Indications for Surgery
Thoracic aortic aneurysms are classified by the portion of aorta involved - ascending, arch, descending thoracic and thoraco-abdominal aorta. 60 per cent of them occur in the ascending aorta and are mostly caused by cystic medial necrosis. Very often aortic root is also involved in this fusiform type of aneuiysm causing aortic regurgitation. Cystic medial necrosis in youilg patients is a manifestation of Marfans syndrome. Instead of classical Marfans syndl-ome a variant or forme frustae of the syndrome can be the aetiological factor. The other common causes are athersclerosis, syphilis or infectious aortitis.
At least 50 per cent of the patients with early thoracic aortic aneurysms are asymptomatic and are picked up during investigations. The sympton~s may be due to aortic regurgitation and heart failure, thrombo embolism causing stroke or lower extremity ischaemia and renal or mesentesic infarction. Dilatalion of the aorta may cause compression of adjacent anatomical sbuctures and cause varied symptoms. These include pain in the neck and jaw, paill in the interscapular area,pleuritic pain, back pain or left shoulder pain. Pressure effects may cause superior venacaval obstruction, hoarseness, dysphagia, dyspnoea, stridor due to compression of trachea, cough and wheezing and collapse of left lung. The most dangerous complication is either leaking or frank rupture causing cardiac tamponade, haemothorax, haeinoptysis or haematemesis. Chest X-ray may reveal the presence of aneurysm. The previous gold standard was aortography, which is now effectively replaced by CT and MRI. Three dimensional imaging and reconstruction give the exact measurements ~lceded for treatment. Transthoracic echocardiogram is useful in measuring the size of proximal aorta but is o l limited accuracy in the rest of thoracic aorta. TEE gives much better information.
Types of Surgery
Technique of surgery and the approach depends on the site of thoracic or thoraco abdominal aortic aneuiysm. Initial preparations are same for all the cases. For monitoring arterial pressure left radian artery is cannulated for ascending aortic and arch aneurysm. For descending thoracic or thoraco abdominal aneurysm, the right radial artery is cannulated. If a left thoracotomy is needed, it is better to use a double lumen endobronchial tube so that single lung anaesthesia can be given.
Insertion of Swan-Ganz pulmonary arterial catheter helps in monitoring cardiac output and pulmonary artery pressure. If circulatory arrest is planned, thermister probes for monitoring nasopharyngeal and rectal temperatures are placed. TEE is another useful monitor for any such case. An aneurysm that has eroded sternum poses problem at the time of opening by a median sternotomy. Then cardio pulmonary bypass can be'instituted by cannulation of femoral artery and vein.Hypothermia and low flow or even circulatory arrest is employed for sternotoiny.
Ascending Aortic Aneurysm
In aneurysms confined to ascending aorta well below the innominate artery origin, the technique used is resection and replacement with a Dacron tube graft.At times the aortic root is also dilated and there is significant aortic regurgitation. Then the technique used is a Bental procedure where the ascending aorta, root and valve we replaced with a composite graft of prosthetic aortic valve'sutured into a suitably sized albumin impregnated dacron graft. First, the conlposite valved graft is sutured to the aortic annulus. Two, coronary arterial buttons are raised and anastomosed to the tube graft. Finally, the distal anastomosis is done below the innominate artery. Deairing is done before releasing aortic clamp and allowing the heart to beat.
Associated coronary artery disease is treated by bypass grafting. Distal nastomoses are done before aneurysm is grafted. Proximal anastomoses are done to the tube graft while rewarming.
Aortic Arch Aneurysm
Hypothermic circulatory arrest and retrograde cerebral perfusion is the most commonly used technique for arch aneurysm. Femoral arteiy cannulation is used for arterial return. Separate SVC and IVC cannulation is done for venous reluin.While cooling on cardio pulmonary bypass, a vent'catheter is introduced through the right superior p u l m o n q vein. Snares are passed arouild superior and inferior vena cavae. After clamping the aorta, antegrade cardioplegia is given through the aortic root. While further cooling is being done, methyl prednisolone and thiopental are administered to enhance the neuroprotective effect of deep hypothermia. Mannitol and frusemide are infused for renal protection. Head is packed with ice bags and continuous EEG monitoiing is done. When nasopharyngeal temperature reaches 12-14°C and rectal temperature is 15°C to 18°C and EEG becomes isoelectric circulatoly arrest can be performed. Thepump is stopped and aortic clamp is removed. Retrograde cerebral perfusion through 'the superior vena caval cannula, is done from the pump at a lower pressure at a rate of 300 to 500 ml/mt,without raising jugular venous pressure above 30 to 35 mm of Hg. The arch aneurysm is opened and as retrograde cerebral perfusion is being done de-oxygenated blood will come out of the carotid arteries. This helps in reducing chances of air embolism to the cerebral arteries. Distal anastomosis of the descending thoracic aorta to an alburnin- impregnated graft is done with continuous sutures. The anastomosis is usually reinforced with a narrow strip of PTFE or Dacron graft. The origins of all the three arch vessels cut out together with margins of aorta are then anastomosed to a suitably sized oval opening made on top of the tube graft. Then slowly arterial pump is re-started and blood is allowed to fill the descending aolta and arch. All air is expelled and the proximal end of the tube graft is clamped and perfusion through brachio - cephalic arteiies are re-established. Retrograde cerebral perfusion is discontinued and noirnal cardio pulmonary perfusion and re-warming started. Proximal anastomosis with the ascending aorta is then completed and complete de-airing of heart and ascending aoita done before heart is allowed to eject and establish effective circulation.
Elephant Trunk Technique
When aneurysm involves arch of the aorta and large portion of descending thoracic aorta, operation has to be done in two stages. This operation is known as elephant trunk technique. The operation is done through median stelnotomy as described in the previous section on arch aneuiysm. The only difference is that a large portion of the graft is left loose in the descending aorta as rill elephant trunk. As a second stage left thoracotomy is done and the end of the graft, which is left as elephant trunk, is anastomosed to descending thorncic aorta below the aneurysm.
Descending Thoracic Aortic and Tltoraco-Abdoininal Arterciysnt
The approach is through n left postcro-lateral thoracotomy. After positioning the patient in the right lateral position with left leg extended and the groin exposed,cardiopulmonasy bypass is instituted by cannulalion of I'emoral artery and vein of the left leg. A well-lubricated venous cannula is inserted and placcd in the middle of right atleiurn. TEE can help in positioning the cannula in the right atrium. Dissection is done above and below the aneurysm 'and tapes passed for clamping the aorta. Femoro-femoral bypass is iilstituted to protect the spinal cord and abdominal viscera. Aorta is clamped above and below and aneurysnl is opened and clots removed. Small intercostal vessels are occluded with sutures.
Other Techniques
A descending thoracic aortic aneurysm can be done by the technique of leIt atrio femoral bypass without an oxygenator or a left vent]-iculo fen~oral bypass without either oxygenator or a pump.A thoraco abdominal aneurysm can also be done on deep hypotherrnia and
circulatory arrest for protection of spinal cord and abdominal organs.
Results
For ascending aorta replacement, the hospital mortality varies from 0 to 9 per cent. Aortic arch has higher risk of 6 to 20 per cent. For descending thoracic aneurysm it varies between 5 and 10 per cent. In case of thoraco abdominal aneurysm it may be as high as 15 per cent.
Thoracic aortic aneurysms are classified by the portion of aorta involved - ascending, arch, descending thoracic and thoraco-abdominal aorta. 60 per cent of them occur in the ascending aorta and are mostly caused by cystic medial necrosis. Very often aortic root is also involved in this fusiform type of aneuiysm causing aortic regurgitation. Cystic medial necrosis in youilg patients is a manifestation of Marfans syndrome. Instead of classical Marfans syndl-ome a variant or forme frustae of the syndrome can be the aetiological factor. The other common causes are athersclerosis, syphilis or infectious aortitis.
At least 50 per cent of the patients with early thoracic aortic aneurysms are asymptomatic and are picked up during investigations. The sympton~s may be due to aortic regurgitation and heart failure, thrombo embolism causing stroke or lower extremity ischaemia and renal or mesentesic infarction. Dilatalion of the aorta may cause compression of adjacent anatomical sbuctures and cause varied symptoms. These include pain in the neck and jaw, paill in the interscapular area,pleuritic pain, back pain or left shoulder pain. Pressure effects may cause superior venacaval obstruction, hoarseness, dysphagia, dyspnoea, stridor due to compression of trachea, cough and wheezing and collapse of left lung. The most dangerous complication is either leaking or frank rupture causing cardiac tamponade, haemothorax, haeinoptysis or haematemesis. Chest X-ray may reveal the presence of aneurysm. The previous gold standard was aortography, which is now effectively replaced by CT and MRI. Three dimensional imaging and reconstruction give the exact measurements ~lceded for treatment. Transthoracic echocardiogram is useful in measuring the size of proximal aorta but is o l limited accuracy in the rest of thoracic aorta. TEE gives much better information.
Types of Surgery
Technique of surgery and the approach depends on the site of thoracic or thoraco abdominal aortic aneuiysm. Initial preparations are same for all the cases. For monitoring arterial pressure left radian artery is cannulated for ascending aortic and arch aneurysm. For descending thoracic or thoraco abdominal aneurysm, the right radial artery is cannulated. If a left thoracotomy is needed, it is better to use a double lumen endobronchial tube so that single lung anaesthesia can be given.
Insertion of Swan-Ganz pulmonary arterial catheter helps in monitoring cardiac output and pulmonary artery pressure. If circulatory arrest is planned, thermister probes for monitoring nasopharyngeal and rectal temperatures are placed. TEE is another useful monitor for any such case. An aneurysm that has eroded sternum poses problem at the time of opening by a median sternotomy. Then cardio pulmonary bypass can be'instituted by cannulation of femoral artery and vein.Hypothermia and low flow or even circulatory arrest is employed for sternotoiny.
Ascending Aortic Aneurysm
In aneurysms confined to ascending aorta well below the innominate artery origin, the technique used is resection and replacement with a Dacron tube graft.At times the aortic root is also dilated and there is significant aortic regurgitation. Then the technique used is a Bental procedure where the ascending aorta, root and valve we replaced with a composite graft of prosthetic aortic valve'sutured into a suitably sized albumin impregnated dacron graft. First, the conlposite valved graft is sutured to the aortic annulus. Two, coronary arterial buttons are raised and anastomosed to the tube graft. Finally, the distal anastomosis is done below the innominate artery. Deairing is done before releasing aortic clamp and allowing the heart to beat.
Associated coronary artery disease is treated by bypass grafting. Distal nastomoses are done before aneurysm is grafted. Proximal anastomoses are done to the tube graft while rewarming.
Aortic Arch Aneurysm
Hypothermic circulatory arrest and retrograde cerebral perfusion is the most commonly used technique for arch aneurysm. Femoral arteiy cannulation is used for arterial return. Separate SVC and IVC cannulation is done for venous reluin.While cooling on cardio pulmonary bypass, a vent'catheter is introduced through the right superior p u l m o n q vein. Snares are passed arouild superior and inferior vena cavae. After clamping the aorta, antegrade cardioplegia is given through the aortic root. While further cooling is being done, methyl prednisolone and thiopental are administered to enhance the neuroprotective effect of deep hypothermia. Mannitol and frusemide are infused for renal protection. Head is packed with ice bags and continuous EEG monitoiing is done. When nasopharyngeal temperature reaches 12-14°C and rectal temperature is 15°C to 18°C and EEG becomes isoelectric circulatoly arrest can be performed. Thepump is stopped and aortic clamp is removed. Retrograde cerebral perfusion through 'the superior vena caval cannula, is done from the pump at a lower pressure at a rate of 300 to 500 ml/mt,without raising jugular venous pressure above 30 to 35 mm of Hg. The arch aneurysm is opened and as retrograde cerebral perfusion is being done de-oxygenated blood will come out of the carotid arteries. This helps in reducing chances of air embolism to the cerebral arteries. Distal anastomosis of the descending thoracic aorta to an alburnin- impregnated graft is done with continuous sutures. The anastomosis is usually reinforced with a narrow strip of PTFE or Dacron graft. The origins of all the three arch vessels cut out together with margins of aorta are then anastomosed to a suitably sized oval opening made on top of the tube graft. Then slowly arterial pump is re-started and blood is allowed to fill the descending aolta and arch. All air is expelled and the proximal end of the tube graft is clamped and perfusion through brachio - cephalic arteiies are re-established. Retrograde cerebral perfusion is discontinued and noirnal cardio pulmonary perfusion and re-warming started. Proximal anastomosis with the ascending aorta is then completed and complete de-airing of heart and ascending aoita done before heart is allowed to eject and establish effective circulation.
Elephant Trunk Technique
When aneurysm involves arch of the aorta and large portion of descending thoracic aorta, operation has to be done in two stages. This operation is known as elephant trunk technique. The operation is done through median stelnotomy as described in the previous section on arch aneuiysm. The only difference is that a large portion of the graft is left loose in the descending aorta as rill elephant trunk. As a second stage left thoracotomy is done and the end of the graft, which is left as elephant trunk, is anastomosed to descending thorncic aorta below the aneurysm.
Descending Thoracic Aortic and Tltoraco-Abdoininal Arterciysnt
The approach is through n left postcro-lateral thoracotomy. After positioning the patient in the right lateral position with left leg extended and the groin exposed,cardiopulmonasy bypass is instituted by cannulalion of I'emoral artery and vein of the left leg. A well-lubricated venous cannula is inserted and placcd in the middle of right atleiurn. TEE can help in positioning the cannula in the right atrium. Dissection is done above and below the aneurysm 'and tapes passed for clamping the aorta. Femoro-femoral bypass is iilstituted to protect the spinal cord and abdominal viscera. Aorta is clamped above and below and aneurysnl is opened and clots removed. Small intercostal vessels are occluded with sutures.
Other Techniques
A descending thoracic aortic aneurysm can be done by the technique of leIt atrio femoral bypass without an oxygenator or a left vent]-iculo fen~oral bypass without either oxygenator or a pump.A thoraco abdominal aneurysm can also be done on deep hypotherrnia and
circulatory arrest for protection of spinal cord and abdominal organs.
Results
For ascending aorta replacement, the hospital mortality varies from 0 to 9 per cent. Aortic arch has higher risk of 6 to 20 per cent. For descending thoracic aneurysm it varies between 5 and 10 per cent. In case of thoraco abdominal aneurysm it may be as high as 15 per cent.
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