Ann Thorac Surg 2006;81:547-553
© 2006 The Society of Thoracic Surgeons
Original article: Cardiovascular
Exploring the Volume-Outcome Relationship for Off-Pump Coronary Artery Bypass Graft Procedures
Mary E. Plomondon, PhD
a
,
Adrianne W. Casebeer, PhD
a
,
b
,
Lynn M. Schooley, MA
a
,
Brandie D. Wagner
a
,
Gary K. Grunwald, PhD
a
,
b
,
Gerald O. McDonald, MD
c
,
Frederick L. Grover, MD
a
,
b
,
A. Laurie W. Shroyer, PhD
a
,
b
,
*
a Eastern Colorado Health Care System, Department of Veterans Affairs Medical Center, Denver, Colorado
b University of Colorado at Denver and Health Sciences Center, Denver, Colorado
c Department of Veterans Affairs Central Office, Office of Patient Care Services, Washington, DC
Accepted for publication August 15, 2005.
* Address correspondence to Dr Shroyer, Cardiac Research, Eastern Colorado Health Care System, Denver VA Medical Center, 820 Clermont St, Suite 120, Denver, CO 80220 (Email: laurie.shroyer{at}med.va.gov; laurie.shroyer{at}uchsc.edu).
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Abstract
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BACKGROUND: The relationship between the surgical case volume and risk-adjusted clinical outcomes has been examined for different surgical specialties. The purpose of this study was to explore the relationship between the off-pump coronary artery bypass graft procedure volumes (OPCABG) with risk-adjusted outcomes within the Department of Veterans Affairs (VA) 44 cardiac surgery programs.
METHODS: Based on VA Continuous Improvement in Cardiac Surgery Program data, the results of 5,076 OPCABG surgical procedures performed between October 1998 and September 2003 were analyzed. Hierarchical logistic regression models evaluated the relationship between OPCABG procedure volume with risk-adjusted 30-day operative mortality, perioperative morbidity, and 180-day mortality. Both a hospital's average OPCABG volume per 6-month period and the hospital's most recent 6-month OPCABG volume were examined.
RESULTS: Hospital OPCABG average volume in a 6-month period ranged from 0.2 to 47.4 procedures; whereas the most recent 6-month OPCABG hospital volume ranged from 0 to 76 OPCABG per site. No relationship between the volume measures and the outcome variables was found.
CONCLUSIONS: We did not find an association between OPCABG volume with short-term mortality, perioperative morbidity, or intermediate-term (180-day) mortality.
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Introduction
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Studies that examine the relationship between the hospital-based volumes for a surgical procedure with clinical outcome are of particular interest to Department of Veteran Affairs (VA) policy makers to evaluate criteria to use on screening for opportunities to assure patient safety and to improve quality of care. Recently, there have been an increasing number of published studies examining these relationships [1–14]. The results of these published studies have been mixed. The purpose of this study was to evaluate for a potential association between hospital off-pump coronary artery bypass graft procedure (OPCABG) volume with 30-day operative mortality, perioperative morbidity, and 180-day mortality; while adjusting for (1) patient characteristics; (2) the hospital where the procedure was performed; and (3) changes in cardiac surgical care practice over time.
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Material and Methods
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Study Population
The VA Continuous Improvement in Cardiac Surgery Program (CICSP) as well as the expansion (CICSP-X) project prospectively collects risk and outcome data on all patients undergoing cardiac surgery at the 44 VA cardiac surgery centers [15]. The present study reviewed all records for veterans undergoing an OPCABG procedure at a VA cardiac surgery center between October 1, 1998, and September 30, 2003 (n = 5,076).
Outcomes
Three outcome variables were evaluated separately for this study, 30-day operative mortality, perioperative morbidity, and 180-day all cause mortality. In CICSP/CICSP-X, the 30-day operative mortality is defined as any death occurring during the index hospitalization or within 30 days after surgery as well as any death occurring more than 30 days after surgery that is the direct result of a perioperative surgical complication. The 180-day all cause mortality is defined as any death occurring within 180 days after surgery. All mortality assessments (both occurrence as well as date of death) were verified using the VA Beneficiary Identification and Records Locator Subsystem (BIRLS). The BIRLS database is both sensitive and specific for determining vital status in Veteran populations [16]. In addition, any death discrepancies identified by the BIRLS match were reconciled by VA personnel using data from VA electronic medical records, cardiology clinics, and contact to verify vital status with the local cardiac surgery program's surgical clinical nurse reviewers. The perioperative morbidity is defined as the presence or absence of any of the following major complications: endocarditis; renal failure requiring dialysis; mediastinitis; reoperation for bleeding; placed on ventilator for longer than 48 hours; repeat cardiopulmonary bypass; stroke; coma for longer than 24 hours; or cardiac arrest requiring cardiopulmonary resuscitation during the inpatient perioperative period or within 30 days after surgery. This complication assessment is based upon the standard definition used for the VA Cardiac Surgery Consultants Committee's national quality assurance endeavors.
OPCABG Procedural Volume
The primary variable of interest was OPCABG procedural volume, which was defined in two ways: (1) average hospital OPCABG procedural volume in a 6-month period; and (2) the most recent 6-month hospital OPCABG volume preceding the surgical procedure. The latter definition for the most recent 6-month period was based on the approach used by Hamilton and Ho [4], who investigated whether "practice makes perfect" for the treatment of hip fracture patients.
Risk Variables
Twenty-four preoperative patient-specific risk characteristics for mortality and morbidity were categorized as either demographic, noncardiac comorbidities, or cardiac-related assessments (related to cardiac disease severity), as listed in Table 1. Although the rates for missing data were extremely low, values for missing risk variables were imputed as the median for continuous variables and the most frequent value for categorical and dichotomous variables. Other variables included in the analysis were time (range, 0 to 10), which was defined based upon the 6-month period in which the procedure was performed (starting from the point of study initiation in October 1, 1998).
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Table 1. Coronary Artery Bypass Graft Surgery (CABG) Patient Characteristics in 44 Department of Veterans Affairs Cardiac Surgery Programs (Mean [SD] or Percent)
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Statistical Analyses
The study population was described by dividing hospitals into four volume groups based on the quartiles of hospital OPCABG procedural volume. Hospital summary statistics were calculated by averaging the 10 6-month hospital averages for continuous variables or the hospital percentages for categorical and dichotomous variables, within each OPCABG procedural volume group, for each risk factor. Tests for equality across volume groups were performed for each risk factor using one-way analysis of variance overall F tests based on these hospital averages across the 44 hospitals.
Using logistic regression, risk estimates for each of the three outcomes, 30-day mortality, perioperative morbidity, and 180-day mortality were constructed using the patient characteristics listed in Table 1. For each of the three study outcomes, backward stepwise selection methods were used to retain only those factors that remained significant at a p level of 0.05. The Appendix contains each model's details. Using these models, a risk estimate for each patient and for each outcome was calculated. These probabilities were summed for each hospital to obtain an expected number of deaths (or morbidities). The ratios of the observed-to-expected (O/E) 30-day deaths, 180-day deaths and 30-day morbidities (separately) were calculated for each VA surgery center. Pearson correlation coefficients were calculated to examine the correlation between the O/E ratio, for each outcome, and the logarithm of average hospital volume. Average hospital volume was log transformed due to its skewed distribution. For interpretability, base-2 logs (log2) were used so that a change in the average volume corresponded to a doubling of average hospital volume.
Finally, hierarchical logistic regression models [17] were developed to examine the relationship between OPCABG procedural volumes with the three study outcomes: 30-day operative mortality, perioperative morbidity, and 180-day mortality while adjusting for both time period and patient risk factors. Each of these three hierarchical models contained the logistic transform of the patient risk estimate, the 6-month period when the surgery occurred, the log of average volume of all CABG procedures, and the log of average hospital OPCABG volume in combination with a random hospital indicator to account for unmeasured differences among hospitals for this hospital level volume analysis. Similar models for each of the study's outcomes were constructed using the log of cardiac surgery procedural volume during the most recent 6-month period also. All statistical analyses were conducted using SAS software (SAS Institute, Cary, North Carolina).
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Results
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Study Population
Table 1 shows summary statistics for all patient risk factors delineated by volume group. Differences in risk factors across volume groups were clinically small and statistically nonsignificant (three-vessel coronary artery disease was borderline significantly different across groups, p = 0.0478). Table 2
shows summary statistics for all complications and outcomes delineated by volume group. Likewise, there were very few differences in complications and mortality across the volume groups. For the entire study, the 30-day operative mortality rate was 2.6%. Overall, the perioperative morbidity rate for these nine major complications was 9.4%. The study's 180-day all cause mortality rate was 4.5%.
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Table 2. Coronary Artery Bypass Graft Surgery (CABG) Patient Complications and Outcomes in the 44 Department of Veterans Affairs Cardiac Surgery Programs (Mean [SD] or Percent)
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Correlation Coefficients
The correlation coefficients are demonstrated in Table 3
for the O/E ratios of the three outcomes and the log average OPCABG procedure volume. None of these correlations was statistically significant, indicating no demonstrable relationship between OPCABG procedure volume and risk-adjusted outcomes.
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Table 3. Relationships of Risk-Adjusted Outcomes (Observed to Expected [O/E] Ratios) With Log of Average OPCABG Surgery Volume in 44 Department of Veterans Affairs Cardiac Surgery Programs
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Hierarchical Logistic Regression Models
The average hospital OPCABG volume odds ratio and 95% confidence interval from the hierarchical logistic regression models are shown in Table 4
for each of the three outcomes: 30-day operative mortality, perioperative 30-day morbidity, and 180-day mortality. The patient risk estimate was the only consistently significant factor found in these analyses for each of the study's three outcomes. Time covariate showed a slightly decreasing trend; however, this trend was not statistically significant for OPCABG procedure.
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Table 4. Hierarchical Logistic Regression Models Estimating Association of Average Department of Veterans Affairs Cardiac Surgery Programs' CABG Volume With Outcomes
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Average hospital OPCABG procedural volume odds ratios were less than one, but were not significant for any of these outcomes. As in the correlation analysis, these results do not demonstrate relationships between average hospital volume and outcomes, adjusting for patient risk and time trends. Finally, as seen in Figure 1, the 180-day risk-adjusted mortality rate varies widely among the lower volume hospitals with cardiac surgery programs without any obvious trend related to volume. The 30-day risk-adjusted mortality and perioperative morbidity figures were similar, with wide variation among the lower volume hospitals and no obvious trend related to volume.
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Fig 1. Risk-adjusted 180-day mortality rate by hospital cardiac surgery program's annual coronary artery bypass graft (CABG) off-pump procedural volume. (Circles = volume group 1; dashes = volume group 2; triangles = volume group 3; crosses = volume group 4.)
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The 95% confidence intervals (CI) for log average volume odds ratios provide precision information that clarifies interpretation. For example, doubling of average hospital volume was associated with a change in the odds ratio for 30-day operative mortality of 0.94 (95% CI: 0.76 to 1.15). Thus, if an association does exist, the odds ratio for a doubling of average hospital volume is unlikely to lie outside of this range. Similar precision results were found for models including hospital OPCABG procedural volume during the previous 6-month period and hence are not reported here.
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Comment
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The objective of this study was to determine if a relationship existed between hospital OPCABG procedural volume and 30-day operative mortality, perioperative morbidity, or 180-day mortality after adjusting for clinical patient risk, trends over time, and hospital-specific effects. No such volume-outcome relationship was demonstrated.
The relationship between CABG procedural volume and outcome has been studied in a variety of settings; however, only one of these studies has specifically looked at the volume of off-pump CABG procedure and outcome [7]. Using administrative data from 72 hospitals, low-volume centers were defined as those with less than 100 off-pump procedures per year (n = 6) and high-volume as those with 100 or more off-pump procedures per year (n = 66). They found no association between volume and mortality. Although an association between volume and morbidity was found, distinguishing between comorbidities and complications is very difficult when using administrative data [18].
The results from studies examining the relationship between volume and outcome among all CABG procedures are mixed. Clinical data from The Society of Thoracic Surgeons (STS) National Cardiac Database was recently used to test the association between hospital CABG volume and all cause mortality [14]. The study included 267,089 procedures among 439 hospitals with average yearly volumes from 39 to 1,754 procedures. The analyses were risk adjusted for both patient and hospital level characteristics. Although a modest association between CABG volume and all cause mortality was found, the association did not remain among certain subpopulations, such as patients younger than 65 and patients with a low preoperative risk. The authors concluded that volume is not an adequate quality indicator for CABG surgery. In another study that used National Medicare claims data, an association between the CABG procedure and mortality was found [8]. However, these results are based on administrative data and can only be generalized to the Medicare population. Lastly, the New York State's Cardiac Surgery Reporting data were used to examine CABG surgery volume and outcome [10]. The analyses were adjusted for both patient clinical characteristics and hospital level data. Mortality rates were lower among hospitals performing between 200 and 800 procedures a year and lower among surgeons who perform between 50 and 200 procedures a year. However, possible selection bias could exist since this study was performed in only one state, and the state has "certificate of need" regulations that decrease the number of low-volume centers. Therefore, there were very few hospitals with less than an average annual volume of 200 CABG procedures. While an association between volume of CABG procedure and mortality was found, again the authors recognize the difficulty in implementing a volume-outcome policy, namely, travel and time constraints and disruption of physician continuity of care.
Although the results of the CABG volume-outcome studies are mixed, many of the authors agree to caution against the implementation of a procedural volume policy. Although procedural volume is easily measurable, it remains a questionable indicator of quality. Attention should remain on collecting clinical data for analyzing risk-adjusted outcomes with the goal of improving care across all surgical centers.
This VA-based study has several inherent limitations. It is possible that the finding of no association between average hospital OPCABG volume and mortality or morbidity may potentially be due to low precision and power. However, the 95% confidence intervals for associations of average hospital volume with outcomes rule out that a large association is likely to exist in VA-based cardiac surgery program settings. Furthermore, VA cardiothoracic surgeons most commonly also perform CABG procedures at their affiliated University hospital. The OPCABG volumes included in this analysis were limited to VA hospital OPCABG procedures performed. Thus, VA hospital OPCABG volume may not adequately measure the number of OPCABG procedures performed by each surgeon. Perhaps the most informative measure of OPCABG volume, which was not available for this study, may be the attending cardiac surgeon-specific OPCABG volume—a variable that is not captured within the VA CICSP/CICSP-X national database. Prior VA-based studies using the CICSP database also found no continuous relationship between volume and risk-adjusted outcome. Inherent variability due to small sample size suggests that risk-adjusted outcomes should be more closely monitored at VA hospitals that perform fewer CABG procedures in a year. Hence, the VA Cardiac Surgery Consultant Committee policies aggregate volume for CABG procedure—but not based on operative approach—for monitoring quality. Owing to data reliability concerns, the off-pump to on-pump as well as on-pump to off-pump conversion rates were unable to be adequately addressed in this analysis. The CICSP/CICSP-X data did not allow for the identification of off-pump cases that were converted to on-pump; therefore, all conversions were defined as on-pump in this study.
Strengths of this study include the use of prospectively collected clinical data for patient risk adjustment and the evaluation of both short-term and intermediate-term mortality, as well as short-term morbidity, as outcomes. Based on the small widths of the confidence intervals for the odds ratios for differences in hospital volume, this study had a large enough study population to ensure adequate precision to rule out a large association between hospital OPCABG procedural volume and 30-day operative mortality, perioperative morbidity, or 180-day mortality.
No associations between VA OPCABG procedural volume and short-term mortality or morbidity or 180-day mortality were found. The Department of Veterans Affairs Central Office's primary focus is the assurance of veteran safety. In context of this goal and the findings of this study, the VA Cardiac Surgery Consultants Committee will continue to monitor OPCABG procedures to assure each cardiac surgery program's quality. Based on this study's findings, however, no minimum volume OPCABG threshold for review is planned to be added to the existing policy thresholds or procedures at this time.
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Appendix
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Logistic Regression Models Used to Estimate Patient Risk
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Acknowledgments
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Funding for this study was provided by VA Health Services Research and Development Grant IHY 99214-1 (Dr Shroyer, Principal Investigator) and by the VA Office of Patient Care Services at VA Headquarters, Washington, DC. University of Colorado at Denver and Health Sciences Center Campus' Graduate School student stipend support was provided under the guidance of Dr Shroyer, in part, to support Dr Plomondon's participation in this project.
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References
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Abstract
Introduction
