Use of compensatory reserve measurements to reassess the sensitivity and specificity of the shock index for early and accurate detection of reduced central blood volume status

Background: Shock index (SI) equals the ratio of heart rate (HR) to systolic blood pressure (SBP) with clinical evidence that it is more sensitive for trauma patient status assessment and prediction of outcome compared to either HR or SBP alone. We used lower body negative pressure (LBNP) as a human model of central hypovolemia and compensatory reserve measurement (CRM) validated for accurate tracking of reduced central blood volume to test the hypotheses that SI: 1) presents a late signal of central blood volume status; 2) displays poor sensitivity and specificity for predicting the onset of hemodynamic decompensation; and 3) cannot identify individuals at greatest risk for the onset of circulatory shock. Methods: We measured HR, SBP and CRM in 172 human subjects (19 to 55 years) during progressive LBNP designed to determine tolerance to central hypovolemia as a model of hemorrhage. Subjects were subsequently divided into those with high (HT; n = 118) and low (LT; n = 54) tolerance based on completion of 60 mmHg LBNP. The time course relationship between SI and CRM was determined and Receiver Operating Characteristic (ROC) Area Under the Curve (AUC) was calculated for sensitivity and specificity of CRM and SI to predict hemodynamic decompensation using clinically defined thresholds of 40% for CRM and 0.9 for SI. Results: The time and level of LBNP required to reach a SI = 0.9 (~60 mmHg LBNP) was significantly greater (P < 0.001) compared to CRM that reached 40% at ~40 mmHg LBNP. SI did not differ between HT and LT subjects at 45 mmHg LBNP levels. ROCAUC for CRM was 0.95 (95%CI = 0.94-0.97) compared to 0.91 (0.89-0.94) for SI (P = 0.0002). Conclusions: Despite high sensitivity and specificity, SI delays time to detect reductions in central blood volume with failure to distinguish individuals with varying tolerances to central hypovolemia. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.