Outcomes of patients receiving a massive transfusion for major trauma

Blood Transfusion for Major Trauma in Emergency Department

Severe bleeding is the leading cause of death in patients with major trauma admitted to the emergency department. It is estimated that about 50% of deaths happen within a few minutes of the traumatic event due to massive hemorrhage; 30% of deaths are related to neurological dysfunction and typically happen within two days of trauma; and approximately 20% of patients died of multiorgan failure and sepsis within days to weeks of the traumatic event. Over the past ten years, there has been an increased understanding of the underlying mechanisms and pathophysiology associated with traumatic bleeding leading to improved management measures. Traumatic events cause significant tissue damage, with the potential for severe blood loss and the release of cytokines and hormones. They are responsible for systemic inflammation, activation of fibrinolysis pathways, and consumption of coagulation factors. As the final results of this (more complex in real life) cascade, patients can develop tissue hypoxia, acidosis, hypothermia, and severe coagulopathy, resulting in a rapid deterioration of general conditions with a high risk of mortality. Prompt and appropriate management of massive bleeding and coagulopathy in patients with trauma remains a significant challenge for emergency physicians in their daily clinical practice. Our review aims to explore literature studies providing evidence on the treatment of hemorrhage with blood support in patients with trauma admitted to the Emergency Department with a high risk of death. Advances in blood transfusion protocols, along with improvements in other resuscitation strategies, have become one of the most important issues to face and a key topic of recent clinical research in this field.

Trends in massive transfusion practice for trauma in Japan from 2011 to 2020: a nationwide inpatient database study

BACKGROUND

Previous studies have reported conflicting results regarding fresh frozen plasma (FFP)-to-red blood cell (RBC) ratio and platelet-to-RBC ratio on outcomes for massive transfusion for trauma. Moreover, nationwide data on massive transfusion practices for trauma in the real-world clinical setting are scarce. This study aimed to examine the nationwide practice patterns and trends in massive transfusion for trauma in Japan using a national administrative, inpatient database.

METHOD

We identified patients who underwent emergency hospitalization for trauma and received massive transfusion, defined as administration of at least 20 units of RBC within the first 2 days of admission, using the nationwide inpatient database, which covers approximately 90% of all tertiary emergency hospitals in Japan, between 2011 and 2020. Trends in the incidence and practice patterns of massive transfusion were described by calendar year. The association of practice patterns with mortality or adverse events was tested.

RESULTS

A total of 3,530,846 trauma hospitalizations were identified, of which 5247 (0.15%) received massive transfusion. A significant declining trend was observed in the incidence of massive transfusion in trauma hospitalizations from 0.24% in 2011 to 0.10% in 2020 (P for trend < 0.001). The FFP-to-RBC ratio rose significantly from 0.77 in 2011 to 1.08 in 2020 (P for trend < 0.001), while the platelet-to-RBC ratio remained virtually unchanged from 0.71 in 2011 to 0.78 in 2020 (P for trend 0.060). Massive transfusion with lower FFP-to-RBC (< 0.75) and platelets-to-RBC ratio (< 1.00) were associated with increased in-hospital mortality compared with those ≥ 1.00, while there were linear increases in adverse events with increasing FFP and platelets ratios.

CONCLUSIONS

This study demonstrated a declining trend in the incidence and a rise in higher FFP-to-RBC ratios in massive transfusion in association with patient outcomes for trauma from 2011 to 2020 in Japan.

Clinical Benefits of Early Concurrent Use of Cryoprecipitate and Plasma Compared With Plasma Only in Bleeding Trauma Patients

OBJECTIVES

The effectiveness of cryoprecipitate (Cryo) in trauma has not been well established; the benefits of Cryo might have been overestimated in previous studies since the difference in the total amount of administered clotting factors was not considered. We aimed to evaluate the benefits of the concurrent use of Cryo in combination with fresh frozen plasma (FFP) for bleeding trauma patients.

DESIGN

Retrospective cohort study.

SETTING

The American College of Surgeons Trauma Quality Improvement Program database between 2015 and 2019.

PATIENTS

Patients who received greater than or equal to 5 units of packed RBCs and at least 1 unit of FFP within the first 4 hours after arrival to a hospital were included and dichotomized according to whether Cryo was used within the first 4 hours of hospital arrival.

INTERVENTION

None.

MEASUREMENTS AND MAIN RESULTS

The outcomes of patients treated with Cryo and FFP were compared with those treated with FFP only using propensity score-matching analysis. The dose of administered clotting factors in each group was balanced. The primary outcome was inhospital mortality, and the secondary outcome was the occurrence rate of adverse events. A total of 24,002 patients (Cryo+FFP group: 6,018; FFP only group: 17,984) were eligible for analysis, of whom 4,852 propensity score-matched pairs were generated. Significantly lower inhospital mortality (1,959 patients [40.4%] in the Cryo+FFP group vs 2,142 patients [44.1%] in the FFP only group; odds ratio [OR], 0.86; 95% CI, 0.79-0.93) was observed in the Cryo+FFP group; no significant difference was observed in the occurrence rate of adverse events (1,857 [38.3%] vs 1,875 [38.6%]; OR, 1.02; 95% CI, 0.94-1.10). Several sensitivity analyses showed similar results.

CONCLUSIONS

Cryo use combined with FFP was significantly associated with reduced mortality in bleeding trauma patients. Future randomized controlled trials are warranted to confirm these results.

Association between the plasma-to-red blood cell ratio and survival in geriatric and non-geriatric trauma patients undergoing massive transfusion: a retrospective cohort study

BACKGROUND

The benefits of a high plasma-to-red blood cell (RBC) ratio on the survival of injured patients who receive massive transfusions remain unclear, especially in older patients. We aimed to investigate the interaction of age with the plasma-to-RBC ratio and clinical outcomes of trauma patients.

METHODS

In this retrospective study conducted from 2013 to 2016, trauma patients who received massive transfusions were included. Using a generalized additive model (GAM),we assessed how the plasma-to-RBC ratio and age affected the in-hospital mortality rates. The association of the plasma-to-RBC ratio [low (< 0.5), medium (0.5-1.0), and high (≥ 1.0)] with in-hospital mortality and the incidence of adverse events were assessed for the overall cohort and for patients stratified into non-geriatric (16-64 years) and geriatric (≥ 65 years) groups using logistic regression analyses.

RESULTS

In total, 13,894 patients were included. The GAM plot of the plasma-to-RBC ratio for in-hospital mortality demonstrated a downward convex unimodal curve for the entire cohort. The low-transfusion ratio group was associated with increased odds of in-hospital mortality in the non-geriatric cohort [odds ratio 1.38, 95% confidence interval (CI) 1.22-1.56]; no association was observed in the geriatric group (odds ratio 0.84, 95% CI 0.62-1.12). An increase in the transfusion ratio was associated with a higher incidence of adverse events in the non-geriatric and geriatric groups.

CONCLUSION

The association of the non-geriatric age category and plasma-to-RBC ratio for in-hospital mortality was clearly demonstrated. However, the relationship between the plasma-to-RBC ratio with mortality among geriatric patients remains inconclusive.

Platelet to erythrocyte transfusion ratio and mortality in massively transfused trauma patients. A systematic review and meta-analysis

BACKGROUND

Platelet transfusion during major hemorrhage is important and often embedded in massive transfusion protocols. However, the optimal ratio of platelets to erythrocytes (platelet-rich plasma [PLT]/red blood cell [RBC] ratio) remains unclear. We hypothesized that high PLT/RBC ratios, as compared with low PLT/RBC ratios, are associated with improved survival in patients requiring massive transfusion.

METHODS

Four databases (Pubmed, CINAHL, EMBASE, and Cochrane) were systematically screened for literatures published until January 21, 2021, to determine the effect of PLT/RBC ratio on the primary outcome measure mortality at 1 hour to 6 hours and 24 hours and at 28 days to 30 days. Studies comparing various PLT/RBC ratios were included in the meta-analysis. Secondary outcomes included intensive care unit length of stay and in-hospital length of stay and total blood component use. The study protocol was registered in PROSPERO under number CRD42020165648.

RESULTS

The search identified a total of 8903 records. After removing the duplicates and second screening of title, abstract, and full text, a total of 59 articles were included in the analysis. Of these articles, 12 were included in the meta-analysis. Mortality at 1 hour to 6 hours, 24 hours, and 28 days to 30 days was significantly lower for high PLT/RBC ratios as compared with low PLT/RBC ratios.

CONCLUSION

Higher PLT/RBC ratios are associated with significantly lower 1-hour to 6-hour, 24-hour, 28-day to 30-day mortalities as compared with lower PLT/RBC ratios. The optimal PLT/RBC ratio for massive transfusion in trauma patients is approximately 1:1.

LEVEL OF EVIDENCE

Systematic review and meta-analysis, therapeutic Level III.

Major Prehospital Trauma and In-Hospital Emergencies: Massive Transfusion Triggers

BACKGROUND

Massive transfusion (MT) in trauma is initiated on the basis of factors of different natures and depending on protocols and scales used both in prehospital and in-hospital care areas.

OBJECTIVE

The main goal was to analyze and relate factors and predictive variables for MT requirements considering both health care areas.

METHOD

This was a retrospective cohort study that included patients who were treated either at the emergency department of a large hospital or through prehospital care before arrival at the hospital. The patients included were adults who received MT, defined as a blood bank request of 10 or more units of red cells in the first 24 hours or 5 or more within 4 hours of trauma, from January 1, 2009, to January 1, 2017. The variables included were individual characteristics and those associated with the trauma, clinical-analytical assessment, resuscitation, timing, and survival.

RESULTS

A total of 52 patients who received MT were included. The average age of the patients was 41.23 ± 16.06 years, a mean of 19.56 ± 12.77 units was administered, and the mortality rate was 21.2%.

DISCUSSION

Injury mechanism, clinical-analytical variables, and resuscitation strategies have a significant influence on the need for MT; therefore, early identification is fundamental for performing quality management and addressing avoidable factors during MT processes.

Age-related differences in the impact of coagulopathy in patients with isolated traumatic brain injury: An observational cohort study

BACKGROUND

Although age and coagulopathy are well-known predictors of poor outcome after traumatic brain injury (TBI), the interaction effect of these two predictors remains unclear.

OBJECTIVES

We assessed age-related differences in the impact of coagulopathy on the outcome following isolated TBI.

METHODS

We conducted a retrospective observational study in two tertiary emergency critical care medical centers in Japan from 2013 to 2018. A total of 1036 patients with isolated TBI (head Abbreviated Injury Scale ≥ 3 and other Abbreviated Injury Scale < 3) were selected and divided into the nonelderly (n = 501, 16-64 years) and elderly group (n = 535, age ≥65 years). We further evaluated the impact of coagulopathy (international normalized ratio, >1.2) on the outcomes (Glasgow Outcome Scale-Extended [GOS-E] scores, in-hospital mortality, and ventilation-free days) in both groups using univariate and multivariate models. Further, we conducted an age-based assessment of the impact of TBI-associated coagulopathy on GOS-E using a generalized additive model.

RESULTS

The multivariate model showed a significant association of age and TBI-associated coagulopathy with lower GOS-E scores, in-hospital mortality, and shorter ventilation-free days in the nonelderly group; however, significant impact of coagulopathy was not observed for all the outcomes in the elderly group. There was a decrease in the correlation degree between coagulopathy and GOS-E scores decreased with those older than 65 years.

CONCLUSION

There was a low impact of coagulopathy on functional and survival outcomes in geriatric patients with isolated TBI.

LEVEL OF EVIDENCE

Therapeutic study, Level IV.

Indications for early plasma transfusion and its optimal use following trauma

Aim

This study aimed to evaluate the effect of plasma transfusion before urgent hemostasis initiation on in‐hospital mortality in hemodynamically unstable patients with severe trauma.

Methods

This retrospective observational study of patients admitted to hospital between January 2011 and January 2019 grouped patients according to whether plasma transfusion was initiated before (Before group) or after (After group) hemostasis initiation. Patients with severe trauma who were unable to wait for plasma transfusion and had started hemostasis before the plasma infusion were excluded. We used multivariable logistic regression analysis to determine the effect of plasma transfusion before the initiation of urgent hemostasis on in‐hospital mortality.

Results

We included 47 and 73 patients in the Before and After groups, respectively. Blunt trauma was more common, and the D‐dimer levels and Injury Severity Score were significantly higher in the Before group than in the After group (median D‐dimer, 57.5 versus 38.1 μg/mL; P = 0.040; median Injury Severity Score, 50 versus 34; P < 0.001). Plasma given before hemostasis initiation was associated with significantly lower in‐hospital mortality (adjusted odds ratio, 0.27; 95% confidence interval, 0.078–0.900; P = 0.033) in contrast with the total plasma volume given in the first 6 or 24 h.

Conclusion

Plasma transfusion before hemostasis initiation could be an important factor for improving outcomes in hemodynamically unstable patients with blunt trauma, high D‐dimer levels, or a high Injury Severity Score.

Impact of a streamlined trauma management approach and determinants of mortality among hemodynamically unstable patients with severe multiple injuries: a before-and-after retrospective cohort study

Background

Trauma management requires a multidisciplinary approach, but coordination of staff and procedures is challenging in patients with severe trauma. In October 2014, we implemented a streamlined trauma management system involving emergency physicians trained in severe trauma management, surgical techniques, and interventional radiology. We evaluated the impact of streamlined trauma management on patient management and outcomes (study 1) and evaluated determinants of mortality in patients with severe trauma (study 2).

Methods

We conducted a retrospective cohort study of 125 patients admitted between January 2011 and 2019 with severe trauma (Injury Severity Score ≥16) and persistent hypotension (≥2 systolic blood pressure measurements <90 mm Hg). Patients were divided into a Before cohort (January 2011 to September 2014) and an After cohort (October 2014 to January 2019) according to whether they were admitted before or after the new approach was implemented. The primary outcome was in-hospital mortality.

Results

Compared with the Before cohort (n=59), the After cohort (n=66) had a significantly lower in-hospital mortality (36.4% vs. 64.4%); required less time from hospital arrival to initiation of surgery/interventional radiology (median, 41.0 vs. 71.5 minutes); and was more likely to undergo resuscitative endovascular balloon occlusion of the aorta (24.2% vs. 6.8%). Plasma administration before initiating hemostasis (adjusted OR 1.49 (95% CI 1.04 to 2.14)), resuscitative endovascular balloon occlusion of the aorta (9.48 (95% CI 1.25 to 71.96)), and shorter time to initiation of surgery/interventional radiology (0.97 (95% CI 0.96 to 0.99)) were associated with significantly lower mortality.

Discussion

Implementing a streamlined trauma management protocol improved outcomes among hemodynamically unstable patients with severe multiple trauma.

Level of evidence

Level III.

Mortality in civilian trauma patients and massive blood transfusion treated with high vs low plasma: red blood cell ratio. Systematic review and meta-analysis

Introduction: Massive bleeding in civilian trauma patients leads to dilutional coagulopathy. Transfusion with high plasma: red blood cell (RBC) ratio has been effective in reducing mortality in war trauma patients. However, in civilian trauma the evidence is controversial. Objective: To assess the impact on mortality of high vs low plasma: RBC ratio transfusion, in civilian trauma patients with massive bleeding. Methods: A systematic review and meta-analysis, including observational studies and clinical trials, was conducted. Databases were systemically searched for relevant studies between January 2007 and June 2019. The main outcome was early (24-hours) and late (30-day) mortality. Fixed and random effects models were used. Results: Out of 1295 studies identified, 33 were selected: 2 clinical trials and 31 observational studies. The analysis of observational trials showed both decreased early mortality (odds ratio [OR] 0.67; 95% confidence interval [CI], 0.60–0.75) and late mortality (OR 0.79; 95% CI, 0.71–0.87) with the use of high plasma:RBC ratio transfusion, but there were no differences when clinical trials were evaluated (OR 0.89; 95% CI, 0.64–1.26). The exclusion of patients who died within the first 24hours was a source of heterogeneity. The Injury Severity Score (ISS) altered the association between high plasma: RBC ratio and mortality, with a reduced protective effect when the ISS was high. Conclusion: The use of high vs low plasma: RBC ratio transfusion, in patients with massive bleeding due to civil trauma, has a protective effect on early and late mortality in observational studies. The exclusion of patients who died within the first 24 hours was a source of heterogeneity.

Prescription of red cell concentrates by emergency physicians

OBJECTIVE

To evaluate the adequacy of the prescription of red cell concentrates by emergency physicians.

METHODS

A cross-sectional study based on the survey of transfusion requests records completed by emergency physicians, from May/2018 to April/2019, in an emergency hospital. Adequacy in the indication, volume, and subtype (filtered, irradiated, and washed) of prescribed erythrocytes were evaluated. To compare the qualitative data, we used the χ2 test. The significance level adopted was 5%.

RESULTS

One thousand and twenty-two transfusions were evaluated. The indication, volume, and subtypes were correct in 72.7%, 45.9%, and 81.6% respectively. Transfusion in symptomatic patients presented superior adequacy when compared to asymptomatic individuals with significant statistical difference (indication: 79,6% vs 67.2%, p <0.001; Volume: 63.5% vs 31.7%, p <0.001; subtype: 85.3% vs 78.7%, p 0.006). Among clinical situations, there were more errors in sepsis (39.7%) and pneumonia (36.3%). More than half of the prescriptions presented excessive volume, raising the risk of circulatory overload, observing that the mean age was 60.6 years. The specific analysis of the prescribed subtypes showed adequacy of 17.9% in the filtered, 1.7% in the irradiated, and none in the washed. Thirty transfusions should have been filtered, but the prescriber did not request the subtype.

CONCLUSION

One hypothesis for the observed inaccuracies is inadequate medical training on the subject, both in undergraduate and medical residency, associated with a lack of continuing education on transfusion protocols. The transfusion Committee received the results of this study with a proposal for continuing education measures on transfusion hemotherapy.

Challenges to improving patient outcome following massive transfusion in severe trauma

Introduction: Uncontrolled hemorrhage with trauma-induced coagulopathy (TIC) still represents the most common cause of preventable death after trauma. Timely diagnosis and treatment including bleeding control and hemostatic resuscitation to correct TIC are important, as death from exsanguination occurs rapidly. Recognizing who requires an early massive transfusion together with the initiation of corresponding massive transfusion protocols (MTPs) is key to outcome.Areas covered: This expert review summarizes the current state of MT including the activation and termination of MTPs, complications of MT, and strategies for refinement in the administration of blood products in order to avoid harmful over-transfusion.Expert opinion: MTPs should be initiated and continued until normal physiologic parameters are reached and definitive control of bleeding is achieved. Hospitals should develop their own MTPs, guided by evidence, and according to local infrastructure, logistics, needs and patient populations. Massive transfusion, defined as > 10 units of packed red blood cell concentrates (pRBCs) within the first 24 hours of hospital admission, can be life-saving, but is not without complications. MTPs are currently being refined through targeted and early goal-directed approaches which include functional coagulation testing assays to better guide the administration of blood products and hemostatic agents once the patient is stabilized.

A Comparative Classification Analysis of Abdominal Aortic Aneurysms by Machine Learning Algorithms

The objective of this work was to perform image-based classification of abdominal aortic aneurysms (AAA) based on their demographic, geometric, and biomechanical attributes. We retrospectively reviewed existing demographics and abdominal computed tomography angiography images of 100 asymptomatic and 50 symptomatic AAA patients who received an elective or emergent repair, respectively, within 1-6 months of their last follow up. An in-house script developed within the MATLAB computational platform was used to segment the clinical images, calculate 53 descriptors of AAA geometry, and generate volume meshes suitable for finite element analysis (FEA). Using a third party FEA solver, four biomechanical markers were calculated from the wall stress distributions. Eight machine learning algorithms (MLA) were used to develop classification models based on the discriminatory potential of the demographic, geometric, and biomechanical variables. The overall classification performance of the algorithms was assessed by the accuracy, area under the receiver operating characteristic curve (AUC), sensitivity, specificity, and precision of their predictions. The generalized additive model (GAM) was found to have the highest accuracy (87%), AUC (89%), and sensitivity (78%), and the third highest specificity (92%), in classifying the individual AAA as either asymptomatic or symptomatic. The k-nearest neighbor classifier yielded the highest specificity (96%). GAM used seven markers (six geometric and one biomechanical) to develop the classifier. The maximum transverse dimension, the average wall thickness at the maximum diameter, and the spatially averaged wall stress were found to be the most influential markers in the classification analysis. A second classification analysis revealed that using maximum diameter alone results in a lower accuracy (79%) than using GAM with seven geometric and biomechanical markers. We infer from these results that biomechanical and geometric measures by themselves are not sufficient to discriminate adequately between population samples of asymptomatic and symptomatic AAA, whereas MLA offer a statistical approach to stratification of rupture risk by combining demographic, geometric, and biomechanical attributes of patient-specific AAA.

Use of a high platelet-to-RBC ratio of 2:1 is more effective in correcting trauma-induced coagulopathy than a ratio of 1:1 in a rat multiple trauma transfusion model

BACKGROUND

Platelet dysfunction importantly contributes to trauma-induced coagulopathy (TIC). Our aim was to examine the impact of transfusing platelets (PLTs) in a 2:1 PLT-to-red blood cell (RBC) ratio versus the standard 1:1 ratio on transfusion requirements, correction of TIC, and organ damage in a rat multiple trauma transfusion model.

METHODS

Mechanically ventilated male Sprague Dawley rats were traumatized by crush injury to the small intestine and liver and a fracture of the femur, followed by exsanguination until a mean arterial pressure (MAP) of 40 mmHg. Animals were randomly assigned to receive resuscitation in a high PLT dose (PLT to plasma to RBC in a ratio of 2:1:1) or a standard PLT dose (ratio of 1:1:1) until a MAP of 60 mmHg was reached (n = 8 per group). Blood samples were taken for biochemical and thromboelastometry (ROTEM) assessment. Organs were harvested for histopathology.Outcome measures were transfusion requirements needed to reach a pretargeted MAP, as well as ROTEM correction and organ failure.

RESULTS

Trauma resulted in coagulopathy as assessed by deranged ROTEM results. Mortality rate was 19%, with all deaths occurring in the standard dose group. The severity of hypovolemic shock as assessed by lactate and base excess was not different in both groups. The volume of transfusion needed to reach the MAP target was lower in the high PLT dose group compared to the standard dose, albeit not statistically significant (p = 0.054). Transfusion with a high PLT dose resulted in significant stronger clot firmness compared to the standard dose at all time points following trauma, while platelet counts were similar. Organ failure as assessed by biochemical analysis and histopathology was not different between groups, nor were there any thromboembolic events recorded.

CONCLUSIONS

Resuscitation with a high (2:1) PLT-to-RBC ratio was more effective compared to standard (1:1) PLT-to-RBC ratio in treating TIC, with a trend towards reduced transfusion volumes. Also, high PLT dose did not aggravate organ damage. Transfusion strategies using higher PLT dose regiments might be a feasible treatment option in hemorrhaging trauma patients for the correction of TIC.

[Platelet function disorder in trauma patients, an underestimated problem? Results of a single center study]

BACKGROUND

Plasmatic coagulation disorders in trauma patients are common and their management is subject to current guidelines. Less evidence exists for platelet function. Although it is known that several trauma-associated factors have a negative influence on platelet function, routine monitoring has not yet become established.

METHODS

A retrospective single center study was carried out at a German level 1 trauma center from 2010 to 2016. In all patients fulfilling the requirements for the German Trauma Society (DGU) Traumaregister® who were admitted directly from the scene of the incident, platelet function was analyzed using the Platelet Function Analyzer (PFA 100®) with adenosine diphosphate (ADP) and epinephrine as activation factors. After exclusion of patients with intake of long-term anticoagulant and antiaggregant medication, possible influencing factors of a reduced platelet function were identified.

RESULTS

The results from 310 patients (44.0 ± 14.7 years, 76% male, Injury Severity Score, ISS 28.4 ± 14.2 points) were available. A delayed platelet activation was found in 25.5% using ADP and 31% using epinephrine. Laboratory parameters indicated a greater blood loss. Prolonged closure times were associated with an increased transfusion rate of packed red blood cell concentrates and a higher mortality rate. Logistic regression revealed hemoglobin (Hb) and fibrinogen levels at admission to be independent predictors for a decreased platelet activation in the assay with ADP (p < 0.001, Cohen's f = 0.61) and with epinephrine (p < 0.001, f = 0.42).

CONCLUSION

Approximately one quarter to one third of primarily admitted trauma patients without long-term anticoagulation medication showed a delayed platelet activation in the PFA-100 test. By considering all trauma patients an even higher rate can be expected. The Hb and fibrinogen levels at admission can be helpful to estimate platelet disorders. The development of platelet assays to guide the resuscitation of individual patients seems to be absolutely necessary. The contribution of platelet disorders to trauma-induced coagulopathy is not sufficiently understood. Regarding the importance assigned to platelet transfusion or administration of desmopressin, these aspects should be the subject of further research.