How To Prevent Session Variables Getting Lost

i'm using session variables for send variables between aspx page and it works fine ...but after some time of inctivity(user) in the page the sessions is los...how can i resolve this issue ? this is my code below how to send session :

protected void Lst_Interv(object sender, EventArgs e)
{
Session["Id"] = gdMedical.SelectedRow.Cells[0].Text.Trim();
Session["Service"] = gdMedical.SelectedRow.Cells[6].Text.Trim();
Session["Nom"] = gdMedical.SelectedRow.Cells[2].Text.Trim();
string pageurl = "LstIntervention.aspx";
Response.Write("<script> window.open('" + pageurl + "','_blank'); </script>");
}