#menu_izq {
    width: 20%;
    position: fixed;    
    left:0;
    bottom:25px;
    top:155px;
    z-index:1;
    margin-right:2.2em;
}

.contenido_menu_iqda{
    /*padding-bottom:10px;*/
    /*margin-right:1em;*/
    background: #ffffff;
    position:absolute;
    right:2.2em;
    top:0;
    left:0;
    bottom:0;
    overflow:auto;
}