A Complication with the Use of a Centrifugal Pump During Thoraco-Abdominal Aortic Surgery

Operative procedures of the thoracic aorta are associated with significant morbidity and mortality because of interruption of perfusion to the kidneys, spinal cord, and splanchnic circulation. Vascular bypass is frequently used when the thoracic aorta is cross-clamped for a significant period of time. The aim of vascular bypass is to prevent distal hypoperfusion during aortic cross-clamping, therefore limiting paraplegia, renal failure, and multi-organ dysfunction caused by ischemia and reperfusion injury (1,2). Historically, this approach included temporary internal shunts or a temporary external conduit. Roller pumps were initially used for distal aortic perfusion but became unpopular because of the requirement for systemic heparinization (3). Centrifugal pumps provide left atrio-femoral bypass with limited heparinization and have proven more efficient in maintaining renal function in comparison with the Gott’s shunt tube and temporary artificial graft placement (4). In this case report, we describe a novel complication that occurred with the use of a centrifugal pump during thoraco-abdominal aortic aneurysm repair. Case Report A 64-yr-old man presented for elective repair of a chronic dissected thoraco-abdominal aneurysm (Stanford type B, De Bakey type IIIb). Previous surgery included an aorto-bifemoral bypass for an infrarenal abdominal aortic dissection. At the time of surgery, it was noted that a small thoracic aneurysm was present, although not involved with the abdominal aortic dissection. One year later, the aneurysm had dissected and increased in size to 8 cm in diameter, originating just beyond the left subclavian artery and continuing to the aorto-bifemoral graft. The patient had a history of poorly controlled hypertension and ischemic heart disease, which had been treated by angioplasty and stenting of the right coronary artery 2 yr before admission. Preoperative medications included labetalol, indapamide, ranitidine, aspirin, and ramipril. His exercise tolerance was moderately limited by claudication. Preoperative electrocardiogram was normal, pulmonary function tests indicated 70%–80% of predicted volumes, and a resting gated cardiac radionucleotide study showed a normal left ventricular ejection fraction. Blood tests, including creatinine, hemoglobin, platelets, coagulation profile, and electrolytes, were within normal physiological range. The proposed procedure was resection and grafting of the aorta from the left subclavian artery to the recently placed aorto-bifemoral graft just below the renal arteries. Because of the size and proximal origin of the aneurysm, the decision was made to use left heart bypass with possible conversion to hypothermic circulatory arrest if proximal control of the aneurysm was difficult. Invasive monitoring included right radial and right pedal artery cannulation (to provide upper and lower limb perfusion pressures) as well as a pulmonary artery catheter. Anesthesia was induced with propofol, midazolam, fentanyl, and cisatracurium and maintained with intermittent boluses of fentanyl, cisatracurium, and inhaled isoflurane, oxygen, and air. A size 39F left double-lumen tube was used to enable one-lung ventilation. A lumbar cerebrospinal fluid (CSF) drain was inserted at the L2-3 vertebral level after the induction of anesthesia, and 25 mL of CSF was removed to achieve a CSF pressure of 10 cm H2O. The patient was turned to the right lateral decubitus position, one-lung ventilation was instituted without event, and an oblique transverse incision was made below the tip of the scapula continuing along the line of the fifth rib on to the left upper quadrant of the abdomen. The aneurysmal segment was identified, distal exposure and control was attained, and proximal control of the aorta was established just above the left subclavian origin. The patient received 10,000 U of heparin IV, a femoral return cannula was placed through the original aorto-bifemoral graft, and the left atrial appendage was opened, and an angled atrial cannula was introduced and secured. After appropriate de-airing of cannulae, Bio-Medicus left atrial femoral bypass was initiated with good flow rates (2–3 L/min) and adequate distal pressures. A proximal cross-clamp was positioned across the aorta between the origins of the left subclavian and left carotid arteries, and the distal cross-clamp was applied 10 cm distally. The section between the two clamps remained firm, and a small aortotomy incision bled briskly. The clamps were removed and reapplied because there was concern regarding their security. During this procedure, a small laceration was made in the pulmonary vein and required oversewing to achieve hemostasis. Salvaged blood as well as 2 U of homologous blood and 1 L of pentastarch were transfused to maintain euvolemia. Contemporaneously, pulmonary artery pressure increased, and cardiac output and proximal arterial pressure decreased. It was also noted that the blood running through the centrifugal pump was extremely dark and seemed deoxygenated. The collapsed lung was re-inflated, and the patient was ventilated using a fraction of inspired oxygen of 1.0. Oxygenation did not improve, and proximal arterial pressures remained low. A trans-esophageal echocardiographic examination showed a previously undiagnosed patent foramen ovale (PFO) with a right to left shunt (visualized with color Doppler examination) and the presence of air in the left atrium and ventricle. The centrifugal pump flow was reduced to increase left atrial pressures, and cardiopulmonary resuscitation was commenced. Normal cardiac function could not be restored despite open cardiac massage, multiple doses of epinephrine, and volume loading. The patient remained profoundly hypoxic, and the cardiac rhythm progressed from electromechanical dissociation to asystole. Discussion The foramen ovale is a fenestration in the septum secundum, which allows blood flow from the right to left atrium in the neonatal period. After birth, the pulmonary vascular resistance decreases, pressure in the right atrium decreases, and pressure in the left atrium increases causing functional closure of the foramen ovale, with permanent closure typically after the first year of life. A PFO is a common finding in 25% of postmortem examinations and can vary in size from 1–9 mm (5). The presence of a PFO has been associated with various disease manifestations such as migraine headache with aura, paradoxical embolism with transient ischemic events, or strokes and persistent hypoxemia (6). In some cases, the patient changing position from supine to upright will induce the shunt. These patients may complain of platypnea-orthodeoxia syndrome, dyspnea, and arterial desaturation in the upright position, relieved by the recumbent position (7,8). Perioperative conditions that change the balance between right and left atrial pressures or cause altered right atrial anatomic relationships with a change in direction of inferior vena cava flow can potentiate a right to left shunt through a PFO. Anatomical relationships are distorted after pneumonectomy or lobectomy, and shunting through a PFO can be triggered in the absence of a pressure gradient between the right and left atria (9,10). During cardiac surgery, especially off-pump surgery, manipulation of the heart can cause the same effect (5). Postoperatively, loculated pericardial effusions have caused right to left shunts through a PFO (11). Size is not the only determinant of clinical significance when evaluating a PFO. The pressure gradient between right and left atrium as well as direction of inferior vena cava flow are important factors when assessing whether deoxygenated blood, air, or emboli are likely to enter the systemic circulation (5). The reference standard for detecting a PFO is trans-esophageal echocardiographic examination with the use of contrast, usually agitated saline (9). The normal inter-atrial pressure gradient (left atrial (LA) pressure higher than right atrial (RA) pressure) has to be reversed by coughing or Valsalva maneuver to allow flow from RA to LA. The presence of an atrial septal aneurysm combined with a PFO increases the risk for stroke significantly (12), as does a PFO larger than 4 mm in diameter (13). Asymptomatic PFOs do not require intervention, but if hypoxemia develops or if the patient experiences paradoxical embolization, treatment is indicated. Long-term management includes anticoagulation if paradoxical emboli are the main concern, but there are insufficient data concerning the efficacy of antiplatelet therapy as opposed to systemic anticoagulation (6). The presence of PFO combined with an atrial septal defect may warrant preventative strategies other than aspirin (12). Persistent hypoxia requires surgical closure either through a percutaneous procedure or open-heart surgery. Immediate therapy is aimed at restoring RA to LA pressure gradient with inotropes and pulmonary vasodilators (5). The most likely cause of the unfortunate outcome in our patient was an undiagnosed PFO that opened during surgery. The combination of a low volume state and high flow through the centrifugal pump could cause the entrainment of air from the damaged pulmonary vein, causing an increase in pulmonary artery pressures and a relative increase in RA versus LA pressure with the resultant shunt through the PFO. Volume loading of the right atrium as well as increased pulmonary vascular resistance because of one-lung ventilation could increase RA pressure in excess of LA pressure resulting in the opening of a PFO. The double insult of air embolism and persistent hypoxemia from a right to left shunt proved fatal in this case. There are case reports describing flow reversal through PFO with a left ventricular assist device in place (14,15), but this complication with a left atrio-femoral bypass centrifugal pump is new, and its importance is highlighted by the associated mortality. We suggest that when a left atrio-femoral centrifugal pump is used, the presence of a PFO should be aggressively investigated. If a PFO is present, the centrifugal pump should be used with caution, maintaining LA pressures in excess of RA pressures and with constant monitoring of shunt flow direction.

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