function callHeartbeat() {
    var callback = {
        success: function() { startHeartbeat(); },
        failure: function() { startHeartbeat(); },
        argument: { }
    };

    YAHOO.util.Connect.asyncRequest('GET', 'heartbeat.php', callback);
}

function startHeartbeat() {
    window.setTimeout('callHeartbeat()', 600000);
}

SafeAddOnload('startHeartbeat');