Blood Flow Versus Hematocrit in Optimization of Oxygen Transfer to Tissue During Fluid Resuscitation

Comparison of whole blood versus red blood cells and plasma to correct trauma-induced coagulopathy ex vivo

BACKGROUND

Early resuscitation is based on platelet-poor components such as red blood cells and plasma (RBC + P), contributing to platelet dilution and worsening of trauma-induced coagulopathy (TIC). We aimed to compare the ability of cold-stored whole blood (WB) versus RBC + P as a single component to correct TIC.

STUDY DESIGN AND METHODS

Blood samples were collected on admission from trauma patients who required activation of the major hemorrhage protocol at a single UK major trauma center in 2021/2022. Samples were spiked ex vivo with volumes equivalent to two, four, or eight units of WB or RBC + P stored for a maximum of 2 weeks. Thromboelastometry, platelet counting, and multiple electrode aggregometry (MEA) were performed.

RESULTS

Samples from 20 adult trauma patients were analyzed. Median age was 32 years (27-42), 89% were male, 70% had platelet dysfunction (tissue factor-activated ROTEM [EXTEM]-tissue factor-activated ROTEM with cytochalasin D [FIBTEM] clot amplitude at 5 min [A5] ≤ 30 mm), 65% were coagulopathic (EXTEM A5 ≤ 40 mm), and 42% died. EXTEM-FIBTEM A5 was higher following spiking with WB than RBC + P (33 mm, 26-33, vs. 27 mm, 24-30, p < .001). WB-spiking corrected platelet dysfunction in 2 patient samples out of 20, whereas RBC + P increased the frequency of platelet dysfunction (1/20 sample) and TIC (4/20 samples). RBC + P was associated with a dose-dependent deterioration in rotational thromboelastometry (ROTEM) clot strength and dynamics, platelet count, and aggregation in response to multiple agonists compared with WB-spiking, which maintained or partially corrected these abnormalities.

CONCLUSION

Compared with RBC + P, WB better preserves ex vivo platelet-related ROTEM parameters, platelet count, and aggregation, but does not fully correct these common derangements of TIC.

Detection of Internal Hemorrhage via Sequential Inference: An In Silico Feasibility Study

This paper investigates the feasibility of detecting and estimating the rate of internal hemorrhage based on continuous noninvasive hematocrit measurement. A unique challenge in hematocrit-based hemorrhage detection is that hematocrit decreases in response to hemorrhage and resuscitation with fluids, which makes hemorrhage detection during resuscitation challenging. We developed two sequential inference algorithms for detection of internal hemorrhage based on the Luenberger observer and the extended Kalman filter. The sequential inference algorithms use fluid resuscitation dose and hematocrit measurement as inputs to generate signatures to enable detection of internal hemorrhage. In the case of the extended Kalman filter, the signature is nothing but inferred hemorrhage rate, which allows it to also estimate internal hemorrhage rate. We evaluated the proof-of-concept of these algorithms based on in silico evaluation in 100 virtual patients subject to diverse hemorrhage and resuscitation rates. The results showed that the sequential inference algorithms outperformed naïve internal hemorrhage detection based on the decrease in hematocrit when hematocrit noise level was 1% (average F1 score: Luenberger observer 0.80; extended Kalman filter 0.76; hematocrit 0.59). Relative to the Luenberger observer, the extended Kalman filter demonstrated comparable internal hemorrhage detection performance and superior accuracy in estimating the hemorrhage rate. The analysis of the dependence of the sequential inference algorithms on measurement noise and plant parametric uncertainty showed that small (≤1%) hematocrit noise level and personalization of sequential inference algorithms may enable continuous noninvasive detection of internal hemorrhage and estimation of its rate.

The physiological basis for individualized oxygenation targets in critically ill patients with circulatory shock

BACKGROUND

Circulatory shock, defined as decreased tissue perfusion, leading to inadequate oxygen delivery to meet cellular metabolic demands, remains a common condition with high morbidity and mortality. Rapid restitution and restoration of adequate tissue perfusion are the main treatment goals. To achieve this, current hemodynamic strategies focus on adjusting global physiological variables such as cardiac output (CO), hemoglobin (Hb) concentration, and arterial hemoglobin oxygen saturation (SaO2). However, it remains a challenge to identify optimal targets for these global variables that best support microcirculatory function. Weighting up the risks and benefits is especially difficult for choosing the amount of oxygen supplementation in critically ill patients. This review assesses the physiological basis for oxygen delivery to the tissue and provides an overview of the relevant literature to emphasize the importance of considering risks and benefits and support decision making at the bedside.

PHYSIOLOGICAL PREMISES

Oxygen must reach the tissue to enable oxidative phosphorylation. The human body timely detects hypoxia via different mechanisms aiming to maintain adequate tissue oxygenation. In contrast to the pulmonary circulation, where the main response to hypoxia is arteriolar vasoconstriction, the regulatory mechanisms of the systemic circulation aim to optimize oxygen availability in the tissues. This is achieved by increasing the capillary density in the microcirculation and the capillary hematocrit thereby increasing the capacity of oxygen diffusion from the red blood cells to the tissue. Hyperoxia, on the other hand, is associated with oxygen radical production, promoting cell death.

CURRENT STATE OF RESEARCH

Clinical trials in critically ill patients have primarily focused on comparing macrocirculatory endpoints and outcomes based on stroke volume and oxygenation targets. Some earlier studies have indicated potential benefits of conservative oxygenation. Recent trials show contradictory results regarding mortality, organ dysfunction, and ventilatory-free days. Empirical studies comparing various targets for SaO2, or partial pressure of oxygen indicate a U-shaped curve balancing positive and negative effects of oxygen supplementation.

CONCLUSION AND FUTURE DIRECTIONS

To optimize risk-benefit ratio of resuscitation measures in critically ill patients with circulatory shock in addition to individual targets for CO and Hb concentration, a primary aim should be to restore tissue perfusion and avoid hyperoxia. In the future, an individualized approach with microcirculatory targets will become increasingly relevant. Further studies are needed to define optimal targets.

Transfusion increased skin blood flow when initially low in volume-resuscitated patients without acute bleeding

Background

Alterations in skin blood flow is a marker of inadequate tissue perfusion in critically ill patients after initial resuscitation. The effects of red blood cell transfusions (RBCT) on skin perfusion are not described in this setting. We evaluated the effects of red blood cell transfusions on skin tissue perfusion in critically ill patients without acute bleeding after initial resuscitation.

Methods

A prospective observational study included 175 non-bleeding adult patients after fluid resuscitation requiring red blood cell transfusions. Using laser Doppler, we measured finger skin blood flow (SBF) at skin basal temperature (SBFBT), together with mean arterial pressure (MAP), heart rate (HR), hemoglobin (Hb), central venous pressure (CVP), lactate, and central or mixed venous oxygen saturation before and 1 h after RBCT. SBF responders were those with a 20% increase in SBFBT after RBCT.

Results

Overall, SBFBT did not significantly change after RBCT [from 79.8 (4.3-479.4) to 83.4 (4.9-561.6); p = 0.67]. A relative increase equal to or more than 20% in SBFBT after RBCT (SBF responders) was observed in 77/175 of RBCT (44%). SBF responders had significantly lower SBFBT [41.3 (4.3-279.3) vs. 136.3 (6.5-479.4) perfusion units; p < 0.01], mixed or central venous oxygen saturation (62.5 ± 9.2 vs. 67.3% ± 12.0%; p < 0.01) and CVP (8.3 ± 5.1 vs. 10.3 ± 5.6 mmHg; p = 0.03) at baseline than non-responders. SBFBT increased in responders [from 41.3 (4.3-279.3) to 93.1 (9.8-561.6) perfusion units; p < 0.01], and decreased in the non-responders [from 136.3 (6.5-479.4) to 80.0 (4.9-540.8) perfusion units; p < 0.01] after RBCT. Pre-transfusion SBFBT was independently associated with a 20% increase in SBFBT after RBCT. Baseline SBFBT had an area under receiver operator characteristic of 0.73 (95% CI, 0.68-0.83) to predict SBFBT increase; A SBFBT of 73.0 perfusion units (PU) had a sensitivity of 71.4% and a specificity of 70.4% to predict SBFBT increase after RBCT. No significant differences in SBFBT were observed after RBCT in different subgroup analyses.

Conclusion

The skin blood flow is globally unaltered by red blood cell transfusions in non-bleeding critically ill patients after initial resuscitation. However, a lower SBFBT at baseline was associated with a relative increase in skin tissue perfusion after RBCT.

The volume of infusion fluids correlates with treatment outcomes in critically ill trauma patients

Background

Appropriate fluid management is essential in the treatment of critically ill trauma patients. Both insufficient and excessive fluid volume can be associated with worse outcomes. Intensive fluid resuscitation is a crucial element of early resuscitation in trauma; however, excessive fluid infusion may lead to fluid accumulation and consequent complications such as pulmonary edema, cardiac failure, impaired bowel function, and delayed wound healing. The aim of this study was to examine the volumes of fluids infused in critically ill trauma patients during the first hours and days of treatment and their relationship to survival and outcomes.

Methods

We retrospectively screened records of all consecutive patients admitted to the intensive care unit (ICU) from the beginning of 2019 to the end of 2020. All adults who were admitted to ICU after trauma and were hospitalized for a minimum of 2 days were included in the study. We used multivariate regression analysis models to assess a relationship between volume of infused fluid or fluid balance, age, ISS or APACHE II score, and mortality. We also compared volumes of fluids in survivors and non-survivors including additional analyses in subgroups depending on disease severity (ISS score, APACHE II score), blood loss, and age.

Results

A total of 52 patients met the inclusion criteria for the study. The volume of infused fluids and fluid balance were positively correlated with mortality, complication rate, time on mechanical ventilation, length of stay in the ICU, INR, and APTT. Fluid volumes were significantly higher in non-survivors than in survivors at the end of the second day of ICU stay (2.77 vs. 2.14 ml/kg/h) and non-survivors had a highly positive fluid balance (6.21 compared with 2.48 L in survivors).

Conclusion

In critically ill trauma patients, worse outcomes were associated with higher volumes of infusion fluids and a more positive fluid balance. Although fluid resuscitation is lifesaving, especially in the first hours after trauma, fluid infusion should be limited to a necessary minimum to avoid fluid overload and its negative consequences.

Pathophysiology, mechanisms, and managements of tissue hypoxia

Oxygen is needed to generate aerobic adenosine triphosphate and energy that is required to support vital cellular functions. Oxygen delivery (DO2) to the tissues is determined by convective and diffusive processes. The ability of the body to adjust oxygen extraction (ERO2) in response to changes in DO2 is crucial to maintain constant tissue oxygen consumption (VO2). The capability to increase ERO2 is the result of the regulation of the circulation and the effects of the simultaneous activation of both central and local factors. The endothelium plays a crucial role in matching tissue oxygen supply to demand in situations of acute drop in tissue oxygenation. Tissue oxygenation is adequate when tissue oxygen demand is met. When DO2 is severely compromised, a critical DO2 value is reached below which VO2 falls and becomes dependent on DO2, resulting in tissue hypoxia. The different mechanisms of tissue hypoxia are circulatory, anaemic, and hypoxic, characterised by a diminished DO2 but preserved capacity of increasing ERO2. Cytopathic hypoxia is another mechanism of tissue hypoxia that is due to impairment in mitochondrial respiration that can be observed in septic conditions with normal overall DO2. Sepsis induces microcirculatory alterations with decreased functional capillary density, increased number of stopped-flow capillaries, and marked heterogeneity between the areas with large intercapillary distance, resulting in impairment of the tissue to extract oxygen and to satisfy the increased tissue oxygen demand, leading to the development of tissue hypoxia. Different therapeutic approaches exist to increase DO2 and improve microcirculation, such as fluid therapy, transfusion, vasopressors, inotropes, and vasodilators. However, the effects of these agents on microcirculation are quite variable.

Current practice and evolving concepts in septic shock resuscitation

Clinical and pathophysiological understanding of septic shock has progressed exponentially in the previous decades, translating into a steady decrease in septic shock-related morbidity and mortality. Even though large randomized, controlled trials have addressed fundamental aspects of septic shock resuscitation, many questions still exist. In this review, we will describe the current standards of septic shock resuscitation, but the emphasis will be placed on evolving concepts in different domains such as clinical resuscitation targets, adequate use of fluids and vasoactive drugs, refractory shock, and the use of extracorporeal therapies. Multiple research opportunities remain open, and collaborative endeavors should be performed to fill in these gaps.

Seven Mathematical Models of Hemorrhagic Shock

Although mathematical modelling of pressure-flow dynamics in the cardiocirculatory system has a lengthy history, readily finding the appropriate model for the experimental situation at hand is often a challenge in and of itself. An ideal model would be relatively easy to use and reliable, besides being ethically acceptable. Furthermore, it would address the pathogenic features of the cardiovascular disease that one seeks to investigate. No universally valid model has been identified, even though a host of models have been developed. The object of this review is to describe several of the most relevant mathematical models of the cardiovascular system: the physiological features of circulatory dynamics are explained, and their mathematical formulations are compared. The focus is on the whole-body scale mathematical models that portray the subject's responses to hypovolemic shock. The models contained in this review differ from one another, both in the mathematical methodology adopted and in the physiological or pathological aspects described. Each model, in fact, mimics different aspects of cardiocirculatory physiology and pathophysiology to varying degrees: some of these models are geared to better understand the mechanisms of vascular hemodynamics, whereas others focus more on disease states so as to develop therapeutic standards of care or to test novel approaches. We will elucidate key issues involved in the modeling of cardiovascular system and its control by reviewing seven of these models developed to address these specific purposes.

The effect of blood transfusion on sublingual microcirculation in critically ill patients: A scoping review

PURPOSE

To investigate the effects of red blood cell (RBC) transfusion on sublingual microcirculation in critically ill patients.

METHODS

Systematic strategy was conducted to search studies that measured sublingual microcirculation before and after transfusion in critically ill patients. This review was reported according to the Preferred Reporting Items for Systematic Review and Meta-Analyses Scoping Review Extension.

RESULTS

The literature search yielded 114 articles. A total of 11 studies met the inclusion criteria. Observational evidence showed diffusive capacity of the microcirculation significantly improved in intraoperative and anemic hematologic patients after transfusion, while the convective parameters significantly improved in traumatic patients. RBC transfusion improved both diffusive and convective microcirculatory parameters in hypovolemic hemorrhagic shock patients. Most of the studies enrolled septic patients showed no microcirculatory improvements after transfusion. The positive effects of the leukoreduction were insufficiently supported. The effects of the storage time of the RBCs were not conclusive. The majority of the evidence supported a negative correlation between baseline proportion of perfused vessels (PPV) and changes in PPV.

CONCLUSIONS

This scoping review has catalogued evidence that RBC transfusion differently improves sublingual microcirculation in different populations. The existing evidence is not sufficient to conclude the effects of the leukoreduction and storage time of RBCs.

Automated quantification of tissue red blood cell perfusion as a new resuscitation target

PURPOSE OF REVIEW

Identification of insufficient tissue perfusion is fundamental to recognizing circulatory shock in critically ill patients, and the primary target to restore adequate oxygen delivery. However, the concept of tissue perfusion remains ill-defined and out-of-reach for clinicians as point-of-care resuscitation target. Even though handheld vital microscopy (HVM) provides the technical prerequisites to collect information on tissue perfusion in the sublingual microcirculation, challenges in image analysis prevent quantification of tissue perfusion and manual analysis steps prohibit point-of-care application. The present review aims to discuss recent advances in algorithm-based HVM analysis and the physiological basis of tissue perfusion-based resuscitation parameters.

RECENT FINDINGS

Advanced computer vision algorithm such as MicroTools independently quantify microcirculatory diffusion and convection capacity by HVM and provide direct insight into tissue perfusion, leading to our formulation a functional parameter, tissue red blood cell (RBC) perfusion (tRBCp). Its definition is discussed in terms of the physiology of oxygen transport to the tissue and its expected effect as a point-of-care resuscitation target. Further refinements to microcirculatory monitoring include multiwavelength HVM techniques and maximal recruitable microcirculatory diffusion and convection capacity.

SUMMARY

tRBCp as measured using algorithm-based HVM analysis with an automated software called MicroTools, represents a promising candidate to assess microcirculatory delivery of oxygen for microcirculation-based resuscitation in critically ill patients at the point-of-care.

Microvascular Dysfunction in the Critically Ill

Microvascular dysfunction is a frequent complication of many chronic and acute conditions, especially in the critically ill. Moreover, the severity of microvascular alterations is associated with development of organ dysfunction and poor outcome. The complexities and heterogeneity of critical illness, especially in the elderly patient, requires more mechanistically oriented clinical trials that monitor the effectiveness of existing therapies and of those to come. Recent advances in the ability to obtain physiologically based assessments of microcirculatory function at the bedside will make microcirculatory-guided resuscitation a point of care reality.