A 45-year-old man presents to the clinic with fatigue. He is concerned about occasional chest palpitation and the blue tinge of his skin, as well as recent onset episodes of fainting. On physical...

A 45-year-old man presents to the clinic with fatigue. He is concerned about occasional chest palpitation and the blue tinge of his skin, as well as recent onset episodes of fainting. On physical exam, you note cyanosis, clubbing of his fingertips, and a loud heart murmur. You suspect a left-to-right shunt that went undiagnosed in his childhood. What are the possible etiologies? What has occurred? What treatment is available?