Aortic Aneurysm & Surgery

§ Surgical repair constitutes the only known effective treatment for thoracic aneurysms, it is an urgent indication in patients with large aneurysms, especially if symptoms suggest expansion or compression of an adjacent structure

§ Cardiac failure from aortic regurgitation may also require surgery

§ Resection is less urgent in small, asymptomatic aneurysms

§ Consideration of the severity of associated diseases is important ie associated coronary or cerebrovascular disease has a greater operative risk and a smaller risk of dying from rupture of the aneurysm before succumbing to the associated vascular disease

§ Surgical treatment consists of replacing the resected aneurysmal segment with a Dacron graft attached to the relatively normal aorta proximally and distally

§ Specific surgical procedures vary with the site of the aneurysm and the need for maintaining circulation to the distal parts of the body during the necessary period of aortic occlusion

Ascending Aorta Surgery

• A left radial artery is used to monitor BP as the right brachiocephalic artery mar be occluded during the procedure
• Median sternotomy
• Venous drainage via single two-stage cannula in right atrium & IVC
• Artery cannulation via distal ascending aorta or proximal aortic arch; if aorta cannot be safely cannulated, use of common femoral arteries [ie dissection]
• CPB perfusate is slowly cooled to 24 — 28° C; haematocrit 20% — 25%
• Aortic clamping just distal to aortic lesion
• Cold cardioplegia via coronary ostia or coronary sinus [if non patent aortic valve] or via aortic root; retrograde perfusion aids in the deairing of the coronary arteries and the aortic root
• Left ventricular venting via superior pulmonary vein
• Note that retrograde flow of aorta via femoral cannulation in ascending aorta dissection may result in compression of the true lumen by the intimal flap thereby compromising blood flow to brain etc — can be determined by monitoring both radial artery pressures and femoral artery pressure

Aortic Arch Surgery

• Median sternotomy
• Artery cannulation via femoral artery
• Singe two-stage venous cannula inserted into right atrial appendage
• Left ventricular vent insertion
• Myocardial protection as for ascending aorta
• Cooling initiated till core temp 15 — 18° C
• Circulatory arrest: use of Trendelenburg position; 25% blood volume drained into venous reservoir [if too much blood is drained, have increased risk of aspiration of air into brachiocephalic arteries]; both venous & arterial cannula are clamped

Descending Thoracic Aorta Surgery

• Distal perfusion with extracorporeal circuit is not universally used
• Some simply clamp aorta without distal perfusion
• Some use passive shunts from proximal to distal aorta
• Concern is with adequate perfusion to spinal cord & abdominal organs — therefore recommend extracorporeal perfusion to distal aorta
• Two methods:

1) Left atrium to femoral artery bypass

§ no oxygenator required

§ low dose heparinisation: useful in traumas

2) Femoral vein to femoral artery

§ if femoral vein gives poor drainage use pulmonary artery

§ oxygenator required

§ full heparinisation

1) Aneurysms of the Ascending Aorta

§ Total cardiopulmonary bypass is required using moderate total body hypothermia (28° C)

§ Cannulation is in the aortic arch or common femoral artery

§ The aorta is cross-clamped distal to aneurysm but proximal to right brachiocephalic artery

§ The myocardium is protected by cold cardioplegia during the period that the coronary ostia are exposed

§ The aneurysm is opened, and a Dacron graft is sutured in place from within the aneurysm with continuous sutures

§ Finally the aneurysmal sac is trimmed and sutured around the graft

§ If the aneurysm is associated with aortic regurgitation, the leaflets are excised and a composite Dacron graft and prothetic valve is sutured into place to the aortic annulus with interrupted sutures, after which the coronary ostia are sutured to the appropriate opening made in the Dacron graft. The distal anastomosis of the graft is performed as previously described

2) Aneurysms of the Transverse Arch of the Aorta

§ Total CPB is required

§ Additionally, profound hypothermia (12 -20° C) is used to protect the brain during temporary absence of circulation to the brachiocephalic vessels

§ Arteriovenous cannulation is made in the common femoral artery & vein ?with subsequent cannulation of SVC

§ Cardiopulmonary bypass & cooling is started during midsternal exposure — total body cooling is performed slowly at a rate ≤ 1°/min to avoid a brain-core gradient [ie brain is warmer than blood]

§ Organ perfusion is enhanced during CPB by SNP infusion

§ Blood sludging is minimised by mild haemodilution

§ A vent is put in the left ventricle through the pulmonary vein

§ The aneurysm within the arch is isolated proximally & distally with aortic cross clamps; additionally, the brachiocephalic arteries may be clamped at their origin from the aorta

§ The Dacron graft is sutured to the relatively normal aorta proximally and distally from within the aneurysm

§ The brachiocephalic, left common carotid and left subclavian arteries are attached individually to appropriate openings in the graft

§ It is often possible to preserve the relatively normal aortic wall segment from which the brachiocephalic vessels arise and simple anastomose this segment to an appropriate opening made in the Dacron graft for this purpose

§ The walls of the aneurysm are trimmed and sutured together around the grafts

§ The period of circulatory arrest is variable ranging from 19 — 75 min

§ Rewarming never exceeds 1° C / 3 minutes to avoid intravascular formation of gas bubbles

3) Aneurysms of the Descending Aorta

§ Aneurysms arising distal to the left common carotid artery do not require CPB

§ Use of left atrial to femoral bypass, femoral-femoral partial CPB, hypothermia or temporary shunts around the aneurysm during the period of aortic occlusion do not prevent spinal cord ischaemia (which occurs in 3-4% of patients)

§ The aortic aneurysm is exposed in the left posterolateral position and by collapsing the left lung using a double lumen ETT

§ The aorta is slowly clamped proximally to the aneurysm while the proximal blood pressure is controlled with SNP

§ The distal clamp may or may not be applied depending on the degree of back bleeding

§ The aneurysm is opened, bleeding intercostal artery orifices are ligated; large intercostal arteries are anastomosed to the graft

§ Finally the walls of the aneurysm are trimmed and sutured around the graft

§ Important to attach 1 or more pairs of lower intercostal arteries to prevent paraplegia

4) Dissections of Type I & II

§ Treatment of dissections and dissecting aneurysms consists of tube graft replacement to prevent progression of the dissecting process and to prevent fusiform dilatation and rupture of the outer wall of the false lumen

§ Procedure consists in transection of ascending aorta with use of CPB

§ Operative procedure is similar for ascending aorta aneurysm except the femoral artery is always the site for arterial cannulation and the side of the best pulsation is selected for cannulation without regard to type & location of dissection

§ If aortic arch involvement, operative procedure is similar for transverse arch aneurysm

§ The false lumen is obliterated by approximation of the inner & outer walls of the dissecting process with a continuous suture proximally & distally, and end-to-end anastomosis of the transected aorta

§ In patients in which this direct repair is not applicable, the proximal segment may need to be resected and replaced by a Dacron graft

§ Aortic valvuloplasty or replacement may be required

5) Dissections of Type III

§ Operative procedure is similar for descending aorta aneurysm

§ Resection of the descending aorta above the level of the origin of the dissection process [usually at or just below the origin of the left subclavian artery]

§ Obliteration of the distal false lumen by suture closure of the inner and outer layers

§ Replacement of the excised segment with an aortic graft

Strategies to protect the brain & spinal cord

Hypothermic Circulatory Arrest

• Hypothermia reduces metabolism and oxygen consumption however the metabolic rate never reaches zero; hypothermic cells still require some oxygen for metabolism
• Because of hypothermia, reduced tissue perfusion is tolerated by all organs with the exception of the brain
• “Safe” duration of circulatory arrest at brain temperatures of 15 — 18°C is possibly less than 45 minutes
• IQ drops 0.5 points per minute of circulatory arrest: some degree of permanent neurological injury probably occurs during HCA
• The degree of neurological damage appears to be related to the length of the arrest period and may be particularly severe after 60 minutes
• Hypothermia has a marked protective effect on the spinal cord during periods of aortic occlusion during operations on the thoracic aorta

Antegrade Cerebral Perfusion

• Used first for resection of aortic arch aneurysm
• Involves use of moderate hypothermia (26-28°C) or profound hypothermia (10-12°C: to “cerebroplege”) with direct cannulation and perfusion of innominate and left carotid artery or the innominate artery alone
• No evidence to indicate that cerebral perfusion provides protection to the brain that is superior to that of HCA alone

Retrograde Cerebral perfusion

• Used during operations on aortic arch
• Is given as a continuous or intermittent retrograde perfusion of the brain via the SVC in combination with HCA
• Unknown what percentage of the retrograde flow via the SVC actually perfuses the brain
• May play a role in deairing the aortic repair

BSL Monitoring

• Increased blood sugar levels are associated with CNS ischaemia or injury at normothermia
• Also, elevated BSL have been associated with increased incidence of CNS dysfunction following HCA
• Should regular monitor BSL and treat hyperglycaemia with insulin

Pharmacological adjuncts

• Mannitol: —?role in reducing cerebral & spinal cord oedema
• Steroids:—? role in reducing cerebral oedema

— given prior to HCA

• Barbiturates— Augments hypothermic effect on brain in reducing cerebral metabolism and oxygen consumption

— given prior to HCA

• Isoflurane— Similar effect to barbiturates

Subarachnoid spinal drainage?

? role in decreasing spinal cord pressure resulting in better perfusion pressures