Experiment 1
A total of 21 animals completed this experimental protocol. Achieving the initial MABP target (35 mm Hg) required phlebotomy of 9 mL to 14 mL of blood, representing 30% to 45% of equivalent blood volume. One animal did not survive the initial 20-minute period and was excluded from the analysis.
Esophageal (eShbO2) and tongue (tShbO2) ShbO2s decreased significantly during the different intervals of hemorrhagic shock (figure 2). eShbO2 decreased from a baseline of 67.4±16.0% to a nadir of 37.8±22.8% (p=0.0004 for change during time) at MABP 15 mm Hg, and tShbO2 decreased from a baseline of 76.1±15.9% to a nadir of 24.9±24.4% (p<0.0001). In the same way, SvO2 (pulmonary artery) decreased from 42.1±7.3% at baseline to 7.1±1.8% at the end of the experiment (p=0.0026). In contrast, hShbO2 (liver) was preserved throughout the hemorrhage period (69.0±13.3% at baseline to 59.3±4.8% at the end of the experiment, p=0.847).
Figure 2Average MABP during the 20-minute intervals (red), pulmonary artery saturation (SvO2) (purple), tissue oxyhemoglobin saturation (ShbO2) in tongue (orange), esophagus (blue), and liver (yellow) in rats undergoing progressive fixed-pressure hemorrhagic shock. Data are means, error bars are SD. eShbO2, oxyhemoglobin saturation in the esophagus; hShbO2, oxyhemoglobin saturation in the liver; MABP, mean arterial blood pressure; tShbO2, oxyhemoglobin saturation in the tongue.
Time-matched eShbO2-SvO2 pairs exhibited a positive correlation (Spearman’s correlation coefficient r=0.705, p<0.0001, figure 3A) and tShbO2-SvO2 correlation (r=0.724, p<0.0001, figure 3B) was strong. However, hShbO2 correlated poorly with SvO2 (r=0.271, p=0.475, figure 3C). The sensitivity and specificity of ShbO2 to categorically detect severe tissue hypoxia (defined as SvO2<20%) was excellent: AUC for eShbO2 was 0.843 (95% CI 0.747 to 0.900, p<0.0001) and for tShbO2 was 0.879 (95% CI 0.789 to 0.969, p<0.0001, figure 3D).
Figure 3ShbO2 is associated with SvO2 during graded hemorrhagic shock. ShbO2 values are average of the oxyhemoglobin measured by RRS during the 2 minutes prior to the SvO2 sample. The line represents a quadratic regression with 95% CI. (A) Time-matched pairs of esophageal ShbO2 and SvO2. Spearman’s correlation coefficient r=0.705, p<0.0001, r2=0.492. (B) Tongue ShbO2. Spearman’s correlation coefficient r=0.724, p<0.0001, r2=0.467. (C) ShbO2 in the liver. Spearman’s correlation coefficient r=0.271, p=0.475, r2=0.089. (D) Defining SvO2<20% as presence of severe shock state, receiver operating characteristics curve analysis of ShbO2 to detect SvO2<20%. Tongue-ShbO2 (orange line) AUC 0.8795 (95% CI 0.789 to 0.969, p<0.0001). Esophageal-ShbO2 (blue line) AUC 0.843 (95% CI 0.747 to 0.9, p<0.0001). eShbO2, oxyhemoglobin saturation in the esophagus; ShbO2, oxyhemoglobin saturation; RRS, resonance Raman spectroscopy; SvO2, mixed venous saturation; tShbO2, oxyhemoglobin saturation in the tongue.
Time-matched eShbOo2-lactate pairs exhibited a strong, inverse correlation (Spearman’s correlation coefficient r=0.708, p<0.0001, figure 4A), as did tShbO2-lactate (r=0.830, p<0.0001, figure 4B). hShbO2 had no correlation with lactate (r=0.247, p=0.033, figure 4C).
Figure 4ShbO2 is associated with lactate during graded hemorrhagic shock. (A) Time-matched pairs of esophageal ShbO2 and lactate. Spearman’s correlation coefficient r=−0.708, p<0.0001, r2=0.478. (B) ShbO2 in the tongue. Spearman’s correlation coefficient r=−0.830, p<0.0001, r2=0.657. (C) ShbO2 in the liver. Spearman’s correlation coefficient r=−0.247, p=0.033, r2=0.111. ShbO2 values are average of the oxyhemoglobin measured by RRS during the 2 minutes prior to the lactate sample. The line represents a quadratic regression with 95% CI. ShbO2, oxyhemoglobin saturation; RRS, resonance Raman spectroscopy.
Experiment 2
A total of 22 animals were instrumented as described; two animals died within 10 minutes of the initial shock and were excluded from the analysis. The phlebotomy volume required to reach the target MABP ranged between 12 mL and 17 mL, approximately 40% to 65% of the total blood volume.
ShbO2 decreased abruptly immediately after the hemorrhage parallel to the MABP (figure 5). Baseline MABP was 59±6 mm Hg, decreasing to 24±6 mm Hg just before death. Accordingly, pulse pressure decreased from 23±4 mm Hg to 9±3 mm Hg and heart rate increased from 207±37 bpm to 231±26 bpm. eShbO2 decreased from 60.8±6.3% to 14.9±12.7% and tShbO2 decreased from 84.0±10.0% to 27.8±22.3%.
Figure 5MABP (mm Hg), pulse pressure (mm Hg), heart rate (beats per minute) and ShbO2 (%) during baseline and shock. Data are 5-minute averages of measurements and presented as mean and error bars as SD. eShbO2, oxyhemoglobin saturation in the esophagus; MABP, mean arterial blood pressure; ShbO2, oxyhemoglobin saturation; tShbO2, oxyhemoglobin saturation in the tongue.
We then assessed the performance of a single point in time ShbO2 measurement as a diagnostic test for impending death, compared with invasive hemodynamic monitoring. Thresholds were established for each variable (online supplemental figure 1): the optimal threshold of eShbO2 was 40%, tShbO2 was 50%, MABP was 30 mm Hg, pulse pressure was 15 mm Hg, and heart rate was 220 bpm. We computed time to death from the first instance (based on a computed 1-minute average) of any of these findings to death and screened the sensitivity of each to detect death within 45 minutes. eShbO2<40% was 83.3% sensitive for death within 45 minutes, tShbO2<50% to 72.2% sensitive, MABP<30 mm Hg was 58.8% sensitive, pulse pressure<15 mm Hg was 50% sensitive, and heart rate>215 bpm was 39% sensitive (figure 6A).
Figure 6(A) After the first occurrence of eShbO2<40% (blue), 83% of the animals died within 45 minutes; in contrast 72% of animals died within the same window after tShbO2<50%. An optimized threshold of MABP<30 mm Hg, pulse pressure<15 mm Hg and heart rate>215 bpm, predicted death within 45 minutes in 59%, 50%, and 39% of animals, respectively, (p=0.004). (B) Down-scale thresholds of eShbO2 to predict death from reaching the cut-off. eShbO2, oxyhemoglobin saturation in the esophagus; MABP, mean arterial blood pressure; tShbO2, oxyhemoglobin saturation in the tongue.
Finally, having observed that eShbO2 was optimally sensitive in detecting impending death, we explored the predictive impact of lower thresholds. Overall, 83.3% of animals with single point measurements of eShbO2<40% died within 45 minutes, 83.3% of animals with single point eShbO2<30% died within 25 minutes, 83.3% of animals with single point eShbO2<20% died within 16 minutes, 80.0% of animals with single point eShbO2<10% died within 8 minutes of the measurement (figure 6B).