Defibrillator - Display message after timeout

I'm using Defibrillator to Keep my ASP .NET Session Alive for 5 minutes, even the user is not doing anything for 5 minutes. After 5 minutes of inactivity, I'm looking display a message/popup to the user as "you session is timed out and you're record is released and so on..". Aspx Code for Defibrillator:

Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        Response.AddHeader("Refresh", Convert.ToString((Session.Timeout*60)-10))    
    End Sub

Added this line of code into the front end WebForm markup language:

<iframe id="Defib" src="../Defibrillator.aspx" frameborder="0" width="0" height="0" runat="server" />

What is the best possible way to do that display the message?? Thanks in Advance