Introduction: The measurement of the CVP is an important way of estimating the preload volume and the intravascular fluid volume for critically ill patients who require emergency attention to their hemodynamic status, this measurement is very important. Recent researches have shown that in severe sepsis and septic shock, CVP <8 mmHg, urgent fluid resuscitation should be considered. Therefore, this study aimed to investigate the ultrasonography examination of the inferior vena cava as well caval index in order to estimate low CVP. Method: This prospective study was performed on 70 patients admitted in ICU of Shahid Bahonar hospital of Kerman who were all reported to utilize central venous catheter. The IVC diameter was measured in 2 cm to 3 cm distance to the right atrium, in subxiphoid area and sagittal views at the end of inspiration and expiration phases. The CVP was measured by a pressure monometer and an indwelling central vein catheter. Caval index, indicating the relative decrease in IVC diameter from inhalation to exhalation, was measured via utilizing CVP monitoring. Results: The results show that there is a positive relationship between the inspiratory phase IVC diameter and the CVP and also between the expiratory phase IVC diameter and the CVP so that increase in the CVP causes increase in inspiratory and expiratory IVC diameters. Also, the caval index >50% has a sensitivity of 94% and the specificity of 97% in the prediction of the low CVP (<8 mmHg). Discussion: It seems that the portable IVC sonography of the emergency patients is non-invasive and a fast way to estimate the CVP and hemodynamic condition of the patient.