简单分析一下美国的还利息前景。假定10年国债利率是美国总债务的平均利率，按现在3％的水平，以后每年光偿还利息就相当于GDP的2.2%(73%X3%)。美国2012年GDP是16.62 trillion，那就等于光还利息要花去350billion（16.62 trillionX2.2%）。如果利率升到4％的水平，以后每年光偿还利息就相当于GDP的2.9%(73%X4%)。那偿还利息要花去470billion（16.62 trillionX2.9%）。从GDP增长的前景看，美国经济需要保持实际年增长率3％以上才不至于被还利息蚕食。同时我们也更加看到10年国债利率的变化对美国财政收支的巨大影响。
现在的两难是：继续QE，债务总额彪升（从2007到2012，债务升了一倍），导致乘法算式的一个因子上升；停止QE，债务上升减缓，但利率上升，乘法公式的另一个因子变大。所以美国负债的前景不好，总债务额和利率当中必然有一个要升。操作不当的话，两个一齐升。



www.marketwatch.com/story/us-debt-now-about-73-of-gdp-cbo-says-2013-09-17-1091240
WASHINGTON (MarketWatch) - The U.S. national debt is now about 73% of gross domestic product, the Congressional Budget Office said Tuesday. The percentage of debt is higher than any point since around World War II, and twice the percentage it was at the end of 2007, the nonpartisan agency said in its long-term budget outlook. If current laws stay in place, debt will decline "slightly" relative to GDP over the next few years, the agency said. But it warned that growing future deficits will push the debt to 100% of GDP 25 years from now.