Five days after a total hip joint replacement a 72-year-old woman becomes acutely short of breath, diaphoretic and hypotensive. Both lung fields are clear to auscultation and percussion, but examination of the neck reveals mild jugular venous distension with prominent A waves. Heart sounds are normal. ECG shows sinus tachycardia with a new right bundle branch block and minor nonspecific ST - T wave changes. The most likely diagnosis is: