Session VII - Pelvic Trauma

Pressure-Volume Characteristics of the Intact and Disrupted Pelvic Retroperitoneum

Kevin A. Thomas, PhD, Mark S. Vrahas, MD, Matthew R. Grimm, MD

Louisiana State University Medical Center, New Orleans, Louisiana, USA

Introduction: Although it is widely accepted that pelvic stabilization helps to control hemorrhage, the concept has not been clearly proven. The vast majority of patients with pelvic fractures do not experience exsanguinating blood loss. Clinical series examining external fixation of the pelvis therefore exclude many patients, while other studies are not randomized with regard to treatment. Some patients exsanguinate even though a fixator has been applied. It is difficult to know whether the application of the fixator has a definite therapeutic benefit or whether other factors may also contribute to the control of blood loss (such as the use of general anesthesia to apply the fixator).

Purpose: The purposes of this study were: (1) to develop a cadaveric model for pelvic fractures, by creating fractures that in vivo would have the potential for exsanguinating blood loss; and (2) to measure the pressure-volume characteristics of the intact and disrupted pelvic retroperitoneum, without and with external fixation. By comparing the pressures required for fluid to flow into the retroperitoneum under these conditions, the effect of external fixation can be assessed.

Materials and Methods: Fluid Inflow and Pressure Measurements: The femoral vein of unembalmed cadaveric specimens was dissected and cannulated, and the external iliac vein was disrupted. An arthroscopic irrigation pump, with a pressure transducer was used to introduce fluid (water) into the retroperitoneum. Radiopaque material was added to the fluid to document the extent of extravasation. After each liter of fluid inflow, the flow was stopped and pressure measurements were made with and without external fixation applied. At the conclusion of the experiment, a laparotomy was performed and retroperitoneal pressure measurements were repeated. Dissection of the specimen verified placement of the catheter for fluid inflow, the retroperitoneal fluid collection, and the extent of the soft tissue injuries caused by the experimental fracture.

Experimental Pelvic Fractures: Steel rods (nominally 1/4" diameter, 16 inches in length) were placed through each pelvis, passing in a straight line entering through the ischial tuberosity and exiting through the anterior superior iliac spine; placed in this orientation the rods passed through the acetabulum and femoral head. The pelvic fractures were created experimentally by application of a load that resulted in simultaneous rapid external rotation of the pelvis around the femoral heads. To apply the loading, a test specimen was held fixed, in a horizontal orientation (i.e., supine), adjacent to the testing machine by a flexible cable attached to one of the pelvic rods. The contralateral pelvic rod was connected to the hydraulic actuator of a mechanical testing machine by a flexible cable passing over a pulley. The loading was applied at a constant displacement rate of 50 mm/second. Control of the testing machine and the real-time data collection was performed using an integrated, computerized control system and software.

Results: In six intact specimens, infusion pressures rapidly rose to approximately 50 mm Hg after inflow of 5 to 8 liters (L) of fluid. With the retroperitoneum intact, pressure averaged 13 mm Hg after 1 L of fluid inflow, and averaged 21 mm Hg after 5 L of fluid inflow. Therefore, in the intact pelvis, significant pressure is required to open the tissue planes to allow fluid flow into the retroperitoneal space.

Open-book fractures (OTA classification 61-B3.1) were created in six specimens. The mean force required to fracture the pelvis was 3481 N (range 2562 N to 5577 N). Dissections revealed that the pelvic floor, sacrotuberous, sacrospinous, and anterior sacroiliac ligaments were disrupted in all cases. After fracture, large volumes of fluid (20 L) could flow into the retroperitoneum at pressures ranging from 11 mm Hg to 38 mm Hg at the highest volumes. External fixation to control the pelvic volume had a moderate effect on the pressures in the retroperitoneal space. Depending on the volume infused, closing the pelvis with an anterior frame increased retroperitoneal pressure approximately 3 mm Hg at lower volumes, to 11 mm Hg at higher volumes. Without the external fixator, pressure within the retroperitoneal space increased an average of 0.74 mm Hg per liter of fluid infused, and with the external fixator, pressure increased an average of 1.04 mm Hg per liter of fluid infused.

Laparotomies were performed on three disrupted specimens following the infusion of fluid. Laparotomy resulted in a drop of pressure from a mean of 35 mm Hg to a mean of 15 mm Hg. Re-application of the external fixator could not restore the pressure to the values measured before laparotomy.

Discussion and Conclusions: The modest effect provided by an anterior external fixator may provide tamponade for venous bleeding, given enough fluid volume is present in the pelvic retroperitoneum. These results suggest that if one was depending upon pressure-induced tamponade, the addition of 3 L of fluid would have the same effect as external fixation. Although this is a substantial volume of fluid, it is relatively small compared to the total volume it is possible to infuse. These results also indicate that even after acute external fixation, there exists a role for selective angiography, ligation, or pelvic packing for patients who do not respond to external fixation alone. Currently, we do not consider external fixation unless the patient has been transfused at least four units of blood and is continuing to bleed.