Factors Affecting Operative Time During Percutaneous Nephrolithotomy: Our Experience with the Complete Supine Position

Abstract

Background and Purpose:

Operative time influences surgical outcomes, operation-related and indirectly anesthesia-related complications. We investigated variables that affect operative time during percutaneous nephrolithotomy (PCNL) with the complete supine position.

Patients and Methods:

We reviewed data of 120 patients who underwent complete supine PCNL by one surgeon. Correlation between age, sex, body mass index (BMI), preoperative serum creatinine and hemoglobin levels, history (stone surgery, shockwave lithotripsy (SWL), hypertension, diabetes), stone (side, opacity, burden, complexity), imaging for access, calix for access, number of tracts, and tubeless approach with operative time were assessed by univariate and multivariate statistical tests.

Results:

Mean operative time was 60.62±30.70 minutes. In univariate analysis, age, sex, groups of BMI, hypertension, diabetes, previous stone surgery and SWL, stone opacity, operation side, number of tracts, and tubeless approach had no effect on operative time. BMI (P=0.029, negative correlation), stone burden (P=0.001, positive correlation), imaging for access (P=0.001, fluoroscopy<ultrasonography), calix for access (P=0.035, upper>lower>middle) and probably complex stones (P=0.057, complex>noncomplex) were effective factors on operative time. Multiple linear regression revealed association between BMI (P=0.000, negative correlation), stone burden (P=0.005, positive correlation), imaging for access (P=0.000, fluoroscopy<ultrasonography), and calix for access (P=0.023) with operative time. In lower calix access, mean operative time was less than upper calix access (P=0.022, significant) and higher than middle calix access (P=0.930, nonsignificant).

Conclusions:

BMI, stone burden, imaging for access, and calix for access were effective parameters on operative time in complete supine PCNL. Groups of BMI, previous stone surgery and SWL, number of tracts, and tubeless approach had no effect on operative time.

Introduction

T oday, percutaneous nephrolithotomy (PCNL) has found a special situation in the management of upper urinary tract stones in comparison with open surgery.^1,2 PCNL is a safe and preferable minimally invasive approach for the majority of renal and upper ureteral stones based on surgical outcomes.^3

–8 PCNL is performed in the prone or supine position.^9
–11 In comparison with the prone position, the complete supine position is a proper situation that is accompanied by higher acceptance and less manipulation of the patient, multiple benefits during anesthesia and operation, and superior outcomes.¹²

PCNL is usually performed with posterior subcostal access because of the safety of passage through posterior access and the Brodel bloodless line and the potential intrathoracic risk of supracostal access.^5,13,14 The urinary collecting system is accessible for PCNL via the upper calix, middle calix, or lower calix.^15

–18 At present, single or multiple percutaneous tracts (for complete removal of stone),^18,19 fluoroscopy or ultrasonography (for guidance during access),^5,15,20,21 pneumatic, ultrasonic, mechanical, electrohydrolic, or lyseric techniques (for lithotripsy), and standard nephrostomy tube insertion or tubeless approach^5,22,23 are used during PCNL.

Operative time is an important factor for judgment and comparison between different surgical techniques^{2,9,12,16,18,19,23} that may increase duration of anesthesia and the risk of postoperative pulmonary complications indirectly²⁴ and influence surgical outcomes (blood loss, hemoglobin drop, and transfusion)^25

–28 and operation-related complications^29,30 in PCNL. In separate studies, Gune and associates,²⁵ Kukreja and colleagues,²⁶ Vorrakitpokatorn and coworkers,²⁷ and Akman and colleagues²⁸ reported that operative time affects blood loss, postoperative hemoglobin drop, and transfusion requirement in PCNL. Also, de la Rosette and associates²⁹ concluded that operative time influences incidence of complications in PCNL, and Özden and coworkers³⁰ inferred that operative time is an independent risk factor for complications in PCNL. Thus, shorter operative time with acquisition of excellent results and outcomes is the desire of each surgeon.

Different parameters may have an impact on operative time during PCNL. In this study, we investigated the variables that affected operative time in PCNL in the complete supine position.

Patients and Methods

We collected and reviewed data of 120 patients with upper urinary tract stones who underwent PCNL in the complete supine position between April 2009 and March 2011. There was neither flank elevation nor change in leg position, and the patients were located at bed edge. One surgeon performed all PCNLs. Preoperative evaluation consisted of laboratory tests (especially coagulation tests, kidney function tests, urinalysis, and urine culture), intravenous urography, ultrasonography and CT (in selected cases). Before PCNL, an antibiotic (third generation cephalosporin) was administered to all patients and was continued after the operation.

General anesthesia and ureteral catheter insertion were applied for all patients. The collecting system was punctured by an 18-gauge needle under fluoroscopic or ultrasonographic guidance and via the posterior auxiliary line and subcostal access. A 0.035 inch J-tip guidewire was inserted after removal of the stylet of the needle. One shot dilation was used in all operations. Dilation was performed initially by a 9F dilator and was continued with a single 28F Amplatz dilator. Then, a 30F Amplatz working sheath was inserted. Rigid nephroscopy and pneumatic lithotripsy were performed in all PCNLs. Multiple tracts and a nephrostomy tube were used in necessary cases based on the surgeon's decision.

The studied parameters comprised age, sex, body mass index (BMI), preoperative serum creatinine and hemoglobin levels, history (stone surgery, shockwave lithotripsy [SWL], hypertension, diabetes melitus), stone (location, opacity, burden, complexity), side of operation, imaging for access during operation, target calix for access, number of tracts, insertion of nephrostomy tube, and operative time.

Software SPSS version 16 was used for analysis. Correlation between recorded variables and operative time was assessed by univariate and multivariate statistical tests. Spearman test, independent t test, Mann-Whitney test, and one-way analysis of variance were used in univariate survey. Then, all factors with P<0.1 and other important variables were analysed by multiple linear regression in multivariate survey.

Operative time was considered as duration from incision or puncture of the skin to the end of the operation and performance of skin suture. Staghorn stones and stones with two or more locations (multiple location stones) were defined as complex stones, and locations consist of upper calix, middle calix, lower calix, pelvis, and upper ureter. Also, BMI was grouped into <25, 25 to 29.9, and ≥30.

Many articles were reviewed concerning this subject. This study was approved by our ethical committee.

Results

The mean age was 50.90±11.63 years (standard error 1.10, range 18–78 y), and mean BMI was 28.58±4.44 kg/m² (standard error 0.42, range 19.36–46.71 kg/m²). Frequency of patients was 18.3% in BMI<25 kg/m², 50.9% in BMI=25–29.9 kg/m², and 30.8% in BMI≥30 kg/m². Male/female ratio was 70(58.3%)/50(41.7%). A history of renal anomaly, hypertension, diabetes melitus, stone surgery, and SWL was present in 3 (2.5%), 40 (33.3%), 23 (19.2%), 39 (32.5%), and 62 (51.7%) patients, respectively. Mean preoperative serum creatinine and hemoglobin levels were 1.20±0.78 mg/dL (standard error 0.07, range 0.5–8.8 mg/dL) and 13.48±1.46 g/dL (standard error 0.13, range 9.6–16.5 g/dl). Eight (6.7%) patients had a preoperative serum creatinine level>1.5 mg/dL.

The mean stone burden was 35.03±20.18 mm (standard error 1.84, range12–200 mm). Radiopaque/radiolucent ratio of stones was 84.2%/15.8%. Locations of stones were only upper calix in 7 (5.8%) patients, only middle calix in 2 (1.7%) patients, only lower calix in 17 (14.2%) patients, only pelvis in 21 (17.5%) patients, only upper ureter in 3 (2.5%) patients, staghorn in 16 (13.3%) patients, and multiple locations in 54 (45%) patients. Seventy (58.3%) patients had complex stones. Right/left ratio of operation side was 60(50%)/60(50%). Mean operative time was 60.62±30.70 minutes (standard error 2.84, range 15–150 min).

Fluoroscopy and ultrasonography were used in 100 (83.3%) and 20 (16.7%) patients during PCNL. Eleven (9.2%) patients were operated by multiple tract access and 109 (80.8%) patients underwent PCNL with single tract access. The target calix for access was the upper calix in 8 (6.7%) patients, middle calix in 43 (35.8%) patients, and lower calix in 69 (57.5%) patients. Thr tubeless technique was performed in 111 (92.5%) PCNL, and 9 (7.5%) patients had nephrostomy tube insertion after operation.

In univariate analysis, no significant correlation was seen between patient age (P=0.422, r−0.078), preoperative serum creatinine level (P=0.435), and preoperative hemoglobin level (P=0.195) with operative time. A significant negative correlation was seen, however, between BMI and operative time (P=0.029). Also, there was a significant positive correlation between stone burden and operative time (P=0.000) (Table 1).

Table 1.

Correlation Between Patient and Stone Parameters with Operative Time During Percutaneous Nephrolithotomy in the Complete Supine Position (Spearman Test)

Parameter	Correlation with operative time	P value	r
Patient age	No correlation	0.422	−0.078
Patient BMI	Negative correlation	0.029	−0.208
Preoperative serum creatinine	No correlation	0.435	0.073
Preoperative hemoglobin	No correlation	0.195	−0.122
Stone burden	Positive correlation	0.000	0.320

BMI=body mass index.

On the other hand, variables such as sex (P=0.168), groups of BMI (P=0.280, df=2), renal insufficiency (preoperative serum creatinine>1.5 mg/dL, P=0.339), renal anomaly (P=0.524), hypertension (P=0.075), diabetes melitus (P=0.671), previous stone surgery (P=0.981), previous SWL (P=0.226), stone opacity (P=0.502), side of operation (P=0.107), number of tracts (P=0.706) and tubeless approach (P=0.191) had no effect on operative time in univariate analysis. Imaging for access, target calix for access and (probably) complex stones affected operative time during PCNL, however. With fluoroscopic guidance (56.01±27.81 minutes), mean operative time was significantly (P=0.001) less than ultrasonographic guidance (83.00±34.81 min). Mean operative time was 83.12±33.16 minutes in upper calix access, 53.15±27.75 minutes in middle calix access, and 62.34±31.61 minutes in lower calix access. These differences were significant among calices for accesses (P=0.035). Also, in the group with complex stones (65.19±33.18 minutes), mean operative time was higher (P=0.057) than in the group with noncomplex stones (54.18±26.29 min) (Table 2).

Table 2.

Effect of Patient, Stone, and Operation-Related Parameters on Operative Time During Percutaneous Nephrolithotomy in the Complete Supine Position

		Operative time
Parameter		Mean±SD	Std. error	Range	P value Statistical test
Sex	Male	64.18±34.68	4.24	15–150	0.168
	Female	56.18±24.04	3.43	20–150	Independent t test
Group of BMI	<25	64.04±32.66	7.13	20–150
	25–29.9	60.87±28.68	3.87	15–150	0.280
	≥30	52.50±27.03	4.64	20–135	One-Way ANOVA
Preop. serum Cr	≤1.5 mg/dL	59.61±29.39	2.83	15–150	0.339
	>1.5 mg/dL	76.88±45.11	15.95	25–150	Mann-Whitney test
Renal anomaly	Yes	48.33±20.21	11.67	30–70	0.524
	No	61.13±31.00	2.92	15–150	Mann-Whitney test
Hypertension	Yes	53.67±23.89	3.82	20–150	0.075
	No	64.42±33.29	3.79	15–150	Independent t test
Diabetes mellitus	Yes	56.43±23.35	5.10	20–110	0.671
	No	61.77±32.21	3.30	15–150	Mann-Whitney test
Previous stone surgery	Yes	60.64±31.29	5.01	20–150	0.981
	No	60.88±30.72	3.50	15–150	Mann-Whitney test
Previous SWL	Yes	57.64±28.90	3.97	15–150	0.226
	No	65.09±33.86	4.65	20–150	Independent t test
Opacity of stone	Radiopaque	67.12±32.10	4.45	20–150	0.502
	Radiolucent	74.50±29.01	9.17	45–120	Independent t test
Complex stone	Yes	65.19±33.18	4.05	15–150	0.057
	No	54.18±26.29	3.75	20–150	Independent t test
Side of operation	Right	65.58±30.49	4.04	20–150	0.107
	Left	56.16±31.16	4.16	15–150	Independent t test
Imaging for access	Fluoroscopy	56.01±27.81	2.82	15–150	0.001
	Ultrasonography	83.00±34.81	7.78	25–150	Mann-Whitney test
Target calix for access	Upper calix	83.12±33.16	11.72	45–150
	Middle calix	53.15±27.75	4.44	15–150	0.035
	Lower calix	62.34±31.61	3.95	20–150	One-Way ANOVA
Number of tracts	Single	59.21±29.93	7.70	15–150	0.706
	Multiple	61.00±24.36	2.93	30–90	Mann-Whitney test
Tubeless approach	Yes	73.67±37.78	12.59	25–150	0.191
	No	58.38±29.39	2.87	15–150	Mann-Whitney test

SD=standard deviation; std.=standard; BMI=body mass index; ANOVA=analysis of variance; Preop=preoperative; Cr=creatinine; SWL=shockwave lithotripsy.

According to the generalized linear model (multiple linear regression), there was an association between patient BMI (P=0.000), stone burden (P=0.005), imaging for access (P=0.000), and target calix for access (P=0.023) with operative time in multivariate analysis (Table 3).

Table 3.

Correlation Between Patient, Stone, and Operation-Related Parameters with Operative Time During Percutaneous Nephrolithotomy According to Multiple Linear Regression

Source	Wald chi-square	df	Sig.
Intercept	23.054	1	0.000
BMI	14.559	1	0.000
Stone burden	7.845	1	0.005
Imaging for access	14.137	1	0.000
Target calix for access	7.511	2	0.023

Dependent variable: Operative time (min).

Sig=significance; BMI=body mass index.

Mean operative time decreases 3.25 minutes with one unit rise in patient BMI and increases 0.84 minute with 1 mm rise in stone burden. Under fluoroscopic guidance, mean operative time was 28.6 minutes less than under ultrasonographic guidance in a similar condition. In upper calix access, mean operative time was significantly (P=0.022) 15.31 minutes higher than in lower calix access. In middle calix access, mean operative time was 0.725 minutes lower than in lower calix access, but this difference was nonsignificant (P=0.930) (Table 4).

Table 4.

Details of Correlation Between Patient, Stone, and Operation-Related Parameters with Operative Time During Percutaneous Nephrolithotomy Based on Multiple Linear Regression

			95% Wald confidence interval
Parameter	B	Std. error	Lower	Upper	Wald chi-square	df	Sig.
Intercept	149.710	27.6950	95.429	203.991	29.221	1	0.000
BMI	−3.251	0.8520	−4.921	−1.581	14.559	1	0.000
Stone burden	0.841	0.3003	0.253	1.430	7.845	1	0.005
Imaging for access:
Fluoroscopy	−28.588	7.6033	−43.490	−13.685	14.137	1	0.000
Ultrasonography	0	–	–	–	–	–	–
Target calix for access:
Upper calix	15.313	6.6664	2.247	28.379	5.276	1	0.022
Middle calix	−0.725	8.2181	−16.833	15.382	0.008	1	0.930
Lower calix	0	–	–	–	–	–	–

Dependent variable: Operative time (min).

Std=standard; sig=significance; BMI=body mass index.

Discussion

PCNL is the choice surgical procedure for the majority of upper urinary tract stones with higher outcomes and cost-effectiveness in comparison with other alternatives.^1

–5 PCNL is usually accomplished in the prone position. In this study, all patients underwent PCNL in the complete supine position. In this position, flank elevation and change in leg situation are not performed, and the patient is located at the bed edge. Also, no change in position is necessary for catheter insertion, intubation, and anesthesia with this technique.

According to our previous randomized clinical trial study, the complete supine position is accompanied by patient and surgeon comfort (sitting condition of the surgeon), better control of the airway by the anesthesiologist, no change of patient position in the operating room, simultaneous performance of PCNL and ureteroscopic interventions, comparable results and similar outcomes to that of the prone position, less operative and anesthesia time, and low colonic complication.¹² De Sio performed PCNL in the modified supine and prone positions in a randomized trial and reported similar outcomes in two positions with lower operative time in the supine position.⁹ For the above reasons and good results and outcomes,³¹ we performed PCNL in the complete supine position for the majority of patients during recent years. With the importance of operative time because of its effects on the duration of anethesia and its related complications,²⁴ the outcomes,^25

–28 and complications^29,30 of PCNL, we assessed parameters that might affect operative time during PCNL in the complete supine position.

Among patient-related parameters, BMI had a significant negative association with operative time in univariate (P=0.029) and multivariate (P=0.000) analysis. There was no significant difference in mean operative time, however, among groups of BMI (<25, 25–29.9, ≥30). In research by El-Assmy and associates,³² the mean operative time was 69.8±32.4 minutes in the group with BMI<25, 71.4±28.7 minutes in the group with BMI=25–29.9, 68.5±29.6 minutes in the group with BMI=30–39.9, and 77.2±32.4 minutes in the group with BMI≥40; no significant difference was seen among these groups concerning operative time (P=0.45).

In our study, renal insufficiency (preoperative serum creatinine level>1.5 mg/dL, P=0.339), renal anomaly (P=0.524), hypertension (P=0.075), and diabetes melitus (P=0.671) had no effect on operative time during PCNL. We had only three patients with renal anomaly. Also there was no significant association between previous stone surgery and previous SWL with operative time in univariate and multivariate analysis. In other experience, we observed no significant difference in mean operative time between groups with (75.41±12.20 min) and without (67.42±21.56 min) previous stone surgery.³³

No significant difference was seen between groups with and without previous stone surgery concerning operative time in studies by Kurtulus and colleagues³⁴ (2.3 vs 2.2 h, P>0.05), Lojanapiwat,³⁵ and Sofikerim and coworkers.³⁶ Tugcu and associates,³⁷ however, observed that in patients with previous open stone surgery (155±30 min), mean operative time was significantly higher than in patients without previous open stone surgery (137±30 min). In a survey by Resorlu and colleagues,³⁸ mean operative time was similar (P>0.05) among a group with previous open stone surgery, a group with previous unsuccessful SWL, and a group without these histories. There was no significant difference concerning operative time between groups with unsuccessful previous SWL (58.5±19.1 min) and without previous SWL (60.7±19.5 min) in an investigation by Yuruk and coworkers.³⁹ Mean operative time per cm² stone, however, was higher in a group with previous SWL (8.6 vs 7.3, P<0.05).

In our study, a significant positive correlation was seen between stone burden and operative time in univariate (P=0.000) and multivariate (P=0.005) analysis. Also in complex stones, mean operative time was higher than in noncomplex stones in univariate analysis (P=0.057). This matter, however, was not confirmed in a multivariate statistical test.

Use of fluoroscopy for access results in radiation exposure to the patient and surgical team during PCNL. In some patients, ultrasonography was used for access instead of fluoroscopy during PCNL based on the surgeon's decision. In this study, the type of imaging for access had a significant association with operative time, so that under fluoroscopic guidance, operative time was significantly lower than under ultrasonographic guidance in univariate (P=0.001) and multivariate (P=0.000) analysis. In our previous nonrandomized study, the mean operative time in the ultrasonographic group (88.92 min) was longer than in the fluoroscopic group (79.28 min), and no significant differences were seen between the two groups concerning outcomes.²¹ We believe that ultrasonographic PCNL is more difficult than fluoroscopic PCNL, but this matter is related to the surgeon's experience.

The target calix for access affected operative time in univariate (P=0.035) and multivariate (P=0.023) statistical tests. Mean operative time was significantly different among the calices that were used for access (upper calix access>lower calix access>middle calix access). Multivariate survey showed that mean operative time in upper calix access was significantly (P=0.022) higher than lower calix access. Also in middle calix access, mean operative time was less than lower calix access. But this difference was nonsignificant (P=0.930). In the experience of Aron and associates,¹⁶ upper calix access (48 min) had significantly (P<0.001) shorter mean operative time than lower calix access (74 minutes) in complex lower pole renal stones. On the other hand, in the results from Netto and colleagues¹⁸ concerning staghorn stones, no significant differences were seen in mean operative time among upper calix access (86.8 min), lower/middle calix access (139.1 min), and multiple access (134.9 min) groups.¹⁸

Some patients were operated on by multiple percutaneous tracts for better access and complete removal of stones based on surgical condition and the surgeon's decision. Although the multiple tract group had a higher mean stone burden and complex stones than the single tract group, the two groups were statistically similar concerning BMI (P=0.673), previous stone surgery (P=0.742), multiple stones (P=0.722), staghorn stone (P=0.648), imaging for access (P=1) and tubeless PCNL (P=1). Mean operative time was 59.21±29.93 minutes in single tract access and 61.00±24.36 minutes in multiple tract access in our survey. No significant association was confirmed between number of tracts and operative time in univariate and multivariate analysis, and we believe that the multiple tract access in some cases can help the surgeon for better visualization to remove the stone or any fragments effortlessly and quickly.

In reports from Netto and coworkers,¹⁸ the mean operative time was statistically similar among upper calix access (single access, 86.8 min), lower/middle calix access (single access, 139.1 min), and multiple access (134.9 min) in staghorn stones. In research from Akman and associates,¹⁹ however, multiple tract access (82.14±32.14 min) had significantly (P<0.0001) higher operative time than single tract access (70.04±27 min) in staghorn stones.¹⁹

Although mean operative time in the tubeless approach (58.38±29.39 min) was lower than the technique with nephrostomy tube insertion (73.67±37.78 min) in our research, this difference was nonsignificant in univariate analysis. In addition, there was no significant association between the tubeless procedure and operative time in multivariate analysis, whereas in another study, the tubeless group (93.76 min) had significantly (P=0.03) lower mean operative time than the group with a nephrostomy tube (109.98 minutes) in staghorn stones.²³

Conclusions

BMI, stone burden, imaging for access, target calix for access, and probably complex stones were effective factors on operative time. No significant association was seen between groups of BMI, hypertension, diabetes melitus, previous stone surgery, previous SWL, stone opacity, number of tracts, and the tubeless approach with operative time. Multivariate analysis revealed that BMI, stone burden, imaging for access, and target calix for access are the most important parameters affecting operative time in PCNL in the complete supine position.

Footnotes

Acknowledgment

The authors acknowledge that Dr. Siavash Falahatkar operated on all patients in this research.

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

References

Brannen

, Bush

, Correa

et al. Kidney stone removal: Percutaneous versus surgical lithotomy. J Urol, 1985; 133:6–12.

Snyder

, Smith

. Staghorn calculi: Percutaneous extraction versus anatrophic nephrolithotomy. J Urol, 1986; 136:351–354.

de la Rosette

, Assimos

, Desai

et al. The Clinical Research Office of the Endourological Society Percutaneous Nephrolithotomy Global Study: Indications, complications, and outcomes in 5803 patients. J Endourol, 2011; 25:11–17.

Galvin

, Pearle

. The contemporary management of renal and ureteric calculi. BJU Int, 2006; 98:1283–1288.

Matlaga

, Kim

, Lingeman

. Improving outcomes of percutaneous nephrolithotomy: access. EAU Update Series, 2005; 3:37–43.

Soucy

, Ko

, Duvdevani

et al. Percutaneous nephrolithotomy for staghorn calculi: A single center's experience over 15 years. J Endourol, 2009; 23:1669–1673.

Darabi Mahboub

, Zolfaghari

, Ahanian

. Percutaneous nephrolithotomy of kidney calculi in horseshoe kidney. Urol J, 2007; 4:147–150.

Segura

. The role of percutaneous surgery in renal and ureteral stone removal. J Urol, 1989; 141:780–781.

De Sio

, Autorino

, Quarto

et al. Modified supine versus prone position in percutaneous nephrolithotomy for renal stones treatable with a single percutaneous access: A prospective randomized trial. Eur Urol, 2008; 54:196–202.

10.

de la Rosette

, Tsakiris

, Ferrandino

et al. Beyond prone position in percutaneous nephrolithotomy: A comprehensive review. Eur Urol, 2008; 54:1262–1269.

11.

Valdivia Uría

, Valle Gerhold

, López López

et al. Technique and complications of percutaneous nephroscopy: Experience with 557 patients in the supine position. J Urol, 1998; 160:1975–1978.

12.

Falahatkar

, Moghaddam

, Salehi

et al. Complete supine percutaneous nephrolithotripsy comparison with the prone standard technique. J Endourol, 2008; 22:2513–2518.

13.

Radecka

, Brehmer

, Holmgren

, Magnusson

. Complications associated with percutaneous nephrolithotripsy: Supra- versus subcostal access. A retrospective study. Acta Radiol, 2003; 44:447–451.

14.

Munver

, Delvecchio

, Newman

, Preminger

. Critical analysis of supracostal access for percutaneous renal surgery. J Urol, 2001; 166:1242–1246.

15.

Nishizawa

, Yamada

, Miyazaki

et al. Results of treatment of renal calculi with lower-pole fluoroscopically guided percutaneous nephrolithotomy. Int J Urol, 2008; 15:399–402.

16.

Aron

, Goel

, Kesarwani

et al. Upper pole access for complex lower pole renal calculi. BJU Int, 2004; 94:849–852.

17.

, He

, Wu

et al. Chinese minimally invasive percutaneous nephrolithotomy: The Guangzhou experience. J Endourol, 2009; 23:1693–1697.

18.

Netto

Jr , Ikonomidis

, Ikari

, Claro

. Comparative study of percutaneous access for staghorn calculi. Urology, 2005; 65:659–663.

19.

Akman

, Sari

, Binbay

et al. Comparison of outcomes after percutaneous nephrolithotomy of staghorn calculi in those with single and multiple accesses. J Endourol, 2010; 24:955–960.

20.

Zhou

, Gao

, Wen

, Xiao

. Clinical value of minimally invasive percutaneous nephrolithotomy in the supine position under the guidance of real-time ultrasound: Report of 92 cases. Urol Res, 2008; 36:111–114.

21.

Falahatkar

, Neiroomand

, Enshaei

et al. Totally ultrasound versus fluoroscopically guided complete supine percutaneous nephrolithotripsy: A first report. J Endourol, 2010; 24:1421–1426.

22.

Lojanapiwat

, Soonthornphan

, Wudhikarn

. Tubeless percutaneous nephrolithotomy in selected patients. J Endourol, 2001; 15:711–713.

23.

Falahatkar

, Khosropanah

, Roshani

et al. Tubeless percutaneous nephrolithotomy for staghorn stones. J Endourol, 2008; 22:1447–1451.

24.

Scholes

, Browning

, Sztendur

, Denehy

. Duration of anaesthesia, type of surgery, respiratory co-morbidity, predict VO₂max and smoking predict postoperative pulmonary complications after upper abdominal surgery: An observational study. Aust J Physiother, 2009; 55:191–198.

25.

Gune

, Ridhorkar

, Sabnis

et al. Factors affecting blood loss during percutaneous nephrolithotomy. Ind J Urol, 1998; 14:9–16.

26.

Kukreja

, Desai

, Patel

et al. Factors affecting blood loss during percutaneous nephrolithotomy: Prospective study. J Endourol, 2004; 18:715–722.

27.

Vorrakitpokatorn

, Permtongchuchai

, Raksamani

, Phettongkam

. Perioperative complications and risk factors of percutaneous nephrolithotomy. J Med Assoc Thai, 2006; 89:826–833.

28.

Akman

, Binbay

, Sari

et al. Factors affecting bleeding during percutaneous nephrolithotomy: Single surgeon experience. J Endourol, 2011; 25:327–333.

29.

de la Rosette

, Zuazu

, Tsakiris

et al. Prognostic factors and percutaneous nephrolithotomy morbidity: A multivariate analysis of a contemporary series using the Clavien classification. J Urol, 2008; 180:2489–2493.

30.

Özden

, Mercimek

, Yakupoğlu

et al. Modified Clavien classification in percutaneous nephrolithotomy: Assessment of complications in children. J Urol, 2011; 185:264–268.

31.

Falahatkar

, Farzan

, Allahkhah

. Is complete supine percutaneous nephrolithotripsy feasible in all patients? Urol Res, 2011; 39:99–104.

32.

El-Assmy

, Shokeir

, El-Nahas

et al. Outcome of percutaneous nephrolithotomy: effect of body mass index. Eur Urol, 2007; 52:199–204.

33.

Falahatkar

, Panahandeh

, Ashoori

et al.

What is the difference between percutaneous nephrolithotomy in patients with and without previous open renal surgery?

J Endourol, 2009; 23:1107–1110.

34.

Kurtulus

, Fazlioglu

, Tandogdu

et al. Percutaneous nephrolithotomy: Primary patients versus patients with history of open renal surgery. J Endourol, 2008; 22:2671–2675.

35.

Lojanapiwat

. Previous open nephrolithotomy: Does it affect percutaneous nephrolithotomy techniques and outcome? J Endourol, 2006; 20:17–20.

36.

Sofikerim

, Demirci

, Gülmez

, Karacagil

. Dose previous open nephrolithotomy affect the outcome of percutaneous nephrolithotomy? J Endourol, 2007; 21:401–403.

37.

Tugcu

, Su

, Kalfazade

et al. Percutaneous nephrolithotomy (PCNL) in patients with previous open stone surgery. Int Urol Nephrol, 2008; 40:881–884.

38.

Resorlu

, Kara

, Senocak

et al. Effect of previous open renal surgery and failed extracorporeal shockwave lithotripsy on the performance and outcomes of percutaneous nephrolithotomy. J Endourol, 2010; 24:13–16.

39.

Yuruk

, Tefekli

, Sari

et al.

Does previous extracorporeal shock wave lithotripsy affect the performance and outcome of percutaneous nephrolithotomy?

J Urol, 2009; 181:663–667.