Discussion
This study suggests that suprahepatic REBOVC may quickly be guided to the correct position using ultrasound in a laboratory porcine model. To our knowledge, no scientific article has previously described the use of ultrasound for the guided positioning of REBOVC. Case reports where REBOVC has been used as a bridge to surgery in hemorrhagic shock and patients in extremis have been published.3 4 27 A recent report from Baltimore, presenting the use of REBOVC in five trauma patients, provides support for successful clinical use of REBOVC in the management of vena cava injuries.5 The use of REBOVC as an adjunct to REBOA in vena cava injuries is a new tool, recommended under very special circumstances. REBOVC may be considered a tool for the hybrid management of vena cava injuries, where vascular control is achieved by combining endovascular techniques with definitive open surgical repair.12 Similar to the REBOA, for the REBOVC to be a clinically available tool, it must be able to be safely, quickly and correctly positioned. In trauma patients, ultrasound provides a rapid, cheap and non-invasive tool.
The subxiphoidal view is one of the standard FAST examination views and although examination of the vena cava is not part of the standard FAST protocol, the inferior vena cava can relatively easily be seen by anyone performing FAST exams with some training.28 In a pilot study by Martin et al, identifying the inferior vena cava by ultrasound was shown to be a skill that is relatively fast to learn.28 Thus, we believe that positioning REBOVC guided by ultrasound may be a skill that can be quickly learned.
For placement of REBOA in less emergent situations, catheter insertion by guidewire and balloon inflation is often guided by fluoroscopy to optimize the position.15 However, in rural emergency departments, prehospital settings and austere military environments, fluoroscopy may not be available.29 In such settings, external anatomical landmarks or standard fixed distance models can guide REBOA placement.15 18 30 31 However, blind placement methods have their limitations.31 Differences in habitus, age, current physiology and pregnancy, vessel morphology and intravascular distances may alter the representativity and the reliability of anatomic landmarks and fixed distances.15–18 Even with the help of guidewires, blind REBOA advancement to the correct position in the vessel may be difficult.17 Incorrect balloon positioning may have devastating consequences.21 Therefore, if possible, radiological or sonographic confirmation of proper balloon placement is recommended for all levels of endovascular balloon placement.17 One animal died of vena cava perforation during catheter insertion. An intravascular resistance was felt at initial catheter insertion but was forced and unintentionally possibly caused the vessel injury. Vessel wall damage is a known complication to all arterial and venous endovascular management, and a risk associated with the method. Venous vessels are thinner, more fragile and may more easily be hurt, which must be considered when dealing with venous endovascular catheters.8 32 That incident happened despite guidance by fluoroscopy. However, considering the limited clinical experience with REBOVC and the results of this study, we do not recommend placement of REBOVC without fluoroscopy in humans. If available, combined usage of ultrasound and portable fluoroscopy would be advisable.
Since endovascular balloon occlusion of the vena cava is a new method to manage inferior vena cava injuries, no ‘standard balloons’ are available. Our choice of the Equalizer balloon was based on our previous experience with this balloon for vena cava occlusion. The Equalizer balloon is made of natural rubber latex, which makes it highly compliant and suitable for suprahepatic venous occlusion.14 This balloon requires a 7 Fr sheath, comparable to other commonly used endovascular balloons, such as the ER-REBOA (Prytime Medical, Texas, USA). However, we used a 10 Fr sheath and thus avoided balloon placement with guidewire insertion, not to facilitate visualization.
The median placement times for fluoroscopy-guided and ultrasound-guided REBOA and REBOVC was short. Importantly, this time does not include gaining femoral access, only balloon positioning. The time differences between fluoroscopy-guided REBOVC and ultrasound-guided REBOVC were small and may be considered negligible in a clinical setting. The number of animals included was small and the study was performed in a controlled laboratory environment, which must also be considered when evaluating the time measurements. In a chaotic emergency setting, the time taken to perform ultrasound-guided or fluoroscopy-guided endovascular placement may differ considerably. In our laboratory, the imaging equipment was set up and ready from start, while in the trauma bay, the ultrasound or fluoroscopy must be set up before examination. However, the primary goal of the study was not to perform a time comparison between ultrasound and fluoroscopy but rather to examine if ultrasound could be used to guide REBOVC placement. The anesthetist performing the ultrasound exam in this study was considered representative to the potential medical staff that may perform ultrasound exams in trauma patients in prehospital and intrahospital settings.
Limitations
This study has some limitations. Although it is based on a porcine model and its applicability to human anatomy is not proven, the vascular anatomy of the pigs is similar to humans with regard to the inferior vena cava and the aorta.33 Pigs have five main hepatic veins, whereas humans have three. Similar to humans, these five porcine hepatic veins have two main drainage locations in the cranial third of the retrohepatic vena cava.34 Through our own experience, we have assessed the porcine suprahepatic vena cava to be similar in length to the corresponding human vessel. Unlike humans, the entry of inferior vena cava to the right atrium is not easily visualized by ultrasound in pigs. However, a too cranially located REBOVC should be seen by ultrasound in humans. No other known major anatomical differences are regarded to have impacted the results in this study.35 In humans, obesity and gas-filled bowels may make visualization by ultrasound more challenging or even impossible.21 36 37 The pigs had a mean weight of 30 kg and thus may be comparable to human children in terms of weight and abdominal wall configuration. The laboratory has a long-time cooperation with this farmer, providing this special breed of pigs. Even though we could have used heavier pigs, at the time of laboratory research, these animals were the ones provided by the local farmer. Our study was performed in normovolemic conditions and not in a state of hemorrhagic shock physiology, when REBOVC is most likely to be used. In hemorrhagic shock, the blood vessels may be more difficult to visualize due to lumen collapse, vasospam and other effects of the cathecholamine response.17 With any endovascular manipulation, there is a risk of injury to the vessel wall. Since venous vessels generally are thinner and more fragile, insertion and inflation of endovascular balloons in veins warrant extra caution. Trauma patients may have occult vessel injuries. Insertion of endovascular catheters in such vessels can worsen those injuries and may be fatal.17 38