Convertino, Víctor A.

This study investigated the first multiomics data for association with tolerance to central hypovolemia in humans. Analyzing gene expression, proteins, and genetic variants in individuals with high and low tolerance to simulated hemorrhage, the findings show that severe blood loss causes widespread transcriptional changes, including the activation of innate immune system pathways and inhibition of gas transport processes. This research can help identify molecular factors that influence individuals’ tolerance levels for progressive blood loss.

Background The baroreflex maintains blood pressure homeostasis by modulating sympathetic activity in response to changes in arterial pressure. Baroreflex sensitivity (BRS) is blunted by activation of either the chemo‐or cardiopulmonary reflex; however, little is known about the interaction of these two reflexes on BRS. We hypothesized that activation of the chemoreflex during stimulation of the cardiopulmonary reflex would attenuate the reduction in BRS. Methods 10 healthy male volunteers (35±3 years, BMI: 25±1 kg/m 2 ) underwent lower body negative pressure (LBNP) at −15mmHg for 5 minutes to stimulate the cardiopulmonary reflex, with chemoreflex activation (hypoxia; arterial oxygen saturation: 85±1%) and without (normoxia; arterial oxygen saturation: 98±0%). Heart rate (HR; ECG), blood pressure (BP; brachial arterial catheter), oxygen saturation (finger pulse oximeter), and ventilation (expired gases) were recorded continuously. BRS was calculated by arterial pressure waveform analysis (spectrum; WinCPRS) during baseline (BL) and LBNP. Results Baseline HR was higher during hypoxia than normoxia (62±2 vs 77±3 bpm, p<0.001) while baseline systolic BP (SBP) was not statistically differentiated between conditions (151±3 vs 149±5 mmHg, normoxia vs hypoxia, p=0.669). During normoxic LBNP, HR increased (62±2 vs 66±3 bpm, p=0.001) and SBP was unchanged (151±3 vs 151±4 mmHg, BL vs LBNP, p=0.464). During hypoxic LBNP, HR remained unchanged (77±3 vs 76±4 bpm, BL vs LBNP, p=0.693) and SBP tended to decrease (149±5 vs 139±6, BL vs LBNP, p=0.067). Baseline BRS was lower during hypoxia than normoxia (15±2 vs 9±1 ms/mmHg, p=0.012). LBNP caused a similar % reduction in BRS during normoxia and hypoxia (−14±9% vs −10±7%, p=0.772). Conclusions Contrary to our hypothesis, chemoreflex activation during LBNP does not attenuate the reduction in BRS. This suggests that the cardiopulmonary reflex and the chemoreflex blunt BRS through independent mechanisms. Support or Funding Information Funding: W81XWH‐13‐2‐0038

Since hemorrhage is a leading cause of preventable death in both civilian and military settings, the development of advanced decision support monitoring capabilities is necessary to promote improved clinical outcomes. The emergence of lower body negative pressure (LBNP) has provided a bioengineering technology for inducing progressive reductions in central blood volume shown to be accurate as a model for the study of the early compensatory stages of hemorrhage. In this context, the specific aim of this study was to provide for the first time a systematic technical evaluation to meet a commonly accepted engineering standard based on the FDA-recognized Standard for Assessing Credibility of Modeling through Verification and Validation (V&V) for Medical Devices (ASME standard V&V 40) specifically highlighting LBNP as a valuable resource for the safe study of hemorrhage physiology in humans. As an experimental tool, evidence is presented that LBNP is credible, repeatable, and validated as an analog for the study of human hemorrhage physiology compared to actual blood loss. The LBNP tool can promote the testing and development of advanced monitoring algorithms and evaluating wearable sensors with the goal of improving clinical outcomes during use in emergency medical settings.

The application of artificial intelligence (AI) has provided new capabilities to develop advanced medical monitoring sensors for detection of clinical conditions of low circulating blood volume such as hemorrhage. The purpose of this study was to compare for the first time the discriminative ability of two machine learning (ML) algorithms based on real-time feature analysis of arterial waveforms obtained from a non-invasive continuous blood pressure system (Finometer®) signal to predict the onset of decompensated shock: the compensatory reserve index (CRI) and the compensatory reserve metric (CRM). One hundred ninety-one healthy volunteers underwent progressive simulated hemorrhage using lower body negative pressure (LBNP). The least squares means and standard deviations for each measure were assessed by LBNP level and stratified by tolerance status (high vs. low tolerance to central hypovolemia). Generalized Linear Mixed Models were used to perform repeated measures logistic regression analysis by regressing the onset of decompensated shock on CRI and CRM. Sensitivity and specificity were assessed by calculation of receiver-operating characteristic (ROC) area under the curve (AUC) for CRI and CRM. Values for CRI and CRM were not distinguishable across levels of LBNP independent of LBNP tolerance classification, with CRM ROC AUC (0.9268) being statistically similar (p = 0.134) to CRI ROC AUC (0.9164). Both CRI and CRM ML algorithms displayed discriminative ability to predict decompensated shock to include individual subjects with varying levels of tolerance to central hypovolemia. Arterial waveform feature analysis provides a highly sensitive and specific monitoring approach for the detection of ongoing hemorrhage, particularly for those patients at greatest risk for early onset of decompensated shock and requirement for implementation of life-saving interventions.

Background Central hypovolemia elicited by hemorrhage triggers sympathetically‐mediated baroreflex responses to maintain organ perfusion through peripheral vasoconstriction; humans with low tolerance (LT) to central hypovolemia have a diminished vasoconstrictor response during progressive reductions in central volume. Using the lower body negative pressure (LBNP) hemorrhage model, we tested the hypothesis that humans with LT to central hypovolemia would display attenuated resting beat‐to‐beat neuro‐hemodynamic transduction of muscle sympathetic nerve activity (MSNA) compared to those with high tolerance (HT). Methods MSNA (microneurography, peroneal nerve) and diastolic arterial pressure (DAP; finger photoplethysmography) were recorded in 33 human subjects (23 men, 10 women, age: 31 ± 9 years, BMI: 26 ± 4 kg/m 2 ) who subsequently underwent LBNP to tolerance (onset of pre‐syncopal symptoms) – Figure 1. Consistent with previous studies, subjects were classified as LT if they experienced pre‐syncopal symptoms prior to completing LBNP of −60 mm Hg, or as HT if they experienced pre‐syncopal symptoms after completing LBNP of −60 mm Hg. We performed linear regression analysis on data of binned MSNA burst areas against DAP; the slope of this regression represents transduction. Results Of the 33 subjects, 8 exhibited LT (time to pre‐syncope for LT: 17 ± 2 min and HT: 27 ± 5 min) Contrary to our hypothesis, resting beat‐to‐beat neuro‐hemodynamic transduction was not different between LT and HT subjects (median 0.09 IQR 0.03 – 0.16 mmHg (%s) − 1 vs. median 0.12 IQR 0.06 – 0.16 mmHg (%s) − 1 , respectively, p = 0.44) and was not related to time to pre‐syncope ( p = 0.70). Conclusion Tolerance to simulated blood loss is not related to resting sympathetic neuro‐hemodynamic transduction. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

We tested the hypothesis that hemodynamic responses would be similar during lower body negative pressure (LBNP) and hemorrhage (HEM). We measured central venous pressure (CVP), mean arterial pressure (MAP), and heart rate (HR) in 6 men during LBNP and HEM. The LBNP protocol consisted of 5‐minute stages at 0, ‐15, ‐30, and ‐45 mmHg of pressure. After a 45 minute break, the HEM protocol began and included 5 minutes at baseline and following three stages of 333 mL of HEM (1 L total). HR during LBNP increased from baseline to ‐45 mmHg (61 ± 3 vs. 86 ± 10 bpm, P < 0.01) while HR was unchanged after 1 L HEM (59 ± 4 vs. 66 ± 5 bpm, P = 0.07). HR was higher during LBNP at ‐30 mmHg vs. HEM at 667 mL (71 ± 7 vs. 63 ± 4 bpm, P = 0.03) and at the final stage (P < 0.01). From baseline, MAP tended to be reduced to a similar degree during ‐45 mmHg LBNP (90.9 ± 3.0 vs. 82.2 ± 4.0 mmHg) and 1 L HEM (89.5 ± 2.1 vs. 80.8 ± 4.0 mmHg) (P = 0.06). From baseline, CVP declined similarly during −45 mmHg LBNP (7.9 ± 1.1 vs. 0.1 ± 1.2 mmHg, P < 0.01) vs. 1 L HEM (5.8 ± 1.1 vs. 1.1 ± 1.4 mmHg, P < 0.01). Using the amalgamated r 2 across each level of LBNP and HEM, HR ( r 2 = 0.98, P < 0.01) and CVP were significantly correlated ( r 2 = 0.98, P < 0.01), but MAP was not ( r 2 = 0.53, P = 0.27). Although LBNP models the changes in MAP and CVP observed in HEM, differences in the HR response to LBNP and HEM require further exploration. Funding: US Army MRMC Combat Casualty Care Research Program Grant # W81XWH‐11–1‐0823.

We tested the hypothesis that erythrocyte count (RBC), hemoglobin (Hgb), and hematocrit (Hct) would be similar during lower body negative pressure (LBNP) and graded blood loss (BL) in humans. Arterial blood pressure, central venous pressure and heart rate were measured in 12 healthy males (88 ± 3 kg) during LBNP and BL. The LBNP protocol consisted of 5 min stages at 0,‐15, ‐30, and ‐45 mmHg. The BL protocol consisted of removal of three 333 mL aliquots of blood (1 L total) which was stored for reinfusion. RBC, Hgb, and Hct were obtained via arterial catheter at baseline and at the end of the final stage for both LBNP and BL. There were no differences (P > 0.05) in baseline values between treatment groups for RBC, Hgb, or Hct. RBC (4.81 ± 0.10 to 4.97 ± 0.10 10 12 /L), Hgb (14.18 ± 0.39 to 14.68 ± 0.36 g/dL), and Hct (40.88 ± 0.82 to 42.36 ± 0.83 %) increased from baseline to final stage (p<0.001 for all) during LBNP, while all values decreased from baseline to final stage (p<0.015 for all) during BL (4.82 ± 0.11 to 4.72 ± 0.11 10 12 /L; 14.33 ± 0.37 to 13.95 ± 0.39 g/dL; 41.09 ± 0.82 to 40.25 ± 0.86 %; respectively). The final stage values were greater in LBNP than BL for RBC, Hgb, and Hct (p< 0.001 for all). LBNP results in relative hemoconcentration while BL results in relative hemodilution. The clinical significance of these findings remains to be elucidated. Grant Funding Source : Supported by U.S. Army MRMC Combat Casualty Care Research Program Grant # W81XWH‐11‐1‐0823.

We tested the hypothesis that the catecholamine response would be similar during lower body negative pressure (LBNP) vs. blood loss (BL). Twelve men underwent randomized LBNP and BL protocols on the same day. LBNP consisted of 5‐minute stages at 0, ‐15, ‐30, and ‐45 mmHg of pressure. The BL protocol included 5 minutes at baseline and then after three stages of 333 ml of whole blood draws (1000 ml total) which were stored for reinfusion. Central venous pressure (CVP) was recorded during each protocol. Blood samples were collected at baseline and at the end of the last stage of each protocol. CVP was greater in LBNP vs. BL at baseline (7.3 ± 0.6 vs. 6.1 ± 0.6 mmHg; P < 0.01) and was lower during LBNP vs. BL (‐0.2 ± 0.6 vs. 1.8 ± 0.8 mmHg; P < 0.01). The increase in epinephrine (EPI) during LBNP (53 ± 7 vs.144 ± 30 pg/ml) and BL (49 ± 7 vs. 103 ± 19 pg/ml; P < 0.01) was similar (P = 0.16). An increase in norepinephrine (NE) during LBNP (148 ± 20 vs. 354 ± 44 pg/ml; P < 0.01) was greater (P < 0.01) than the trend to increase during BL (155 ± 22 vs. 211 ± 29 pg/ml; P = 0.09). Individual slopes of catecholamines vs. CVP were calculated to determine if the responses had similar trajectories between the two protocols. The EPI vs. CVP slope was steeper in LBNP vs. BL (‐13.5 ± 4.6 vs. ‐0.6 ± 0.1 pg/ml/mmHg; P < 0.01). The NE vs. CVP slope was steeper in LBNP vs. BL (‐27.8 ± 5.6 vs. ‐8.7 ± 5.9 pg/ml/mmHg; P = 0.01). LBNP to ‐45 mmHg appears to be a greater sympathetic stressor than 1000 ml of BL. Grant Funding Source : Supported by US Army MRMC Combat Casualty Care Research Program Grant # W81XWH‐11‐1‐0823

Lower body negative pressure (LBNP) is often used to simulate blood loss in humans. However, it is unknown if the effects of actual blood loss (BL) on cerebral blood flow regulation is analogous to simulated blood loss during LBNP. Nine healthy men (32±6 y) were studied at baseline, during 3 levels of LBNP (5 min each at ‐15, ‐30, ‐45 mmHg), and during 3 levels of blood loss (5 min each at 333, 667, 1000 mL). LBNP and BL conditions were randomized. Intra‐arterial mean arterial pressure (MAP) was similar during LBNP compared with BL (p>0.20). Central venous pressure (CVP; 2.8±0.7 vs. 4.0±0.8, 1.2±0.6 vs. 3.5±0.8, 0.2±0.9 vs. 2.1±0.9 mmHg for level 1, 2, and 3; p<0.01) and stroke volume (71±4 vs. 80±3, 60±3 vs. 74±3, 51±2 vs. 68±4 mL for level 1, 2, and 3; p<0.01) were lower during LBNP compared with BL. Despite differences in CVP, middle cerebral artery velocity (MCAv) and cerebrovascular conductance index (CVCi) were similar between LBNP and BL at each level (MCAv at level 3: 62±6 vs. 66±5 cm/s; p=0.37; CVCi at level 3: 0.72±0.05 vs. 0.73±0.05 cm/s/mmHg; p=0.29). Furthermore, the slope of the relationship between MAP and MCAv was not different between LBNP and BL (1.06 vs. 1.07 cm/s/mmHg). Our results suggest that the cerebral hemodynamic responses to LBNP are similar to actual blood loss. Grant Funding Source : Supported by U.S. Army MRMC Combat Casualty Care Research Program Grant # W81XWH‐11‐1‐0823