In our study, the xiphoid process and the umbilicus has been demonstrated to be involved in REBOA positioning in zone III with acceptable predicted accuracy (right: 86.0%, left:84.2%). FA-Xi was longer than FA-LRA and FA-Ui was shorter than the FA-AB. Hence, the length of REBOA catheter positioning in zone III should range from FA-Xi to FA-Ui. The combination of xiphoid process and umbilicus can be used to guide the placement of REBOA in zone III without fluoroscopy.
At present, studies have shown that severe pelvic fractures are regularly accompanied by arterial injury, and the mortality rate can achieve 32% when hemorrhagic shock occurs [12]. Timely and effective treatment of vascular hemorrhage is critical to prevent death from hemorrhagic shock. Clinically, it is possible to intervene early through pelvic packing, and then be treated with interventional embolization to further control catastrophic hemorrhage [13]. However, it is almost unrealistic to achieve complete hemostasis by means of vascular embolization in the pre-hospital environment. REBOA can achieve rapid hemostasis in the pre-hospital settings to maintain central arterial pressure and stabilize vital signs. But due to the limited transport times and pre-hospital environment, the placement of REBOA in zone III is still not ideal at present.
Although it has been proposed the estimation of the catheter length positioning in zone III by torso height, the result was just passable [6]. Wessels et al tried to apply radiofrequency identification to predict the location of balloon in zone III on the cadaver with an accuracy of only 16% [14]. It is also recommended to assemble various individual information (gender, smoking, diabetes mellitus, etc.) to estimate catheter length through multi-factor correlation analysis [15]. Considering the integrity of information and the accuracy of calculation, it may hamper its wide application in the pre-hospital environment. Recently, New technologies such as Gamma probe and the magnetically trackable device have been applied into the balloon placement of the REBOA without fluoroscopy, yet these studies are still in the animal experimental stage [16, 17]. Thus, it is unclear whether these findings are applicable to human being. The positioning of the balloon guided by anatomical structures has attracted much attention. Measuring the distance between external landmarks can directly determine the inserted catheter length for the sake of balloon positioning, which is a practical approach.
Some researchers have struggled to utilize anatomical structures or other strategies to guide the right placement of REBOA. Martinelli et al. proposed that the catheter was inserted 50 cm toward the cephalic, filled with the saline, withdrawn until it was wedged in the aortic bifurcation, and then moved 5 cm to the cephalic [18]. However, it should be noted that the withdrawal process may provoke vascular endothelial damage and the relevant imaging equipment is required for real-time positioning. Recently, It has been proposed to estimate the balloon length by torso height. The accuracy of balloon placement in the target region is appreciable—the accuracy of zone I and zone III are 100%,74%, respectively [6]. We have to admit, however, that the accuracy of the catheter length positioning in zone III is not very satisfactory. In our study, the catheter length positioning in zone III can be quickly estimated according to the external landmarks. Of these, we found that the aortic length was shorter in those patients with short stature. If the fixed catheters continued to be applied in zone III at this time, it may cause long-term ischemic necrosis of vital organs. Inaba and Okada proposed that sternum was likely to be used to guide the catheter length positioning in zone I [9, 10], yet there is no further study on the relevant anatomical structure of zone III.
Through analysis of the CTA measurement data, our study further verified that the accuracy of the catheter length positioning in zone I was up to be 100% by the midpoint of the sternum. Besides, we utilized three measurement methods to compare the difference of predicted accuracy in zone III and found that the midpoint of the xiphoid process and the umbilicus, 7 cm or 9 cm above the umbilicus, and 9.5 cm below the xiphoid process were proven no significant difference. Considering the characteristics of simple operation and quick location in the pre-hospital environment, the midpoint of the xiphoid process and the umbilicus is the preferred method to predict the appropriate catheter length. The optimal distances above umbilicus on the left and right sides are not the same, which may be explained by the difference in bilateral anatomical structures. Interestingly, the optimal distance below the xiphoid process and above the umbilicus is close to half of the distance between the xiphoid process and the umbilicus (17.9 cm (SD = 2.1)). Thus, we have reason to believe that the midpoint of the umbilicus and xiphoid process may be a useful external landmark for the rapid judgment of catheter length positioning in zone III.
Rapid determination of the catheter length positioning in zone III is significantly imperative for the pre-hospital patients who suffered from catastrophic hemorrhage due to the lower limb and pelvis. Although the position of the balloon can be confirmed by X-ray and other imaging examinations after REBOA balloon being filled, it is impractical to achieve this strategy in the pre-hospital environment. For patients with non-compressible torso hemorrhage, the long-time malposition of balloon not only fail to achieve effective hemostasis in the target region, but also largely cause balloon-related complications. As the White Book states, the maximum transportation time for a major trauma patient from the scene should not exceed 30 minutes [19]. We found that the right FA-mZIII and FA-mXU were shown no significant difference in statistical analysis, and FA-mXU could be used to guide the inserted catheter length positioning in zone III. It is well known that the length of zone III is about 8–9 cm without a large fault-tolerance range as zone I. "Too long" and "too short" are likely to lead to disastrous consequences. "Too long" means that the inferior edge of the catheter balloon is easy to cross the lowest renal artery and block the blood supply of vital organs, which immensely reduces the blocking time of REBOA and drives visceral ischemic necrosis and reperfusion injury in severe cases. "Too short" means that the superior edge of the catheter balloon fails to exceed the aortic bifurcation, which can easily lead to vascular dissection or even amputation, thus fail to take full advantage of REBOA in the rapid control of catastrophic hemorrhage. As a result, the appropriate catheter length positioning in zone III is essential to rapid hemostasis in a critical situation.
Finally, several limitations to this study should be considered. First, these conclusions were based on the responses of CT three-dimensional reconstruction and might not vividly reflect measurement processes on the intact body. The positioning of body surface markers (such as umbilicus) may be affected by body shape and body position.
For special groups, such as obesity and pregnant patients, some extra imaging methods must be equipped to avoid possible injuries and confirm precise positioning before inflation. Second, the catheter length estimated by the external landmark may be different from the actual length running in the vascular. The actual catheter length may also be affected by the blood flow velocity and the degree of vascular curvature. Third, our subjects were Chinese population, so it is not known whether our results apply to other groups. Larger studies with longer follow-up are needed to study whether exists other factors affecting the prediction of catheter length in zone III.