Question category: Cardiology
A 65 year old male has a history of ischaemic heart disease and mitral regurgitation. He undergoes insertion of a permanent pacemaker for sick sinus syndrome. A few hours later you are called as he has become agitated and very short of breath. On examination he is tachycardic, hypotensive and tachypnoeic. His heart sounds are quiet and his hands are cold and clammy and there is evidence of a raised JVP and bibasal crackles. An ECG shows small QRS complexes. Which of the following is the most likely diagnosis?