The investigators hypothesized that the serum B-type Natriuretic Peptide (BNP) level may be correlated with global end diastolic volume (GEDV) as measured by transpulmonary thermodilution technique (PiCCO). The rationale is that some animal studies have proven that BNP is released from ventricular myocardium in response to physical expansion. Such physical expansion can be measured by GEDV. If the hypothesis can be confirmed with our study, the serum level BNP can be used to monitor volume status of critically ill patient, instead of the invasive monitoring system.