reverse proxy: how to make it to pass headers to backend server?

Hello,

I have an Apache 1.3.19 server configured in reverse proxy, I would
like it to pass some additional HTTP headers to the backend server.

I tried the 'Header' directive but it seems it will add only to HTTP
responses back to the browser and not to HTTP requests forwarded to
the backend server.

Any suggestion?

Thanks,

Gael