In septic shock, a mixed venous O2 saturation (SO2) of 84% indicates that

A high mixed venous O2 saturation (SO2) indicates that the tissue cells are not extracting enough oxygen from the blood, which can be due to impaired cellular metabolism or increased oxygen delivery. In septic shock, there is often a mismatch between oxygen delivery and consumption, resulting in a high SO2 despite tissue hypoxia. Therefore, SO2 alone is not a reliable indicator of tissue perfusion in septic shock.

References:

Mixed venous oxygen saturation (SvO2) monitoring • LITFL • CCC

SvO2 to monitor resuscitation of septic patients: let’s just understand the basic physiology | Critical Care | Full Text

Reference Guide for CCRN (Adult), page 14.