Blood pressure variability, heart functionality, and left ventricular tissue alterations in a protocol of severe hemorrhagic shock and resuscitation

Investigating the variability in pressure-volume relationships during hemorrhage and aortic occlusion

Introduction

The pressure-volume (P-V) relationships of the left ventricle are the classical benchmark for studying cardiac mechanics and pumping function. Perturbations in the P-V relationship (or P-V loop) can be informative and guide the management of heart failure, hypovolemia, and aortic occlusion. Traditionally, P-V loop analyses have been limited to a single-beat P-V loop or an average of consecutive P-V loops (e.g., 10 cardiac cycles). While there are several algorithms to obtain single-beat estimations of the end-systolic and end-diastolic pressure-volume relations (i.e., ESPVR and EDPVR, respectively), there remains a need to better evaluate the variations in P-V relationships longitudinally over time. This is particularly important when studying acute and transient hemodynamic and cardiac events, such as active hemorrhage or aortic occlusion. In this study, we aim to investigate the variability in P-V relationships during hemorrhagic shock and aortic occlusion, by leveraging on a previously published porcine hemorrhage model.

Methods

Briefly, swine were instrumented with a P-V catheter in the left ventricle of the heart and underwent a 25% total blood volume hemorrhage over 30 min, followed by either Zone 1 complete aortic occlusion (i.e., REBOA), Zone 1 endovascular variable aortic control (EVAC), or no occlusion as a control, for 45 min. Preload-independent metrics of cardiac performance were obtained at predetermined time points by performing inferior vena cava occlusion during a ventilatory pause. Continuous P-V loop data and other hemodynamic flow and pressure measurements were collected in real-time using a multi-channel data acquisition system.

Results

We developed a custom algorithm to quantify the time-dependent variance in both load-dependent and independent cardiac parameters from each P-V loop. As expected, all pigs displayed a significant decrease in the end-systolic pressures and volumes (i.e., ESP, ESV) after hemorrhage. The variability in response to hemorrhage was consistent across all three groups. However, upon introduction of REBOA, we observed significantly high levels of variability in both load-dependent and independent cardiac metrics such as ESP, ESV, and the slope of ESPVR (Ees). For instance, pigs receiving REBOA experienced a 342% increase in ESP from hemorrhage, while pigs receiving EVAC experienced only a 188% increase. The level of variability within the EVAC group was consistently less than that of the REBOA group, which suggests that the EVAC group may be more supportive of maintaining healthier cardiac performance than complete occlusion with REBOA.

Discussion

In conclusion, we successfully developed a novel algorithm to reliably quantify the single-beat and longitudinal P-V relations during hemorrhage and aortic occlusion. As expected, hemorrhage resulted in smaller P-V loops, reflective of decreased preload and afterload conditions; however, the cardiac output and heart rate were preserved. The use of REBOA and EVAC for 44 min resulted in the restoration of baseline afterload and preload conditions, but often REBOA exceeded baseline pressure conditions to an alarming level. The level of variability in response to REBOA was significant and could be potentially associated to cardiac injury. By quantifying each P-V loop, we were able to capture the variability in all P-V loops, including those that were irregular in shape and believe that this can help us identify critical time points associated with declining cardiac performance during hemorrhage and REBOA use.

Hemodynamic Effects of Cardiovascular Medications in a Normovolemic and Hemorrhaged Yorkshire-cross Swine Model

The Yorkshire-cross swine model is a valuable translational model commonly used to study cardiovascular physiologyand response to insult. Although the effects of vasoactive medications have been well described in healthy swine, the effects of these medications during hemorrhagic shock are less studied. In this study, we sought to expand the utility of the swine model by characterizing the hemodynamic changes that occurred after the administration of commonly available vasoactive medications during euvolemic and hypovolemic states. To this end, we anesthetized and established femoral arterial,central venous, and pulmonary arterial access in 15 juvenile Yorkshire-cross pigs. The pigs then received a series of rapidlymetabolized but highly vasoactive medications in a standard dosing sequence. After completion of this sequence, each pigunderwent a 30-mL/kg hemorrhage over 10 min, and the standard dosing sequence was repeated. We then used standard statisticaltechniques to compare the effects of these vasoactive medications on a variety of hemodynamic parameters betweenthe euvolemic and hemorrhagic states. All subjects completed the study protocol. The responses in the hemorrhagic state wereoften attenuated or even opposite of those in the euvolemic state. For example, phenylephrine decreased the mean arterialblood pressure during the euvolemic state but increased it in the hemorrhagic state. These results clarify previously poorlydefined responses to commonly used vasoactive agents during the hemorrhagic state in swine. Our findings also demonstratethe need to consider the complex and dynamic physiologic state of hemorrhage when anticipating the effects of vasoactivedrugs and planning study protocols.

Principles for minimizing oxygen debt: can they translate to clinical application and improve outcomes?

Oxygen delivery is dependent on pulmonary gas exchange, cardiac output, blood oxygen-carrying capacity, and tissue oxygen extraction. Reduction in oxygen delivery or higher oxygen consumption can initiate complex protective cellular processes precipitating oxygen debt. In critically ill and potentially surgical patients, stress and consequent hormonal or metabolic changes can trigger oxygen debt which is associated with worse morbidity and mortality. Increase in oxygen delivery by augmenting cardiac output or by increasing fraction of inspired oxygen (FiO₂) can help reduce oxygen debt. However, the extent of oxygen debt in an individual patient is poorly defined and difficult to measure. Furthermore, large heterogeneity in clinical trials assessing outcomes benefit of increasing oxygen delivery limits our ability to recommend goal directed fluid therapy aimed at increasing cardiac ouput or higher FiO₂. To understand and prevent oxygen debt in critically ill and surgical patients, we need to develop continuous monitoring techniques to assess the balance of oxygen delivery and consumption. Furthermore, methods of increasing oxygen delivery like goal-directed fluid therapy, higher FiO₂ and anemia prevention should be rigorously evaluated with focus on establishing outcomes benefit.

The autonomic nervous system in septic shock and its role as a future therapeutic target: a narrative review

The autonomic nervous system (ANS) regulates the cardiovascular system. A growing body of experimental and clinical evidence confirms significant dysfunction of this regulation during sepsis and septic shock. Clinical guidelines do not currently include any evaluation of ANS function during the resuscitation phase of septic shock despite the fact that the severity and persistence of ANS dysfunction are correlated with worse clinical outcomes. In the critical care setting, the clinical use of ANS-related hemodynamic indices is currently limited to preliminary investigations trying to predict and anticipate imminent clinical deterioration. In this review, we discuss the evidence supporting the concept that, in septic shock, restoration of ANS-mediated control of the cardiovascular system or alleviation of the clinical consequences induced by its dysfunction (e.g., excessive tachycardia, etc.), may be an important therapeutic goal, in combination with traditional resuscitation targets. Recent studies, which have used standard and advanced monitoring methods and mathematical models to investigate the ANS-mediated mechanisms of physiological regulation, have shown the feasibility and importance of monitoring ANS hemodynamic indices at the bedside, based on the acquisition of simple signals, such as heart rate and arterial blood pressure fluctuations. During the early phase of septic shock, experimental and/or clinical studies have shown the efficacy of negative-chronotropic agents (i.e., beta-blockers or ivabradine) in controlling persistent tachycardia despite adequate resuscitation. Central α-2 agonists have been shown to prevent peripheral adrenergic receptor desensitization by reducing catecholamine exposure. Whether these new therapeutic approaches can safely improve clinical outcomes remains to be confirmed in larger clinical trials. New technological solutions are now available to non-invasively modulate ANS outflow, such as transcutaneous vagal stimulation, with initial pre-clinical studies showing promising results and paving the way for ANS modulation to be considered as a new potential therapeutic target in patients with septic shock.

High-Resolution Mass Spectrometry-Based Approaches for the Detection and Quantification of Peptidase Activity in Plasma

Proteomic technologies have identified 234 peptidases in plasma but little quantitative information about the proteolytic activity has been uncovered. In this study, the substrate profile of plasma proteases was evaluated using two nano-LC-ESI-MS/MS methods. Multiplex substrate profiling by mass spectrometry (MSP-MS) quantifies plasma protease activity in vitro using a global and unbiased library of synthetic peptide reporter substrates, and shotgun peptidomics quantifies protein degradation products that have been generated in vivo by proteases. The two approaches gave complementary results since they both highlight key peptidase activities in plasma including amino- and carboxypeptidases with different substrate specificity profiles. These assays provide a significant advantage over traditional approaches, such as fluorogenic peptide reporter substrates, because they can detect active plasma proteases in a global and unbiased manner, in comparison to detecting select proteases using specific reporter substrates. We discovered that plasma proteins are cleaved by endoproteases and these peptide products are subsequently degraded by amino- and carboxypeptidases. The exopeptidases are more active and stable in plasma and therefore were found to be the most active proteases in the in vitro assay. The protocols presented here set the groundwork for studies to evaluate changes in plasma proteolytic activity in shock.

Vascular Decoupling in Septic Shock: The Combined Role of Autonomic Nervous System, Arterial Stiffness, and Peripheral Vascular Tone

BACKGROUND

Acute inflammation and sepsis are known to induce changes in vascular properties, leading to increased arterial stiffness; at the same time, the autonomic nervous system (ANS) also affects vascular properties by modulating the arterial smooth muscle tone, and it is widely reported that sepsis and septic shock severely impair ANS activity. Currently, clinical guidelines are mainly concerned to resuscitate septic shock patients from hypotension, hypovolemia, and hypoperfusion; however, if the current resuscitation maneuvers have a beneficial effect also on vascular properties and autonomic functionality is still unclear. The objective of this work is to assess the effects of standard resuscitation at vascular level and to verify if there is any association between alterations in vascular properties and ANS activity.

METHODS

Six pigs underwent a protocol of polymicrobial septic shock and resuscitation (fluids and noradrenaline). The arterial blood pressure (ABP) waveform was recorded in the central aorta and in the peripheral radial and femoral artery. The characteristic arterial time constant was computed at the three arterial sites based on the two-element Windkessel model, to characterize the overall arterial vascular tree. Moreover, independent estimates of total arterial compliance (AC) and total peripheral resistance (TPR) were performed. Baroreflex sensitivity (BRS), low frequency (LF, 0.04-0.15 Hz) spectral power of diastolic blood pressure, and indices of heart rate variability (HRV) were computed to assess ANS functionality.

RESULTS

Septic shock induced a severe vascular disarray, decoupling the usual pressure wave propagation from central to peripheral sites; this phenomenon appeared as an inversion of the physiological pulse pressure (PP) amplification, with a higher PP in the central aorta than in the peripheral arteries. The time constant was decreased, together with AC and TPR. ANS dysfunction was described by a reduced BRS, decreased LF power, and suppressed HRV. This compromised condition was not resolved by administration of fluids and noradrenaline. Thus, a persistent vascular and autonomic dysfunction were reported also in the resuscitated animals, and they were found to be significantly correlated.

CONCLUSION

Measures of vascular function and ANS activity could add information to standard hemodynamic and clinical markers, and the current resuscitation strategies could benefit from the adjunction of these additional functional indices.

Tachycardia control in septic shock with esmolol and ivabradine: a comparison on heart function

Persisting tachycardia is often observed in resuscitated septic shock patients, and it is an independent risk factor for increased mortality. Recently, several drugs, such as esmolol and ivabradine, have been proved to be beneficial in HR control, but their overall impact on cardiac functions needs further investigation. The aim of this study is to study the effects of the two drugs on heart function in a protocol of polymicrobial septic shock and resuscitation. Twelve pigs were divided into three experimental groups: the esmolol-treated group (n=4), the ivabradine-treated group (n=5) and the control group (n=3). Cardiac autonomic activity was estimated by heart rate variability (HRV) indices and baroreflex sensitivity (BRS). The Buckberg index was adopted to evaluate myocardial oxygenation efficiency. Septic shock induced a severe autonomic dysfunction and a lower cardiac efficiency, not resolved by fluids resuscitation. The administration of the drugs improved both the HRV and the BRS, but this favourable condition was preserved after noradrenaline administration only in the esmolol group. The interaction of esmolol with the autonomic system is beneficial in septic shock to restore an improved condition of HRV and control, while ivabradine is not as effective when administered in adjunction to noradrenaline.

Reducing tachycardia in septic shock patients: do esmolol and ivabradine have a chronotropic effect only?

An elevated heart rate (HR) often persists in resuscitated septic shock patients, increasing the risk of mortality. Several drugs for HR control, such as esmolol and ivabradine, have been tested in the recent years, but their benefit on the overall cardiovascular system is still under investigation. The aim of this study is to investigate the hemodynamic effects of the two drugs in a protocol of polymicrobial septic shock and resuscitation, mainly focusing on the vascular function. Twelve pigs were divided into three experimental groups: the esmolol-treated group (n=4), the ivabradine-treated group (n=5) and the control group (n=3). The characteristic arterial time constant τ was computed on aortic arterial pressure (AoP), together with estimates of total arterial compliance and peripheral resistance. Power spectral analysis of aortic and radial diastolic BP oscillations was performed to estimate the sympathetic autonomic control of vascular tone. Septic shock induced a severe cardiac and vascular disarray, only partially resolved by resuscitation. The administration of esmolol, but not ivabradine, was beneficial both for cardiac and vascular function, thereby its adjunction to standard therapies could help to improve patient's condition and optimize the resuscitation strategies.Clinical Relevance-This study shows a potential beneficial effect of esmolol on the arterial tree.

A Mathematical Model of dP/dt Max for the Evaluation of the Dynamic Control of Heart Contractility in Septic Shock

OBJECTIVE

Septic shock (SS) patients often show elevated heart rate (HR) despite resuscitation, and this condition is considered an early manifestation of myocardial dysfunction due to an impairment of autonomic nervous system (ANS). We aimed at proposing a mathematical model to assess the autonomic control of ventricular contractility (VC) and HR to track changes in heart functionality during an experimental animal model of SS and resuscitation.

METHODS

SS was induced in six adult swine by polymicrobial peritonitis. We analyzed the beat-to-beat variability of the maximum positive time derivative of left ventricular pressure (dP/dt max), heart period (HP), and aortic blood pressure (ABP). We identified the transfer functions relating fluctuations in ABP and HP to dP/dt max to characterize the static and dynamic properties of the arterial baroreflex and the force-frequency relation mechanisms, respectively. Standard indices of autonomic dysfunction have also been considered as HR variability (HRV) and baroreflex sensitivity (BRS).

RESULTS

During baseline, the baroreflex is predominant in controlling VC with a gain value of -5.8 (-7.5,-3) s^-1, compared to -1.2 (-1.9,-0.5) mmHg/s ms^-1 of the force-frequency autoregulation. During shock, both mechanisms increase their extent in VC control (higher gains and slightly faster dynamics for the baroreflex). After resuscitation, the physiological control of VC is not restored and all the animals still exhibit high HR and reduced HRV and BRS.

CONCLUSION

A condition of cardiovascular inefficiency is persistent after resuscitation and this could be due to autonomic dysfunction.

SIGNIFICANCE

The ANS in SS is crucial to restore homeostasis. Our model could be used to evaluate the efficacy of treatments on VC and related control mechanisms.