Keywords
aortic root, cannulation, cardiopulmonary bypass, dissection, perfusion
Complications of Aortic Root Cannulation: Acute Aortic Root Dissection
Case Synopsis
A 75-year-old man with a history of calcific aortic stenosis was scheduled for valve replacement. Induction of anesthesia, sternotomy, and insertion of perfusion cannulas were uneventful. However, the aortic tissue appeared thin and calcified. The aortic purse strings and cannula appeared well placed, but when cardiopulmonary bypass (CPB) was initiated, the pump arterial line pressure increased, and systemic blood pressure (radial artery) decreased. The aorta appeared acutely dilated and bluish.
Problem Analysis
Definition
Acute aortic dissection is a very serious complication of aortic cannulation for bypass. Although rare, it can occur at any time, even in experienced hands. It is fatal unless a diagnosis is made early; thus it requires a high degree of vigilance. Dissection can also occur during CPB or after decannulation.
For dissection to occur, blood under pressure must gain access to the media of the aortic wall. In this case the cannula orifice was unintentionally placed within the media of the arterial wall rather than the true lumen during cannulation. When CPB was begun, the resulting increase in pressure created a dissection.
Additional manipulation of the ascending aorta (e.g., placement of the aortic cross-clamp, antegrade cardioplegia line, or proximal bypass grafts) may increase the risk of dissection. Predisposing factors that also increase the risk of acute aortic dissection include conditions that weaken the aortic wall, such as the following:
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Cystic medial necrosis or other genetically related tissue weakness
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Elastic or medial degeneration associated with aging
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Thin or friable aorta tissue, as with poststenotic dilation (aortic stenosis)
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Atheromatous disease
Dissection can also occur spontaneously in the operating room or during the postoperative period in the intensive care unit.
Recognition
In cases of intraoperative dissection, a sudden, unexplained decrease in mean arterial pressure and venous return is usually seen, along with an acute elevation in arterial line pressure as measured at the pump and bluish discoloration and enlargement of the aortic root. Myocardial ischemia, aortic insufficiency, or both may develop, and signs of organ hypoperfusion (including oliguria and pupil asymmetry) may be present if the dissection extends to other major arterial vessels. If transesophageal echocardiography is used, dissection may be evident on examination of the thoracic aorta ( Fig. 81.1 ).
Management
Once dissection is recognized, CPB must be discontinued immediately. The surgeon must then either reposition or replace the arterial cannula so that it is inside the true lumen at a more distal site on the aortic arch or (more often) switch to femoral artery cannulation. Surgical repair of the aortic dissection is almost always necessary; such repair should include coronary artery reimplantation if the patency of the coronary arteries is compromised.
Prevention
Measures that may be effective in reducing the risk of aortic dissection during cannulation include the following:
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Blood pressure control (to avoid hypertension) at the time of cannulation
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Inserting the cannula at a right angle to the aorta to prevent dissection of tissue planes
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Special care in positioning the tip in the true lumen of the aorta
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Blood pressure reduction when the aortic cross-clamp is applied or removed
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Use of atraumatic clamps, with as few applications to the aorta as possible
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Continuous monitoring of arterial cannula pressure by the perfusionist
Innominate or Carotid Artery Hyperperfusion
Case Synopsis
A 58-year-old woman underwent CPB for coronary artery bypass grafting. After successful aortic and venous cannulation by the surgeon, the anesthesiologist noted right-sided facial blanching. Further examination showed the presence of a right carotid thrill.
Problem Analysis
Definition
Pump flow can be directed primarily into the carotid or innominate artery instead of the aorta. This can result in cerebral edema or arterial rupture due to high perfusion pressure or the creation of an intimal flap that obstructs arterial flow.
Recognition
Signs of innominate artery cannulation include ipsilateral facial blanching, pupillary dilation, and conjunctival chemosis. Hypotension may be detected with a left radial or femoral artery catheter, but a right radial artery catheter may show hypertension.
Prevention
Using a short aortic cannula with a flange to prevent insertion too far into the aorta is usually effective. The anesthesiologist can check for bilateral carotid pulses without thrills after cannulation and initiation of CPB, but this may not reliably detect problems caused by carotid or innominate artery hyperperfusion. With certain types of cannulas, transesophageal echocardiography may be useful in determining position.
Obstruction of Venous Return to the Pump
Case Synopsis
In a 60-year-old man, CPB was initiated after uneventful aortic cannulation and insertion of a single venous cannula into the right atrium. There was an immediate decrease in pump-oxygenator venous reservoir volume and thus the pump’s ability to maintain a normal flow rate. Adding volume to the circuit did not resolve the issue. Obvious venous engorgement in the patient’s face and neck was noted on examination.
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