Compensatory Reserve Index as a Hemodynamic Status Evaluation Tool in Patients

Assessment of patients in the emergency department, ICU or surgical department can be a challenging process. This assessment includes collecting data regarding injury mechanism, vital signs, and physical examination findings before proceeding to other ways of evaluating the patient.

These data are then integrated in an attempt to form an accurate patient status and to determine the urgency of treatment and evacuation to the next echelon of care when necessary. When caring for multiple casualties, this process takes on an even greater importance because the care of one patient can delay the care of others.

Triage and monitoring of patients consists of several vital sign measurements including blood pressure, oxygen saturation, and heart rate. These measurements show varying correlation with patient survival, Injury Severity Score, and the need for life-saving intervention. The most significant disadvantage of their use as part of patient triage is that they are all retrospective by nature, and a change in these indices occur only after substantial hemodynamic compromise and failure of compensatory mechanisms when life-saving interventions might be too late.

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.

The Compensatory Reserve Index (CRI) represents a new paradigm for measuring the physiological reserve of integrated cardiopulmonary mechanisms (e.g., tachycardia, vasoconstriction, breathing) that compensate for reduced central blood volume. Advanced sensor technologies such as photoplethysmography enable noninvasive recordings of analog arterial waveforms. Using a model that induces stepwise reduction of central blood volume (lower body negative pressure [LBNP]) in volunteering young healthy human test subjects, through application of negative pressure to the lower body, feature-extraction and machine-learning techniques were used to reveal subtle changes in waveform features that are associated with a declining volume. This approach enables simultaneous abstraction and normalization of various characteristics of the arterial waveform. As such, the CRI aims to reflect the capacity of all factors contributing to physiological compensatory mechanisms, including compensatory reflexes, various muscle contractions, and respirations, among others. Compensatory Reserve Index values range from 0 (complete decompensation) to 1 (full compensatory reserve available). The device itself is compact, light, and can be placed on the patient's finger, and the test can be performed within 30 s, making the measurement of CRI theoretically feasible in almost any setting. The approach was designed to prospectively identify ongoing loss of central blood volume and thus estimate the point at which individuals will experience hemodynamic decompensation (onset of shock) well in advance compared with changes in standard or "legacy" vital signs.

The CRI has been shown to correlate with central blood volume changes in human subjects in laboratory conditions however, few published data regarding its use in other experimental models or its ability to detect actual blood loss exist. The current investigation represents the first effort to apply a small pulse oximeter unit to test the CRI on human subjects admitted to hospital.

The purpose of the study is to test the hypothesis that a novel noninvasive CRI monitoring algorithm would demonstrate greater sensitivity and specificity compared with standard vital signs for identifying patients with blood loss, SIRS, sepsis thus enabling appropriate measures to be taken.