Shock index: a re-evaluation in acute circulatory failure

To evaluate the relationship between the shock index SI (ratio of heart rate to systolic arterial pressure) and cardiac function and oxygen transport in an experimental model of hemorrhage and clinical septic shock.

This study was conducted in a hypovolemic circulatory failure model; 40% hemorrhage in the anesthetized pig and normovolemic hyperdynamic septic patients in the intensive care unit (ICU). Hemodynamic and oxygen transport variables were measured and their relationships to SI was examined. SI was inversely related to blood loss, cardiac index (CI), stroke volume (SV), mean arterial pressure (MAP) and left ventricular stroke work (LVSW) (r = -0.73, -0.75, -0.89 and -0.75, respectively P less than 0.01) following hemorrhage in the anesthetized pig. Oxygen transport variables, i.e. oxygen delivery (DO2) and mixed venous oxygen saturation (SvO2P) (r = -0.68 and -0.74, respectively, P less than 0.01) were also inversely related to the SI. Oxygen consumption (VO2) increased initially with increasing SI and fell when SI was greater than 3.0. In clinical septic shock and following blood volume expansion, the SI was not correlated to CI, SVI, MAP or systemic vascular resistance (SVR) (r = -0.01, -0.47, -0.34 and -0.14, respectively, P-value NS) but was inversely related to LVSWI (r = -0.68, P less than 0.01). There were no relationships between the SI and oxygen transport variables (DO2, SvO2) (r = -0.02 and -0.17, P-value NS) in septic shock.

SI provides a non-invasive means to monitor deterioration or recovery of LVSW during acute hypovolemic and normovolemic circulatory failure and its therapy. SI may be of limited value in the assessment of systemic oxygen transport and response to therapy in clinical shock.