The Compensatory Reserve For Early and Accurate Prediction Of Hemodynamic Compromise

Abstract

Shock is deadly and unpredictable if it is not recognized and treated in early stages of hemorrhage. Unfortunately, measurements of standard vital signs that are displayed on current medical monitors fail to provide accurate or early indicators of shock because of physiological mechanisms that effectively compensate for blood loss. As a result of new insights provided by the latest research on the physiology of shock using human experimental models of controlled hemorrhage, it is now recognized that measurement of the body's reserve to compensate for reduced circulating blood volume is the single most important indicator for early and accurate assessment of shock. We have called this function the "compensatory reserve," which can be accurately assessed by real-time measurements of changes in the features of the arterial waveform. In this paper, the physiology underlying the development and evaluation of a new noninvasive technology that allows for real-time measurement of the compensatory reserve will be reviewed, with its clinical implications for earlier and more accurate prediction of shock.

Specificity of Compensatory Reserve and Tissue Oxygenation as Early Predictors of Tolerance to Progressive Reductions in Central Blood Volume

“…Perhaps as important to low sensitivity and specificity in SmO 2 is the inherent assumption of homogeneity across all tissues represented by a sensor that is placed on a single small area of muscle. In contrast, the sum total of mechanisms involved in compensation to blood loss can be measured by features of the arterial waveform that will involve changes in both total vascular resistance (reflective wave) and cardiac output (ejective wave) (6). Not only do changes in reflective waveform features represent the impact of endocrine-and sympathetically-mediated vascular tone as well as oxygen delivery and utilization at the local tissue, but changes in ejective waveform features reflect integration of mechanisms underlying chronotropic and inotropic responses that control the heart.…”

“…Our experimental approach also provided the opportunity to test the hypothesis that hemorrhagic fever upon hospital admission (6). Perhaps as important is that once bleeding is stopped and/or resuscitation has been initiated, continued monitoring of the CRI provides the clinician with feedback that their applied interventions have been effective.…”

“…Accurate assessment of patient status may be delayed because of cardiovascular compensatory mechanisms that prevent early changes in primary indicators of shock such as severe hypotension (3). Vasoconstriction is one such mechanism that can be an early compensatory event, resulting in reduced peripheral tissue oxygenation (4,5) and alterations in features of the arterial waveform (6). We previously reported that noninvasive measurements of deep muscle oxygen saturation (SmO 2 ) and compensatory reserve based on changes in arterial waveform features using a specificallycomputed machine-learning based compensatory reserve index (CRI) provide earlier indications of central blood volume loss compared to standard vital sign measurements (6)(7)(8)(9)(10)(11)(12).…”

“…Vasoconstriction is one such mechanism that can be an early compensatory event, resulting in reduced peripheral tissue oxygenation (4,5) and alterations in features of the arterial waveform (6). We previously reported that noninvasive measurements of deep muscle oxygen saturation (SmO 2 ) and compensatory reserve based on changes in arterial waveform features using a specificallycomputed machine-learning based compensatory reserve index (CRI) provide earlier indications of central blood volume loss compared to standard vital sign measurements (6)(7)(8)(9)(10)(11)(12). However, like any clinical measurement, for either SmO 2 or CRI to prove effective for real-time monitoring and identification of patients at risk for hemodynamic instability resulting from hemorrhage, they must demonstrate both high sensitivity and specificity when applied to individual patients.…”

“…One prominent observation is the existence of a subset of individuals who have relatively low tolerance to reduced central blood volume (6,(13)(14)(15)(16). It is critical to assess physiological measures that can provide identification of individual patients with low tolerance to hemorrhage because they are inherently at highest risk for earlier onset of shock.…”