Resuscitative balloon occlusion of the aorta (REBOA) in zone 1 [between the left subclavian artery (LSCA) and the celiac trunk (CeT)] is useful for maintaining hemodynamic stability during hemorrhagic shock following a torso injury [1]. This procedure can be simply and rapidly implemented in the emergency department and is a suitable alternative to open aortic cross-clamping [1]. However, REBOA can cause lower limb ischemia [2]. Further, inappropriate balloon placement above zone 1 may induce brain ischemia, and misplacement in front of CeT may cause total occlusion of the celiac arterial flow and severe visceral organ ischemia [1]. It is preferable to perform REBOA using fluoroscopy, although this can delay definitive surgery. Ongoing research seeks a safe method to implement REBOA in zone 1 without using fluoroscopy [3–5]. However, most of these studies focused on male combatants (not representative of the general population) or required additional information, such as body mass index (BMI), cardio vascular risk factor (smoking and diabetes mellitus), and images from contrast-enhanced ultrasonography, which are rarely available in acute settings. Pezy et al. [6] reported a fixed-distance model that defines zone 1 as 414–474 mm from the upper border of the symphysis pubis in a French civilian population, although it seems to be difficult to implement the balloon within this narrow range in emergency settings and may depend on the target population. Thus, a simple and patient-based method is still needed to guide REBOA implementation in zone 1 and facilitate tis safe use in the emergency department or pre-hospital settings. A cadaver study [7] has indicated that mid-sternum (the mid point between the xiphoid process and sternal notch) may be useful for implementing REBOA. However, these findings may not extrapolate well to trauma patients because cadavers are typically representative of older patients and may also exhibit precise morphological differences from living humans [7]. In Japan, some acute care surgeons use similar methods involving SSN and Xi for implementing REBOA in zone 1, although there is no clear evidence to support this approach. We sought to determine if we could use these anatomical landmarks to safely implement REBOA in zone 1 without using fluoroscopy.

Our study showed that FA–LCSA was significantly longer than FA–SSN and FA–CeT was significantly shorter than FA–Xi. Based on these findings, the REBOA balloon catheter should be shorter than FA–SSN, and longer than FA–Xi to avoid placement outside zone 1 (Fig. 3). External anatomical landmarks can be useful referents for safely implementation of REBOA in zone 1 without radiographic guidance.

This method for determining the target catheter length has several advantages compared with other previously reported methods. The first advantage is that it can rapidly and easily predict a safe balloon catheter length because it only requires external measurements from the common FA to XI and to SSN. Other methods for placement of REBOA require additional equipment, information, and time. For example, contrast-enhanced ultrasonography is useful for implementing REBOA in zone 3 [3]. Although this approach is relatively non-invasive, the need for contrast medium requires additional equipment. In addition, ultrasonography has limited ability in evaluating the descending aorta, thereby precluding its use in zone 1. Another study has constructed a morphometric map for predicting the distance from the right common FA to the major aortic branches [4], although it involves complex calculations that require additional information, such as the patient’s medical history and BMI. Thus, this approach cannot be simply and rapidly used in chaotic trauma resuscitation settings.

The second advantage of this method is that it accounts for the patient’s torso height. In the fixed-distance model, zone 1 was defined as 414–474 mm from the symphysis pubis in a population of trauma patients from an urban area in France. However, this model was not based on each patient’s individual torso height. Moreover, placement can be difficult in such a narrow range. In contrast, anatomical landmarks reflect each patient’s individual torso height. This method may, therefore, give us a wider safety margin to ensure placement in zone 1. We believe that the external anatomical landmark method is a simple and patient-specific method for safely implementing REBOA in zone 1.

A similar approach was proposed in the previous cadaver study [7]. However, we believe that our study is more suited for application with general trauma victims, in urban areas, comparing with the cadaver study. In the cadaver study, older age [median age: 70 (range: 43–75) years], and smaller BMI (mean ± SDBMI: 19.4 ± 3.1 kg/m²) of cadavers limits the ability to translate these results to typical trauma patients [7]. Changes in aortic morphology in the cadavers may further limit to translate these findings to younger patients [7]. It is unknown if the cadaver study findings are representative of younger trauma victims.

We selected actual adult blunt trauma victims from an urban area. Our study population was younger [median age: 47 (range: 21–83) years] with an average BMI (mean ± SDBMI: 22.4 ± 2.26 kg/m²) and may, therefore, be more suitable for application in the general trauma victim population. Our results build upon those of the cadaver study to confirm the acceptability of external landmarks for implementation of REBOA in zone 1.

This study had several limitations. First, it is unclear if our population was representative of all trauma patients because we only evaluated Japanese adults. Additional studies with different populations (particularly those with higher BMI and other nationalities) are needed to confirm if our approach is appropriate for all trauma patients. Second, we do not know if our measurements reflect the clinical requirements of living humans because the advancement and placement of the balloon catheter can be affected by the cardiac output.

Lastly, we believe that this method may be useful for safely implementing REBOA in emergency settings, without radiographic guidance. Importantly, our findings do not mean that it is unnecessary to confirm balloon position by other methods, such as portable chest X-ray in the trauma bay. We should always consider anatomical abnormalities and unexpected misplacement, and radiologically confirm the position as soon as possible to maximize patient safety.

FA–LCSA was significantly longer than FA–SSN, and FA–Xi was significantly longer than FA–CeT. External anatomical landmarks (SSN and Xi) can be used to determine the required balloon catheter length (shorter than FA–SSN and longer than FA–Xi). These anatomical landmarks are useful for safe implementation of REBOA in zone 1 without radiographic guidance.