Because of the limitations inherent to these vital signs, several calculated indices have been suggested in an attempt to integrate a few vital signs into more sensitive metrics for prediction of patient outcomes. The most frequently described metric is shock index (SI), which is calculated as the ratio between heart rate and systolic blood pressure (normal values, 0.5 Y 0.7), and has demonstrated superiority over other indices. Heart rate variability has also been frequently suggested as a calculated vital sign, but its clinical utility in the acute blood loss setting has proven to be limited as a result of its high interpatient and intrapatient variability.