Increasing severity of shock (worsening tissue hypoperfusion) is associated with increasing heart rate (HR), worsening hypotension and worsening metabolic acidosis.

Physiological compensation to maintain adequate tissue perfusion and oxygen delivery results in measurable changes in heart rate. Once these compensatory mechanisms are exhausted a progressive decline in blood pressure can be measured. Multiple confounding factors may also affect these vital signs.

Traditional vital signs (HR and SBP) have limitations as indicators of blood loss.