Verses with the word fixed in the Old Testament (3 verses):

Psalms 57:7
My heart is fixed, O God, my heart is fixed: I will sing and give praise.

Psalms 108:1
O god, my heart is fixed; I will sing and give praise, even with my glory.

Psalms 112:7
He shall not be afraid of evil tidings: his heart is fixed, trusting in the LORD.