refer a column as a varible name?

Hi all. I have troubles trying to achieve this assignment:
suppose the

type mycolumn as (field1, varchar, field2 varchar)
and

field_name = ''field1''

and returnValue declared as mycolumn
...
can i say returnValue.$field_name = ''ok''?
There is a way to achieve this piece of code?

Thanks  a lot!

-- 
Gerardo Herzig
Direccion General de Organizacion y Sistemas
Facultad de Medicina
U.B.A.