Many countries use flexible ureteroscopy as the primary treatment for upper urinary tract stones. At present, Ho:YAG laser lithotripsy is still regarded as the gold standard for flexible surgery for stone disease. The use of laser energy in the urethra generates heat, and if the temperature is not controlled, it can be too high and damage the kidneys. Some studies have suggested that perfusion can improve temperature increases. However, this inevitably leads to an increase in intrarenal pressure (IRP), which can lead to "sepsis, pain, and even kidney damage". As a result, many studies have advocated measuring changes in intrarenal pressure (IRP) during flexible endoscopic surgery. IRP is usually defined as the pressure in the renal pelvis and calyces (produced during ureteroscopy due to the balance between perfusion fluid inflow and outflow). Under normal physiological conditions, the variation range of IRP is 0~20 cm H2O, and renal pelvicular regurgitation occurs when the IRP is between 27~41 cm H2O. When the IRP is between 41~68 cm H2O, it will lead to renal pelvic venous reflux, and when the IRP is raised above 81 cm H2O, renal fornix rupture may occur. Lildal et al. used MRI to confirm that more than half of the kidneys would be affected by pelvic venous reflux when the IRP was set to 58 cm H2O. The exact reflux depends not only on the IRP itself, but also on how long the IRP remains stable.
Alternative treatment options for maintaining low intrarenal pressure (IRP) include "reducing perfusion volume, using a ureteral introducer sheath (flexible sheath), and improving access to the ureteroscope (laser fiber or stone basket)". However, the main problem is that although some measures have been taken, it is still not possible to assess the intrarenal pressure (IRP) during the flexible endoscopic surgery, and it is not possible to change the treatment parameters at any time to further improve the treatment environment. The study, published by Canadian and European physicians, aimed to systematically review the "measurement of intrarenal pressure (IRP) using non-invasive methods (e.g., endoscopic treatment) during flexible surgery".
Methods:
Systematic review analyses were performed according to the Preferred Reporting Item for Systematic Reviews and Meta-Analyses (PRISMA). A single author (FP) searched the literature using the MEDLINE (PubMed) database by entering the keywords "soft mirror", "pressure" and "AND". The time range of the search was 'before 21 March 2023'. Duplicate literature is removed, and "reviews, editorials, case reports, and abstracts" are removed. "Studies using percutaneous methods to measure IRP" were also not included in the scope of the study. The study involved literature in all languages. Qualitative analysis was performed of the included study literature. Literature analysis was performed by two authors (FP and EV), and the other two authors (BC and NB) made the final selection. There was considerable heterogeneity in the included studies in terms of study design and findings, and the authors reported the results narratively.
Synthesis of evidence:
Figure 1 shows the PRISMA flow chart of the literature search. A total of 275 research articles were involved in the whole literature study. A total of 19 studies were included in the review after applying the predetermined inclusion criteria. Ultimately, the researchers identified a total of four non-invasive treatments that could be used to monitor IRP (see Table 1 for details).
The first study dates back to 2008 and deals with the placement of a ureteral catheter (4~5 Fr) connected to a sensor into the renal pelvis. The baseline pressure range is 10~12 mmHg, and the intrarenal pressure during flexible endoscopic surgery can reach up to 33 mmHg. It wasn't until 2016 that researchers took this method a step further, after in vivo and ex vivo tests of a suction introducer sheath with pressure measurement capabilities. The device consists of a soft mirror sheath (12~14) and interacts with the irrigation pump to be able to set the IRP threshold and sound an alarm when the threshold is exceeded. Other innovative technologies include sensors with pressure recording (soft guidewires), which are often used for interventional cardiovascular surgery. It is important to note that the device has not been approved for endoluminal urological procedures, so in these studies, the above techniques are considered to be outside the scope of licensing. At the beginning of the procedure, the sensor was inserted into the renal pelvis. The method of placement of the sensing guidewire is not the same. Patel et al. used a two-channel ureteroscopy approach to perform a flexible endoscopic procedure, with the perfume passing through the primary working channel and the sensor passing through the secondary working channel to determine the IRP. Other authors have also proposed to "make the sensor pass through the only working channel and maintain this situation during the procedure". A more practical approach is to place the sensor wire next to or through the ureteroscope/flexible sheath to keep the working channel open. The fourth method is to measure the patient's intrarenal pressure (IRP) using a perfusion system. The device was shown to be strongly correlated with the "measurement of the patient's IRP in the kidneys" by a non-invasive method by placing a pressure sensor around the perfusion system.
Discuss:
Sepsis remains a major problem with elevated intrarenal pressure (IRP) due to high "morbidity and mortality". To determine that IRP is within a safe range, the investigators have made several treatment recommendations. However, to date, researchers have not recorded changes in patients' intrarenal pressure (IRP) through a "validation method" or tracked during endoscopic surgery. Doizi et al. performed surgery with a "perfusion pressure of 80 cm H2O and a flexible sheath of 10~12 or 12~14", and reported that during laser lithotripsy, the patient's basal IRP value was 4~6 cm H2O, the mean IRP was 115 cm H2O, and the peak IRP was more than 400 cm H2O. There were no complications during surgical treatment. However, due to the limited sample size of the study, it is unclear "the extent to which IRP fluctuations and peaks affect clinical symptoms".
Advantages and limitations of IRP measurement methods
Each IRP measurement system has its inherent advantages and limitations. The "transducer-fitted ureteral catheter" is an easy-to-operate, low-cost treatment option, but its main problem is the limited availability of flexible surgical procedures within the ureteral and renal systems. This means that ureteral sheaths of any size cannot be used for the same reason, thus losing the advantage of operating with ureteral sheaths.
A flexible sheath that integrates "perfusion, aspiration, and determination of IRP function" appears to be a better treatment option, and the technique includes both "variables such as perfusate inflow and perfusion outflow". Nevertheless, we must also point out some problems with the device. The outer diameter of the introducer sheath (14~15 Fr) is large. Traxer et al. reported that 46.5% of patients with 12/14 Fr implantation had symptoms of ureteral injury to varying degrees. One way to avoid this risk is to systematically pre-place stents, which reduces the risk of ureteral injury by a factor of seven. Notably, up to 22.5% of patients were unable to have a new ureteral introducer sheath (flexible sheath) pre-placed without a stent, and only 50% of patients in the same study series had a ureteral stent pre-placed. Another study using the same treatment technique claimed that 100% of patients had ureteral stents pre-placed. Although not all ureteral injuries lead to ureteral strictures, we recommend that they should be avoided as much as possible.
Another problem with this new ureteral introducer sheath (flexible sheath) is that the device is located in the upper urinary tract. Whenever IRP measurement and aspiration of residual stone fragments are required, we recommend placing the flexible sheath above the ureteral junction (UPJ), which is inconsistent with the traditional site of insertion, further increasing the risk of UPJ injury and ureteroscopic injury. In addition, the device may not be very helpful in the treatment of ureteral stones. In addition, when using a coarse soft sheath (14~15Fr), it is easy to mislead the situation of low IRPs.
Operation with a thin sensor guidewire appears to overcome these limitations. First, the treatment device can be used regardless of whether the ureteral introducer sheath (flexible sheath) is in place; Secondly, the advantage of this therapeutic device is that it can measure IRP during the treatment of ureteral stones; Third, it does not take up valuable space within the working channel of the ureteroscope. The disadvantage of this approach is that the device has not been officially promoted. Another limiting factor of the sensor is the measurement of IRP at its tip. The tip of the device is placed in a static position (usually at the upper pole of the ureter) and does not necessarily reflect the intrarenal pressure (IRP) at the site of laser lithotripsy. For example, in the treatment of lower calytic stones, the IRP of the upper calypium (where the sensor is placed) will be lower than that of the lower calyptic site. Pressure sensors with IRP measurement are a technique for evaluating IRP and correlate well with real IRP, but there is a lag when the perfusion velocity changes (i.e., in the case of perfusion). In addition, when a laser fiber is introduced into the working channel, the linear relationship between its proximal pressure and IRP is weakened. These two constraints make it less useful to use the system for continuous determination of the true IRP situation.
Future outlook
There is increasing concern about intrarenal pressure (IRP) and intrarenal temperature (IRT) during endoluminal urological procedures. Even if there is tissue burning on the surface of the urothelium, the kidney can be damaged if the intrarenal temperature (IRT) is too high. The ideal treatment system should be able to detect the patient's intrarenal pressure (IRP) and intrarenal temperature (IRT) in real time and be able to sound an alarm that, like a dashboard in an airplane cockpit, accurately informs the urologist that something is above a given threshold. Recently, the robotic platform used to perform flexible ureteroscopy surgery (flexible endoscopy) has integrated temperature and pressure sensors to display clinical treatment data in the main console. It helps the surgeon to better understand and control the relevant variables (perfusion inflow and outflow), and can provide the urologist with a true IRP value with better therapeutic value. A better understanding of the IRP and other intervention factors (e.g., "calyceal anatomy or type of anesthesia") can help maintain IRP values within safe limits. To date, we have not conducted a direct comparative analysis of these treatment systems, so it is not possible to determine the superiority of a single technology.
When we need to take into account all the factors that may affect the IRP, we should pay special attention to the aspiration situation. Overall, during the flexible endoscopic procedure, the degree of visualization and stone removal can be further improved by aspiration. We consider stone fragments ≤ 250 μm as stone powder. Larger stone fragments (500 μm) can be removed with a suction sheath, but this often depends on the "ratio of ureteral introducer sheath to ureteroscope". Compared to the Ho:YAG technology, new technologies such as fiber thulium lasers can produce finer stone powder (which is more conducive to aspiration). Theoretically, aspiration can further improve perfusion without affecting IRP and reduce the incidence of fever/sepsis after flexible endoscopic surgery.
As mentioned earlier, the ureteral introducer sheath (flexible sheath) with the integration of "suction, perfusion, and IRP measurement functions" has a better application prospect, but due to the above problems, further research and analysis are needed. Recently, a new ureteral introducer sheath (flexible sheath) with a bendable head and suction function has been developed, which can protect the ureteroscope when bent and further improve aspiration, but further research and development is needed. Unfortunately, this new flexible sheath does not have IRP measurement.
There is also a new technology, a disposable flexible ureteroscope with a pressure sensor (LithoVueTM Elite - Boston Scientific) at the end of the ureteroscope. We did not include full-text studies as we did not have access to them. The maneuverability of this new flexible scope is similar to that of other endoscopic treatment instruments, but with a wider surgical field of view and better clarity. There is a good correlation between the soft mirror and the pressure sensor. One disadvantage of this device is that IRP cannot be assessed during ureteral stone treatment. The endoscope was approved by the FDA in February 2023. The average IRP value of the patient was 40 cmH2O, and the peak IRP was 240 cmH2O, but the intrarenal pressure (IRP) was kept below 81 cmH2O for most of the time (91.3%) during the operation. One of the main advantages of the mirror is its ability to monitor intrarenal pressure (IRP) in real time at the surgical site (i.e., "laser lithotripsy"). Therefore, the mirror can provide us with accurate information about the intrarenal pressure (IRP) during flexible endoscopic surgery. Ongoing clinical trials will further answer the clinical significance of these clinical treatment data. The study is expected to be completed by June 2025. The trial will compare the "soft scope with pressure monitoring function and the soft scope without pressure monitoring function", and mainly evaluate and analyze the occurrence of serious adverse reactions.
Nowadays, we all know that both the patient's "IRP and IRT values" should be kept low. We all know ways to lower IRP and temperature, but in clinical practice, there is a lack of methods that can show the "accuracy or safety" of these clinical treatments. The development of new technologies should include treatment systems that can accurately "monitor IRP and IRT" so that urologists can treat correctly and safely based on more clinical treatment data. Ideally, in order to balance the "inflow and outflow of perfusate and the power of the treatment", we should use a "treatment system that integrates perfusion, aspiration, IRP, and IRT" for flexible surgery.
At present, the clinical evidence related to the "IRP assay" is constrained by a variety of factors, such as heterogeneity of treatment methods, small sample size of patients, diversity of trial designs, and lack of comparative analysis of different treatment systems.
Although clinicians are unclear about the clinical significance of measuring IRP, we suggest that some treatment should be taken whenever possible to maintain IRP within a safe range of variation. The system provides urologists with real-time IRP information and can further improve the safety and understanding of flexible surgery. With this data, the flexible endoscopic procedure can be adjusted based on the patient's clinical characteristics, thereby keeping the IRP within a safe range.
Conclusions:
This review provides a comprehensive overview of the surgical treatment systems that can measure IRP during flexible surgery. At present, researchers have not yet developed a perfect treatment system, but urologists will soon be able to monitor IRP in real time. It is unclear "the clinical significance of this clinical information during surgical treatment". However, the principle of safety first should always be paramount, and doctors should keep the "laser power, IRP and IRT" as safe as possible during the procedure. The ideal therapeutic device should be able to integrate functions such as "perfusion, aspiration, determination of IRP and IRT".
----Pauchard F, Bhojani N, Chew B, Ventimiglia E. How to measure intra-renal pressure during flexible URS: Historical background, technological innovations and future perspectives. Actas Urol Esp (Engl Ed). 2023