OUP user menu

Nationwide quality improvement of cholecystectomy: results from a national database

Kirstine M. Harboe, Linda Bardram
DOI: http://dx.doi.org/10.1093/intqhc/mzr041 565-573 First published online: 4 July 2011

Abstract

Objective To evaluate whether quality improvements in the performance of cholecystectomy have been achieved in Denmark since 2006, after revision of the Danish National Guidelines for treatment of gallstones.

Design A national database that monitors the quality of cholecystectomy was established, and registration of all cholecystectomies in Denmark was mandatory since 1 January 2006. Indicators describing the operation, the postoperative course, the surgical outcome and various risk factors were followed for 4 years. Results from 2006 were defined as reference values and indicator values, and covariates were stratified by year and tested for trend. Logistic regression models were used to adjust for changes in the prevalence of risk factors/covariates in the study period.

Setting Nationwide, prospective clinical database in Denmark. Data from 2006 to 2009.

Participants 23 672 patients undergoing cholecystectomy where a laparoscopic procedure was considered the standard operation according to national guidelines.

Main outcome measures The rate of conversion from laparoscopic to open operation, the frequency of primary open operations where laparoscopic procedure was the standard, length of postoperative stay including frequency of same-day surgery, additional surgical procedures within 30 days, readmission and mortality.

Results Conversion rate and frequency of primary open cholecystectomy were reduced in the study period. Same-day surgery increased by 14.6%, without an increase in readmission rate (9.4%). The frequency of ‘additional procedures within 30 days’ was also reduced (2.8%). The frequency of injuries requiring reconstructive bile-duct surgery was unaffected (0.15%).

Conclusion The study demonstrates nationwide quality improvements of cholecystectomy in Denmark from 2006 to 2009.

  • quality improvement
  • quality indicators
  • readmissions
  • re-operations
  • digestive diseases
  • hospital care
  • surgery
  • cholecystectomy

Introduction

Gallstones are common in Denmark and other Western countries [1], and the treatment of symptomatic gallstones is the surgical removal of the gall bladder (cholecystectomy) [2]. In the early 1990s, the laparoscopic approach rapidly replaced open surgery as the standard procedure [3]. The laparoscopic procedure was found to cause less scarring, shorter hospital stay and faster recovery than the open procedure, but probably at the expense of a higher rate of bile duct injuries [4],—especially in the surgeon's learning phase of the procedure [5]. Laparoscopic cholecystectomy is one of the most frequent surgical procedures in Western hospitals today and is a routine element of programs that monitor the quality of surgery [69].

In January 2006, The Danish Surgical Society—in cooperation with the National Institute of Health—published the second edition of a national consensus with guidelines regarding the treatment of gallstone disease, including the standard procedures for cholecystectomy [10]. The guidelines state among other things that the laparoscopic cholecystectomy is the standard surgical procedure for the treatment of symptomatic gallstone disease; and also in the case of acute or chronic cholecystitis, after recent acute pancreatitis and after previous abdominal surgery. Also in the elderly patients with comorbidity and for the obese, the laparoscopic operation is recommended as the gold standard.

The guidelines also include detailed technical recommendations of how to perform the operation and especially how to avoid bile duct injuries. The aims were: (i) to reduce the number of primary open cholecystectomies in patients, where the laparoscopic procedure is the standard procedure according to the national consensus; (ii) to decrease hospital stay and increase the frequency of same-day surgery without increasing the readmission rate and (iii) to keep the frequency of bile duct injuries and postoperative complications on acceptable levels.

Nationwide outcome data after cholecystectomy in Denmark were published in the early period after the introduction of the laparoscopic technique [11, 12], but since the 1990s we have only had reports from single centers. The national quality of cholecystectomy in Denmark was therefore unknown. Thus a national database that monitors the quality of cholecystectomy by indicators was established—also to monitor how well the new guidelines were followed. Since 1 January 2006, registration of all cholecystectomies in the Danish Cholecystectomy Database (DCD) has been mandatory. The outcome measures of the DCD are quality indicators [13], and a variety of risk factors that might influence outcome have been registered as well. The national results as well as results from each identifiable department are published in an annual report [14] and local results are presented every quarter on-line to the individual heads of departments and hospital managers. The indicators are presented—in relation to the national average—by means of statistical process control [15]. In a validation study, the data quality has been found to be high and the indicators found to have close correlation to clinical and surgical quality [16]. Recently, the overall national results and the significant influence of the risk factors on outcome have been thoroughly analyzed [17].

The aim of the present study was to evaluate whether quality improvements in the performance of cholecystectomy had been achieved in Denmark since 2006, when the Danish National Guidelines for treatment of gallstones were revised and published in its second edition.

Methods

Data from the DCD were used. The database has been described in detail previously [16]. It has two data sources: clinical data entered into a secure web site by the surgeon and administrative data from the National Patient Registry (NPR) (Box 1). The two data sets are merged by the unique personal identification number assigned to all residents in Denmark.

Study population

The DCD was queried for all data from 1 January 2006 to 31 December 2009 and stratified by year. The data were extracted 26 May 2010.

A total of 28 379 patients underwent a cholecystectomy in the study period (Fig. 1).

Box 1

Data in the Danish Cholecystectomy Database

Clinical data entered by the surgeon

The operation

  Personal identification number

  Date of surgery

  Type of cholecystectomy (laparoscopic, open, conversion of laparoscopic)

  Reason for primary open operation (cancer, hepato-biliary surgery etc.)

  Length of incision (<8 cm defines a minicholecystectomy)

  Reason for conversion (complication or difficult anatomy)

  Perioperatively acknowledged bile duct injury (y/n, need for reconstructive surgery y/n)

  Method of cystic duct closure

  Completion of cholangiography

Risk factors

  ASA-score (I–V)

  Height and weight—BMI

  Acute cholecystitis (y/n)

  Previous acute cholecystitis (chronic cholecystitis) (y/n)

  Recent acute pancreatitis (y/n)

  Previous upper abdominal surgery (y/n)

  Concomitant laparoscopic removal of bile duct stones (y/n)

  Surgeon's experience (<50, 50–200, >200 cholecystectomies)

  Operation used for education (y/n)

Administrative data from the National Patient Registry

Admission

  Personal identification number

  Date of surgery

  Length of postoperative stay (LOS)

  Readmission within 30 days (y/n)

  Complications within 30 days

  Codes for reconstructive surgery of the bile duct

  Codes for other (‘non-reconstructive’) surgery of the bile duct

  Codes for additional procedures

  Death within 30 days (y/n)

Figure 1

Flowchart of the study population. Patients in the Danish Cholecystectomy Database, 2006–09.

Of these, 4139 (14.6%) were excluded due to missing registration or incomplete data. Patients in whom primary open cholecystectomy was performed and considered the standard procedure according to Danish consensus [10] were excluded as well (n = 568). The key indications for these open cholecystectomies were: part of hepatic or pancreatic operation due to malignancies or chronic pancreatitis, coincident removal of bile duct stones or a simultaneous other abdominal operation that demanded an open approach. This left 23 672 patients in whom laparoscopic operation was considered to be the standard and who were registered with both clinical and administrative data.

Of the 23 672 patients, 307 were excluded in the calculation of the conversion rate, as the cholecystectomy was not initiated as a traditional laparoscopic procedure: 268 had a primary open cholecystectomy, 33 had a minicholecystectomy and 6 patients had a cholecystectomy performed as single incision laparoscopic surgery. This left 23 355 patients with an intended laparoscopic procedure (Fig. 1). Over the entire study period, 2632 patients were not entered into the database by the surgeons and therefore missed clinical data about the course of the operation and about risk factors. They were however registered in the NPR with the administrative data, which allowed for the evaluation of the outcome. To ensure that our final conclusion about national quality improvement was not compromised by the exclusion of these patients, we also analyzed the outcomes of all patients registered in the NPR.

Measures

The quality indicators included in the analyses are listed in Box 2.

Covariates/risk factors

The registered risk factors are listed in Box 1. It is generally agreed that risk adjustment of quality indicators is essential. We have identified the risk factors that significantly affected the quality indicators [17] and to exemplify, the influence of the risk factors on length of stay, readmission and on the conversion rate is illustrated in Fig. 2. Patient's sex is determined from the unique personal identifier, Age is grouped in to three categories in the analyses: ‘< 40 years’, ‘40–60 years’ and ‘> 60 years’. The American Society of Anesthesiology (ASA) score (ASA I–V) is a marker of comorbidity [17, 18]. ASA III–V are grouped in the analyses. Definitions of the following categories of peroperative findings are available in the database in a drop-down box: Acute cholecystitis: clinical and ultrasonic (or scintigraphic) signs of acute cholecystitis immediately before the operation and the finding at operation of an inflamed, distended gallbladder with edema or suppuration; previous acute cholecystitis/chronic cholecystitis: the finding at the operation of a thick-walled gallbladder with fibrosis

Box 2

Quality indicators and outcome measures in the Danish Cholecystectomy Database

Conversion rate. The proportion of intended laparoscopic procedures converted to open cholecystectomy

Frequency of primary open cholecystectomy where laparoscopic procedure is standard. The proportion of primary open operations where laparoscopic procedure is the standard according to consensus. An indicator of how well guidelines are followed

LOS. Length of the postoperative stay where the day of operation is Day 0

Readmission within 30 days. Includes readmission to any Danish hospital within 30 days after the operation

Frequency of same-day surgery. Monitors high quality of both surgery and organization (when it is without readmission)

LOS ≤1 day and no readmission. The quality indicator in the database of an event free uncomplicated cholecystectomy

LOS >3 days. A quality indicator of a prolonged admission following cholecystectomy

LOS >3 days and/or readmission within 30 days: The quality indicator in the database of some kind of problem or complication after the cholecystectomy

Additional procedures within 30 days. The quality indicator of surgical complications or procedures due to postoperative bile duct stones. The procedures monitored by this indicator are listed in Box 1

Bile duct injuries. Two indicators monitor the incidences of bile duct injuries: the more severe transections or occlusions of the main bile ducts that need reconstructive surgery and minor lesions that can be treated with other surgical procedures (see Box 1)

Death within 30 days. Death of any cause, including death out of hospital within 30 days after the cholecystectomy

and dense adhesions; recent acute pancreatitis: clinical evident acute pancreatitis within the last 3 months before operation; previous upper abdominal surgery: prior operations that have resulted in an incision between the umbilicus and the xiphoid process,—an incision in the lower region extending a few centimeter over the umbilicus is not included. Surgeon's experience, ‘operation used for training’, ‘completion of cholangiography’ and Body Mass Index (kg/m2) (BMI) were found to affect the outcome indicators only to a minor extent in the adjusted analyses, [17] and are therefore not included in the present analyses.

Figure 2

Influence of the individual risk factors on the quality indicators: (a) length of stay (LOS) and readmission and (b) conversion rate.

Analytical methods

The prevalence of indicators and risk factors is reported stratified by year and tested with the Cochran-Armitage test for trend. We report the two-sided P-values, as we—before the analyses—did not know whether the quality would improve or worsen. We report the mean LOS after exclusion of one patient operated in 2007 with a LOS of 735 days. Change per year of LOS (mean) was tested in a general linear model. A logistic regression model was used to examine the influence of surgical year on the quality indicators with adjustment of previously shown important confounders and with 2006 as reference year. By including the risk factors in the analyses, we also adjust the outcomes for any difference in risk factor prevalence over the time period. Changes in the odds ratios from 2006 to 2009 were tested for linearity with departure from trend tests using a likelihood ratio test. Surgical year was then included in the model as a numeric variable and the annual change in odds ratio reported both as crude and adjusted values. All analyses were performed with SAS software, version 9.1. (SAS Institute, Cary, NC, USA).

Results

The 23 672 patients were operated in 35 different institutions. The median number from each institution was 428, but with a wide range from 3 to 2526 procedures in the study period. The mean age was 49, range 4–101 years and 73% of the patients were women. The characteristics of the patients are presented in Table 1, where the covariates are distributed over year of surgery. The distribution of sex and age did not change, but the prevalence of patients with ASA-score II increased. The prevalence of acute cholecystitis was unchanged, whereas the prevalence of chronic cholecystitis, previous upper abdominal surgery and recent acute pancreatitis increased. The number of patients with an open cholecystectomy fell from 10.3 to 7.1%. This was due to a decrease in both the conversion rate (from 8.8 to 6.0%) and the rate of primary open cholecystectomies.

View this table:
Table 1

Risk factor prevalence in the Danish Cholecystectomy Database in 2006–09 (n = 23 672)

Risk factor2006 (n = 5873)2007 (n = 6175)2008 (n = 5409)2009 (n = 6215)P-value
Male sex1606 (27.4)1589 (25.7)1533 (28.3)1675 (27.0)0.6007**
Age
 <401864 (31.7)2021 (32.7)1737 (32.1)1948 (31.3)0.5646*
 40–602356 (40.1)2479 (40.2)2151 (39.8)2551 (41.1)
 >601653 (28.1)1675 (27.1)1521 (28.1)1716 (27.6)
ASA-score
 I3735 (63.6)3956 (64.1)3315 (61.3)3746 (60.3)<0.0001*
 II1839 (31.3)1974 (31.9)1863 (34.4)2200 (35.8)
 III–V299 (5.1)245 (4.0)231 (4.3)269 (4.4)
BMI >35509 (9.1)596 (10.4)516 (10.2)597 (10.1)0.1150**
Acute cholecystitis885 (15.1)891 (14.4)931 (17.2)883 (14.2)0.9652**
Chronic cholecystitis1351 (23.0)1663 (26.9)1465 (27.1)1635 (26.3)<0.0001**
Previous upper abdominal surgery159 (2.7)181 (2.9)188 (3.5)233 (3.8)0.0003**
Recent acute pancreatitis275 (4.7)304 (4.9)324 (6.0)356 (5.7)0.0014**
Open cholecystectomy602 (10.3)556 (9.0)442 (8.2)440 (7.1)<0.0001**
  • Data are given as numbers (%).

  • *χ2-test.

  • **Two-sided P-value (Cochran-Armitage test for trend).

The quality indicator values distributed by year of surgery are listed in Table 2. The mean LOS fell from 1.7 to 1.2 days. The median LOS remained unchanged and was 1 day. The frequency of same-day surgery increased by 14.6–45.9% in 2009. The total readmission rate remained stable ∼9.5%, and readmission rates for the patients with same-day surgery were 7.9 in 2006 and 7.5 in 2009. There was a significant fall in the number of patient having additional procedures within 30 days (from 5.8 to 2.8%). The two indicators of bile duct injuries did not change, but the prevalence of both indicators was low (0.1–0.2%). Mortality rates were unchanged as well.

View this table:
Table 2

Quality indicators stratified by operation year 2006–09 and tested for trend (n = 23 672)

Quality indicator2006200720082009P-valueb
Number of cholecystectomies5873617554096215
Conversion of laparoscopic cholecystectomya508 (8.8)476 (7.8)377 (7.1)368 (6.0)<0.0001
Primary open where laparoscopic procedure is standard77 (1.3)74 (1.2)60 (1.1)57 (0.9)0.0361
LOS mean (SD)1.7 (3.4)1.5 (3.1)1.4 (2.7)1.2 (2.7)<0.0001
Same day surgery (LOS =0)1840 (31.3)2281 (36.9)2137 (39.5)2854 (45.9)<0.0001
LOS ≤1 day and no readmission3929 (66.9)4258 (69.0)3835 (70.9)4570 (73.5)<0.0001
LOS >3 days669 (11.4)588 (9.5)465 (8.6)500 (8.1)<0.0001
LOS >3 days and or readmission1114 (19.0)1095 (17.7)869 (16.1)1013 (16.3)<0.0001
Readmission within 30 days551 (9.4)610 (9.9)484 (9.0)591 (9.5)0.7633
Reconstructive bile duct surgery within 30 days11 (0.19)6 (0.10)13 (0.25)9 (0.14)0.9585
Other bile duct surgery within 30 days9 (0.15)12 (0.19)8 (0.15)7 (0.11)0.4419
Additional procedures within 30 days331 (5.6)351 (5.7)186 (3.4)173 (2.8)<0.0001
Death within 30 days17 (0.29)24 (0.39)7 (0.13)22 (0.35)0.8832
  • LOS, length of stay; SD, standard deviation.

  • Data are presented as the number of patients (%) who flag the indicator in the respective year. LOS is presented as mean (standard deviation) with exclusion of one outlier (LOS =735 days).

  • an = 23 355, 317 patients with primary open cholecystectomy, minicholecystectomy or single incision surgery are excluded; bTwo-sided P-value from test for trend.

In Table 3 we present the risk-adjusted indicators for each year with 2006 as reference year. The indicators with a linear trend, statistically or graphically, over the study period are presented in Table 4 where the odds ratios (presented both unadjusted and adjusted for risk factors) show the estimates of annual change. The conversion-rate decreased with 16% [odds ratio 0.84 (95% CI 0.80–0.88)] every year in the study period, and the proportion of patients having same-day surgery, increased with 27% [odds ratio 1.27 (95% CI 1.24–1.30)]. The readmission rate was unchanged [odds ratio 0.99 (95% CI 0.95–1.03)] and there is a significant decrease from 2006 to 2009 in the proportion of patients who needed additional procedures within 30 days [odds ratio 0.76 (95% CI 0.71–0.80)].

View this table:
Table 3

Risk-adjusteda quality indicators in the Danish Cholecystectomy Database from 2007 to 2009 with ‘2006’ as reference year

Year200720082009
Quality indicatorsOdds ratio95% CIOdds ratio95% CIOdds ratio95% CIP-valueDeparture from trendb
Conversion of laparoscopic cholecystectomy0.850.74–0.980.660.57–0.770.600.52–0.69<0.0001P > 0.2
Same day surgery (LOS =0)1.321.22–1.431.601.48–1.742.061.90–2.23<0.0001P > 0.2
Primary open cholecystectomyc0.990.71–1.390.740.52–1.070.650.45–0.940.0484P > 0.2
LOS ≤1 day and no readmission1.111.02–1.211.371.25–1.501.491.36–1.62<0.0001P > 0.2
LOS >3 days0.830.73–0.940.620.54–0.720.640.56–0.73<0.0001P > 0.05
LOS >3 days and/or readmission0.930.85–1.030.760.68–0.840.810.74–0.90<0.0001P < 0.01
Readmission within 30 days1.060.94–1.200.930.82–1.061.000.89–1.130.2608P > 0.1
Additional procedures within 30 days1.000.86–1.170.560.46–0.670.460.38–0.56<0.0001P < 0.01
Death within 30 days1.660.88–3.140.460.19–1.131.340.70–2.560.0247P < 0.01
  • LOS, length of stay; CI, confidence intervals.

  • aAdjusted for the following risk factors: ASA-score, age, sex acute and chronic cholecystitis, previous upper abdominal surgery, recent acute pancreatitis and open cholecystectomy; bThe change in odds ratio shows linearity if the result is insignificant (see Table 4); cPrimary open procedure performed, where a laparoscopic procedure is standard.

View this table:
Table 4

Quality improvement per year of the quality indicators in the Danish Cholecystectomy Database from 2006 to 2009

UnadjustedAdjusteda
Odds ratio95% CIP-valueOdds ratio95% CIP-value
Conversion of laparoscopic cholecystectomy0.870.84–0.91<0.00010.840.80–0.88<0.0001
Same day surgery (LOS =0)1.221.19–1.25<0.00011.271.24–1.30<0.0001
Primary open cholecystectomyb0.890.80–0.990.03690.860.77–0.960.0074
LOS ≤1 day and no readmission1.111.08–1.14<0.00011.151.12–1.18<0.0001
LOS >3 days0.880.85–0.92<0.00010.850.81–0.89<0.0001
LOS >3 days and/or readmission0.940.91–0.96<0.00010.92c0.89–0.95<0.0001
Additional procedures within 30 days0.770.73–0.81<0.00010.76c0.71–0.80<0.0001
  • The odds ratios estimate the change in the indicator with the increase in time of 1 year.

  • LOS; length of stay, CI; confidence intervals.

  • aAdjusted for the following risk factors: age, sex, ASA-score, acute and chronic cholecystitis, previous upper abdominal surgery, recent acute pancreatitis and open cholecystectomy; bPrimary open procedure performed, where laparoscopic procedure is standard; cCaution in the interpretation of the estimates as the departure from trend was significant.

The supplementary analysis of all the patients in the NPR (n = 27 789), including those not entered into the database with clinical data, confirmed the results of national quality improvement with an increase in the proportion of patients with same-day surgery from 29.2 to 41.5%, a decrease in the number of open cholecystectomies from 13.1 to 9.8%, and a decrease in the frequency of additional procedures from 6.2 to 3.6% (detailed data not shown).

Discussion

This study demonstrates quality improvement of cholecystectomy in Denmark during the years 2006–09 after the introduction of revised national guidelines in 2006. The introduction of a national database (DCD) made it possible to monitor the development in performance by quality indicators.The frequency of open cholecystectomy—where laparoscopic procedure is the standard—decreased, both in regard to primary open procedures and to laparoscopic surgery being converted to open surgery. The odds ratio of conversion annually decreased by 16% (adjusted for known risk factors of conversion) and the overall conversion rate at the end of the study period was 6.0%. Other studies from single centers have found elective laparoscopic cholecystectomy to be a safe and cost-effective outpatient procedure [1921], and we show that this is also possible on a nationwide scale. The frequency of same-day surgery increased in the study period without a compensatory increase in the readmission rate, and in 2009 almost 46% percent had same-day surgery and 73.5% of the patients had a LOS ≤1 day with no readmission, synonymous with an event-free, uncomplicated cholecystectomy of high quality. In the validation study of the DCD, we found that the indicator ‘an additional procedure within 30 days’ correlated well to surgical complications. [16] This frequency decreased from 5.6 to 2.8% although the prevalence of patients with chronic cholecystitis and recent acute pancreatitis—both proven risk factors for additional procedures [17]—increased in the study period. After adjustment for risk factors there is a considerable annual relative decrease of additional procedures in the study period of 24%. The frequency of serious complications, measured by the quality indicator ‘reconstructive surgery of the bile duct’, was low (0.18%) and comparable with international standards [22, 23]. It was unchanged during the study period, but lower than in the early Danish experience from 1991 to 1995, where it was 0.74% [11].

Only a few large-scale surgical registries have been implemented [2427], and few have reported improvements in quality indicators [2831]. The Veteran Affairs hospitals in the USA have documented quality improvement with decrease in both postoperative mortality and morbidity monitored in the National Surgical Quality Improvement Program (NSQIP) [31] and from Australia, Collopy has—by means of the Care Evaluation Program (CEP)—demonstrated examples of quality improvement, including a fall in the number of unplanned readmission after cholecystectomy after the procedure was restricted to surgeons with appropriate training [29].

The present project, in which guidelines for a specific procedure were implemented and the results monitored in a national database, was designed to improve the quality on a national scale. There was no contemporary control group and we can only speculate about how to explain the changes in the indicator values during the study period. Probably several factors have contributed to the quality improvements. The Danish Surgical Society had in 2005 brought course and outcome after cholecystectomy into great focus, and several local publications from single centers reported about the quality of care and growing experience with same-day surgery [20]. These results and the new guidelines were thoroughly discussed and emphasized during national surgical meetings. In every surgical course about laparoscopic cholecystectomy (theoretical as well as practical), the surgical principles were emphasized to trainees and specialists. Thus, a great awareness about the quality in both course and outcome was present when the DCD was established. The quality indicator results, nationwide as well as for each identifiable department, were hereafter published and discussed with the representatives from the departments once a year during the study period. These factors may all play a role in the general improvement of the nationwide quality. And finally, some quality improvement might be due to the Hawthorne effect,—that measurement alone improves quality [32].

There are no formal requirements of laparoscopic surgical training in Denmark and no system of professional credentials or hospital privileging. Owing to public policies the Danish healthcare system experienced a major reorganization in the study period—especially in regard to a centralization of the management of highly specialized procedures in cancer surgery and—to a lesser extent—in other procedures. Some public departments merged and at the same time, new private clinics performing laparoscopic cholecystectomy increased. Thus the total number of departments and hospitals performing laparoscopic cholecystectomy was not reduced. These changes have made it difficult and uncertain to identify specific departments for targeted improvements and necessitated our focus on the national results. Furthermore, as studies from the NSQIP have demonstrated, implementation of risk-adjusted indicators is essential for a true comparison of departments [7]. We have identified significant risk factors of outcomes [17] and the indicator values need to be risk adjusted in order to correctly identify hospitals with quality problems [17, 31, 33, 34]. Efforts and resources are now allocated to perform a proper statistical risk adjustment to rank the hospitals.

In conclusion, the quality of cholecystectomy in Denmark has gradually improved after the introduction of the National Guidelines in 2006, and the DCD has been—and will be—a strong tool to monitor significant development in the quality indicators—both nationwide and within the individual departments.

Funding

K.M.H. was employed by Hvidovre Hospital as a research assistant in the laparoscopic unit from 2006–2010.

Acknowledgement

The authors are thankful for the invaluable help of data manager Susie Lendal Antvorskov.

References

View Abstract