We previously reported that measurements of muscle oxygen saturation (SmO2) and the compensatory reserve index (CRI) provided earlier indication of reduced central blood volume than standard vital signs (e.g., blood pressure, heart rate, arterial oxygen saturation). In the present study, we hypothesized that the CRI would provide greater sensitivity and specificity to detect progressive decrease in central circulating blood volume compared with SmO2. Continuous noninvasive measures of CRI (calculated from feature changes in the photoplethysmographic arterial waveforms) were collected from 55 healthy volunteer subjects before and during stepwise lower body negative pressure (LBNP) to the onset of hemodynamic decompensation. Near infrared spectroscopy was used on the forearm to obtain deep SmO2, hydrogen ion concentration ([H+]), and hemoglobin volume (HbT; decreases reflect vasoconstriction). CRI decreased by 97% in a linear fashion across progressive blood volume loss, with no clinically significant alterations in vital signs. The receiver operating characteristic (ROC) area under the curve (AUC) for the CRI was 0.91, with a sensitivity of 0.87 and specificity of 0.80, when predicting decompensation at progressive levels of LBNP. In comparison, SmO2, [H+], and HbT had significantly lower ROC AUC, sensitivity and specificity values for detecting the same outcome. Consistent with our hypothesis, CRI detected central hypovolemia with significantly greater specificity than measures of tissue metabolism. Single measurement of CRI may enable more accurate triage, while CRI monitoring may allow for earlier detection of casualty deterioration.
United States Army Institute of Surgical Research, JBSA Fort Sam Houston, San Antonio, Texas
Address correspondence to Jeffrey T. Howard, PhD, 3698 Chambers Pass, Bldg. 3611, JBSA Fort Sam Houston, San Antonio, TX 78234. E-mail: firstname.lastname@example.org.
Received 4 February, 2016
Revised 23 February, 2016
Accepted 4 April, 2016
The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.
Support and funding for this study was provided in part by an appointment to the Internship/Research Participation Program at the United States Army Institute of Surgical Research, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and EPA, and a grant from the US Army Medical Research and Materiel Command Combat Casualty Care Research Program (D-009-2014-USAISR).
The authors report no conflicts of interest.