What is the most likely cause of hypotension in a trauma patient with negative FAST and imaging studies?

In the scenario of a trauma patient who presents with hypotension but has negative FAST (Focused Assessment with Sonography for Trauma) and imaging studies, a retroperitoneal bleed is a likely cause of the hypotension.

This situation suggests that there may be internal bleeding occurring outside of the peritoneal cavity, which would not be detected by a FAST scan or typical imaging studies like CT if the bleeding is limited to the retroperitoneal space. Retroperitoneal hemorrhages often arise from injuries to solid organs like the kidneys or major vascular structures (like the aorta) and can lead to significant blood loss without immediate evidence seen on initial imaging.

In contrast, cardiac tamponade would typically present with characteristic signs and symptoms such as becks triad, and could be detected on ultrasound or imaging in most cases, whereas intra-abdominal hemorrhages would usually result in positive findings on FAST if they are substantial. Liver lacerations also tend to present with visible bleeding or fluid in the abdominal cavity on imaging. Therefore, when hypotension presents without findings in the usual areas assessed, a retroperitoneal bleed becomes a stronger consideration.