ANAESTHESIA FOR THORACO-ABDOMINAL

AORTIC SURGERY

-Dr Raunaq Chakraborty

PGT Department of anaesthesiology and Critical care.

NEIGRIHMS Shillong

TOPICS

Etiology and classification

Morbidity and Mortality

Pre-operative preparation and monitoring

Anaesthetic management

ETIOLOGY

Atherosclerotic degenerative disease (80%)

Chronic Aortic Dissection(17%)

Trauma

Connective tissue diseases(Marfan

syndrome,Takayasu arteritis,Syphilitic aortitis)

Enlargement tends to be progressive and non-operative management is generally associated with a poor prognosis.

With progressive enlargement, nutritional blood flow to the aorta is compromised.

The increasing diameter is associated with increased wall tension, even when arterial pressure is constant (law of Laplace).

The high incidence of associated systemic hypertension enhances aneurysm enlargement

CLASSIFICATION

The Crawford classification.Type I aneurysms involve all or most of the descending thoracic aorta and the upper

abdominal aorta;type II aneurysms involve all or most of the descending thoracic aorta and all or most of

the abdominal aorta;type III aneurysms involve the lower portion of the descending thoracic aorta and most

of the abdominal aorta; andtype IV aneurysms involve all or most of the abdominal aorta, including the visceral

segment.

CRAWFORD CLASSIFICATION (1986)

Types II and III are the most difficult to repair because they involve both the thoracic and the abdominal segments of the aorta.

Crawford type II aneurysms are at greatest risk for paraplegia and renal failure from spinal cord and kidney ischemia during cross-clamping.

Even with extracorporeal circulatory support, an obligatory period occurs when blood flow to these organs is interrupted because the origin of the blood flow is between the cross-clamps.

For this reason, protective measures to prevent ischemic injury are important in reducing morbidity

DE-BAKEY CLASSIFICATION FOR AORTIC ANEURYSMS

The DeBakey classificationType I has an intimal tear in the ascending aorta with dissectionextending down the entire aorta. Type II has an intimal tear in the ascending aorta with dissection limited to the ascending aorta. Type III has an intimal tear in the proximal descending thoracic aorta withdissection either limited to the thoracic aorta (type IIIA)or extending distally to the abdominal aorta or aorto-iliac bifurcation (type IIIB).

DE-BAKEY CLASSIFICATION

Type 1 – Repaired with a two staged approach

Type 1 and type 2 both cause some degree of Aortic regurgitation and may involve the Ostia of coronary arteries

OTHER CLASSIFICATIONS

Stanford classification

1. Involving ascending aorta (type A)

2. Not involving ascending aorta(type b)

According to duration

1. Acute (< 2 weeks)

2. Chronic(>2 weeks)

Mortality implications as acute has significant more mortality

• Acute aortic dissection involving the ascending aorta (DeBakey types I and II, Stanford type A) is a surgical emergency - requires immediate cardiac surgical repair.

• Acute dissections involving the descending aorta (DeBakey type III, Stanford type B) are most often treated conservatively (i.e., arterial blood pressure, heart rate, and pain control) - surgical repair has no proved benefit over medical or interventional treatment in stable patients.

MORTALITY AND MORBIDITY

In a large state-wide series, the mortality with elective TAA repair was 19% at 30 days and 31% at 365 days.

The incidence of paraplegia or paraparesis in patients undergoing surgical repair of TAA is reported to be 3.8%to 40%.

Renal failure occurs in 3% to 30% of patients, Overall, approximately 6% of patients need postoperative dialysis after TAA repair, which is associated with high mortality (30% to 60%).

Gastrointestinal complications occur in approximately 7% of patients and are associated with a mortality approaching 40%.

Incidence of postoperative pulmonary insufficiency approaches 50%, with 8% to 14% of patients requiring tracheostomy.

As with all other vascular surgical procedures, cardiac complications are common and a leading cause of perioperative mortality.

PRE-OPERATIVE PREPARATION AND MONITORING

Discussion between Anaesthesiologist and surgeon

1. extent of the aneurysm and technique of surgical repair,

2. plans for distal aortic perfusion,

3. monitoring for spinal cord ischemia,

4. renal and spinal cord protection,

5. hemodynamic monitoring, and

6. ventilation strategy.

Things to be kept ready in OR

1. Large bore IV access( author routinely inserts three 8.5-Fr

catheters into the internal jugular and antecubital veins. One of theseaccepts a pulmonary artery catheter, and the other two are connectedto a rapid infuser system that allows the delivery of up to 1500

mL/minute of blood products at a temperature of 37° to 38° C).

2. Blood products ( Author recommends15 units of packed RBCs and

15 units of thawed fresh frozen plasma immediately available in the operating room, and additional units must be readily obtainable).

MONITORING

Arterial Catheterisation

1. Radial and femoral arterial pressure should be simultaneously displayed on the anaesthesiologist's monitor and a monitor visible to the surgeons and the perfusionists.

2. Placement of a right radial arterial catheter.

3. Distal aortic perfusion techniques – Femoral artery catheter

Trans oesophageal echocardiography

1. Properly trained individual

2. Left –end diastolic volume

3. Myocardial ischemia

4. Valvular function

Ventilation Strategy

1. One-lung ventilation provides optimal visualization and reduces retraction related trauma to the left lung.

2. Left-sided endo-bronchial tube is optimal because it is easily positioned and less likely to become mal-positioned.

3. Occasionally, a Right sided tube is necessary when the left main stem bronchus is compressed by a large aneurysm.

4. Double lumen tube changed to single lumen before shifting to ICU

Monitoring For spinal cord ischemia

1. Somatosensory evoked potentials (SSEPs)

1. Trans cranial Motor evoked potentials (MEPs)

Temperature Monitoring

1. Body temperature should be monitored at two sites(core and peripheral) to assess cooling and warming when bypass techniques are used.

2. With full bypass, perfusion is usually into the ascending aorta, and typically the upper body core temperature (i.e., nasopharynx or esophagus) cools and warms fastest, whereas the lower body temperature changes more slowly.

3. With partial bypass, the opposite is true. The blood from bypass is returned into the femoral artery, and the lower part of the body (i.e., rectum or bladder) changes before the upper part changes.

4. This lagging temperature should be the end point for cooling and warming.

SURGICAL TECHNIQUES

Simple aortic –cross clamping (without extra corporeal support)

1. Clamp-and-sew” technique has had relatively favorable outcomes,

2. But these cases are from institutions with extensive clinical experience and the shortest cross-clamp times.

3. Clamp times < 20 - 30 mins are associated with almost no paraplegia. When clamp times are between 30 -60 mins (the vulnerable interval), the incidence of paraplegia increases from approximately 10% to 90% as time progresses.

Left Heart Bypass

1. Heparin is not needed because the circuit is coated with heparin. The typical heparin dose for partial bypass is 100 units/kg.

2. With this technique, an oxygenator is unnecessary because only the left side of the heart is bypassed.

3. Insertion of a heat exchanger into the circuit allows cooling and warming, which is beneficial but not absolutely essential.

Deep –Hypothermic Circulatory arrest (DHCA)

Complex aneurysms involving the aortic arch often require elective cardiopulmonary bypass with an interval of deep hypothermic (15° C) circulatory arrest (DHCA) because cerebral blood flow is transiently interrupted during surgery.

DHCA also may be necessary whenever the location, extent, or severity of aortic disease precludes placement of a proximal aortic clamp during thoracic or thoraco-abdominal aortic repair.

Some centres also use anterograde (i.e.,innominate artery) or retrograde (i.e., internal jugular vein) selective cerebral perfusion with cold oxygenated blood to extend the safe maximum duration of circulatory arrest.

Without this technique, 45 to 60 minutes is thought to be the safe limit of DHCA, but 90 minutes has been reported with selective cerebral perfusion.

ANAESTHETIC TECHNIQUE

Balanced Anaesthesia combination of an opioid, a low-dose potent volatile anaesthetic, a benzodiazepine, and a muscle relaxant.

A total IV technique may be optimal if trans-cranial MEP monitoring is used.

Hypertension should be avoided because acute stress on the aneurysm can cause rupture.

The heart rate should be maintained at or below baseline

COMPLICATIONS

Spinal cord Ischemia and protection

CSF Drainage :

Spinal cord perfusion pressure is defined as distal mean aortic pressure minus CSF pressure or central venous pressure, whichever is greatest.

Auto-regulation of spinal cord blood is relatively constant over the range of 50 to 125 mm Hg.

During hypoxia or hypercapnia, auto-regulation is lost, and flow becomes linearly related to perfusion pressure.

Drainage of CSF is important because CSF pressure often increases (by 10 to 15 mm Hg) with cross-clamping of the descending thoracic aorta.

1. Clinical use of this technique is controversial.

2. Coselli and colleagues offered the strongest evidence supporting the efficacy of CSF drainage.

3. Potential complications include headache,meningitis, chronic CSF leakage, spinal or epidural hematoma, and subdural hematoma.

Temperature

1. Hypothermia is probably the most reliable method of neuro-protection from ischemic injury.

2. By reducing O2 requirements by approximately 5% for each degree centigrade, a twofold prolongation of tolerated cross-clamp time is achieved by cooling even to mild hypothermia (34° C).

3. Regional cooling of the spinal cord by cold perfusion of the GRA with blood or crystalloid provides significant protection during spinal ischemia in animal models. Regional cooling is beneficial in humans who received epidural infusions of 4° C saline.

Drugs

1. Barbiturates

2. Corticosteroids provide protection in dogs but were beneficial in humans only when they were combined with CSF drainage.

3. Dextrorphan (a non-competitive NMDA receptor antagonist) shows promise during spinal cord ischemia.

4. Magnesium, another NMDA receptor antagonist, improves recovery from spinal cord ischemia in rat and dog models when administered intrathecally.

5. Naloxone also shows promise when combined with CSF drainage in patients undergoing TAA repair.

6. Intrathecal papaverine appears to be protective, especially when combined with CSF drainage.

7. Other agents under investigation include levosimendan, allopurinol, adenosine, ziconotide, activated protein C, and desferrioxamine.

Preoperative spinal cord angiography has been used in patients with TAA.

Rationale for this highly invasive angiographic procedure is that precise identification of inter-costal arteries giving rise to the GRA will allow focused re-implantation of these vessels during surgical repair and help prevent spinal cord injury.

Renal Ischemia and protection

1. Retrograde distal aortic perfusion techniques

2. Adequate bypass flow

3. Systemic and regional hypothermia,

4. Mannitol 12.5 to 25 g/70 kg is often given before cross-clamping.

5. Loop diuretics are sometimes given, but these drugs have been less effective than mannitol in experimental models.

6. Dopamine given in low doses (1 to 3 μg/kg/minute) dilates renal blood vessels.However beneficial effects are not clear.

7. Fenoldopam mesylate, a selective dopamine type 1 agonist that preferentially dilates the renal and splanchnic vascular beds, has shown some promise as a reno-protective drug.

At the present time, optimal renal protection during TAA surgery should rely on hypothermia, mannitol, and prevention of hypotension and hypoperfusion of the kidneys.

Coagulation and Metabolic Management

Causes:-

1. Dilutional coagulopathy

2. Residual heparin

3. Ischemia of the liver

4. Persistent hypothermia

Management :-

1. Cryoprecipitate –better than FFP (Hypervolemia)

2. Antifibrinolytic therapy like ε-aminocaproic acid

3. Desmopressin can be given to increase circulating levels of von Willebrandfactor and factor VIII.

4. Normothermia should be achieved by by increasing ambient temperature after separation from bypass, and by forced air warming over the upper body skin surface.

5. Sodium bicarbonate should be given to treat the metabolic acidosis and other electrolyte drangements to be corrected.