Hypoxia Caused by Persistent Left Superior Vena Cava Connecting to the Left Atrium A Rare Clinical Entity

Abstract

We relate the case of a 40-year-old man with a history of premature birth and dextroposition of the heart who presented for an evaluation of persistent hypoxia. An unrevealing pulmonary evaluation and agitated-saline echocardiogram led to cardiac magnetic resonance imaging. This revealed a very unusual finding: a persistent left superior vena cava with insertion into the left atrium and a small connecting vein between the right and left superior venae cavae. The implications, embryology, and pathogenesis of this rare condition are discussed.

Hypoxia as a clinical entity often has a very clear and easily identifiable cause. Here we describe the case of a patient who presented with resting hypoxia, the cause of which remained unclear after a thorough pulmonary examination and agitated-saline echocardiography. Eventually, a cardiology opinion was sought and cardiac magnetic resonance (CMR) revealed a persistent left superior vena cava (SVC). Surprisingly, this left SVC connected not to the coronary sinus, but led directly to the left atrium; the result was mild, asymptomatic hypoxia.

This SVC variant with a left atrial connection is exceedingly rare—found in approximately 0.03% of the population.¹ This case and its accompanying CMR capture the variant in a fantastically clear way.

Case Report

A 40-year-old man presented for evaluation of low pulse oximeter readings. He had been referred by his primary care physician after an incidental note of hypoxia appeared during a physical examination and persisted on multiple follow-up examinations. The patient related a history of premature birth and cardiac dextroposition. He underwent a thorough pulmonary evaluation that included pulmonary function testing (normal result), exercise tolerance testing (normal result), and computed tomography of the lung parenchyma (this showed underdevelopment of a small portion of the right lung). He also underwent an agitated-saline echocardiogram (a “bubble study”), which was of poor quality and revealed no obvious shunt when saline was injected from the right antecubital vein. In short, no cause for the patient's hypoxia could be identified. His electrocardiogram showed a rightward axis with inverted P waves in lead I and biphasic in lead II (Fig. 1). An anteroposterior computed tomographic scan of the chest (a scout shot) confirmed the patient's report of dextroposition (Fig. 2). In light of his dextroposition and its association with other congenital anomalies, CMR was ordered.

Fig. 1 Standard 12-lead electrocardiogram shows unusual P-wave access and reverse R-wave progression in the chest leads in this patient with dextroposition.

Fig. 2 Computed tomographic scout film (anteroposterior view) shows presumed dextroposition.

The CMR (Figs. 3 and 4) revealed a persistent left SVC with connection to the left atrium (instead of the coronary sinus). There was a small connecting vein between the right and left SVCs. The coronary sinus appeared to be minuscule. Of note, the patient's dextroposition appeared to have been caused by right-lung underdevelopment (mild hypoplasia), and the heart was reported to have partially shifted (to be in isolated dextroposition) in the chest. There were no other abnormal locations of organs or vessels, no abnormalities in systemic arteries arising from the descending thoracic or abdominal aorta, and no bronchiectasis.

Fig. 3 Cardiac magnetic resonance (sagittal oblique view) shows the persistent left superior vena cava (arrow) connecting to the left atrium.

Fig. 4 Cardiac magnetic resonance (4-chamber view) shows an intact atrial septum and dextroposition of the otherwise anatomically normal heart.

On follow-up, the patient was told that the finding posed no immediate danger, but that prolonged hypoxia could eventually result in cardiovascular dysfunction. In addition, he was made aware of the need for an air filter should intravenous lines be placed in his left arm (due to the possible introduction of air into the systemic circulation). In analyzing his CMR, we felt that the small connecting vein would be adequate to prevent SVC syndrome, should ligation of the persistent left SVC be attempted. He was offered a percutaneous coil embolization of the persistent left SVC to left atrial connection; but because he had no symptoms, he decided to wait.

Discussion

Discovered in 1738,² persistent left SVC is the most common variation of the thoracic venous system,^3,4 prevalent in 0.3% of the population.¹ The vast majority (90%) of persistent left SVCs drain into the coronary sinus, with a small portion of the remainder (just 0.03% of the population) draining into the left atrium.¹ Drainage into the left atrium is often associated with atrial septal defect and can be seen in the absence of a coronary sinus,⁵ with the orifice of the coronary sinus persisting as an interatrial communication.⁶ Persistent left SVC is associated with varied cardiac structural anomalies, including bicuspid aortic valve, atrial septal defect, atresia of the coronary sinus ostium, cor triatriatum, and coarctation of the aorta.⁷ Approximately 20% of patients with tetralogy of Fallot and 8% of patients with Eisenmenger syndrome have evidence of persistent left SVC.⁸ The condition is usually an incidental finding during central venous access for device therapy or during echocardiography, when an enlarged coronary sinus appears unexpectedly. It is sometimes found with cyanosis or hypoxia but can also be seen without evidence of hypoxia.⁹ Occasionally, these anomalous persistent left SVCs are associated with stroke, and closure is pursued.¹⁰

The thoracic embryonic venous system comprises 2 pairs of large veins: the left and right superior cardinal veins, which return blood from the cranial aspect of the embryo, and the left and right inferior cardinal veins, which return blood from the caudal aspect. These veins form the common cardinal veins before entering the heart.¹¹

The left common cardinal vein eventually forms the coronary sinus.¹² Drainage of the persistent left SVC into the left atrium, as in our patient, can alter the coronary venous drainage, often with split venous drainage or (most often) with no coronary sinus. During week 8 of development, an anastomotic connection develops between the right and left superior cardinal veins, developing the brachiocephalic vein. Parts of the superior cardinal veins (cephalic to the brachiocephalic vein) form the internal jugular veins. Next, the right SVC is formed from the caudal portion of the right cardinal vein, while regression on the left leaves the ligament of Marshall.¹³ When normal regression of the left superior cardinal vein fails, a persistent left SVC remains.

In our patient, in the absence of evidence of stroke or other abnormality, watchful waiting was a reasonable choice in management. Should he develop emboli, atrial fibrillation, stroke, or other problems, action should be taken to close this abnormal connection. It should be observed that this diagnosis could have been reached sooner, had saline injection been performed from both arms during echocardiography. Perhaps, when the causes of hypoxia are unclear, saline injection from both arms should be strongly considered.