title: Detection of low-volume blood loss: compensatory reserve versus traditional vital signs.
abstract: BACKGROUND
  Humans are able to compensate for low-volume blood loss with minimal change in traditional vital signs. We hypothesized that a novel algorithm, which analyzes photoplethysmogram (PPG) wave forms to continuously estimate compensatory reserve would provide greater sensitivity and specificity to detect low-volume blood loss compared with traditional vital signs. The compensatory reserve index (CRI) is a measure of the reserve remaining to compensate for reduced central blood volume, where a CRI of 1 represents supine normovolemia and 0 represents the circulating blood volume at which hemodynamic decompensation occurs; values between 1 and 0 indicate the proportion of reserve remaining.
  
  
  METHODS
  Subjects underwent voluntary donation of 1 U (approximately 450 mL) of blood. Demographic and continuous noninvasive vital sign wave form data were collected, including PPG, heart rate, systolic blood pressure, cardiac output, and stroke volume. PPG wave forms were later processed by the algorithm to estimate CRI values.
  
  
  RESULTS
  Data were collected from 244 healthy subjects (79 males and 165 females), with a mean (SD) age of 40.1 (14.2) years and mean (SD) body mass index of 25.6 (4.7). After blood donation, CRI significantly decreased in 92% (α = 0.05; 95% confidence interval [CI], 88-95%) of the subjects. With the use of a threshold decrease in CRI of 0.05 or greater for the detection of 1 U of blood loss, the receiver operating characteristic area under the curve was 0.90, with a sensitivity of 0.84 and specificity of 0.86. In comparison, systolic blood pressure (52%; 95% CI, 45-59%), heart rate (65%; 95% CI, 58-72%), cardiac output (47%; 95% CI, 40-54%), and stroke volume (74%; 95% CI, 67-80%) changed in fewer subjects, had significantly lower receiver operating characteristic area under the curve values, and significantly lower specificities for detecting the same volume of blood loss.
  
  
  CONCLUSIONS
  Consistent with our hypothesis, CRI detected low-volume blood loss with significantly greater specificity than other traditional physiologic measures. These findings warrant further evaluation of the CRI algorithm in actual trauma settings.
  
  
  METHODS
  Diagnostic study, level II.
authors.affiliation: From the Department of Surgery (C.L.S., S.L.M.), School of Medicine, University of Colorado; and Flashback Technologies, Inc. (J.M., G.Z.G., S.L.M.), Boulder; and Division of Pediatric Surgery (S.L.M.), Children's Hospital of Colorado, Aurora, Colorado; and US Army Institute of Surgical Research (V.A.C.), Fort Sam Houston, San Antonio, Texas.
authors.forename: Camille L
authors.initials: CL
authors.lastname: Stewart
authors.forename: Jane
authors.initials: J
authors.lastname: Mulligan
authors.forename: Greg Z
authors.initials: GZ
authors.lastname: Grudic
authors.forename: Victor A
authors.initials: VA
authors.lastname: Convertino
authors.forename: Steven L
authors.initials: SL
authors.lastname: Moulton
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D001794:Blood Pressure / Q000502:physiology; D001810:Blood Volume / Q000502:physiology; D001811:Blood Volume Determination / Q000379:methods; D002302:Cardiac Output / Q000502:physiology; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006470:Hemorrhage / Q000175:diagnosis* / Q000503:physiopathology; D006801:Humans; D008297:Male; D008875:Middle Aged; D017156:Photoplethysmography; D012680:Sensitivity and Specificity; D013318:Stroke Volume / Q000502:physiology; D055986:Vital Signs / Q000502:physiology; D055815:Young Adult