Have you ever been severely jabbed in the kidney, or been on the receiving end of a door opening into the side your lower back? While that injury may entitle you to some financial compensation, who are you going to call to the stand at the personal injury trial to make that happen? Your local urologic surgery is a one-stop shop for all significant surgical needs for kidney, ureter, bladder, and urethral conditions. Kidneys and other ductwork, given their fragility and importance to excretion, as well as maintaining blood salinity, require a host of specialized medical tools for conducting effective surgeries safely. My rotation group watched one of these surgeries at the UCI Medical Center, a percutaneous nephrolithotomy. Through the lens of this single surgery, we can begin to grasp the state of the art of biomedical engineering technology in urological surgery environments with a particular focus on Holmium laser lithotripters, and general anesthesia equipment.
Percutaneous nephrolithotomy, or PNCL, is a minimally invasive surgery for removing kidney stones larger than 2 cm in diameter. PNCL is also used when kidney stones block a collecting duct or ureter, if the kidney is anatomically abnormal or if the stone is smaller but does not respond to ultrasound. [1, 2] To conduct the procedure, the patient lays prone, and receives local anesthesia in the back above the kidney. The surgeon then punctures the back with a needle to the calyx of the target kidney to make a 1.3 cm incision, using an Xray . [2] The patient is then placed under general anesthesia, or a more powerful regional anesthesia. The incision is dilated with Teflon dilators. The last dilator is fitted with a sheath, which in the surgery we watched had a notable resemblance to a boba straw. [2] The sheath is used to allow access to a Holmium laser lithotripter to fragment or pulverize the kidney stone. The patient is also fitted with a catheter through the urethra, the bladder and to the ureter. [1] This catheter allows a nephoscope fitted with a camera, irrigation fluid and X-ray contrast to reach the target area without interfering with the laser lithotripter. The nephoscope can also grab some smaller stones. Lastly, the surgeon might place a nephrostomy tube to drain urine post-operatively. [1] In the procedure I attended, this process took about 2-3 hours, with most of the time dedicated to directing the two laparoscopic tools with X-ray contrast, and making sure all large pieces were removed. The patient was not fitted with a nephrostomy tube and did not have any complications. Complications to the procedure could include hemorrhage from damaging blood vessels, sepsis from damaging the kidney, and pleural puncture injury from accidentally hitting the lung during the initial needle entry. Patient death is uncommon, but can happen to anywhere between 1 in 1000 to 7 in 1000 patients, usually caused by sepsis. [2] Device development must minimize these critical risks before getting nice-to-have results, and if possible reduce the patient mortality rate to make the procedure even safer.
Holmium laser lithotripters make sure all kidney stone chunks are either removed or small enough to pass safely out with urine, otherwise a second surgery may be needed. Currently used lasers are effective, but require careful surgical use to keep track of fragmentation and to not burn the kidney. Holmium laser lithotripters work by shining a powerful, focused ray of infrared light at the kidney stone. The stone heats up from light exposure and builds up pressure inside small air pockets in the stone, causing it to fracture apart. Under the correct conditions, this fracture is so numerous and widespread that the targeted is turned into dust. [3] Softer stones are easier to “dust”, and harder stones typically fragment. Soft stones sometimes also fragment into 2 mm size chunks, so the dusting technique still requires a thorough check for straggler pieces. [3] The search for extra pieces is not always enough given the small field-of-view on the cameras and X-ray tracer missing small chunks, and post-operative stays are necessary to verify the patient’s urine flow returns. This risk of needing to undergo a second procedure shows a medical need for a better mechanism or laser setting to pulverize harder stones into dust, and make dusting more consistent. This pulverization could be done with current tools, but the biggest limiting factor from achieving this change is the trade off between laser power and tissue damage. Currently, two factors protect the surrounding tissue from damage from the laser. First, the infrared wavelength the laser uses, 2120 nm, is absorbed by water in the urine and irrigation fluid around the laser. Second, the laser light is pulsed at 80 Hz, so quickly that the urinary tract wall are only superficially damaged, with a tissue penetration of 0.4 mm. [3] Increasing the power of the laser or the wavelength carries risk of non-superficial tissue damage, and by extension increased risk of sepsis. Avenues for future development focus on altering pulse frequency, as increasing pulse frequency can alter dusting behavior of stones without increasing tissue damage. Recent experiments into higher frequency lithotripsy use frequencies up to 500 Hz, and may provide improved capability in the near future. [3]
The role of anesthesia throughout this surgery should not go unnoticed, not only because my team stood closer to the anesthesiologist through the procedure, but also due to the importance of anesthetic monitoring in catching complications early, thereby reducing patient mortality in urologic surgery. From the very start of the procedure, general anesthesia monitoring tools help to minimize damage from complication. Since general anesthesia equipment can allow the anesthesiologist to control oxygen content reaching the patient, during the access puncture, the anesthesiologist can flood the patient with extra oxygen if there is an accidental pleural puncture. [2]
When PCNL surgeries target kidney stone high up the back on the kidney general anesthesia is the safest option, but comes with its own risks. Using endotracheal tubes in a prone position can cause the tubes to kink and choke patient, and pressure on patient eyes from the prone position can also cause vision loss from pressure on the optic nerve. Anesthesiologists also monitor for volume overload, which can damage the kidneys further by forcing the kidneys to process more fluids than they can handle. Volume overload is more difficult to monitor in patients under general anesthesia and is mainly visible from ECG changes. [2] With so many issues to consider, easy access to the patient’s head is important to ensure airflow and post-operative vision are not lost due to the prone position of the patient. Currently, ECG and EEG monitoring tools obscure the patient with as many as a dozen wires to connect electrodes to the computer. When asked what would make his job easier, the anesthesiologist overseeing our procedure suggested we investigate wireless options to keep the space around the patient’s head free of obstruction. The technology currently exists to make this switch, but there are some trade-offs that may not be optimal for the operating room. Wireless EEG systems consist of a battery, an electrode cap, and a transmission method, typically either using Wi-Fi or Bluetooth. [4] The operating setting already uses a wired electrode cap, so the only additions needed would be a long-lasting battery and a WiFi transmitter. The three possible downsides are electronic interference, and bandwidth of WiFi signals, battery weight on the head, and battery recharging. Niso et al. 2023 notes that WiFi can carry 2 Gigabytes per second, but the main two concerns are if 2 Gbps is enough to transfer dense EEG data, and if an X-ray in the room interferes with WiFi. [4] I was able to use my phone to access the internet in the OR, so the ability for the WiFi to transfer the data with high resolution is the biggest technical impediment. Handling the battery would also be a significant undertaking to make sure it would be light enough to attach to the cap and not clutter the area around the head. The biggest issue for this wireless design is battery life. Currently available options can run for 24 hours continuously, but with multiple operations a week each taking 3 hours without complications, the cap would need consistent recharging. [4] While technically feasible, the risk of forgetting to recharge the cap halting the surgery may be more significant than the risks associated with a cluttered area.
Biomedical engineering tools lay the foundation to the techniques and best practices used in urologic surgery today. Modern methods of large kidney stone removal are much safer today due to advancements in tools like laser lithotripsy, or would simply not be possible, like general anesthesia enabling removal of kidney stones at the top of the kidney. The direction of biomedical engineering research into clinical needs in urologic surgery suggest highly feasible improvements to patient treatment effectiveness and complication mitigation can be met with new medical devices in the near-term future.
[1] “Percutaneous nephrolithotomy,” Mayo Clinic, https://www.mayoclinic.org/testsprocedures/percutaneous-nephrolithotomy/about/pac-20385051 (accessed Jun. 13, 2023).
[2] I. Malik and R. Wadhwa, “Percutaneous nephrolithotomy: Current clinical opinions and anesthesiologists perspective,” Anesthesiology Research and Practice, vol. 2016, pp. 1–7, 2016. doi:10.1155/2016/9036872
[3] L. Tzelves, B. Somani, M. Berdempes, T. Markopoulos, and A. Skolarikos, “Basic and advanced technological evolution of laser lithotripsy over the past decade: An educational review by the European Society of Urotechnology Section of the European Association of Urology,” Türk Üroloji Dergisi/Turkish Journal of Urology, vol. 47, no. 3, pp. 183–192, 2021. doi:10.5152/tud.2021.21030
[4] G. Niso, E. Romero, J. T. Moreau, A. Araujo, and L. R. Krol, “Wireless EEG: A survey of systems and studies,” NeuroImage, vol. 269, p. 119774, 2023. doi:10.1016/j.neuroimage.2022.119774