Ultrasonographic evaluation of the inferior vena cava (IVC) is widely used in emergency and critical care settings for noninvasive assessment of intravascular volume status. The subcostal (SC) window is the standard approach; however, it is frequently limited by obesity, bowel distension, or postoperative anatomical changes. In such cases, the right coronal (RC) and anterior transhepatic (aTH) windows have been proposed as alternatives.

The existing literature presents conflicting results regarding the interchangeability of these windows. Several studies report significant discrepancies between SC and RC measurements, while others demonstrate strong agreement. Direct comparison of the SC and aTH windows has received considerably less attention, with only a single prospective study to date reporting reliable agreement between these approaches under standardized imaging criteria.

Methodological heterogeneity, most likely arising from an unstandardized imaging technique, underlies much of the observed discordance. As a result, clinicians lack clear guidance on whether alternative windows can reliably substitute for the SC approach across diverse clinical settings.

Inclusion Criteria:

• Adult patients (≥18 years) requiring ultrasonographic assessment of intravascular volume status will be consecutively enrolled.

Exclusion Criteria:

• Pregnancy, refusal to participate, and inability to obtain any IVC image via the SC approach.

Adult patients (≥18 years) requiring ultrasonographic assessment of intravascular volume status in the emergency departments, intensive care units, and other acute care settings.