John Calvin Commentary Isaiah 21:4

"My heart fluttereth, horror hath affrighted me; the twilight that I desired hath been turned into trembling unto me." — Isaiah 21:4 (ASV)

My heart was shaken. Others correctly translate it, “my heart wandered;” for excessive terror moves the heart, as it were, out of its place. He declares how sudden and unexpected will be the destruction of Babylon, for a sudden calamity makes us tremble more than one which has been long foreseen and expected. Daniel relates that what Isaiah here foretells was accomplished, and that he was an eyewitness. Belshazzar had prepared a magnificent banquet that night when the Persians suddenly rushed upon him, and nothing was further from his expectation than that he would be slain. High delight was thus suddenly changed into terror. (Daniel 5:30)