Cardiac & Resuscitation, Columns, Patient Care, Special Topics

Compensatory Reserve Index Can Aid in Early Shock Detection

Issue 8 and Volume 40.

EARLY SHOCK DETECTION

Convertino VA, Howard JT, Hinojosa-Laborde C. Individual-specific, beat-to-beat trending of significant human blood loss: The compensatory reserve. Shock. Jan. 6, 2015. [Epub ahead of print.]

Hemorrhagic shock is the leading cause of death in trauma. The challenge is detecting shock early enough to intervene clinically. The human body is masterful at compensating with a variety of seemingly undetectable mechanisms, such as autonomic activity, vasoconstriction, increased stroke volume, improved cardiac filling and more efficient breathing. So by the time we see alterations in vital sign metrics, the patient may have lost as much as a third of their circulating blood volume. What if there was a way to measure accurate beat-to-beat assessment of hemodynamics?

Background: Former NASA physiologist and EMS 10 award-winner Victor Convertino, PhD, who now works in the United States Army Institute of Surgical Research, has been working toward being able to better and more accurately detect when the body may be going into hemorrhagic shock. After years of not finding any helpful patterns in the metrics, Convertino and his team looked at arterial waveforms, finding that they’re made up of two distinct waves: the ejected wave (the contraction of the heart) and the reflected wave (pressure being reflected back from the arterial vasculature).

With the help of robotics experts, Convertino and his team were able to use self-learning algorithms to learn from an individual’s own waveform how the body is using their compensatory mechanisms. With each beat the algorithm becomes more accurate in predicting the body’s ability to protect itself or begin to decompensate.

The Compensatory Reserve Index (CRI) is a “gas gauge” analysis that incorporates all of the metrics available in the wave form, and normalizes the data into a 0–100% value. At 100% the body’s full compensatory mechanisms (e.g., baroreceptors, respiration) are available to help maintain adequate perfusion (or blood flow and oxygen delivery) of the body’s tissues.

The CRI acts as a quick “gas gauge” analysis showing the body’s ability to compensate for blood loss. Photo courtesy Victor Convertino

As the body’s reserve begins to run dry, the ability to compensate decreases and the gas gauge begins to drop as the body is plunging deeper into shock.

Methods: In this study, 25 healthy volunteers were attached to pulse oximetry, 3-lead ECG monitoring and a photoplethysmograph monitor, which offers real-time arterial pressure via a sensor wrapped around a finger. Subjects then underwent controlled hemorrhage via an IV drained into a blood collection bag while their vitals were measured minute by minute until estimated blood loss reached around 20%, or no more than 1,333 mL. At the completion of the study, participants had their blood volume replaced.

Results: Five patients were excluded from the study due to corrupt data or clinical deterioration, leaving 20 patients to be studied. Even with moderate hemorrhage, the systolic blood pressure, diastolic blood pressure and mean arterial pressure of the patients remained within normal limits during the four measurement periods. The heart rate of all patients had a slight increase but would not likely alert us to shock.

Each patient’s CRI, on the other hand, decreased reliably at each measurement, from an average of 0.92 (92%) at baseline to 0.62 (62%) at the final measurement, with 1.0 (100%) considered a normal measurement. When plotted as a linear model, the CRI correlated to blood loss. When blood volume was replaced in the patients, the CRI returned to baseline in all 20 patients.

Discussion: The CRI is a powerful new tool that integrates measurement of compensatory mechanisms as well as hemodynamics and uses a novel mathematical model to deliver a statistically unbiased and individually tailored prediction of circulatory compromise. The specificity and sensitivity of the tool are unlike anything we’ve seen in this field, and the algorithm can be easily integrated into any monitor that generates an arterial waveform. We look forward to using this tool and watching more research done with this metric.

BOTTOM LINE

What we already know: The body’s ability to compensate during blood loss makes it hard to detect when a patient is going into hemorrhagic shock.

What this study adds: Using the new CRI metric, we may soon be able to earlier detect hemorrhagic shock.