US corporations have loaded up on a lot of debt since the financial crisis.

In fact, America's corporations have doubled their total debt levels, according to a note Tuesday from analysts at Goldman Sachs.

The debt has been raised by American firms to fund mergers and acquisitions and to buy back their own shares.

And despite seven years of zero-interest rates and quantitative easing, interest payments have climbed by nearly 40% since 2008. The rate of interest paid, however, has fallen to around 4% from nearly 6% since 2009.

Goldman Sachs

Of course, this isn't the whole story, as debt is only important when compared to a company's ability to service and repay it. For corporate America's real debt burden to have doubled, earnings would have had to have stayed still.

But even when it's compared to EBITDA — or Earnings Before Interest, Taxes, Depreciation, and Amortization — net debt is 30% higher than the average over the last 10 years.

Though interest rates are likely to stay low for a long time even after the Federal Reserve starts hiking rates for the first time since the financial crisis, the extra debt accumulated will be an increasing burden if earnings don't keep up.

In its note, Goldman writes:

Following the crisis, imbalances of all types have been created. Chief among them, in our view, is the re-leveraging of America and the quiet growth of goodwill, as a percentage of assets on balance sheets. While neither poses an immediate terminal risk to the health of corporate America the changing nature of corporate balance sheets does raise the question, again, about the lack of organic growth and reinvestment post the crisis. Taken a step further, the spectre of rising rates, potential global disinflation (dare we say "deflation"?), declining operating profits and wider credit spreads continues to create near-term consternation for weak balance sheet stocks.