use of carbon dioxide gap and arterio venous oxygen content to trace the effect of preload challenge by giving via a specific venous line , first 150 ml is infused over one minute, after stroke volume at one minute measure the remaining 350 ml is infused over 14 minutes at constant rate , the fluid challenge performed with albumin 5% , fluid responsiveness is an increase in stroke volume by 15% after infusion of 500ml albumin 5%.