The middle column indicates the room between a country's current debt-to-GDP ratio and the maximum sustainable ratio, according to this analyis. And the right column indicates the maximum yield which, if sustained, will not result in a spiral of collapse (in which rising interest costs make debt less affordable, contributing to rising interest costs). One can quibble with the results; I'm sure there are many people who think markets would abandon America long before its debt-to-GDP ratio topped 200%. But the yield calculations are very interesting.Greece, Portugal, Italy, and Ireland already sport 10-year yields above the make-or-break level. Now, these yields don't bring everything crashing down right away. They have to be sustained, so as to have an impact on actual debt costs as countries are forced to roll over existing debt as it comes due. Spanish yields have been above the critical threshold at times but have since fallen back below it.