well as central arterial perfusion has also been reported [19]. We have used this approach in our most recent experience (10/147: 7%) of open thoracic and thoraco-abdominal aneurysm repairs. For partial CPB target (achieved) pump flow was 2.1±0.2 l/min (3.9±0.6 l/min) vs. 3.8±0.3 l/min (4.1±0.6 l/min) for full CPB with one death and no paraplegia so far.
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Discussion
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The various adjuncts which can be used during repair of descending thoracic and thoraco-abdominal aortic aneurysm repair in addition to spinal fluid drainage are summarized in Table 1. As outlined above, we have used all of these techniques in order to improve the outcome of our patients undergoing extensive open aneurysm repair. With the advent of endovascular aneurysm repair [21], which allows for a simplified treatment of smaller aneurysms with well-defined necks, an optimal surgical strategy for open repair of the even more complex remaining and/or recurring aneurysms is mandatory. Partial cardiopulmonary bypass for proximal unloading and distal protection relying on heparin coated perfusion equipment and low systemic heparinization has served us well. Active cooling during repair of thoraco-abdominal aortic aneurysms allows for longer cross-clamp times in order to perform more complex aortic reconstructions and improves outcome.
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Table 1 Overview of the various strategies used during repair of descending thoracic and thoraco-abdominal aortic aneurysms and recommended target ACT levels (all techniques relying on ACT levels below 400 s require perfusion equipment with improved thrombo-resistance, permanent flow in the perfusion system, and ACT checks at short intervals: [13,14,15,16, 19])
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It is interesting to note that ventricular fibrillation can occur during partial cardiopulmonary bypass for descending thoracic aortic aneurysm repair with and without active cooling [17]. It is under such circumstances where the advantages of systematic smart femoral cannulation allowing for adequate venous drainage for full cardiopulmonary bypass [20] and the possibility of rapid conversion for combined peripheral and central (e.g. trans-apical; see also footnote 1) arterial perfusion are most evident.
Regarding per-procedural device complications related to perfusion with low systemic heparinization, we have to mention occasional cardiotomy reservoir as well as red cell spinning device reservoir occlusions which are handled by immediate reservoir changeover. Thrombotic material originating from the thoraco-abdominal aortic aneurysms as well as major quantities of air/blood mixtures are certainly limiting the performance of cardiotomy reservoirs although those used in this setting were all heparin surface coated. In order to reduce the risk of cardiotomy reservoir occlusion, we recommend to increase the target ACT to >350 s if excess blood volume has to be stored there temporarily.
The mortality (4%) of our group with hypothermic perfusion [17] compares favorably with other reports on repair of thoraco-abdominal aortic aneurysms. Although parapareses and paraplegias occurred in 8% of the patients with normothermic perfusion as well as in 8% of the patients with hypothermic perfusion, these results were achieved with a 52% longer cross-clamp time in the latter group. Furthermore, some additional neural chord injuries may have remained hidden in the group with normothermic perfusion which had a higher mortality (15% vs. 4% for hypothermia).
Interestingly enough, there were less revisions for bleeding and less revisions for distal vascular problems in the patients who had hypothermic perfusion and, therefore, longer cross-clamp and longer perfusion times. Although this is not a randomized study, the overall outcome, considering freedom from negative events including death, paraparesis, paraplegia, and surgical revision for bleeding or distal vascular problems, was significantly better for patients who underwent thoraco-abdominal aneurysm repair with hypothermic perfusion and active core cooling.
We expect further improvements of the outcome in open repair of advanced descending thoracic and thoraco-abdominal aneurysms with the recently introduced peripheral smart venous cannulation technique allowing for full pump flow with gravity drainage alone in combination with the possibility for rapid conversion of dual, peripheral and central, arterial perfusion.
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Footnotes
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 L.K. von Segesser is founder and shareholder of Smartcanula LLC, Lausanne, Switzerland. 
1 von Segesser LK. Partial cardiopulmonary bypass for spinal chord protection. Aortic symposium X, New York 2006, Syllabus page: 80.
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References
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Summary
Introduction
