Discussion
Rapid hemorrhage control is the top priority in NCTH, and yet current treatment modalities are limited. To address some of these concerns, it has previously been demonstrated that a novel retrievable Rescue stent graft can provide rapid hemorrhage control and improved survival compared with REBOA.6 A vascular morphometry study later detailed an algorithm not only for patient triage to presized and retrievable damage control stent grafts based on height and gender, but also the critical relationship of major visceral aortic branches to palpable bony landmarks.17 Numerous efforts have been made to address the issue of placement of endovascular NCTH control in the austere or emergency environment. Methods using standardized positions,10 RF identification,9 14 and even magnetic sensors12 have been explored. However, the aforementioned vascular morphometry study suggests that variability of aortic lengths can complicate device positioning as premeasured metrics can be impacted by body size.17 This morphometric diversity demands a method to track the intravascular device to a target. Although fluoroscopy is a foundation of accurate elective endovascular interventions, the bulky nature and inherent radiation risks to bystanders in an emergency are impractical for the logistics of exsanguinating hemorrhage.
Instead, we investigated a magnetic sensor approach for emergent endovascular device placement. Rezende-Neto et al12 proposed a similar magnetically trackable positioning system for the REBOA, though it was limited by the fact that the positioning system provided only auditory and visual feedback rather than precise distance measurements to localize the distance of the sensor from the magnet. Moreover, their device, although tested in a hemorrhagic porcine model, was not validated under the environmental “noise” conditions as with this study.12
A particular advantage of our approach, contrasting to the use of a permanent magnet in the Rezende-Neto study, is our use of an electromagnet. As the electromagnet can be cycled “on” and “off,” baseline EMFs can be sampled by the magnetic sensor and accounted for, similar to the “white balance” calibration of a photographic camera. This offers a particular “noise cancelling” advantage for the austere situation where environmental noise such as metallic shrapnel, vibration during transportation, or electromagnetic interference from nearby medical devices may negatively impact accuracy. We reasoned such self-calibration may also account for changing physiologic parameters such as extremes in heart rate and blood pressure. Finally, the rapid cycling of the magnet at 0.04 seconds to 0.08 seconds allows for rapid assessment of location for the magnetic sensor during intravascular placement.16
A difference of ±0.5 cm in the fluoroscopy versus magnetic sensor readings was analyzed to be clinically acceptable. This device is envisioned for potential use with a retrievable damage control stent graft, either as a single tier stent graft14 or three-tier Rescue stent graft, as previously reported,6 which consists of a proximal and distal covered zone to control injuries to the descending thoracic and infrarenal abdominal aorta, respectively, whereas a bare-metal middle tier facilitates continued perfusion to the visceral branches of the abdominal aorta. Proper alignment of the bare metal section is crucial to prevent organ ischemia.
The magnetic sensor was least accurate further away (+12 cm) from the target and more accurate as it approached the target, such that at 0 cm and +4 cm; the mean difference in accuracy for both sensors was within±0.5 cm, except for one condition when MSIS with hemorrhage revealed a mean of 0.51 cm, a mere millimeter over our target. This value is well within the tolerable margin of error highlighted in the aforementioned human morphometry study.17 The reduced accuracy at +12 cm is of less importance clinically since it does not define final device placement. Rather the accuracy improves once the sensor makes the final approach on the actual target; as a result the accuracy at +12 cm is probably not significant to the clinical utility.
The sensor, both alone and integrated with the retrievable stent, showed no clinically significant difference under any experimental condition when compared with “control” conditions at 0 cm and +4 cm (table 2), indicating improvement in accuracy with increasing proximity to the target.
The magnetic sensor was maintained under conditions that might otherwise seem disruptive to sensors, including increased aortic impulse (tachycardia and hypertension). Although we did not specifically examine under conditions of bradycardia or asystole, these represent low impulse states and are less likely to impact magnetic sensor performance. Future investigations should include formal evaluation under all physiologic and environmental conditions expected in an environment where torso hemorrhage occurs.
Our study was particularly reassuring in the presence of ferrous shrapnel, which might intuitively seem prohibitive for a magnetic positioning system. This latter finding highlights an especially important feature of this system. By cycling the electromagnet target on and off, the system is continuously sampling the environment, which allows accuracy in the face of electromagnetic noise. The frequent sampling of the environment at 0.04 seconds to 0.08 seconds ensures the system is accounting for dynamic changes in environmental noise and rapid feedback of position to the care provider.
Notably, in this study, the electromagnet was placed posterior to the animal, aligned with the xiphoid, a surrogate for major vascular branches of the aorta. Arguably the magnet could also be placed anterior to the xiphoid, and we found the sensor to be effective at up to 26 cm from the aorta in this position. The reason we elected not to use a front-placed magnet was because the deep chested porcine model created parallax effect with the magnet angled cephalad, thereby creating an error between the “true” and magnetic sensor position. Posterior placement perpendicular to the sensor path therefore produced greater consistency in the experimental design. Ultimately, parallax effect may be a limitation of this device in humans, with the back potentially providing a more consistent surface for magnet placement to avoid the angle deviation that accompanies different types of body habitus at the xiphoid (eg, scaphoid vs obese abdomen). Although our study demonstrated good accuracy within a consistently sized porcine model with a depth averaging up to 10 cm from skin to aorta from a magnet placed on the back, future examination across a wide spectrum of body habitus types in humans will be an important validation. Drift is the tendency of a sensor to adversely bias accuracy with the passing of time or changes in temperature, for instance. Importantly, our study revealed no significant loss of accuracy over either condition, although temperature extremes such as a desert or arctic environment certainly would need evaluation prior to clinical use in such environments. Although we had some initial concerns that the accuracy of the sensor might decrease due to rising magnet temperature with prolonged use in our in vivo model, ex vivo testing confirmed there was no significant relationship between rising difference in true and sensor readings as temperature increased. Nevertheless, future iterations of this system should include electromagnets with optimized wire gauge to reduce heat generation. It is also noteworthy that although passage of endovascular devices without fluoroscopy would seem treacherous due to aneurysms and arterial occlusions, in fact most civilian and battlefield NCTH occur in a young demographic free of these barriers to intravascular device passage. Moreover, these small risks may be justified given the otherwise certain death from rapid exsanguination. The use of a 10 French sheath in this study was largely related to the large gauge wires to which the otherwise small sensor was attached. Future iterations will certainly reduce the wire gauge and are expected to reduce the delivery profile further, as well as produce a more portable delivery system (eg, battery powered electromagnet to eliminate the need for DC power supply). Moreover, while this study examined the magnetic sensor integrated with a retrievable stent graft, the technology certainly should be compatible with other intravascular hemorrhage control approaches. Although this study depicts a porcine model with depths comparable to the human condition, the model was relatively consistent contrasting to a diverse body type of the human population. Rigorous testing in a human cadaver model with varied degree of adipose tissue and size will be critical prior to human use. Moreover, although several categories of simulated environmental noise were examined, testing of all implants both metallic and those generating EMFs will be equally important in the next phases of this technology.
In conclusion, this pilot study demonstrated that a magnetic sensor and electromagnet can be used in conjunction with fixed palpable anatomic landmarks to accurately localize endovascular hemorrhage control devices in emergency settings where use of fluoroscopy would otherwise be prohibitive. Importantly, this was a pilot study of a novel approach for device placement and will require further refinement, validation, regulatory approval, and human clinical trials before it can be considered for human clinical care.