For nearly two years, the massive interventions of central banks have repeatedly pulled a fundamentally weak and debt-burdened global economy from the brink of resumed recession. The Federal Reserve's balance sheet is now leveraged 52-to-1, with assets having an average duration of over 5 years, suggesting that if those assets were marked-to-market, an interest rate increase of less than 50 basis points would wipe out the Fed's entire capital base. Of course, the Fed takes no marks on its assets when it reports its balance sheet, though it does occasionally take down the value of the securities in the Maiden Lane shell companies that it illegally set up to bail out Bear Stearns and other entities (in violation of Section 13(3) of the Federal Reserve Act, which Congress had to amend and spell out like a See-Spot-Run book as a result).
At a 10-year Treasury yield of 1.7%, interest on reserves of 0.25%, and a monetary base now at about 18 cents per dollar of nominal GDP (see Run, Don't Walk), further purchases of long-term Treasury securities by the Fed would produce net losses for the Fed in any scenario where yields rise more than about 20 basis points a year, or the Fed ever has to unwind any portion of its already massive positions. So further QE by the Fed would effectively amount to fiscal policy. Moreover, the benefits of central bank interventions are becoming progressively smaller and short-lived (nearly log-periodic in fact, to borrow a term from crash dynamics). None of this restricts the Fed from embarking on further interventions. It just emphasizes how far the Fed has already descended into the deep.