Proximal clamping of the descending thoracic aorta or distal arch presents several major dif- ficulties

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Contents

14.1 Introduction . . . . 153 14.2 Operative Strategies . . . . 153 14.3 Methods . . . . 154

14.1 Introduction

Aneurysms of the descending thoracic and thoracoabdominal aorta range from focal outpouchings of the thoracic aorta to extensive degenerative aneurysms en- compassing the entire thoracoabdominal aorta. Similar- ly, treatment strategies range from simple clamp techniques, unsupported by distal perfusion, the ªclamp- and-goº technique, to more complex techniques utiliz- ing distal perfusion, separate visceral perfusion, and hypothermia as an adjunct for preservation of spinal cord function. This treatise advances our rationale for a more aggressive perfusion strategy that affords optimal protection for CNS, spinal cord, and abdominal viscera.

Although the simplicity of a clamp-and-go approach is attractive, there are severe perturbations that argue against its utility. Proximal clamping of the descending thoracic aorta or distal arch presents several major dif- ficulties. First, in many instances with diffuse atheromatous change, clamping of the distal arch may be unsafe, with central embolization of atheromatous debris into the cerebral vasculature. Second, proximal clamping may not afford sufficient normal proximal aorta to allow a secure proximal anastomosis. Third, proximal clamping also produces severe hemodynamic distur- bances, including proximal hypertension, an abrupt increase in afterload, a decrease in distal perfusion pressure, and an increase in cerebrospinal fluid (CSF) pressure [1]. In addition to a profound sympathetic stimulation, there is also requisite distal ischemia, with secondary and injurious metabolic changes, increasing the likelihood for spinal cord and visceral end-organ injury.

Similarly, with the release of the aortic cross-clamp, further hemodynamic perturbations are again pro-

duced, many of which will aggravate a previous insult, including proximal hypotension, washout of ischemic metabolites, and reperfusion injury, all compounding the effects of the prior ischemic injury.

Distal aortic perfusion techniques may substantially reduce many of these ill effects, albeit at the price of in- troducing negative attributes essential for extracorpore- al perfusion, including anticoagulation, stimulation of the inflammatory response, and alterations in coagulation.

In the absence of distal aortic perfusion, the technique of simple aortic clamping is limited to a ªsafe ischemic period,º unpredictable, and likely highly vari- able for different vascular beds. Historically, spinal cord complications have been noted to rise significantly after 30±35 min of ischemia, with similar end-organ dysfunc- tion becoming apparent after slightly longer ischemic intervals for kidneys and liver. In an effort to minimize these secondary perturbations, distal perfusion techniques have been developed to allow longer periods of ischemia and more complex reconstructions [2]. It is our contention that femoral±femoral bypass provides a stable operative environment, facilitates surgical reconstruction, provides good end-organ preservation, and affords valuable flexibility for improved surgical results, with only modest negative repercussions.

14.2 Operative Strategies

A thorough preoperative assessment, including physical examination, assessment of physiologic cardiac, pulmo- nary, and renal reserve, and scrutiny of available imaging modalities precedes operative intervention. Surgical as well as perfusion strategies are then decided.

The major downside of femoral±femoral bypass is the necessity for full heparinization as a membrane oxygenator is an integral part of the perfusion circuit.

Usually 300 IU/kg is sufficient to maintain an activated clotting time of more than 400 s. Fully heparin coated circuitry may allow a lower systemic heparin level, usually 100 IU/kg heparin and an activated clotting

Femoral Bypass and Hypothermia for the Treatment

of Thoracoabdominal Aneurysms

R. Scott Mitchell

14

Chapter

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time of more than 180 s. Although expensive, fully coated circuitry may be associated with less blood loss, reduced transfusion requirements, and perhaps a decrease in the systemic inflammatory response. Similarly, the use of aprotinen (Trasolol, Bayer Pharmaceuticals) has been associated with decreased blood loss and a decreased requirement for transfusion products, but re- quires ascertainment of adequate heparin effect, namely a Kaolin-determined activated clotting time of more than 400 s, and a heparin concentration of more than 3.5 mg/kg.

The major advantage of femoral±femoral bypass is flexibility. For thoracic or thoracoabdominal aneurysms in which the distal arch is unsuitable for clamping (large diameter, severe calcification, intraluminal atherogenic debris, proximal extension of dissection flap), hypothermic circulatory arrest (HCA) is the method of choice for constructing a secure proximal anastomosis and avoiding cerebral atheroembolic injury.

Similarly, for patients in whom sequential cross-clamping of the aorta is not possible (chronic dissection, large mural thrombus, or generalized aneurysmal en- largement without focal narrowing), HCA is an excellent method of spinal cord protection, coupled with distal aortic perfusion and CSF drainage [3±5]. The oxygenator also adds flexibility, such as with the patient with poor oxygenation who may become a ventilatory problemduring single lung ventilation.

14.3 Methods

Preoperatively, an epidural catheter and a CSF drain are placed prior to operative intervention. Full intraopera- tive monitoring is employed, including radial and femoral artery pressure monitoring, large-bore intravenous access, and transesophageal echo (TEE). A double-lu- men endotracheal tube facilitates operative exposure and minimizes operative trauma to the left lung. Both femoral arterial and venous access are attained via an oblique supra-inguinal crease incision.

Long, flexible, thin-walled venous catheters are available with tapered over-the-wire dilators that almost uni- formly assure access to the right atrium. Endovascular access through the femoral vein can be ascertained by TEE visualization of the guide wire emerging from the inferior vena cava, traversing the right atrium, and en- tering the superior vena cava. If passage of a soft ªJº- tipped guide wire is not successful, a floppy-tip Glide- wire (Terumo, Tokyo, Japan) and a Benson catheter frequently assure passage.

Arterial cannulation is either via a transverse arter- iotomy with a 22-F arterial catheter, or over a guide wire using a 17-F or a 19-F catheter (DLP).

Successful cannulation is assured by easy aspiration of venous blood fromthe venous catheter, and pulsatile

flow fromthe arterial catheter with a pulsatile wave formin the cardiopulmonary bypass pump circuitry.

Full thoracoabdominal exposure is attained in the routine manner. Proximally, the distal arch and proximal descending thoracic aorta are circumferentially dissected, with careful sharp dissection of the phrenic, va- gus, and recurrent laryngeal nerves. After careful palpa- tion, inspection, and perhaps interrogation by transesophageal echocardiography, and/or epiaortic ultrasound, a decision is made whether the aorta can be safely clamped to allow a secure proximal anastomosis. If not, femoral±femoral bypass is used to cool the patient to 16±188C, and an open proximal anastomosis is constructed to the full-thickness divided aorta of normal caliber. Great care is taken to prevent any atheromatous debris fromfalling into the dependent aortic arch, and retrograde cerebral perfusion through the long venous cannula can be used as a partial flush of the arch vessels. Following completion of the anastomosis, the new graft is cannulated and clamped distally, and retrograde arterial perfusion of the heart and great vessels is re- sumed.

If retrograde perfusion fromthe femoral artery is thought to entail significant risk for retrograde embolization, separate antegrade cerebral perfusion can be performed from a previously placed 6-mm Dacron side limb sewn to the left common carotid artery established prior to thoracotomy.

If the aorta is focally narrowed so as to allow sequential cross-clamping, then that is done serially down the aorta, allowing retrograde femoral perfusion to pro- vide visceral perfusion until that aortic segment is opened. Initially, a distal clamp at mid-chest allows retrograde perfusion of critical intercostal arteries. Then, clamping just above the celiac axis allows identification of large paired intercostals at the T8±T12 level which need to be attached to the graft. Decision-making about which arteries to reattach is guided by the identification of the anterior spinal artery on computed tomography or magnetic resonance imaging [6, 7], loss of either evoked sensory or motor potentials, and by their gener- al suitability at the time of operation. It should be noted that while hypothermia is an excellent adjunct for spinal cord protection, it also effectively silences neural response, thus limiting the utility of either evoked sensory or motor potentials.

After attachment of the intercostal artery pairs, at- tention is directed toward the visceral vessels, which are now individually cannulated with balloon-tipped perfusion catheters and perfused with cold blood. Depending on the local anatomy, and the quality of this portion of the aorta, this reconstruction may utilize the island technique, frequently incorporating the celiac axis, superior mesenteric artery, and right renal artery as one button, and the left renal artery as a second button. Al- ternatively, if the quality of the aortic tissues is poor, individual grafts may be led separately to each visceral III. Treatment of Thoracic Degenerative Aortic Aneurysms

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vessel orifice. Preloading short Dacron graft segments on the balloon catheters prior to initiating perfusion greatly facilitates this reconstruction, and minimizes visceral ischemic time.

Distally, after exposure of the aorta in the retroperi- toneal plane, the distal end point is determined, and the aorta is again circumferentially dissected.

Again, if this can be safely clamped, and a secure anastomosis obtained, then distal aortic perfusion is used distal to the aortic clamp. If a satisfactory length of normal aorta is not available, then distal perfusion is temporarily discontinued, and an open distal full-thickness anastomosis is constructed.

The beauty of hypothermia is that it allows the safe conduct of these multiple previous maneuvers for all patients, regardless of the extent of aortic disease, and without the risk of atheroembolization. Cardiopulmo- nary bypass is initiated, and the proximal and distal dissection completed while cooling proceeds to 16±

188C, with assurance of EEG silence. After equilibration of a core temperature of 16±188C for at least 5 min, cardiopulmonary bypass is discontinued in the head-down position, and retrograde perfusion through the venous catheter is commenced at approximately 500 ml/min to continuously flush the cerebral vasculature. The aneu- rysmis incised to the level of the renal arteries, and visceral artery perfusion is instigated with balloon- tipped catheters. The proximal aorta is transected to allow construction of a full-thickness anastomosis to normal aorta, after which the graft is then cannulated, clamped distally, and retrograde arterial perfusion established to cardiac and brachiocephalic vessels. Next, selective intercostal artery reimplantation is effected to large patent intercostal arteries in the critical zone. Pre- operative localization of contributing intercostal pairs to the anterior spinal artery can be obtained with computed tomographic or magnetic resonance imaging. Ex- cellent spinal cord protection is afforded during this in- terval by generalized hypothermia and perfusion of col- laterals fromthe left vertebral and hypogastric systems.

The paravisceral aorta is then assessed. For good- quality aorta in the absence of connective tissue disease, aortic island reconstruction is effected, usually with the celiac axis, superior mesenteric artery, and right renal artery as one island, and the left renal artery as a separate full-thickness button. Alternatively, especially for Marfan patients in whomaneurysmal dilation of the visceral island has been noted, individual branch vessel reconstruction with 6-, 8-, or 10-mm Dacron grafts can be accomplished during continuous perfusion via balloon-tipped catheters. An open distal anastomosis is then completed, allowing restoration of femoral perfu-

sion after clamping of the graft. Individual visceral branch vessel grafts can then be reimplanted into the central aorta graft between clamps, allowing continuous perfusion of cardiac, cerebral, and intercostal arteries, and pelvic circulation during reimplantation and warm- ing. Visceral ischemic time is limited to the time necessary to reimplant Dacron side limbs into a central aortic graft, and its effect is minimized by the now regional hypothermia.

Although full heparinization is necessary, cardiac, central nervous system, and spinal cord protection is assured, and the hepatic contribution to postoperative coagulation is preserved by hypothermia and continuous perfusion.

Although there is a requirement for full heparinization in order to use an oxygenator, this coagulopathy may be more than offset by avoiding the use of rein- fused washed red cells, with their own inherent coagu- lopathic tendencies. Integrity of all anastomoses can be easily assured during the period of rewarming, and the patient can then be weaned fromcardiopulmonary bypass. Although a somewhat prolonged cardiopulmonary bypass run may be necessary, especially in the obese patient, good end-organ preservation is assured, and the risk of atheroembolism minimized. With more tar- get-specific anticoagulation, coagulopathies secondary to a prolonged cardiopulmonary bypass run can be minimized, and excellent end-organ preservation can be achieved by these techniques.

References

1. Gelman S. The pathophysiology of aortic cross-clamping and unclamping. Anesthesiology 1995; 82:1026±1060.

2. Von Segesser LK, Killer I, Jenni R, et al. Improved distal circulatory support for repair of descending thoracic aortic aneurysms. Ann Thor Surg 1993; 56:1373±1380.

3. Kouchoukos NT, et al. Hypothermic cardiopulmonary bypass and circulatory arrest for operations on the descending thoracic and thoracoabdominal aorta. Ann Thorac Surg 2002; 74:S1885±1887.

4. Colon R, et al. Hypothermic regional perfusion for protection of the spinal cord during periods of ischemia. Ann Thor Surg 1987; 43:639±643.

5. Robertson CS, et al. Protection against experimental ischemic spinal cord injury. J Neurosurg 1986; 64:633±642.

6. Kawaharada N, et al. Thoracoabdominal or descending aortic aneurysmrepair after preoperative demonstration of the Adamkiewicz artery by magnetic resonance angiography.

Eur J Cardiothorac Surg 2002; 21:970±974.

7. Jacobs MT, et al. Spinal cord blood supply in patients with thoracoabdominal aortic aneurysms. J Vasc Surg 2002;

35:30±37.

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