Variables That Influence Operative Time During Percutaneous Nephrolithotomy: An Analysis of 1897 Cases

Abstract

Background and Purpose:

Operative time is an important perioperative factor and is related to postoperative complications and procedural cost-effectiveness. There are few studies, however, investigating factors that affect operative time during percutaneous nephrolithotomy (PCNL). In this study, patient and kidney stone-related factors that influence operative time were analyzed.

Materials and Methods:

Clinical records from 1897 patients with renal calculi who underwent PCNL were retrospectively reviewed, and these patients were divided into two groups based on their median operative time (group 1: ≤60 min; group 2: >60 min). Multivariate analyses as well as univariate analyses including chi-square, Fisher exact and Mann Whitney U tests were used to investigate the effects of independent variables on operative time, including patient and kidney stone-related factors such as age, sex, body mass index, history of ipsilateral open surgery, shockwave lithotripsy, presence of hydronephrosis, stone burden, stone type and opacity, and surgical experience.

Results:

The mean operative time was 64.9±27.6 minutes (range 10–220 min). Univariate analyses determined that a history of open surgery, presence of hydronephrosis, stone type and size, and surgical experience correlated with operative time (P<0.05 for all). Multivariate analyses revealed that presence of hydronephrosis (odds ratio [OR]: 1.44, 95% confidence interval [CI]: 1.17–1.78, P=0.002), stone type (OR: 2.12, 95% CI: 1.69–2.70, P<0.0001), stone burden (OR: 2.44, 95% CI: 1.85–3.12, P<0.0001), and surgical experience (OR: 0.56, 95% CI: 0.46–0.70, P<0.0001) were significant independent factors in influencing operative time.

Conclusions:

Presence of hydronephrosis, renal stone size and type significantly affect the operative time during PCNL. The duration of the operation was also observed to decrease with increased surgical experience.

Introduction

P ercutaneous nephrolithotomy (PCNL) plays an important role in the management of upper urinary tract stones and has been found to result in decreased mortality, shorter hospital stays, and decreased time lost from work. American Urological Association and European Association of Urology (EAU) guidelines specifically recommend PCNL for stones >2 cm or staghorn calculi.^1,2 Several investigators around the globe have reported excellent outcomes using this minimally invasive procedure.^3
–5 Despite technologic advances and growing experience with PCNL, complications from this technique include blood loss, perforation of the pelvicaliceal system, injuries to adjacent organs, sepsis, urine leakage from the nephrostomy tract, renal loss; even mortality can still be seen.⁶

The duration of the procedure is as an important perioperative factor in determining intraoperative and postoperative complications. The correlation between operative time and perioperative blood loss in PCNL has been reported by many studies.^7,8 Furthermore, operative time is important because cost-effectiveness is an issue in helping to contain costs and manage health care economics⁹.

Therefore, operative time during PCNL is an important outcome parameter. To our knowledge, however, there are few studies investigating factors that affect operative time.¹⁰ In this study, patient-related, renal stone-related, and procedure-related factors that influence operative time were evaluated.

Materials and Methods

Data were collected and retrospectively analyzed for patients who underwent PCNL for renal stones between October 2002 and December 2010 at our institution. All patients were evaluated preoperatively with intravenous urography (IVU) and/or CT. Demographic characteristics collected on patients in the study included age, sex, body mass index (BMI), history of extracorporeal shockwave lithotripsy (SWL), and previous kidney surgery. Preoperative laboratory tests included serum creatinine, hemoglobin measurements, platelet counts, coagulation tests, and urine cultures. All patients included in the study had sterile urine cultures, and patients with urinary tract infections were treated with a complete course of culture-specific antibiotics.

Stone types were categorized as either staghorn calculi (partial or complete) or other types, including pelvic stones and multiple or isolated caliceal stones. Complete staghorn calculi were defined as stones occupying ≥80% of the renal pelvis and all of the caliceal system or occupying the renal collecting system.³ Partial staghorn calculi were defined as stones occupying the renal pelvis or occupying at least two calices.³ Stone size was assessed by calculating surface area according to EAU guidelines.² The operative time was defined as the time from puncture for an access tract to the final placement of the nephrostomy tube. PCNL procedures were divided into two groups based on surgical experience and were classified as either being performed before or after 2006.

An attending urologist obtained access to the selected calix by using an 18-gauge needle under fluoroscopic guidance with the patient in the prone position, as described in detail previously.¹¹ The access tract was dilated with a balloon dilator (Nephromax,^™ Microvasive Boston Scientific Corporation, Natick, MA), and a 30F Amplatz sheath was inserted. Nephroscopy was performed with a rigid 26F nephroscope. Fragmentation of the stone burden was accomplished using a pneumatic (Vibrolith,^® Elmed, Ankara, Turkey) or ultrasonic lithotripter (Swiss LithoClast,^® EMS Electro Medical Systems, Nyon, Switzerland). Forceps were used to remove stone fragments. Additional tracts were created when indicated during the same session. Flexible nephroscopy was used at the conclusion of the procedure for cases with suspected residual stones. A 14F nephrostomy tube was placed within the renal pelvis or the involved calix at the conclusion of the majority of cases.

Radiography of the kidneys, ureters, and bladder was performed on each patient on the first postoperative day. The nephrostomy tube was removed on postoperative day 2 after obtaining antegrade nephrostography demonstrating patency of the ureteral drainage system to the level of the bladder. Repeated PCNL, ureteroscopy, and SWL were considered as accessory treatment alternatives when indicated.

All cases were evaluated by spiral CT or IVU 3 months postoperatively. The procedure was considered successful if the patient was either stone free or had only clinically insignificant residual fragments (CIRFs). Results were classified as stone free, CIRFs, and unsuccessful (residual stones) at the third month of the follow-up. CIRFs were considered to be ≤4 mm, nonobstructing, noninfectious, and asymptomatic residual fragments.¹¹

A total of 1897 patients were categorized into two groups according to median operative time (median=60 min) (group 1: ≤60 min; group 2: >60 min). Data were expressed as the mean±standard deviation. Patient and kidney stone-related factors such as age, sex, BMI, history of previous open renal surgery or SWL, presence of hydronephrosis, stone burden, stone opacity, and surgical experience were analyzed by univariate and multivariate tests using SPSS 16 Windows software (SPSS Inc, Chicago, IL). Chi-square or Fisher exact tests/Mann-Whitney U tests were used for univariate analyses. Odds ratios and statistical estimates were calculated and expressed with 95% confidence intervals. Also, binary logistic regression models were performed for further investigation if any parameter was found to be significant with a univariate test. P values less than 0.05 were regarded as statistically significant.

Results

Mean age and BMI were 43.2±14.6 (7–83) years and 25.9±4.7 (12.2–50.8) kg/m², respectively. Multiple accesses were necessary in 371 (19.6%) patients. The mean operative time and mean length of hospitalization were 64.9±27.6 (median: 60; range 10–220) minutes and 2.87±1.75 (median: 2; range 1– 21) days, respectively. Overall success rates of 85.6% including CIRFs were observed in 15.1% of patients after a single session of PCNL. As an auxiliary treatment, SWL was performed in 162 patients, re-PCNL in 91 patients, and ureterorenoscopy in 28 patients.

Preoperative demographic data from the two study groups are shown in Table 1. Patient-related factors such as mean age, mean BMI, sex, and history of SWL did not differ between the two groups. The incidence of previous ipsilateral open surgery and hydronephrosis were higher in group 2, according to univariate analysis. In addition, the mean stone burden was 642.1±418.6 (100–2800) mm² and 976.6±665.2 mm² (100–4500 mm) in groups 1 and 2, respectively (P<0.0001). Operative time was significantly increased for stones >10 cm² compared with stones measuring <10 cm². Group 2 also had a significantly higher incidence of staghorn calculi (42.2% vs 19.0%, P<0.0001). Of 1897 procedures, 651 (34.4%) were performed before 2006 and 1246 (65.7%) after 2006. Patients who underwent PCNL either before or after 2006 presented with similar demographic data with regard to sex, stone size and type, degree of hydronephrosis, and access number. The mean operative time prior to 2006, however, was 1.75-fold longer than after 2006 (P<0.0001).

Table 1.

Comparison of Patient and Renal Stone-Related Factors that Affect Operative Time

Parameters	Group 1	Group 2	P value	OR (95% CI)
Mean age	43.6±14.9	42.6±14.0	0.15
Sex			0.13
Female	49.4 (549)	53.1 (417)
Male	50.6 (562)	46.9 (369)
Mean BMI	25.7±4.6	26.0±4.8	0.14
Previous open surgery			0.03	1.26 (1.02–1.56)
(−)	79.2 (880)	75.2 (591)
(+)	20.8 (231)	24.8 (195)
History of SWL			0.53
(−)	74.3 (825)	75.4 (593)
(+)	25.7 (286)	24.6 (193)
Hydronephrosis			<0.0001	1.84 (1.51–2.24)
Nil or mild	42.0 (467)	28.2 (222)
Moderate or severe	58.0 (644)	71.8 (564)
Stone opacity			0.78
Opaque	97.1 (1079)	97.3 (765)
Nonopaque	2.9 (32)	2.7 (21)
Stone type			<0.0001	3.11 (2.53–3.83)
Coraliform	81.0 (900)	57.8 (454)
Other stone types	19.0 (211)	42.2 (332)
Stone burden			<0.0001	3.51 (2.79–4.41)
≤1000 mm²	87.0 (967)	65.6 (516)
>1000 mm²	13.0 (144)	34.4 (270)
Operation date			<0.0001	0.57 (0.47–0.69)
<2006	29.0 (322)	41.9 (329)
≥2006	71.0 (789)	58.1 (457)

OR=odds ratio; CI=confidence interval; BMI=body mass index; SWL=shockwave lithotripsy.

Multivariate analyses demonstrated that the presence of hydronephrosis, stone type, stone burden, and surgical experience were the most important factors in determining operative time (Table 2).

Table 2.

Predictive Factors for Operative Time During Percutaneous Nephrolithotomy: Results of Multivariate Stepwise Regression Analyses

Parameters	Significance	OR (95% CI)
Hydronephrosis	0.002
Nil or mild (reference)		1
Moderate or severe		1.44 (1.17–1.78)
Stone type	<0.0001
Coraliform		2.12 (1.69–2.70)
Other stone types (reference)		1
Stone burden	<0.0001
≤1000 mm² (reference)		1
>1000 mm²		2.44 (1.85–3.12)
Surgical experience	0.0001
<2006 (reference)		1
≥2006		0.56 (0.46–0.70)

OR=odds ratio; CI=confidence interval.

Blood loss necessitating transfusion during the operation occurred in 84 (7.6%) patients in group 1 and in 125 (15.9%) patients in group 2 (P<0.0.0001). The need for blood transfusion increased 2.3-fold when the operative time was longer than 60 minutes. Angioembolization was also indicated in three and two patients in groups 1 and 2, respectively (P=0.7). Furthermore, two patients in each group underwent nephrectomy secondary to intractable bleeding and unstable hemodynamics. During the study, sepsis developed in seven (0.6%) patients in group 1 and six (0.7%) patients in group 2; sepsis was defined by the presence of two or more of the following parameters: Temperature >38°C or <36°C, heart rate >90 beats/min, respiratory rate >20/min, or white blood cell count >12,000/mm³ or <4000/mm³ (P=0.9). Two patients in group 1 died on the second and third postoperative day from sepsis. Hydrothorax or hemothorax necessitating chest tube insertion developed in nine (0.8%) patients in group 1 and six (0.7%) patients in group 2 (P=0.8). In group 1, open surgery was performed in one patient because of the presence of renopleural fistula. Patients in group 2 were found to have longer hospital stays when compared with patients in group 1 (3.1±2.0 days vs 2.7±1.6 days, P<0.0001).

Discussion

Today, the effectiveness and safety of PCNL in the management of renal stones have been proven. Despite high success rates, concerns with percutaneous renal surgery include serious complications, such as blood loss necessitating transfusion, fever, urinary tract infection, and adjacent organ injury. Aside from potential complications, an important consideration with PCNL is its relative cost-effectiveness. The cost-effectiveness of PCNL correlates with stone-free rates, operative time, length of hospitalization, and major complications. Therefore, the operative time of PCNL has an important role in decreasing perioperative bleeding and in the cost-effectiveness of the procedure.

Although studies investigating the effects of hydronephrosis on PCNL are reported in the literature,^7,8,10 there are only a few trials that explore the impact of hydronephrosis on the operative time.¹⁰ Olbert and associates¹⁰ could not show the presence of hydronephrosis to be an independent predictor of operative time. In the present study, hydronephrosis increased the operative time 1.84-fold in univariate analysis. Among patient-related factors, only hydronephrosis was significantly correlated with operative time by multivariate regression analysis. This correlation may be explained by relatively higher rates of stone fragment mobility between calices in hydronephrotic kidneys. In severely hydronephrotic kidneys, tissue may contract after percutaneous access, which complicates stone fragment detection and may contribute to increasing the operative time.

Various clinical studies have demonstrated that PCNL may be performed safely and with similar success in patients with a history of SWL or open surgery compared with patients without such a history.^12
–14 In our study, the relationship between operative time and history of open surgery was analyzed, and the percentage of patients with a history of open surgery was higher in group 2 based on univariate analyses; however, in multivariate analyses, this correlation was not revealed. Margel and colleagues¹⁵ compared PCNL in patients who did or did not have previous open surgeries and reported that the operative time, percentage of secondary procedures, and attempts to gain access were significantly increased in patients who had previously undergone open renal stone surgery. According to this study, the use of flexible nephroscopy is recommended to avoid additional access, which has the potential to prolong operative time in this group.

Tugcu and coworkers¹⁶ found that the mean operative time was significantly longer in patients who underwent previous open surgery compared with those who did not. This relationship may be explained by the development of postoperative adhesions, and therefore Amplatz dilators are better to be used instead of balloon dilatators. Recent reports regarding new higher pressure balloon (30 ATM – Bard X-force) may allow for safe balloon dilation even in patients with open surgery and should be considered an alternative to Amplatz in these patients.¹⁷ Nonetheless, various studies have indicated that previous renal surgery does not affect either operative time or success rates.

The impact of BMI on complications and outcomes of PCNL has been investigated in various studies.^18,19 Carson and colleagues¹⁹ did not determine any significant difference in operative time, length of hospitalization, requirement for secondary procedures, complications, or success rates between obese and nonobese patients. Similarly, Koo and coworkers²⁰ and Pearle and associates²¹ did not detect any impact of BMI on PCNL outcomes, including stone-free rates, blood loss, analgesic usage, hospital stay, complications, and operative time. Similar to other studies, no correlation was found between BMI and operative time in the present study.

The operative time for stones >1000 mm² was three-fold greater than smaller stones (≤1000 mm²) in the present study. Olbert and colleagues¹⁰ analyzed preoperative factors correlating with short-term perioperative outcomes and found that larger stone size significantly predicted a greater operative time. With increasing renal stone size and complexity, the necessity for multiple accesses increases to achieve complete stone removal. Concerns regarding multiple tracts include greater blood loss and higher complication rates. Furthermore, blood loss during operation, which impaired visual acuity during surgery, was also greater in multitract PCNL.

Our findings demonstrate a negative correlation between operative time and surgical experience. Despite similarities between stone size and types, patient-related demographic factors, and access number, the mean operative time of PCNL before 2006 was 1.75-fold longer than procedures after 2006. de la Rosette and coworkers²² also investigated morbidities associated with PCNL in 244 patients using the Clavien classification system of surgical complications. This study divided patients into two groups based on those who were operated on before (group 1) or after 2002 (group 2) and determined that the impact of operative time on complication rate weakens significantly with increasing surgical experience. Surgical experience as well as a reduction in operative time may be explained by developments in endoscopic instruments and techniques.

In another study investigating the effects of surgical experience on operative time, Allen and associates²³ proposed a prerequisite of 60 PCNL procedures to gain surgical competence and 115 procedures for surgical excellence. Tanriverdi and coworkers²⁴ also investigated the learning curve in PCNL training. The authors found that the mean operative time gradually decreased after the first 60 cases performed and then reached a plateau.

One of neglected factors for operative time is the experience of the operating room staff. During the recent 10 years, we have been working with the same two operating room staffs responsible for endourologic cases; thus, these staff members' experience in instrument handling, C-arm positioning, resolving various problems in the operating room have also affected the operation times.

The retrospective nature of this study has several limitations. First, because our center is one of the biggest training centers for endourology in our country, our data reflect results from several different attending surgeons working at an academic training program with fellow and resident case participation. Second, data on the instruments used for stone disintegration is limited. Finally, we were not able to evaluate the effects of mini-PCNL on operative time during PCNL because of the routine use of 30F in all cases.

Conclusions

The present study demonstrates that when PCNL treatment is applied at a reference center for stone disease, operative time significantly decreases. Other factors influencing operative time include presence of hydronephrosis, stone size, and type. Any relationships between operative time and patient age, sex, BMI, history of open kidney surgery and SWL, or stone opacity, however, have not been detected by multivariate analysis.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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