## 4 Discussion  
The enormous technological advances in endourology that have occurred over the past two decades of the 21 st century have led to the dominance of minimally invasive methods in the catalogue of surgical treatments for kidney stones. The challenge of modern endoscopic urology in the treatment of urolithiasis is to establish a universally applied method for the real-time evaluation of physical conditions in the upper urinary tract during surgery. The consequences of increased IPP are urinary tract infections, increased risk of bleeding, and damage to the renal parenchyma [12; 13]. Heretofore published studies  
Figure 4: The IPP means of conditional effects between the categorical predictor _group_ and the _parameter_ based on the fitted regression model (the thick solid line at the top of the bars stands for 95% _CI_). \begin{table}
\begin{tabular}{c|c c c c c c} \hline \hline \multirow{2}{*}{_Parameter_} & \multicolumn{3}{c}{_Contrasts_} & \multicolumn{3}{c}{_The Effect Size_} \\ \cline{2-7}  & _Difference_ & **95\% CI** & _SE_ & \(p\) & \(d\) & _SE_ & **95\% CI** \\ \hline
12 W 15 Hz 0.8 & 138.76 & [138.42, 139.09] & 0.17 & \textless{}0.001 & 2.18 & \textless{}0.01 & [2.18, 2.19] \\
15 W 15 Hz 1.0 & 113.66 & [113.33, 113.99] & 0.17 & \textless{}0.001 & 1.79 & \textless{}0.01 & [1.78, 1.79] \\
15 W 50 Hz 0.3 & 158.13 & [157.83, 158.42] & 0.15 & \textless{}0.001 & 2.49 & \textless{}0.01 & [2.48, 2.49] \\
18 W 15 Hz 1.2 & 164.92 & [164.61, 165.64] & 0.16 & \textless{}0.001 & 2.59 & \textless{}0.01 & [2.59, 2.60] \\
18 W 30 Hz 0.6 & 182.19 & [181.89, 182.50] & 0.16 & \textless{}0.001 & 2.86 & \textless{}0.01 & [2.86, 2.87] \\
20 W 10 Hz 2.0 & 143.36 & [143.07, 143.65] & 0.15 & \textless{}0.001 & 2.25 & \textless{}0.01 & [2.25, 2.26] \\
25 W 10 Hz 2.5 & 155.81 & [155.48, 156.15] & 0.17 & \textless{}0.001 & 2.45 & \textless{}0.01 & [2.44, 2.46] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of contrast analysis with effect sizes between groups (10/12 Fr-12/14 Fr) for individual parameters. _p_—the _p_-value of the statistical test; _SE_—the standard error; _d_—[PERSON]’s \(d\) effect size; 95% _CI_—95% confidence interval. have indicated that, regardless of the IPP measurement method, the equipment used, or the instrument's size, the pelvic pressure generated can exceed the safe threshold value of 40 cmH\({}_{2}\)O a few times. The morphological consequences for the kidney as a result of high IPP were revealed in animal studies. High pressure elicited diffuse denudation and flattening of the calyx urothel, submucosal oedema and congestion, which were not observed in calyxes undergoing low-pressure irrigation. Four to six weeks later, a higher incidence of columnar metaplasia, subepithelial nests, and periurethral vasculitis was observed in high-pressure treated calyces compared to low-pressure irrigated ones [14]. A factor with a protective function for the upper urinary tract during flexible URS is UAS. First applied in 1974 by [PERSON] and [PERSON] [15], UAS is currently a standard option when performing flexible URS. Based on the results of the animal study reported by [PERSON] et al., a larger UAS (12/14 Fr and 14/16 Fr) mitigated intrarenal pressure, while smaller access sheaths (9.5/11.5 Fr) provided inadequate drainage ureteroscopy [16]. Surgeons prefer a 10/12 Fr and 12/14 Fr size in most cases [1]; therefore, the primary objective of our study was to analyse the IPP according to these sizes of the UAS used. In addition, some studies have shown that a reduction in the size of the ureteroscope used for fixed-size UAS also results in a reduction in IPP [17; 18; 19]. The evidence on changes in IPP in the pyelocalycesal system during flexible URS is mainly based on single animal model studies published hitherto. Animal studies evaluated IPP using a wire inserted through a nephrostomy tube (in some studies, using urodynamic devices). The cutting-edge study was published in 2021 by [PERSON] et al. [6].