Atrial Septal Aneurysm with Right-to-Left Interatrial Shunting

Abstract

Interatrial shunting in the presence of an atrial septal aneurysm is an uncommon but well recognized abnormality. Previous case reports have demonstrated that elevated right atrial pressure secondary to pulmonary embolism or right ventricular infarction may cause right-to-left interatrial shunting in the presence of an atrial septal aneurysm. We describe a unique situation in which an atrial septal aneurysm was associated with a right-to-left shunt secondary to severe systemic hypotension and normal right atrial pressure. In this patient, we used midodrine, an oral alpha-1 agonist, to increase systemic arterial pressure, decrease the severity of the shunt, and treat the severe hypoxemia. This case establishes that right-to-left interatrial shunting can result from a decrease in left ventricular afterload with normal right atrial pressure. Oral alpha-1 agonist therapy can be used successfully to treat patients such as ours and possibly others with similar functional abnormalities. (Tex Heart Inst J 2003;30:68–70)

Interatrial shunting in the presence of an atrial septal aneurysm (ASA) is an uncommon but recognized abnormality: its clinical significance has yet to be fully determined. ¹ Case reports in the literature have shown that elevated right atrial pressure secondary to pulmonary embolism or right ventricular myocardial infarction can cause right-to-left interatrial shunting in the presence of an ASA. We report a unique situation in which ASA was associated with a right-to-left shunt secondary to severe systemic hypotension.

Case Report

In March 2000, a 68-year-old man with non-Hodgkin's lymphoma presented at the emergency room of our institution with a history of progressive left leg weakness. He had been diagnosed with lymphoma the previous year, and had received 4 cycles of CHOP chemotherapy (cyclophosphamide, doxorubicin, vincristine, and prednisone); the last cycle had been given 2 months earlier. Since his last course of chemotherapy, he had been feeling generally unwell, with increasing dyspnea on minimal exertion, palpitations, and a 10-pound weight loss. He had also been having weakness in his left leg, which had progressed to the point where he was unable to stand. The patient had no history of trauma and no associated neurologic symptoms. His wife had been measuring his blood pressure and temperature at home for the past several months and reported “extremely low” blood pressures that week. Physical examination revealed a blood pressure of 90/54 mmHg with the patient in the supine position, decreased hearing bilaterally, and decreased strength in his left lower limb. Oral medications included prednisone, 25 mg per day, and allopurinol, 300 mg per day. The patient was admitted to the hospital for further examination.

Within 48 hours of admission, the patient's condition began to deteriorate. He became severely hypotensive and hypoxemic, with increasing respiratory distress. He was admitted to the intensive care unit with a blood pressure of 80/40 mmHg. Arterial blood gases on a 10-L non-rebreathe mask were pH 7.43; PaO₂, 66 mmHg; PaCO₂, 25 mmHg; plasma bicarbonate concentration (HCO₃), 16 mEq/L. A Swan-Ganz catheter was positioned and revealed a pulmonary artery pressure of 24/10 mmHg, a pulmonary capillary wedge pressure of 6 mmHg, and a right atrial pressure of 8 mmHg. The cardiac output could not be measured, because the patient developed arrhythmias and the Swan-Ganz catheter needed to be removed. A chest radiograph showed a small right-sided pleural effusion, and thoracentesis revealed a transudate with no malignant cells. There was no sepsis. A spiral computed tomographic (CT) scan of the lungs showed no thromboembolic disease. We noted that the patient's hypoxemia was related to his hypotension: higher blood pressures resulted in better arterial oxygen saturation. At this time, we thought that the patient's symptoms were likely secondary to an intracardiac shunt. Contrast transthoracic echocardiography revealed an atrial septal aneurysm associated with a right-to-left shunt through multiple perforations in the aneurysm. In addition to vasopressor therapy with intravenous norepinephrine, we administered midodrine, an oral alpha-1 agonist, in order to maintain adequate systemic pressure and to increase left ventricular afterload. The patient was soon discharged to the general medicine ward. His blood pressures remained in the range of 90–100 mmHg systolic and 60–70 mmHg diastolic. A follow-up echocardiogram 2 weeks later showed that the shunt was still present, although greatly diminished. The patient improved while in the hospital but died 2 months later due to central nervous system infiltration by lymphoma.

Discussion

An atrial septal aneurysm (ASA) is a thin, localized segment of the atrial septum that bulges into the right or left atrium. It is often fenestrated, and it can be found as an isolated lesion or in association with other congenital heart anomalies such as patent foramen ovale, atrial septal defect, or mitral valve prolapse. ^2–5 Such aneurysms are readily detected by 2-dimensional and transesophageal echocardiography. They are mobile and can be seen moving between the atria during the cardiac cycle. ³ Most ASAs are clinically silent. In fact, they are an incidental finding in 1% of the general population. ⁶ Although an ASA is considered a clinically benign entity, potential clinical consequences, such as cardiogenic embolism, can occur in conjunction with an ASA. ³ Embolism can result from thrombus formation within the aneurysm or occur as a paradoxical embolism. ⁷ Another complication is a right-to-left interatrial shunt, which occurs in 90% of ASAs. ³ Previously published case reports have shown that elevated right atrial pressure from either right ventricular dysfunction or elevated pulmonary arterial pressures sometimes leads to right-to-left shunting, which in turn results in arterial desaturation. ⁷

Previous studies have reported that in certain situations, an ASA can lead to the development of a hemodynamically significant right-to-left shunt. One group ² described a patient with an acute right ventricular myocardial infarction in whom an ASA was associated with an interatrial right-to-left shunt, causing hypoxemia. ² Others ⁸ have reported 3 cases in which patients experienced severe hypoxemia and pulmonary embolism, complicated by a right-to-left shunt associated with an ASA and patent foramen ovale. ⁸ In addition, different groups ^9–11 have described patients who had undergone a right pneumonectomy; they subsequently developed dyspnea and hypoxemia secondary to a right-to-left shunt through a previously asymptomatic ASA or patent foramen ovale. Surgical repair of the defects led to resolution of symptoms. ^9–11 The pathophysiologic mechanism described in these patients is that right ventricular myocardial infarction, pulmonary embolism, and pneumonectomy all elevate right ventricular pressure, leading to a right-to-left shunt through the ASA and the associated interatrial defect. ⁸

Our case demonstrates a unique situation in which vasogenic hypotension secondary to autonomic neuropathy (a well recognized side effect of vincristine chemotherapy) caused a right-to-left shunt through a fenestrated ASA, which in turn led to hypoxemia and respiratory distress. There are 3 mechanisms of interatrial shunt: 1) an increase in pulmonary vascular resistance (pulmonary embolism) increases right atrial pressure relative to left atrial pressure, thereby leading to a right-to-left shunt; 2) an increase in systemic vascular resistance increases left atrial pressure relative to right atrial pressure, resulting in a left-to-right shunt; and 3) a decrease in systemic vascular resistance (vasogenic shock) decreases left atrial pressure relative to right atrial pressure, resulting in a right-to-left shunt.

In our patient, severe hypotension and vasogenic shock resulted in a decrease in left ventricular afterload and an increase in left ventricular compliance. Therefore, the left atrial pressure decreased relative to the right atrial pressure, resulting in a right-to-left shunt through the fenestrated ASA. Furthermore, a strong direct correlation between the patient's hypoxemia and lowering of the blood pressure was exhibited: a lower blood pressure was associated with lower oxygen saturation and vice versa. With the use of intravenous norepinephrine, initially, and then midodrine ¹² (an alpha-1 agonist agent often used to treat severe orthostatic hypotension ¹³), we were able to increase the left ventricular afterload and decrease the left ventricular compliance. This therapy successfully decreased the magnitude of the right-to-left shunt, and the patient's hypoxemia resolved.

Right-to-left interatrial shunting as a complication of atrial septal aneurysm has been described in previous case reports. However, those cases demonstrated right-to-left shunting only in the presence of elevated right atrial pressure secondary to pulmonary embolism or right ventricular myocardial infarction. To our knowledge, this is the 1st report confirming a right-to-left interatrial shunt that occurred in the presence of severe hypotension and normal right atrial pressure. With the use of an oral alpha-1 agonist, we were able to treat our patient's hypoxemia with success. This treatment should be considered as a possible form of therapy for patients with similar physiologic abnormalities in the future.