I was always taught that:

Luke 21:26 Men’s hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken.

Men's hearts failing them for fear, meant that they have heart attacks and die of fear.

But what about:

Genesis 42:28 And he said unto his brethren, My money is restored; and, lo, it is even in my sack: and their heart failed them, and they were afraid, saying one to another, What is this that God hath done unto us?

Jospeh's brethren were afraid,  and it said their heart failed them, but they didn't die of heart attacks.

And what about the following two?

Psalms 40:12 For innumerable evils have compassed me about: mine iniquities have taken hold upon me, so that I am not able to look up; they are more than the hairs of mine head: therefore my heart faileth me.

Psalms 73:26 My flesh and my heart faileth: but God is the strength of my heart, and my portion for ever.

These weren't heart attacks, either, were they?

If not, then what does it actually mean, "hearts failing them"?