Comparison of renal pelvic pressure and postoperative fever incidence between standard‐ and mini‐tract percutaneous nephrolithotomy

Abstract

This study was proposed to compare the clinical effectiveness of mini‐tract percutaneous nephrolithotomy (MPCNL) with standard‐tract percutaneous nephrolithotomy (SPCNL) and verify whether MPCNL is associated with both higher renal pelvic pressure (RPP) and incidence of postoperative fever. A total of 228 patients with kidney stone were randomly allocated to the MPCNL group (n = 114) and SPCNL group (n = 114). Both intraoperative and postoperative indexes along with the incidence of complications were compared between the two treatment groups. RPP was measured using a baroreceptor which was connected to an open‐ended ureteric catheter during the operation of percutaneous nephrolithotomy. The MPCNL group exhibited significantly longer average operation time, more average amount of flush water, and lesser average amount of bleeding during the operation than the SPCNL group (p < 0.05). Moreover, significantly lesser average amount of postoperative serum creatinine, shorter average hospital stay, and more average amount of postoperative hemoglobin were observed in the MPCNL group than in the SPCNL group (p < 0.05). MPCNL were more applicable to clear caliceal stones (p < 0.05), whereas SPCNL were more effective for the removal of simple pelvic stones. The difference in the incidence of postoperative fever between the two treatment groups also appeared to be significant (p < 0.05). Logistic regression provided solid evidence that both RPP and its accumulation time at which RPP ≥ 30 mmHg significantly affected the incidence of postoperative fever. MPCNL was correlated with both higher RPP and increased likelihood of postoperative fever compared with SPCNL.

Introduction

Kidney stone is one of the most severe disease occurring in the urinary system, and its incidence is steadily rising; 10.6% of men and 7.1% of women in the United States are affected by kidney stones according to the National Health and Nutrition Examination Surgery [1]. In addition, this disease is usually observed with an unfavorable recurrence rate, for instance, about 50% of patients encounter recurrence within 5 years [1]. Several risk factors have been verified for the induction of kidney stone, including hyperoxaluria, hypercalciuria, hyperuricosuria, hypocitraturia, undue urinary acidity, cystinuria, and low urine volume [2]. Furthermore, kidney stone may be accompanied by disorders, including full‐blown renal colic and dysuria and urinary tract blockage and infection, which may finally result in renal failure [3].

The currently available diagnosis of nephrolithiasis generally indicates abnormality in uric elements; therefore, treatment strategies are mainly concentrated to target uric calcium, uric sodium, and uric acid [2]. To be specific, extracorporeal shock wave lithotripsy has been developed to disintegrate kidney stones by applying shock waves in the upper urinary tract and kidney, yet it is limited by the relatively low cure rate of kidney stone [2]. Instead, other interventions with minimum invasiveness, such as percutaneous nephrolithotomy (PCNL), have been widely accepted as the preferred first‐line treatment for patients with kidney stone, especially for those with large and complicated renal calculi [4]. In particular, standard‐tract PCNL (SPCNL) and mini‐tract PCNL (MPCNL) therein are safe and efficacious approaches for the elimination of kidney stones [5]. For one thing, SPCNL is usually dilated up to 24–26F with larger channel and lower perfusion pressure, yet blood loss and blood vessel rupture are two common adverse effects. For another, MPCNL with smaller service aisle between 16 and 18F may overcome the above limitations and reduce damage to kidney at the cost of extended operation time [6].

Accumulating evidence has shown that intrapelvic pressure, which is remarkably increased during the operation of PCNL, is closely related to the irrigation pressure and the location of kidney stones [7]. Systemic absorption of irrigation fluid, which may contain bacteria or endotoxins caused by high renal pelvic pressure (RPP), may lead to postoperative fever [8]. Although technological improvement and increase in surgical practices have reduced the risk of complications, such as bleeding, fever, infection, and kidney loss [9], PCNL operation factors including tract size, tract number, operation time, and tract dilation approach may still influence the incidence of postoperative complications [9].

To the best of our knowledge, there are only few researches that are able to clarify how surgical treatment conditions for managing kidney stones are related to the incidence of postoperative complications. Therefore, this study attempted to unveil such an association by comparing the status of RPP and postoperative complications between SPCNL and MPCNL.

Materials and methods

Subjects

This retrospective study included 228 adult patients who underwent PCNL in DongTai People's Hospital, Dongtai Affiliated Hospital of Nantong University from March 2014 to July 2015. These 228 patients were randomly allocated to the MPCNL group (n = 114) and SPCNL group (n = 114). PCNL operations were performed by surgeons who had at least an experience of 5 years. All the patients were diagnosed with plain film X‐rays, computed tomography, or intravenous pyelogram. The patients were excluded if: (1) they were < 18 years old; (2) they underwent a surgery on the ipsilateral kidney; (3) they had nephrostomy tube placed on the ipsilateral kidney before the surgery. Moreover, patients were diagnosed with sepsis if they suffered from life‐threatening organ dysfunctions caused by abnormal host responses to infections [10]. Specifically, the systemic inflammatory response syndrome was confirmed if at least two of the following conditions were present: (1) body temperature > 38°C or < 36°C; (2) heart rate > 90 beats/min; (3) respiratory rate > 20 times/min or PaCO₂ < 32 mmHg (4.3 kPa); (4) white blood cell (WBC) count > 12000/mm³ or < 4000/mm³, or neocyte (i.e. neutrophilic stab granulocyte) >10% [11]. The experimental procedures were approved by the ethics committee of the DongTai People's Hospital, Dongtai Affiliated Hospital of Nantong University. All the patients and their relatives provided signed informed consents prior to the trial.

PCNL operation procedure

The operation was initiated when patients were under general anesthesia. A 6‐Fr external ureteral catheter (Boston Scientific Corporation, Miami, FL) was retrogradely placed into the pelvicalyceal system. Next, patients were placed in the prone position and areas under pressure were protected by pads. The appropriate calyx for puncture was selected using an 18G coaxial needle (Cook Inc., USA), which was guided by a 5‐multicolor ultrasound instrument (Hitachi‐Aloka Medical, Japan) based on the anatomy of the intrarenal collecting system. Once the pelvicalyceal system was successfully entered, a first guidewire (Boston Scientific Corporation, USA) was manipulated down the ureter through the needle sheath. A security thread was inserted along the needle core, and the security thread was subsequently moved around to handle the sensation of resistance produced by the contact between bending section of thread and renal cortex. After the exit of puncture needle, F8 fascial dilator was applied to gradually dilate the tract, and F16 work sheath was subsequently placed throughout the tract. For the MPCNL group, 8.0/9.8F ureteroscope (Richard Wolf GmbH, Knittlingen, Germany) was employed to detect the situation within kidney. Besides, the tracts of SPCNL group were dilated with Amplatz (Cook Inc., USA), and F24 work sheath as well as 20.8F nephroscope (Richard Wolf GmbH, Knittlingen, Germany) were placed within the tract. After successful establishment of F18 and F24 renal puncture tracts, stones were fragmented using the Ultrasonic Lithotripsy System (Cybersonics Inc., USA). The perfusion pump was applied to clean up calculus, and large stones were drawn out using lithotomy forceps. The stones were removed bilaterally if necessary. After removing the stones, 6‐Fr external ureteral catheter was first pulled out, and 5F or 6F double‐J catheter was subsequently inserted. Postoperatively, a clamped 20‐ or 14‐Fr Foley catheter, the nephrostomy tube, was clipped for 30–60 minutes to stop bleeding. Finally, the double‐J tube was removed in about 1 month.

RPP measurement

In the first place, the end of the aforementioned 6‐Fr ureteral catheter was connected to Advanti 57 Urodynamic Analyzer (Laboria Cor., America). Subsequently, the upper end of ureteral catheter was fixed within pelvis, and baroreceptor was placed on the horizontal plane of the kidney. According to operating instructions, air and liquid in the ureteral catheter were pumped with sterile syringe (20 mL) so that the system parameters were adjusted to zero. During the operation, ureteral catheter was intermittently flushed to prevent blocking of powder‐like gravels and bloodshots, which might affect accurate detection of pressure. Intrapelvic pressure and accumulated time were recorded instantaneously. The perfusion pumps for medical intracavity within SPCNL and MPCNL were set as 200–250 revolutions per minute (rpm) and 150–200 rpm, respectively. Moreover, the average perfusion flow for SPCNL and MPCNL were controlled to be 350–450 mL/min and 280–380 mL/min, respectively. Pressure changes of the two channels were instantaneously and sustainably recorded and were then introduced to the computing system for procession (1 cm H₂O = 0.76 mmHg, 1 mmHg = 0.133 kPa). It was most appropriate to form a successive flow with strong pulse, and the pressure was adjusted according to the magnitude of the resistive force as well as the thickness of ureteral lumen. Besides, we classified average RPP of 20 mmHg, maximum RPP of ≥ 30 mmHg, and accumulated time of ≥ 20/60 seconds as the dividing points in order to analyze the correlation between RPP and postoperative fever.

Recording of observation indicators

Kidney stone size, expressed as stone diameter, was measured in accordance with the European Association of Urology guideline [12]. The operation time was recorded since the commencement of cystoscopy for the purpose of ureteral catheter insertion until nephrostomy placement was completed. The plain films of X‐rays were taken to measure the status of kidney stone clearance on the next day. Patients whose postoperative residual fragments were less than 4 mm were defined as stone‐free status (clinically insignificant residual fragments). Stone clearance rate was equivalent to the ratio of number of patients without calculi and number of total patients.

Serum electrolytes (Na⁺, K⁺, and Cl^–), serum creatinine (SCr), procalcitonin (PCT), and interleukin‐6 (IL‐6) were, irrespectively, measured. Blood urea nitrogen (BUN), routine urine leukocyte count, and abnormal elevated rate of white blood cell (WBC) (level > 10.0 × 10⁹/L) were evaluated using the UF‐100 automated urine analyzer (Kyoto Medical Corporation, Japan). Intraoperative blood loss was assessed based on the following formula: $\frac{\text{sample}\text{hemoglobin}\text{concentration}\left(\text{g/L}\right)\times \text{total}\text{amount}\text{of}\text{rinse}\text{solution}\left(\text{mL}\right)}{\text{pre}‐\text{operative}\text{hemoglobin}\text{concentration}\left(\text{g/L}\right)}$.

Using spiral computed tomography scanning and reconstructed multiple planar, the thickness of renal parenchyma was measured in the central kidney for three times, and the average value was subsequently derived.

Statistical analysis

All statistical analyses were performed using SPSS software package (version 13.0, SPSS, Inc., Chicago, III., USA). Categorical variables were demonstrated as counted data and compared using the Chi‐square test, whereas continuous variables were presented as mean ± standard deviation and compared using the unpaired t test, provided all data were complied with statistical model assumptions. An unconditional logistic regression model, which incorporated the odds ratio (OR) and the corresponding 95% confidence interval (CI), was used to determine significant risk factors for the incidence of postoperative fever. A p value of < 0.05 was representative of statistical significance.

Results

Characteristics of patients who underwent PCNL

There were 114 (69 male and 45 female) patients in MPCNL group with mean age of 47.6 ± 8.2 years and another 114 (68 male and 46 female) patients with mean age of 48.1 ± 7.9 years in SPCNL group (Table 1). MPCNL and SPCNL groups were well matched in sex, average age, average BMI, stone location, stone type, and stone size (all p > 0.05).

Table 1.

Comparison of basic situation before operation between mini‐tract percutaneous nephrolithotomy (MPCNL) group and standard‐tract percutaneous nephrolithotomy (SPCNL) group.

Variables	MPCNL	SPCNL	p
Sex
Male	69 (60.53)	68 (59.65)	0.892
Female	45 (39.47)	46 (40.35)
Mean age (y)	47.6  ±  8.2	48.1  ±  7.9	0.633
BMI (kg/m2)	23.0  ±  2.7	22.8  ±  2.8	0.587
Location of kidney calculi
Left side	55 (48.25)	59 (51.75)	0.596
Right side	59 (51.75)	55 (48.25)
Type of kidney calculi
Caliceal type	53 (46.49)	51 (44.74)	0.966
Staghorn type	34 (29.82)	35 (30.70)
Simple pelvic type	27 (23.69)	28 (24.56)
Stone diameter (cm)	3.4  ±  1.0	3.3  ±  1.1	0.761

BMI  =  body mass index.

Data are presented as n (%) or mean  ±  SD.

Operative indexes of patients who underwent PCNL

Although duration of operation varied with types of stone, MPCNL group was generally associated with significantly longer operation time than SPCNL group (p < 0.05) (Table 2). Moreover, MPCNLs were more applicable to clear caliceal stones than SPCNLs (p < 0.05), whereas SPCNLs were more effective in the removal of simple pelvic stones than MPCNLs. With respect to efficacy in the elimination of staghorn stones, MPCNLs and SPCNLs were fairly close (72.9% vs. 74.2%). Nonetheless, MPCNL group required a bigger amount of flush water than SPCNL group, and SPCNL group could lead to more serious bleeding than MPCNL group (p < 0.05). Apart from that, MPCNL group was associated with significantly shorter average hospital stay period than SPCNL group; however, the determination of hospital stay was somewhat subjective (p < 0.05).

Table 2.

Comparison of operation situations between mini‐tract percutaneous nephrolithotomy (MPCNL) group and standard‐tract percutaneous nephrolithotomy (SPCNL) group.

Variables	MPCNL	SPCNL	p
Total amount of tracts	117	116	—
Duration of operation (min)
Caliceal type	89.1  ±  31.9	69.8  ±  20.8	< 0.001
Staghorn type	72.8  ±  15.4	62.9  ±  13.6	< 0.001
Simple pelvic type	83.7  ±  27.3	71.3  ±  21.6	< 0.001
Stone clearance rate (%)
Staghorn stone	72.9	74.2	0.747
Simple pelvic stone	76.5	91.9	0.003
Caliceal stone	92.0	74.8	0.001
Nephrostomy tube retention time (d)	5.1  ±  1.4	5.3  ±  1.5	0.299
Amount of flush water (mL)	22400.0  ±  1360.0	13800.0  ±  1700.0	< 0.001
Amount of bleeding (mL)	58.8  ±  6.7	67.9  ±  7.2	< 0.001
Mean hospital stay (d)	3.9  ±  1.1	5.2  ±  1.0	< 0.001

Comparison of postoperative infections and complications between the two groups

The incidence of abnormal elevated rate of WBC in MPCNL group was more than that in SPCNL group (p < 0.05) (Table 3). And obviously MPCNL group possessed higher concentrations of PCT, IL‐6, and Hb than SPCNL group (p < 0.05). However, these two rendered similar positive rates of urine culture for patients (p > 0.05).

Table 3.

Comparison of postoperative infections and complications between mini‐tract percutaneous nephrolithotomy (MPCNL) group and standard‐tract percutaneous nephrolithotomy (SPCNL) group.

Variables	MPCNL	SPCNL	p
Abnormal elevated rate of UWBC (%)	47 (41.23)	28 (24.56)	0.007
Average renal pelvic pressure (mm Hg)	25.6  ±  5.3	13.8  ±  4.1	< 0.001
Procalcitonin (ng/mL)*	0.17 (0.04–29.75)	0.08 (0.03–2.12)	< 0.001
IL‐6 (pg/mL)*	46.2 (0.21–4851)	18.50 (0.19–1609)	< 0.001
Hb (g/L)	134.1  ±  8.3	107.6  ±  8.1	< 0.001
Positive urine culture	28 (24.56)	30 (26.31)	0.761
Fever	28 (24.56)	15 (13.16)	0.028
Bleeding	2 (1.75)	6 (5.26)	0.150
Interventional embolization	2 (1.75)	2 (1.75)	1.000
Combined with perforation	2 (1.75)	3 (2.63)	0.650
Sepsis	3 (2.63)	1 (0.88)	0.310

UWBC  =  urinary white blood count; IL‐6  =  interleukin‐6; Hb  =  hemoglobin;

* Values are expressed as median (95% confidence interval).

Data are presented as n (%) or mean  ±  SD.

With respect to postoperative complications, MPCNL group (24.56%) appeared to suffer from fever more easily than SPCNL group (13.16%) (p = 0.028). Nonetheless, possibilities of bleeding, interventional embolization, combination with perforation, and sepsis were evenly distributed between both MPCNL and SPCNL groups (p > 0.05).

Association between RPP and incidence of postoperative fever among the two groups

A total of 42.98% (98/228) patients had an average RPP ≥ 20 mmHg and 43.86% (100/228) patients had a maximum RPP ≥ 30 mmHg during the operation. Besides, 15.35% (35/228) patients had an accumulated time of RPP ≥ 30 mmHg for more than 60 seconds (Table 4). Intriguingly, patients with the average RPP > 20 mmHg were more likely to encounter fever after undergoing MPCNL than those with the average RPP < 20 mmHg (p = 0.024). More than that, higher‐level RPP (≥ 20 mmHg) also rendered patients undergoing SPCNL to be infected with fever more readily than lower‐level RPP (< 20 mmHg) (p = 0.037). The above inter‐relationships could equally be applied to patients who were grouped in light of maximum RPP (≥ 30 mmHg and < 30 mmHg). Among patients with RPP ≥ 30 mmHg, the accumulated time with the cutoff point of 20 seconds was hardly related with incidence of fever either in MPCNL group or SPCNL group (p > 0.05), but the accumulated time that was longer than 60 seconds appeared as a risk factor for the development of postoperative fever among MPCNL group (p < 0.001).

Table 4.

Relation between renal pelvic pressure (RPP) and postoperative fever between mini‐tract percutaneous nephrolithotomy (MPCNL) group and standard‐tract percutaneous nephrolithotomy (SPCNL) group.

Items		MPCNL	SPCNL	χ2	p
Average RPP
≥ 20 mmHg	fever  <  38.5°C	48	19	0.90	0.342
	fever  ≥  38.5°C	25	6
< 20 mmHg	fever  <  38.5°C	35	59	1.24	0.266
	fever  ≥  38.5°C	6	5
χ2		5.10	4.35
p		0.024	0.037
Maximum RPP
≥ 30 mmHg	fever  <  38.5°C	49	17	0.48	0.487
	fever  ≥  38.5°C	23	11
< 30 mmHg	fever  <  38.5°C	37	83	3.41	0.065
	fever  ≥  38.5°C	5	3
χ2		5.75	5.01
p		0.017	<0.001
Accumulated time of RPP  ≥  30 mmHg
< 20 s	fever  <  38.5°C	7	10	1.64	0.200
	fever  ≥  38.5°C	9	5
≥ 20 s	fever  <  38.5°C	30	9	1.05	0.305
	fever  ≥  38.5°C	26	4
χ2		0.48	0.021
p		0.488	0.885
< 60 s	fever  <  38.5°C	32	13	0.008	0.927
	fever  ≥  38.5°C	14	6
≥ 60 s	fever  <  38.5°C	7	6	4.52	0.033
	fever  ≥  38.5°C	19	3
χ2		12.17	0.009
p		< 0.001	0.926

Determination of potential parameters that were independently related to postoperative fever

In addition, the unconditional logistic regression (Table 5) also enabled us to conclude that postoperative fever was not significantly correlated with sex, age, stone location, stone type, and stone size (all p > 0.05). By contrast, higher average RPP, longer accumulated time of RPP ≥ 30 mmHg for more than 60 seconds, and employment of MPCNL appeared to be significantly associated with an increased incidence of postoperative fever (all p < 0.05).

Table 5.

The unconditional logistic regression analysis of characteristics related to postoperative fever.

Characteristics	p	OR	95% CI
			lower limit	upper limit
Sex	0.197	0.576	0.249	1.332
Age	0.925	1.002	0.952	1.055
Location of stone	0.558	0.783	0.345	1.777
Type of stone	0.888	0.943	0.418	2.127
Stone diameter	0.631	0.906	0.605	1.356
Average RPP  ≥  20 mmHg	0.048	1.150	1.010	1.310
Maximum RPP  ≥  30 mmHg	0.966	1.033	0.228	4.680
Accumulated time (RPP  ≥  30 mmHg) ≥ 20 s	0.769	1.236	0.300	5.083
Accumulated time (RPP  ≥  30 mm Hg) ≥ 60 s	0.044	3.680	1.040	13.160
Type of PCNL (SPCNL vs. MPCNL)	0.008	0.189	0.055	0.650

CI  =  confidence interval; MPCNL  =  mini‐tract percutaneous nephrolithotomy; OR  =  odds ratio; RPP  =  renal pelvic pressure; SPCNL  =  standard‐tract percutaneous nephrolithotomy.

Comparison of renal function‐relevant parameters between MPCNL and SPCNL groups

MPCNL group and SPCNL group showed hardly statistical difference in the levels of serum electrolytes (Na⁺, K⁺, and Cl^–), SCr, glomerular filtration rate (GFR), and BUN until performance of surgery (Table 6). To be specific, when compared with preoperative status, levels of Na⁺, Cl^–, SCr, and BUN increased significantly during postoperative 1–5 days (p < 0.05), whereas serum K⁺ and GFR exhibited notably opposite trends (p < 0.05). Furthermore, MPCNL group was correlated with the larger rising amplitude of the above indicators than SPCNL group (p < 0.05).

Table 6.

Comparison of renal function‐relevant parameters between mini‐tract percutaneous nephrolithotomy (MPCNL) group and standard‐tract percutaneous nephrolithotomy (SPCNL) group.

Variables	MPCNL	SPCNL	p
SCr (μmol/L)
Pre‐operation	97.6  ±  16.2	99.8  ±  13.1	0.261
1 day after operation	115.4  ±  12.8*	102.6  ±  16.7*	< 0.001
3 days after operation	117.7  ±  17.9	111.8  ±  16.1	0.009
1 month after operation	99.3  ±  13.3	114.6  ±  12.8	< 0.001
GFR (mL/min/1.73 m2)
Pre‐operation	98.8  ±  18.0	101.7  ±  10.8	0.142
1 day after operation	67.1  ±  18.8*	86.4  ±  20.2*	< 0.001
3 days after operation	102.1  ±  23.2	105.6  ±  17.9	0.230
1 month after operation	100.6  ±  22.7	104.5  ±  15.6	0.132
Na+ (mmol/L)
Pre‐operation	137.5  ±  2.55	137.3  ±  2.45	0.546
1 day after operation	137.4  ±  2.43	142.5  ±  2.51*	< 0.001
3 days after operation	138.3  ±  2.64	148.1  ±  2.97	< 0.001
5 days after operation	137.3  ±  2.56	140.4  ±  2.38	< 0.001
K+ (mmol/L)
Pre‐operation	3.82  ±  0.25	3.85  ±  0.28	0.394
1 day after operation	3.60  ±  0.37*	3.15  ±  0.39*	< 0.001
3 days after operation	3.65  ±  0.26	3.12  ±  0.28	< 0.001
5 days after operation	3.76  ±  0.27	3.49  ±  0.31	< 0.001
Cl– (mmol/L)
Pre‐operation	101.1  ±  4.73	102.2  ±  4.65	0.078
1 day after operation	102.0  ±  4.57	104.5  ±  3.97	< 0.001
3 days after operation	102.9  ±  4.32	106.6  ±  4.15	< 0.001
5 days after operation	101.3  ±  4.56	104.7  ±  4.34	< 0.001
BUN (mmol/L)
Pre‐operation	4.91  ±  1.17	4.88  ±  1.14	0.845
1 day after operation	5.05  ±  1.88	5.61  ±  2.02*	0.031
3 days after operation	5.02  ±  2.18	5.65  ±  2.63	0.050
5 days after operation	4.95  ±  1.72	5.44  ±  1.99	0.048
Thickness of the operated renal parenchyma
Pre‐operation	13.6  ±  2.1	13.9  ±  1.2	0.187
Immediate post‐operation	15.8  ±  1.9*	17.0  ±  1.6*	< 0.001
1 day after operation	15.3  ±  2.3	16.3  ±  1.6	< 0.001
3 days after operation	14.2  ±  2.3	14.9  ±  1.5	0.007
5 days after operation	13.7  ±  2.0	14.2  ±  1.3	0.026

* p  <  0.05 compared with pre‐operation.

BUN  =  blood urea nitrogen; GFR  =  glomerular filtration rate; SCr  =  serum creatinine.

The patients' lateral renal parenchyma thickened for 0.1–5.9 mm after PCNL when compared with their preoperative status (Table 6). Besides, performances of PCNL were all accompanied with perirenal fluid collection and even extension of perirenal hematoma to pelvic cavity occurred. Incrassation of renal parenchyma became more obvious with rise of RPP (p < 0.05), indicating that high perfusion pressure contributed to remarkable reflux and long recovery.

Discussion

PCNL is an established urologic procedure that can manage kidney stones safely and effectively, and it is also superior to open renal operations for its rapid recovery, high stone‐free rate, and low incidence of complications. However, the conduction of PCNL is also accompanied by severe complications. For instance, since the operation of PCNL requires continuous washing to maintain a clear vision field, increased incidence of septicemia could be induced [13]. Furthermore, a certain degree of bleeding is unavoidable with PCNL since kidney is a vascular organ [14]. Unlike SPCNL, MPCNL is equipped with a smaller access tract and does not require serial dilatation, and patients receiving MPCNL may experience smaller Hb drop, decreased trauma, and reduced bleeding amount [15]. Nevertheless, limitations of MPCNL also existed, such as the need for specialized equipment, increased operation time, and usage restriction (e.g. not fit for kidney stone with a diameter > 2 cm) [16].

Consistent with our result, RPP during the operation of ureteroscopic lithotripsy was increased when compared with its preoperative values, which could be attributed to certain factors, including actual urine flow, ureteropelvic junction compliance, pelvic wall tension, actual system capacity, and external pressures [17]. Notably, the increase in RPP was associated with both the position of kidney stone and the irrigation pressure, which was dependent on irrigation outflow that mainly passed through ureter and the gap between peel‐away sheath and scope [8]. As mentioned by Tepeler et al., the absence of a sheath in MPCNL restricted efficient drainage of irrigation fluid and thus increased RPP [18]. The above evidence may imply a crucial linkage between RPP and tract size used in PCNL. Moreover, Zhong et al. found a significant interaction between fever incidence and mean RPP, suggesting that an unexpectedly higher RPP was associated with an increased incidence of postoperative fever [8]. They also concluded that the proportion of patients with RPP ≥ 30 mmHg was 83.75% (67/80), and 17.9% (12/67) of these patients had postoperative fever [8]. Apart from that, their analysis revealed that postoperative fever was not associated with RPP ≥ 30 mmHg during the operation [8]. Thus, high RPP could be associated with postoperative fever under certain circumstances. It is suggested that RPP should be observed carefully during and after operations. Besides, once RPP increases rapidly, more attention should be paid to avoid serious postoperative complications.

PCNL‐induced postoperative fever may be related to operation conditions, such as backflow during the surgery [19]. RPP remains lower than the backflow level in the course of MPCNL, despite the presence of various factors that are able to affect the fluctuation of RPP [8]. A recent research conducted by Dogan et al. reported that the proportion of patients with postoperative fever was 21% (17/81), and the absorption of perfusion fluid was identified as a significant risk factor [20]. As suggested by Hinman et al., pyelovenous backflow usually appeared when the renal pressure is between 30 mmHg and 35 mmHg [21]. Intrapelvic pressure of less than 30 mmHg during percutaneous chemo dissolution of upper tract stones is recommended as a criterion in clinical practices [22]. Factors that triggered poor drainage could temporarily increase RPP to a level of higher than 30 mmHg, and this may result in systemic absorption of irrigation fluid containing endotoxins or bacteria, further causing postoperative fever and sepsis [8]. This study employed PCT and IL‐6 as two key biomarkers for sepsis because they were closely related with severity and prognosis of sepsis [23]. To be specific, healthy individuals were accompanied with low PCT concentrations which remained steady at pictogram levels [24], whereas sustained elevations of plasma PCT were found in sepsis patients [25]. Additionally, no other biomarkers can compete against PCT in differentiation of bacterial‐inducing infections from infections that originated from non‐bacterial causes [26]. On account of the above proof, PCT has been announced by International Sepsis Definitions Conference to be a candidate diagnostic criterion for sepsis in 2001 [27]. Besides, it was documented that expression levels of PCT and IL‐6 within non‐survivors due to delayed treatment of sepsis were notably greater than those within survivors, implying that PCT and IL could serve as critical prognostic factors for sepsis patients [28].

In addition, indicators that signified alterations of renal functions were also evaluated, including Na⁺, Cl^–, SCr, and BUN. We detected the renal function because the occurrence of postoperative complications was indeed associated with changes of renal functions. In other words, deteriorating renal function is a critical parameter for increased occurrence of postoperative complications, including fever. Nevertheless, one major limitation of our study is the relatively small sample size due to resource constraint; thus, large‐scale randomized control trials are encouraged to be designed so that the above conclusions can be verified with an increased statistical power. On the other hand, efficient drainage procedures with minimum invasiveness should be developed to optimize the application of MPCNL.

In conclusion, this study provided a notion that MPCNL was associated with higher RPP and an increased incidence of postoperative fever in comparison to SPCNL. Besides, irrespective of tract size, accumulated time > 60 seconds with RPP above 30 mmHg also forecasted fever.

Supporting information

Supplementary data

Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.kjms.2016.10.012.