Moulton SL, Mulligan J, Grudic GZ, et al. Running on empty? The compensatory reserve index. J Trauma Acute Care Surg. 2013;75(6):1053–1059.
The EMS Science
The authors of this study placed 184 healthy volunteers into a machine that generates a negative pressure environment from the waist down. This machine has been used extensively by NASA and the military to induce a hypovolemic state on the volunteer. By creating negative pressure, the blood pools in the lower extremities and stimulates the body to respond as if it
The subjects had routine vital signs continuously monitored using highly sensitive blood pressure cuffs, a heart rate monitor and pulse oximeter. Data was compiled as the subjects were physiologically stressed. Using learning algorithms, the authors were able to calculate what they term as the “compensatory reserve index” (CRI), where a CRI of 1 represents full circulatory volume and a CRI of 0 represents a completely empty vascular volume. They hypothesized this dynamic learning algorithm could analyze human vital sign waveform data and reveal the subtle waveform features that trend and correspond with the compensatory phase
For all 184 subjects, the CRI value dropped to less than 0.3 before the subject went into collapse. The correlation between predicted and actual lower-body negative pressure level for hemodynamic decompensation was 0.89.
Their conclusion states, “Machine modeling can quickly and accurately detect and trend central blood volume reduction in real time during the compensatory phase of hemorrhage as well as estimate when an individual is ‘running on empty’ and will decompensate (CRI 0), well in advance of meaningful changes in traditional vital signs.”
Doc Keith Wesley Comments
Karen and I have previously discussed the poor correlation between conventional vital signs and the presence of shock. We must often rely on level of consciousness, yet waiting until that’s altered may be too late to prevent cardiovascular collapse. The fundamental problem with heart rate, respiratory rate and blood pressure is that no two persons are identical in their ability to compensate for shock.
Someone in excellent cardiovascular condition is not going to generate significant tachycardia until late into the progression of shock, while children generate tachycardia at the sight of a new video game. The elderly have blunted adrenergic response due to aging and the effect of blood pressure medications such as beta-blockers and ACE inhibitors.
Therefore, the best means of predicting shock is one that bases its finding on individual responses. This is the patient’s compensatory reserve. The authors are currently engaged in phase 1 trials with a device called the CipherOx that can, over the course of the first 30 heartbeats, calculate an individual’s CRI. The device then continues to hone the accuracy of the CRI with each subsequent heartbeat. The CipherOx is applied to a finger just like a standard pulse oximeter and provides the heart rate and oxygen saturation in addition to the CRI. The device also includes a user-friendly “bar” that moves up or down and changes color with patient status: adequate compensation (green: CRI > 0.6), moderately compromised (amber: CRI = 0.6–0.3) and unstable (red: CRI < 0.3).
If this device proves as reliable in the field with unhealthy patients as it does in this study, we may very well be on the brink of a revolution in patient monitoring.
Medic Karen Wesley Comments
I’m fortunate to be able to travel to national EMS conferences and tour the vendor area looking for something new. While on my usual scouting mission at the 2014 EMS Today Conference and Exposition, I had the opportunity to listen to a podcast explaining the principles of the CipherOx.
With 34 years in EMS, I tend to only find devices that improve on past creations—new products are few and far between. But in this case I scored! I was so excited I had to find and bring Doc Wesley to the booth to speak with Vincent Convertino, an EMS 10 Award winner and a primary author of this research. I usually get an eye roll from Doc when I find the latest and greatest, but this time he was as fascinated as I was.
If the CipherOx works as tested, it will surely make a difference in not only indicating that your patient is in shock, but providing a warning before he or she clinically deteriorates. Although the study was limited to a specific set of patients, the outcome of the measurement of progressive shock was impressive.
Once further studies on different age groups and disease states are complete, I can see this device being invaluable to EMS in the battle of shock management.