Autonomic compensation to simulated hemorrhage monitored with heart period variability:

Comparisons of heart rate variability responses to head-up tilt with and without abdominal and lower-extremity compression in healthy young individuals: a randomized crossover study

Introduction: Abdominal and lower-extremity compression techniques can help reduce orthostatic heart rate increases. However, the effects of body compression on the cardiac autonomic systems, which control heart rate, remain unclear. The primary objective of this study was to compare heart rate variability, a reflection of cardiac autonomic regulation, during a head-up tilt test with and without abdominal and lower-extremity compression in healthy young individuals. The secondary objective was to conduct a subgroup analysis, considering participant sex, and compare heart rate and heart rate variability responses to head-up tilt with and without compression therapy. Methods: In a randomized crossover design, 39 healthy volunteers (20 females, aged 20.9 ± 1.2 years) underwent two head-up tilt tests with and without abdominal and lower-extremity compression. Heart rate and heart rate variability parameters were measured during the head-up tilt tests, including the Stress Index, root mean square of successive differences between adjacent R-R intervals, low- and high-frequency components, and low-to-high frequency ratio. Results: Abdominal and lower-extremity compression reduced the orthostatic increase in heart rate (p < 0.001). The tilt-induced changes in heart rate variability parameters, except for the low-frequency component, were smaller in the compression condition than in the no-compression condition (p < 0.001). These results were consistent regardless of sex. Additionally, multiple regression analysis with potentially confounding variables revealed that the compression-induced reduction in Stress Index during the head-up tilt position was a significant independent variable for the compression-induced reduction in heart rate in the head-up tilt position (coefficient = 0.411, p = 0.025). Conclusion: Comparative analyses revealed that abdominal and lower-extremity compression has a notable impact on the compensatory sympathetic activation and vagal withdrawal typically observed during orthostasis, resulting in a reduction of the increase in heart rate. Furthermore, this decrease in heart rate was primarily attributed to the attenuation of cardiac sympathetic activity associated with compression. Our findings could contribute to the appropriate application of compression therapy for preventing orthostatic tachycardia. This study is registered with UMIN000045179.

Assessing the Quality of Heart Rate Variability Estimated from Wrist and Finger PPG: A Novel Approach Based on Cross-Mapping Method

The non-invasiveness of photoplethysmographic (PPG) acquisition systems, together with their cost-effectiveness and easiness of connection with IoT technologies, is opening up to the possibility of their widespread use. For this reason, the study of the reliability of PPG and pulse rate variability (PRV) signal quality has become of great scientific, technological, and commercial interest. In this field, sensor location has been demonstrated to play a crucial role. The goal of this study was to investigate PPG and PRV signal quality acquired from two body locations: finger and wrist. We simultaneously acquired the PPG and electrocardiographic (ECG) signals from sixteen healthy subjects (aged 28.5 ± 3.5, seven females) who followed an experimental protocol of affective stimulation through visual stimuli. Statistical tests demonstrated that PPG signals acquired from the wrist and the finger presented different signal quality indexes (kurtosis and Shannon entropy), with higher values for the wrist-PPG. Then we propose to apply the cross-mapping (CM) approach as a new method to quantify the PRV signal quality. We found that the performance achieved using the two sites was significantly different in all the experimental sessions (p < 0.01), and the PRV dynamics acquired from the finger were the most similar to heart rate variability (HRV) dynamics.

Neuromodulation as a new avenue for resuscitation in hemorrhagic shock

Hemorrhagic shock (HS), a major cause of early death from trauma, accounts for around 40% of mortality, with 33–56% of these deaths occurring before the patient reaches a medical facility. Intravenous fluid therapy and blood transfusions are the cornerstone of treating HS. However, these options may not be available soon after the injury, resulting in death or a poorer quality of survival. Therefore, new strategies are needed to manage HS patients before they can receive definitive care. Recently, various forms of neuromodulation have been investigated as possible supplementary treatments for HS in the prehospital phase of care. Here, we provide an overview of neuromodulation methods that show promise to treat HS, such as vagus nerve stimulation, electroacupuncture, trigeminal nerve stimulation, and phrenic nerve stimulation and outline their possible mechanisms in the treatment of HS. Although all of these approaches are only validated in the preclinical models of HS and are yet to be translated to clinical settings, they clearly represent a paradigm shift in the way that this deadly condition is managed in the future.

Lower Body Negative Pressure: Physiological Effects, Applications, and Implementation

This review presents lower body negative pressure (LBNP) as a unique tool to investigate the physiology of integrated systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia in humans. An early review published in Physiological Reviews over 40 yr ago (Wolthuis et al. Physiol Rev 54: 566-595, 1974) focused on the use of LBNP as a tool to study effects of central hypovolemia, while more than a decade ago a review appeared that focused on LBNP as a model of hemorrhagic shock (Cooke et al. J Appl Physiol (1985) 96: 1249-1261, 2004). Since then there has been a great deal of new research that has applied LBNP to investigate complex physiological responses to a variety of challenges including orthostasis, hemorrhage, and other important stressors seen in humans such as microgravity encountered during spaceflight. The LBNP stimulus has provided novel insights into the physiology underlying areas such as intolerance to reduced central blood volume, sex differences concerning blood pressure regulation, autonomic dysfunctions, adaptations to exercise training, and effects of space flight. Furthermore, approaching cardiovascular assessment using prediction models for orthostatic capacity in healthy populations, derived from LBNP tolerance protocols, has provided important insights into the mechanisms of orthostatic hypotension and central hypovolemia, especially in some patient populations as well as in healthy subjects. This review also presents a concise discussion of mathematical modeling regarding compensatory responses induced by LBNP. Given the diverse applications of LBNP, it is to be expected that new and innovative applications of LBNP will be developed to explore the complex physiological mechanisms that underline health and disease.

Spectral analysis of heart rate variability predicts mortality and instability from vascular injury

BACKGROUND

Spectral analysis of continuous blood pressure and heart rate variability provides a quantitative assessment of autonomic response to hemorrhage. This may reveal markers of mortality as well as endpoints of resuscitation.

METHODS

Fourteen male Yorkshire pigs, ranging in weight from 33 to 36 kg, were included in the analysis. All pigs underwent laparotomy and then sustained a standardized retrohepatic inferior vena cava injury. Animals were then allowed to progress to class 3 hemorrhagic shock and where then treated with abdominal sponge packing followed by 6 h of crystalloid resuscitation. If the pigs survived the 6 h resuscitation, they were in the survival (S) group, otherwise they were placed in the nonsurvival (NS) group. Fast Fourier transformation calculations were used to convert the components of blood pressure and heart rate variability into corresponding frequency classifications. Autonomic tones are represented as the following: high frequency (HF) = parasympathetic tone, low frequency (LF) = sympathetic, and very low frequency (VLF) = renin-angiotensin aldosterone system. The relative sympathetic to parasympathetic tone was expressed as LF/HF ratio.

RESULTS

Baseline hemodynamic parameters were equal for the S (n = 11) and NS groups. LF/HF was lower at baseline for the NS group but was higher after hemorrhage and the resuscitation period indicative of a predominately parasympathetic response during hemorrhagic shock before mortality. HF signal was lower in the NS group during the resuscitation indicating a relatively lower sympathetic tone during hemorrhagic shock, which may have contributed to mortality. Finally, the NS group had a lower VLF signal at baseline (e.g., [S] 16.3 ± 2.5 versus [NS] 4.6 ± 2.9 P < 0.05,) which was predictive of mortality and hemodynamic instability in response to a similar hemorrhagic injury.

CONCLUSIONS

An increased LF/HF ratio, indicative of parasympathetic predominance following injury and during resuscitation of hemorrhagic shock was a marker of impending death. Spectral analysis of heart rate variability can also identify autonomic lability following hemorrhagic injuries with implications for first responder triage. Furthermore, a decreased VLF signal at baseline indicates an additional marker of hemodynamic instability and marker of mortality following a hemorrhagic injury. These data indicate that continuous quantitative assessment of autonomic response can be a predictor of mortality and potentially guide resuscitation of patients in hemorrhagic shock.

Validation of a noninvasive monitor to continuously trend individual responses to hypovolemia

BACKGROUND

Humans are able to compensate for significant blood loss with little change in traditional vital signs, limiting early detection and intervention. We hypothesized that the Compensatory Reserve Index (CRI), a new hemodynamic parameter that trends changes in intravascular volume relative to the individual patient's response to hypovolemia, would accurately trend each subject's progression from normovolemia to decompensation (systolic blood pressure < 80) and back to normovolemia in humans.

METHODS

Men and women, ages 19 years to 36 years, underwent stepwise (~333 mL aliquot) removal and replacement of 20% blood volume (men, 15 mL/kg; women, 13 mL/kg) via a large bore intravenous (i.v.) line. During each experiment, subjects were monitored with four CipherOx CRI Tablets. Withdrawn blood was reinfused at the end of each experiment.

RESULTS

Forty-two subjects (24 men; 18 women) were enrolled in the study, of which 32 completed the protocol. Seven subjects became symptomatic and collapsed (systolic blood pressure < 80), six never achieving maximum blood loss; each was rescued with a saline infusion followed by reinfusion of their stored blood. The mean CRI at baseline for all 42 subjects was 0.9 ± 0.04. The mean CRI for the 32 subjects while asymptomatic at maximum blood loss was 0.611 ± 0.028. For the asymptomatic subjects, the average blood loss volume was 1018 mL ± 286 mL. In comparison, the mean CRI at maximum blood loss for the seven subjects who collapsed was 0.15 ± 0.007 and their average blood loss volume was 860 ± 183 mL. Mean CRI after reinfusion of blood was 0.89 ± 0.02. In addition symptomatic subjects demonstrated three times larger average decrease in CRI per liter of blood removed, 0.85 versus 0.28 for asymptomatic subjects.

CONCLUSION

CRI trends change in intravascular volume relative to an individual's response to hypovolemia and is sensitive to the differing risks associated with individuals' differing tolerance to volume loss.

LEVEL OF EVIDENCE

Prognostic study, level II.

Heart Rate Variability Analysis in an Experimental Model of Hemorrhagic Shock and Resuscitation in Pigs

Background

The analysis of heart rate variability (HRV) has been shown as a promising non-invasive technique for assessing the cardiac autonomic modulation in trauma. The aim of this study was to evaluate HRV during hemorrhagic shock and fluid resuscitation, comparing to traditional hemodynamic and metabolic parameters.

Methods

Twenty anesthetized and mechanically ventilated pigs were submitted to hemorrhagic shock (60% of estimated blood volume) and evaluated for 60 minutes without fluid replacement. Surviving animals were treated with Ringer solution and evaluated for an additional period of 180 minutes. HRV metrics (time and frequency domain) as well as hemodynamic and metabolic parameters were evaluated in survivors and non-survivors animals.

Results

Seven of the 20 animals died during hemorrhage and initial fluid resuscitation. All animals presented an increase in time-domain HRV measures during haemorrhage and fluid resuscitation restored baseline values. Although not significantly, normalized low-frequency and LF/HF ratio decreased during early stages of haemorrhage, recovering baseline values later during hemorrhagic shock, and increased after fluid resuscitation. Non-surviving animals presented significantly lower mean arterial pressure (43±7vs57±9 mmHg, P<0.05) and cardiac index (1.7±0.2vs2.6±0.5 L/min/m², P<0.05), and higher levels of plasma lactate (7.2±2.4vs3.7±1.4 mmol/L, P<0.05), base excess (-6.8±3.3vs-2.3±2.8 mmol/L, P<0.05) and potassium (5.3±0.6vs4.2±0.3 mmol/L, P<0.05) at 30 minutes after hemorrhagic shock compared with surviving animals.

Conclusions

The HRV increased early during hemorrhage but none of the evaluated HRV metrics was able to discriminate survivors from non-survivors during hemorrhagic shock. Moreover, metabolic and hemodynamic variables were more reliable to reflect hemorrhagic shock severity than HRV metrics.

Greater autonomic modulation during post-exercise hypotension following high-intensity interval exercise in endurance-trained men and women

PURPOSE

An acute reduction in blood pressure observed after a single bout of exercise is termed post-exercise hypotension (PEH). In contrast to moderate intensity aerobic exercise, little is known about the PEH response following high-intensity interval exercise. The present purpose is to assess how sex and training status impact PEH following high-intensity interval exercise.

METHODS

Cardiac volumes and function via echocardiography were measured in 40 normotensive, endurance-trained (ET) and normally active (NA) men and women (Age ± SD = 30.5 ± 5.7) following high-intensity interval cycle exercise. Continuous measurements of ECG and beat-by-beat blood pressure were collected before and 30 min post-exercise for determination of cardiovagal baroreflex function (BRS and αLF), spectral analysis of heart rate and systolic blood pressure (SBP LF).

RESULTS

Post-exercise systolic BP was significantly reduced from baseline, occurring to a greater degree in ET compared with NA (-12.9 vs. -5.3 mmHg, P = 0.008), while mean arterial pressure was similarly reduced in all groups (-4.6 mmHg, P = 0.003). Despite reduced SVI and TPRI, CI was increased post-exercise (P < 0.01). ET experienced a greater decrease in αLF (P = 0.037) and increase in SBP LF (P = 0.017) than NA. Lean body mass was a significant predictor of change in SBP LF (Std. β = 0.735, P = 0.008).

CONCLUSIONS

These results characterize greater depressions in cardiovagal baroreflex function, and increased sympathetic activity, following vigorous exercise in endurance-trained individuals compared with normally active participants. This heightened sympathovagal balance after high-intensity exercise may be a compensatory mechanism in response to greater peripheral blood flow demands following vigorous exercise.

Heart rate variability in patients being treated for dengue viral infection: new insights from mathematical correction of heart rate

INTRODUCTION

Severe dengue hemorrhagic fever (DHF) is a viral infection that acts to increase permeability of capillaries, resulting in internal hemorrhage. Linear frequency domain Fourier spectral analysis represents the most published noninvasive tool for diagnosing and assessing health status via calculated heart rate variability (HRV). As such, HRV may be useful in assessing clinical status in DHF patients, but is prone to erroneous results and conclusions due to the influence of the average HR during the time period of HRV assessment (defined as the "prevailing" HR). We tested the hypothesis that alterations in HRV calculated with linear frequency analysis would be minimal when mathematically corrected for prevailing HR following dengue viral infection.

METHODS

Male (N = 16) and female (N = 11) patients between the ages of 6 months and 15 years of age (10 ± 6 SD years) were tracked through the progression of the dengue viral infection with treatment following the abatement of a fever (defervescence). Electrocardiographic recordings were collected and analyzed for HRV.

RESULTS

High frequency (HF), low frequency (LF), and LF/HF ratio were unaffected by correction for prevailing HR.

CONCLUSION

HRV corrected for changes in HR did not alter the interpretation of our data. Therefore, we conclude that cardiac parasympathetic activity (based on HF frequency) is responsible for the majority of the HR reduction following defervescence in patients with dengue viral infection.

Peripheral neural detection of danger-associated and pathogen-associated molecular patterns

Objective:

Bidirectional links between the nervous and immune systems modulate inflammation. The cellular mechanisms underlying the detection of danger-associated molecular patterns and pathogen-associated molecular patterns by the nervous system are not well understood. We hypothesized that the carotid body, a tissue of neural crest origin, detect pathogen associated molecular patterns and danger associated molecular patterns via an inflammasome-dependent mechanism similar to that described in immune cells.

Design:

Randomized, controlled laboratory investigation.

Setting:

University laboratory.

Subjects:

C57Bl/6J mice; juvenile Sprague-Dawley rats, primary human neutrophils.

Interventions:

Rat carotid body chemosensitive cells, and human neutrophils, were treated with TLR agonists to activate inflammasome-dependent pathways. In mice, systemic inflammation was induced by the pathogen associated molecular pattern zymosan (intraperitoneal injection; 500 mg/kg). Isolated carotid body/carotid sinus nerve preparations were used to assess peripheral chemoafferent activity. Ventilation was measured by whole-body plethysmography.

Measurements and Main Results:

Chemosensitive carotid body glomus cells exhibited toll-like receptor (TLR-2 and TLR-4), NLRP1, and NLRP3 inflammasome immunoreactivities. Zymosan increased NLRP3 inflammasome and interleukin-1β expression in glomus cells (p < 0.01). Human neutrophils demonstrated similar LPS-induced changes in inflammasome expression. Carotid body glomus cells also expressed IL-1 receptor and responded to application of IL-1β with increases in intracellular [Ca²⁺]. Four hours after injection of zymosan carotid sinus nerve chemoafferent discharge assessed in vitro (i.e., in the absence of acidosis/circulating inflammatory mediators) was increased five-fold (p < 0.001). Accordingly, zymosan-induced systemic inflammation was accompanied by enhanced respiratory activity.

Conclusions:

In carotid body chemosensitive glomus cells, activation of toll-like receptors increases NLRP3 inflammasome expression, and enhances IL-1β production, which is capable of acting in an autocrine manner to enhance peripheral chemoreceptor drive.

Heart rate variability analysis during central hypovolemia using wavelet transformation

Detection of hypovolemia prior to overt hemodynamic decompensation remains an elusive goal in the treatment of critically injured patients in both civilian and combat settings. Monitoring of heart rate variability has been advocated as a potential means to monitor the rapid changes in the physiological state of hemorrhaging patients, with the most popular methods involving calculation of the R-R interval signal's power spectral density (PSD) or use of fractal dimensions (FD). However, the latter method poses technical challenges, while the former is best suited to stationary signals rather than the non-stationary R-R interval. Both approaches are also limited by high inter- and intra-individual variability, a serious issue when applying these indices to the clinical setting. We propose an approach which applies the discrete wavelet transform (DWT) to the R-R interval signal to extract information at both 500 and 125 Hz sampling rates. The utility of machine learning models based on these features were tested in assessing electrocardiogram signals from volunteers subjected to lower body negative pressure induced central hypovolemia as a surrogate of hemorrhage. These machine learning models based on DWT features were compared against those based on the traditional PSD and FD, at both sampling rates and their performance was evaluated based on leave-one-subject-out fold cross-validation. Results demonstrate that the proposed DWT-based model outperforms individual PSD and FD methods as well as the combination of these two traditional methods at both sample rates of 500 Hz (p value <0.0001) and 125 Hz (p value <0.0001) in detecting the degree of hypovolemia. These findings indicate the potential of the proposed DWT approach in monitoring the physiological changes caused by hemorrhage. The speed and relatively low computational costs in deriving these features may make it particularly suited for implementation in portable devices for remote monitoring.

Sympathetic responses to central hypovolemia: new insights from microneurographic recordings

Hemorrhage remains a major cause of mortality following traumatic injury in both military and civilian settings. Lower body negative pressure (LBNP) has been used as an experimental model to study the compensatory phase of hemorrhage in conscious humans, as it elicits central hypovolemia like that induced by hemorrhage. One physiological compensatory mechanism that changes during the course of central hypovolemia induced by both LBNP and hemorrhage is a baroreflex-mediated increase in muscle sympathetic nerve activity (MSNA), as assessed with microneurography. The purpose of this review is to describe recent results obtained using microneurography in our laboratory as well as those of others that have revealed new insights into mechanisms underlying compensatory increases in MSNA during progressive reductions in central blood volume and how MSNA is altered at the point of hemodynamic decompensation. We will also review recent work that has compared direct MSNA recordings with non-invasive surrogates of MSNA to determine the appropriateness of using such surrogates in assessing the clinical status of hemorrhaging patients.

Autonomic control mechanism of maximal lower body negative pressure application

Autonomic control mechanisms during progressive hemorrhage in humans remain complex and unclear. The present study investigates the autonomic reflexes during maximal application of lower body negative pressure (LBNP) that mimics severe hemorrhage in conscious human subjects (n=10) using analyses of heart rate variability (HRV) and systolic blood pressure variability (BPV) and baroreflex sensitivity. Spectral analysis of HRV included linear power spectral density (PSD), and nonlinear principal dynamic modes (PDM) methods. The maximal LBNP application decreased (P&#60;0.01) the systolic and pulse pressures (PP), root mean square successive differences, normalized high frequency (HF) power of HRV, and transfer function gains at low frequency (LF) and HF bands. Meanwhile, increases (P&#60;0.05) in heart rate, diastolic blood pressure (DBP), LFHRV, LF/HFHRV, and sympathetic activity of HRV using PDM were observed during maximal LBNP tolerance. After the termination of LBNP, no significant changes (P>0.05) were found in all the parameters except DBP and PP between recovery and baseline conditions. Rapid application of maximal LBNP that simulated severe hemorrhage was found to be associated with unloading of baroreflex mediated increased sympathetic reflex.

Heart Rate Variability during Simulated Hemorrhage with Lower Body Negative Pressure in High and Low Tolerant Subjects

Heart rate variability (HRV) decreases during hemorrhage, and has been proposed as a new vital sign to assess cardiovascular stability in trauma patients. The purpose of this study was to determine if any of the HRV metrics could accurately distinguish between individuals with different tolerance to simulated hemorrhage. Specifically, we hypothesized that (1) HRV would be similar in low tolerant (LT) and high tolerant (HT) subjects at presyncope when both groups are on the verge of hemodynamic collapse; and (2) HRV could distinguish LT subjects at presyncope from hemodynamically stable HT subjects (i.e., at a submaximal level of hypovolemia). Lower body negative pressure (LBNP) was used as a model of hemorrhage in healthy human subjects, eliciting central hypovolemia to the point of presyncopal symptoms (onset of hemodynamic collapse). Subjects were classified as LT if presyncopal symptoms occurred during the −15 to −60 mmHg levels of LBNP, and HT if symptoms occurred after LBNP of −60 mmHg. A total of 20 HRV metrics were derived from R–R interval measurements at the time of presyncope, and at one level prior to presyncope (submax) in LT and HT groups. Only four HRV metrics (Long-range Detrended Fluctuation Analysis, Forbidden Words, Poincaré Plot Descriptor Ratio, and Fractal Dimensions by Curve Length) supported both hypotheses. These four HRV metrics were evaluated further for their ability to identify individual LT subjects at presyncope when compared to HT subjects at submax. Variability in individual LT and HT responses was so high that LT responses overlapped with HT responses by 85–97%. The sensitivity of these HRV metrics to distinguish between individual LT from HT subjects was 6–33%, and positive predictive values were 40–73%. These results indicate that while a small number of HRV metrics can accurately distinguish between LT and HT subjects using group mean data, individual HRV values are poor indicators of tolerance to hypovolemia.

Arterial pressure oscillations are not associated with muscle sympathetic nerve activity in individuals exposed to central hypovolaemia

The spectral power of low frequency oscillations of systolic arterial pressure (SAP(LF)) has been used as a non-invasive surrogate of muscle sympathetic nerve activity (MSNA) in both experimental and clinical situations. For SAP(LF) to be used in this way, a relationship must exist between SAP(LF) and MSNA within individuals during sympathetic activation. Using progressive central hypovolaemia to induce sympathetic activation, we hypothesised that SAP(LF) would correlate with MSNA in all subjects. ECG, beat-by-beat arterial pressure and MSNA were recorded in humans (n = 20) during a progressive lower body negative pressure (LBNP) protocol designed to cause presyncope in all subjects. Arterial pressure oscillations were assessed in the low frequency (LF; 0.04-0.15 Hz) domain using a Fourier transform. For the entire group, SAP(LF), MSNA burst frequency, and total MSNA increased during LBNP. Values for coefficients of determination (r(2)) describing the linear associations of SAP(LF) with MSNA burst frequency and total MSNA were 0.73 and 0.84, but rose to 0.89 and 0.98 when curvilinear fits were used, indicating that the relationship is curvilinear rather than linear. Associations between SAP(LF) and MSNA within each individual subject, however, varied widely for both MSNA burst frequency and total MSNA, whether derived by linear (r(2) range, 1.7 × 10(-6) to 0.99) or polynomial (r(2) range, 0.09 to 1.0) regression analysis. Similar results were obtained when relationships between low frequency oscillations in diastolic arterial pressure and MSNA were evaluated. These results do not support the use of low frequency oscillations in arterial pressure as a non-invasive measure of sympathetic outflow for individual subjects during sympathetic activation.

Clinical applications of heart rate variability in the triage and assessment of traumatically injured patients

Heart rate variability (HRV) is a method of physiologic assessment which uses fluctuations in the RR intervals to evaluate modulation of the heart rate by the autonomic nervous system (ANS). Decreased variability has been studied as a marker of increased pathology and a predictor of morbidity and mortality in multiple medical disciplines. HRV is potentially useful in trauma as a tool for prehospital triage, initial patient assessment, and continuous monitoring of critically injured patients. However, several technical limitations and a lack of standardized values have inhibited its clinical implementation in trauma. The purpose of this paper is to describe the three analytical methods (time domain, frequency domain, and entropy) and specific clinical populations that have been evaluated in trauma patients and to identify key issues regarding HRV that must be explored if it is to be widely adopted for the assessment of trauma patients.

Combat casualty care research at the U.S. Army Institute of Surgical Research

The Institute of Surgical Research is the U.S. Army's lead research laboratory for improving the care of combat casualties. The Institute follows a rigorous process for analyzing patterns of injury and the burden of disease to determine where research can be conducted in order to positively impact care. These analyses led the ISR to focus research on: preventing death from bleeding; developing improved pain control techniques; developing improved vital signs analysis techniques; improving the treatment of extremity injuries; preventing burn injuries on the battlefield; and improving critical care for combat casualties. This process has resulted in numerous improvements in care on the battlefield. Highlights include development, fielding, and efficiency testing of tourniquets and improved dressings for bleeding control. Significant progress has also been made in the resuscitation of combat casualties using blood products instead of crystalloid or colloid solutions. Improvements in pain control include assessments of the effect of perioperative anaesthetics on the development of post-traumatic stress disorder [PTSD]. Novelvital signs analyses have been successful in identifying promising techniques which may improve the medic's ability to accurately triage patients. Current research in extremity injuries has focused on optimizing the use of negative pressure wound therapy for contaminated wounds. Burn research has focused on improving personnel protective equipment and implementing continuous renal replacement therapy. This research program is soldier focused and addresses care from self aid and buddy aid through all echelons of care. Many of these advances have been adopted in civilian medical centres as well, benefiting not only the military trauma patient, but also the civilian trauma patient.

Hemodynamic and autonomic response to acute hemorrhage in streptozotocin-induced diabetic rats

Background

The various autonomic control systems lead to characteristic changes in heart rate (HR) and blood pressure (BP) during acute hemorrhage. However, cardiovascular autonomic neuropathy due to diabetes mellitus may interfere with the normal compensation for hemorrhage.

Materials and methods

A controlled graded bleeding (6 - 36% loss of estimated total blood volume: ETBV) was performed in streptozotocin-induced diabetic rats (STZ rats) under a conscious state. Hemodynamic and autonomic responses to acute hemorrhage were examined using analysis of BP-HR variability. The effects of dextran treatment after hemorrhage were also examined.

Results

A significant reduction in mean arterial pressure began at 12% ETBV loss in STZ rats and 18% in the control rats, respectively. When blood loss reached 18% of TEBV, the decrease in HR was prominent in STD rats due to the activation of a parasympathetic drive, as indicated by the increase in high frequency (HF; 0.75~3.0 Hz) power in HR variability, while in the control rats this response was not observed. The administration of dextran prevented the activation of the parasympathetic drive in STZ rats during hemorrhaging. In the control rats, the dextran treatment sustained the initial increase in HR with reduced HF power in HR variability.

Conclusion

STZ rats showed different hemodynamic and autonomic responses to acute hemorrhage from the control rats. STZ rats were prone to develop bradycardiac hypotension characterized by marked parasympathetic activation during hemorrhaging. This finding suggests enhancement of the Bezold-Jarisch reflex in STZ rats. Dextran treatment to maintain a normovolemic hemorrhage state inhibits this reflex.

Persistence of muscle sympathetic nerve activity during vasovagal syncope

AIMS

To determine whether sympathetic nerve firing to skeletal muscle vasculature is withdrawn during vasovagal syncope (VVS) precipitated by passive graded head-up tilt (HUT) table testing.

METHODS AND RESULTS

We performed passive graded HUT table testing in 18 patients with a history of recurrent postural VVS whom we evaluated during the syncopal event. All patients developed typical VVS during testing. Blood pressure was measured continuously via intra-brachial arterial line. Muscle sympathetic nerve activity (MSNA) was measured using an electrode in the peroneal nerve. Passive graded HUT was then applied. No pharmacological agents were used to provoke syncope. The recording site was maintained through the syncopal event in 10 of 18 patients and we were able to demonstrate persistence of MSNA during syncope in 9. The predominant haemodynamic pattern of syncope in this cohort was mixed--hypotension and bradycardia, with heart rate not falling <40 b.p.m. (n = 10).

CONCLUSION

Our data challenge the established view that the final trigger for human orthostatic vasovagal reactions is sympathetic nervous system inhibition. Efferent sympathetic nerve traffic to the skeletal muscle vasculature was nearly always maintained through the faint. This finding supports an alternative viewpoint, that vasodilator mechanisms underlie the blood pressure fall in VVS.

Muscle sympathetic nerve activity during intense lower body negative pressure to presyncope in humans

Activation of sympathetic efferent traffic is essential to maintaining adequate arterial pressures during reductions of central blood volume. Sympathetic baroreflex gain may be reduced, and muscle sympathetic firing characteristics altered with head-up tilt just before presyncope in humans. Volume redistributions with lower body negative pressure (LBNP) are similar to those that occur during haemorrhage, but limited data exist describing arterial pressure-muscle sympathetic nerve activity (MSNA) relationships during intense LBNP. Responses similar to those that occur in presyncopal subjects during head-up tilt may signal the beginnings of cardiovascular decompensation associated with haemorrhage. We therefore tested the hypotheses that intense LBNP disrupts MSNA firing characteristics and leads to a dissociation between arterial pressure and sympathetic traffic prior to presyncope. In 17 healthy volunteers (12 males and 5 females), we recorded ECG, finger photoplethysmographic arterial pressure and MSNA. Subjects were exposed to 5 min LBNP stages until the onset of presyncope. The LBNP level eliciting presyncope was denoted as 100% tolerance, and then data were assessed relative to this normalised maximal tolerance by expressing LBNP levels as 80, 60, 40, 20 and 0% (baseline) of maximal tolerance. Data were analysed in both time and frequency domains, and cross-spectral analyses were performed to determine the coherence, transfer function and phase angle between diastolic arterial pressure (DAP) and MSNA. DAP-MSNA coherence increased progressively and significantly up to 80% maximal tolerance. Transfer functions were unchanged, but phase angle shifted from positive to negative with application of LBNP. Sympathetic bursts fused in 10 subjects during high levels of LBNP (burst fusing may reflect modulation of central mechanisms, an artefact arising from our use of a 0.1 s time constant for integrating filtered nerve activity, or a combination of both). On average, arterial pressures and MSNA decreased significantly the final 20 s before presyncope (n = 17), but of this group, MSNA increased in seven subjects. No linear relationship was observed between the magnitude of DAP and MSNA changes before presyncope (r = 0.12). We report three primary findings: (1) progressive LBNP (and presumed progressive arterial baroreceptor unloading) increases cross-spectral coherence between arterial pressure and MSNA, but sympathetic baroreflex control is reduced before presyncope; (2) withdrawal of MSNA is not a prerequisite for presyncope despite significant decreases of arterial pressure; and (3) reductions of venous return, probably induced by intense LBNP, disrupt MSNA firing characteristics that manifest as fused integrated bursts before the onset of presyncope. Although fusing of integrated sympathetic bursts may reflect a true physiological compensation to severe reductions of venous return, duplication of this finding utilizing shorter time constants for integration of the nerve signal is required.

Heart-rate complexity for prediction of prehospital lifesaving interventions in trauma patients

BACKGROUND

Traditional vital signs often fail to identify critically injured patients soon enough to permit timely intervention. To improve our ability to forecast the need for prehospital lifesaving interventions (LSIs), we applied heart-rate complexity (HRC) analysis to the electrocardiogram (ECG) of patients en route to trauma centers.

METHODS

Analysis of ECG and clinical data from 374 patients en route by helicopter to three urban Level I trauma centers was conducted. Waveforms from 182 patients were excluded (because of ectopy, noise, or inadequate length). Of the remaining 192 patients, 54 received 66 LSIs in the field (LSI group): intubation (n = 52), cardiopulmonary resuscitation (n = 5), cricothyroidotomy (n = 2), and pneumothorax decompression (n = 7); 138 patients did not (non-LSI group). In the field, heart rate, blood pressure, and the Glasgow Coma Scale score (GCS(TOTAL)) and its motor component (GCS(MOTOR)) were recorded. ECG was recorded during flight. Ectopy-free, 800-beat sections of ECG were identified off-line and analyzed by HRC methods including Sample Entropy (SampEn) and Detrended Fluctuations Analysis (DFA).

RESULTS

There was no difference between LSI and non-LSI patients in heart rate or blood pressure. SampEn was lower in LSI than in non-LSI (0.88 +/- 0.03 vs. 1.11 +/- 0.03), as was DFA (1.09 +/- 0.05 vs. 1.33 +/- 0.03) and GCS(MOTOR) (3.4 +/- 0.4 vs. 5.7 +/- 0.1) (all p < 0.0001). By logistic regression, SampEn, DFA, and GCS(MOTOR) were independently associated with LSIs (area under the receiver operating characteristic curve, 0.897).

CONCLUSIONS

Decreased HRC is associated with LSIs in prehospital trauma patients. HRC may be useful as a new vital sign for identification of the severely injured.