Page 22 - Journal of Special Operations Medicine - Spring 2016

Compensatory Reserve for Early and
Accurate Prediction of Hemodynamic Compromise

Case Studies for Clinical Utility in Acute Care and Physical Performance

Camille L. Stewart, MD; Corinne D. Nawn, BS; Jane Mulligan, PhD;
Greg Grudic, PhD; Steven L. Moulton, MD; Victor A. Convertino, PhD

ABSTRACT

Background: Humans are able to compensate for sig- a review of traditional vital signs, especially some com-
nificant loss of their circulating blood volume, allowing bination of heart rate (HR), blood pressure (BP), respi-
vital signs to remain relatively stable until compensatory ratory rate, oxygen saturation, and mental status. These
mechanisms are overwhelmed. The authors present sev- vital signs can prove to be unreliable. This is espe-
2–6
eral clinical and performance case studies in an effort to cially true for young healthy people (e.g., children, ath-
demonstrate real-time measurements of an individual’s letes, military personnel), who are able to compensate
reserve to compensate for acute changes in circulat- for comparatively larger volume losses without evidence
ing blood volume. This measurement is referred to as of compromise. When individuals do decompensate,
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the Compensatory Reserve Index (CRI). Methods: We the process can be sudden and unpredictable. 4,7
identified seven clinical and two physical performance
conditions relevant to military casualty and operational Technology to detect and track compensated hemor-
medicine as models of intravascular volume compro- rhage that is more sensitive and specific than “legacy”
mise. Retrospective analysis of photoplethysmogram vital signs remains a capability gap in military emergency
(PPG) waveform features was used to calculate CRI, medicine. In an effort to address this gap, photoplethys-
where 1 represents supine normovolemia and 0 repre- mogram (PPG) waveform signals were recorded from
sents hemodynamic decompensation. Results: All cases a large cohort of healthy volunteers by investigators at
had CRI values suggestive of volume compromise (<0.6) the US Army Institute of Surgical Research (USAISR)
not otherwise evident by heart rate and systolic blood to study individual responses to hemorrhage-like reduc-
pressure. CRI decreased with reduced central blood tions in central blood volume. Advanced signal pro-
8,9
volume and increased with restored volume (e.g., fluid cessing and machine-learning techniques were used by
resuscitation). Conclusion: The results from these case researchers at the University of Colorado to analyze the
studies demonstrate that machine-learning techniques entirety of millions of PPG waveforms generated during
can be used to (1) identify a clinical or physiologic status the USAISR experiments. This work led to the discovery
of individuals through real-time measures of changes in of multiple, previously unidentified waveform features,
PPG waveform features that result from compromise to which represent the integration of all mechanisms that
circulating blood volume and (2) signal progression to- enable a human to compensate for acute reductions in
ward hemodynamic instability, with opportunity for early central blood volume (e.g., hemorrhage, dehydration).
and effective intervention, well in advance of changes in This physiologic phenomenon is described as the com-
traditional vital signs. pensatory reserve, and the algorithm used to calculate
8
this reserve capacity is called the Compensatory Reserve
Keywords: Compensatory Reserve Index; machine learn- Index (CRI). 5,8,10 The CRI acts like a “fuel gauge,” in-
ing; photoplethysmography; shock; testing, orthostatic; dicating the proportion of additional circulating blood
physical exercise volume loss a patient can tolerate before the onset of
hemodynamic decompensation. CRI values range from
1 to 0 and correspond with the body’s ability to com-
pensate for acute changes in intravascular volume. 8,10
Introduction
When a patient loses intravascular volume due to bleed-
Continuous assessment of clinical or performance con- ing or dehydration, the “fuel tank” begins to empty and
sequences related to reduced circulatory blood volume CRI goes down. We thus hypothesized that CRI values
is one of the most difficult tasks in civilian medicine, as would parallel changes in central blood volume status
well as military clinical and operational medicine. This over time specific to individuals whose physiologic re-
1
assessment is usually made by physical examination and serve to compensate had been compromised. As an initial

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