Hemodynamic study of splanchnic circulation for the pathophysiology and early diagnosis of dengue hemorrhagic fever, and the use of cardiac ultrasound for management of shock in patients with dengue hemorrhagic fever. Objective 1. To study the dynamic of splanchnic venous pooling in the early stage of dengue virus infection (febrile and early toxic stage) and the utility of using portal vein congestion index (or other indices of splanchnic venous pooling) as a predictor and/or monitoring tool for DHF/DSS. 2. To study the association between the severity of dengue virus infection and vasoconstictive mediators (endothelin-1, vasopressin) and pretease (MMP-2 and MMP-9). 3. To study the utility of echocardiographic assessment as hemodynamic monitoring for low cardiac output syndrome and shock in patients with dengue shock syndrome (to find out the least acceptable end-diastolic volume and/or cardiac index which may be used to guide fluid management in patients with dengue shock syndrome).