Learn English – What does “quick on his toes” mean

If you're looking for something humorous, then you'll have to create your own translation, in the spirit of Arthur Waley, the great American translator of poetry from Chinese and Japanese to English. Waley was criticized, unjustly, IMHO, for his brilliant translation of The Tale of Genji by a contemporary translator, Edward Seidensticker, touted as "the best translator of Japanese that has ever lived", but, frankly, I found his "faithful to the original text" translation an utter bore compared with Waley's. Waley was a poet; Seidensticker was a translator. There's a world of difference.

Enough background. I'm suggesting that you create your own phrase. I'm not a poet, but I do have a suggestion. Why not say something on the order of "my heart dropped down my leg and into my shoe, rolled between my toes, then stopped for a full five seconds until I caught my breath again"? You can change things to say what you think will amuse your readers or listeners. Everything else that's been suggested here, including my earlier suggestion of "my heart lept into my mouth", is merely a cliché. Avoid clichés whenever possible.

The human mind has a natural tendency to make metaphors to describe abstract concepts. In English, one such metaphor is

"up" = good, in working order
"down" = bad, broken

Examples:

• Is the mail server up?
  The server went down yesterday when the hard disk crashed.

• The dictatorship rose to power in 1969.
  The regime fell as a result of the coup d'état in 1974.

• In the heyday of the motor industry, things were looking up.
  However, since the collapse of the motor industry, Detroit has gone downhill.

• He hit rock bottom when an alcoholic binge landed him in the emergency room.
  He has since rebounded.

• Management approved proposal A, but gave a thumbs down to proposal B.

Combine that with the arbitrary convention to orient maps with north pointing up and south pointing down, and you get an idiomatic expression "going south", meaning "turning bad".

Note that these metaphors are not always generalizable. For example, going uphill is not the opposite of going downhill. ("Uphill" usually refers to some kind of struggle.) Similarly, "going north" is not a commonly accepted expression. However, it is possible to say

Profits are expected to be north of $3 million this year.

… meaning "above 3 million dollars".