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.