The current study specified the lengths of REBOA-related Zone II (36.4 ± 8.1 mm) and Zone III (94.7 ± 9.9 mm), which were similar to those in previous studies (13, 15, 16). The CFA terminus of both sides was commonly located beneath the femoral head, which was consistent with Garrett’s report(17). Despite exhaustive anatomical studies of REBOA, parameters related to the Chinese population are first revealed, which is important for enriching the ethnic database and guiding endovascular treatments involving Chinese patients.
This research focuses on a series of details highly related to the application and redesign of EHCDs. Therefore, it can be reasonably deemed the first applied anatomical study of EHCD-related arteries as well as the first surface localization strategy for external NCTH control.
1. The umbilicus is a reliable surface landmark for the AB. Gender, age and BMI can affect their positional relationship
The AB and umbilical subpoints are mainly located at the lower median 1/3 of L4, and the AB is nearly 10 mm left to the latter, which is similar to the literature(18–21) but higher than L5, as reported by Pirró(22). The angle of the AB in this study (47.1 ± 10.5°) was slightly smaller than measurements from cadavers (54 ± 16°)(23). Vascular deformation caused by aging, embalmment and intravascular fillings may account for the discrepancy(24). In addition, a negative correlation between BMI and the AB angle was found in this study (r=-0.245, p = 0.000), and the possibility that different body shapes among living patients and cadavers may affect the results should also be considered.
A series of studies have confirmed that the umbilicus is a reliable surface landmark for the AB(25–29), though the study populations were predominantly female(25–27, 29). The current study found that the vertical distance of the umbilicus-AB in women or skinny patients was smaller than that in men or obese people. It is rational to decrease the compression strength for the former when the umbilicus is used as the landing point of EHCDs to avoid extra damage. According to Attwell(28), the positional relationship of the AB and the umbilicus is not significantly correlated among patients aged under 65, but a positive correlation of the umbilicus-AB vertical distance was revealed by this study (r = 0.161, p = 0.022). Abdominal fat accumulation resulting from hypokinesia and metabolic dysfunction associated with aging may be the main cause for this phenomenon(30). However, aging is also naturally accompanied by decreases in abdominal muscle thickness and strength(31, 32); thus, age and body shape should only be used as a reference for predicting pressurization. The actual pressure and the end point of pressurization should still be determined by hemostasis or the pressure limit of EHCDs. A downward decreasing trend of the artery-surface distance is also detected. The relationship between the depth of arteries and pressurized degree has been preliminarily investigated in humans (34, 35).
The AB is the largest part among EHCD-related arteries, and the current result (17.8 ± 2.7 mm) lies between the previous data (14.0 mm-22.1 mm)(22, 33). Because the difference among CIA/EIA/CFA diameters is only a millimeter in magnitude, the author considers that a uniform size may be acceptable in designing pressurized components (PCs) for iliac and femoral arteries to reduce the time wasted by frequent PC replacement and resultant delays in hemostatic rescue.
Despite the close relationship among the AB, the umbilical subpoint and the L4 vertebra(20), a downward trend of AB position with aging and anatomical variation in the lumbosacral spine(21), the accuracy and clinical value of the L4 alone in localizing the AB declined. Hemostasis is a competition with time. The time to effective hemostasis will be undoubtedly extended by turning the wounded and counting the vertebrae for AB localization. In addition, it is difficult to turn over an individual when they are trapped in a narrow space, and rescuers may apply EHCDs only by their experience if no alternative localization method is available, thus increasing the risk of death from hemostatic failure. Therefore, anterior landmarks taken for artery localization are more intuitive and convenient.
2. The surface localization strategy is accurate to determine the position of EHCD-related arteries
Animal experiments have confirmed that EHCDs can cause serious organ damage with application times of as long as 4 hours(8). However, in extreme environments such as scenes of fierce fighting and austere situations or when transportation is damaged by earthquakes, the EHCD application time must be further extended, thus increasing the incidence and severity of subsequent ischemic damage. In addition, the existing EHCD applications heavily rely on clinical experience(36). Rough arterial localization may result in excessive pressurized areas or exceeding hemostatic segments, which thereby account for unnecessary peripheral tissue injuries. In contrast, the wounded may suffer a high incidence of prehospital death if medical workers try to minimize complications by reducing the application time. Redesign of EHCDs based on refinement of arterial targeted occlusion is helpful for disequilibrating the above balance.
Bland-Altman analysis evaluates the consistency of the new method with the “gold standard” by analyzing their difference value and LoA(37). The actual subpoint printed on the dummy surface is equal to the standard ultrasound localization method. Although the boundary of the LoA looks large, the AAS/SL-ASIS distance of the AB/CIA/EIA/CFA is commonly within ± 10 mm (75%-90%), the length between the AAS/SL of the EIA and the ASIS even fluctuates within ± 5 mm (left: 70%, right: 80%), and the relative distance error of the AAS/SL-ASIS is less than 9.1%. Combined with the average difference between AAS/SL-ASIS (ranging from − 0.1 mm-3.9 mm), data from this study suggest a promising potential surface localization strategy for NCTH external control. The depth of arteries and body surface morphology may intervene in the hemostatic effect. Thus, desired hemostasis is possibly achieved by the meticulous design of the PCs. The size of PCs may be several times the arterial diameter to maintain the balance of hemostasis and minimize organ damage. The vertical distance of the AAS-SL is no more than 4.1 mm, and such a tiny positional deviation is covered by the PCs. This conception can reduce the possibility of mispositioning and resultant hemostatic failure. Theoretically, the surface localization method offers an opportunity for refining NCTH control. The incidence and severity of EHCD-related damage can be reduced, a longer application time will be obtained, and eventually, the prehospital survival rate of NCTH will be improved. In addition, localization training is beneficial for understanding relevant anatomical knowledge and for improving the efficiency of prehospital first aid.
When applying EHCDs for shock patients resulting from blunt injury of the lower abdomen or pelvic fracture, the end of the AA or the AB is the acceptable landing point for PCs due to the unknown location of the damaged artery. However, given the significantly greater transverse diameter of the AB, the investigators suggest that 10 mm above the AB may be a better occlusion point for the purpose of rapid and definitive hemostasis. The scope and severity of ischemia will be limited when the pressurized segment approaches the bleeding artery(38). The confirmation of the location of the iliac and femoral artery termini is helpful to guide the appropriate pressuring site. When compressing the EIA, the occlusion position should be as close to the terminus as possible, but inguinal ligament coverage should be avoided. Similarly, the PC should be located close to or even partially across the CIA terminus when CIA compression is needed. When targeting the narrow inguinal region, pressure transmission will be disturbed because of mispositioning the PC onto the ligament. Genitalia compression may also lead to disastrous outcomes. Therefore, distal and lateral PC placement should be attempted under the premise of effective hemostasis.
The PC is the key for the redesign of EHCDs. The main defect of existing EHCDs is the inability to precisely control the damaged arteries within the lower abdomen and relevant junctional regions(39). Based on the current study, the authors propose a proposal for EHCD improvement. The PCs are composed of 3 pressurized balloons and one spherical balloon 10 mm above the AB, and the size should be adequate for bleeding occlusion but not excessive to interrupt the lowest RA. The other two rectangle-like PCs target the CIA/EIA/CFA; the length can be designed based on the CFA length (the shortest distance), and the width is referred to by the CIA diameter (the widest distance). The optimal material and size need to be clarified in a follow-up study. The PC is fixed with a tourniquet by nylon stickers, and the position is flexible according to the shape of the body, the pressurized site and the targeted artery. The balloon is pumped independently to facilitate different pressure intensities for different arteries. The pumping starts from the lowest bleeding site; if the CFA or EIA bleeding is not well controlled, then the CIA or the end of the AA should be pressurized. Since the downstream blood flow has been partially controlled in advance, further restriction of upstream flow and velocity will result in acceptable hemostasis. Therefore, compared with occluding the AA alone, the novel strategy is expected to reduce the risk of extensive and severe EHCD-related ischemic injury. Relevant studies have been carried out in succession.
There are several limitations in the current study. Patients older than 65 years were excluded from the analysis of anatomical parameters. Frontline forces are mainly constituted by young adults, and multiple epidemiologic studies have indicated that trauma is the leading cause of death among people under 65 who are prone to lose their lives before arriving at the MTF (40–42). Therefore, it is reasonable to believe that the included subjects have already covered the majority of people susceptible to traumatic hemorrhage. The first part of the study belongs to the category of observational research, and no intervention existed during recruitment. The differences in BMI and age between sexes not only reflect the nature of non-artificial data but also manifest the authenticity of the data. Due to the COVID-19 pandemic(43), researchers decided to replace volunteers with 3D-printed dummies to reduce person-to-person infection(44, 45). Since the data originated from live people, the printed models and relevant artery subpoints were qualified alternatives to “gold standard”-ultrasound vessel localization. In addition, the dummies can be repeatedly used in NCTH training, which is resource-friendly and beneficial in hemostatic training.