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Ann Thorac Surg 2001;72:S1009-S1015
© 2001 The Society of Thoracic Surgeons

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Supplement: Cardiothoracic techniques and technologies

Comparing clinical outcomes in high-volume and low-volume off-pump coronary bypass operation programs

^a HCA—The Healthcare Company, Nashville, Tennessee, USA
^b Rollins School of Public Health at Emory University, Atlanta, Georgia, USA
^c Cardiac Data Solutions, Inc, Atlanta, Georgia, USA

Address reprint requests to Dr Becker, Rollins School of Public Health at Emory University, 1518 Clifton Rd NE, Atlanta, GA 30322
e-mail: ebeck01{at}sph.emory.edu

Presented at the Seventh Annual Cardiothoracic Techniques and Technologies Meeting 2001, New Orleans, LA, Jan 24–27, 2001.

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. A growing body of data suggests that physicians or medical centers performing a higher volume of patient services achieve better outcomes. We hypothesized that an important dimension of coronary artery bypass graft (CABG) volume is the cardiovascular surgical team experience with performing off-pump coronary artery bypass (OPCAB) procedures, as well as the team’s ability to provide the optimal approach for the patient’s clinical condition. Teams performing 100 or more OPCAB operations have improved clinical decision-making processes and technical skills regarding treatment of all CABG patients, regardless of whether cardiopulmonary bypass is used. We hypothesized that this experience and choice of approaches for CABG procedures translates into better clinical outcomes.

Methods. Using data on 16,988 consecutive patients in 72 hospitals from the HCA—The Healthcare Company casemix database, we identified high- and low-volume OPCAB sites and then analyzed the patient and hospital characteristics that had an impact on clinical outcomes.

Results. The mortality rates for the high- and low-volume OPCAB facilities both averaged 2.9% (p = NS). Patients at the high-volume OPCAB facilities had significantly lower rates of major complications (shock/hemorrhage, neurologic, renal, and cardiac) than those at the low-volume OPCAB facilities. Of the seven minor complications, rates for six were lower in the high-volume OPCAB facilities, but none of the differences reached statistical significance. High-volume OPCAB sites were significantly more likely to discharge their patients directly home than were low-volume OPCAB sites (80% versus 66%; p = 0.001).

Conclusions. The results suggested that surgical team experience and choice of approaches to performing CABG had an impact on patient outcomes.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
A growing body of literature suggests that physicians or medical centers performing higher volumes of surgical procedures may achieve improved patient outcomes [1–10]. In their comprehensive review of coronary artery bypass graft (CABG) operation guidelines, Eagle and associates [11] concluded "that survival after CABG is negatively affected when carried out in institutions that perform fewer than a threshold number of cases annually." Although the inherent logic in "practice makes perfect" may seem inescapable, important caveats can be raised relevant to the introduction and diffusion of new cardiovascular technologies such as off-pump coronary artery bypass (OPCAB) operations. One of these caveats relates to surgical team experience in performing new OPCAB techniques. Surgical team members, especially those in anesthesia and nursing, need to be especially alert regarding the physiologic management of patients when bypass is being performed off-pump. How do these new clinical skills and the volume of cases performed off-pump impact patient outcomes?

It has been suggested that the growing use of minimally invasive off-pump methods in CABG operations offers the potential for further reducing CABG mortality and morbidity rates. Patients undergoing OPCAB operations have experienced reduced mortality and complication rates, reduced lengths of hospital stay, and reduced costs at selected sites, subgroups, and centers [12–17]. However, to date, there is a relative paucity of literature describing the relation between hospital and patient volumes and differences in off-pump and on-pump CABG operation outcomes.

We have speculated that as the cardiovascular surgical team experience with performing off-pump CABG procedures increases, changes occur in their knowledge base, clinical decision-making processes, and technical skills regarding treatment of all CABG patients, regardless of whether or not cardiopulmonary bypass is used. That is, as the surgical team becomes more comfortable with the off-pump technique and the number of their off-pump patients increases, they become better prepared with every CABG patient to select either an off-pump or on-pump technique. The decision rests primarily with the surgeon about which approach will optimize the patient’s outcome. This ability to choose between the two approaches especially for surgical teams doing higher volumes of off-pump procedures may equate to better overall clinical outcomes and the ability to treat patients who previously would not have been candidates for surgical treatment.

Based on the literature and our discussion, a number of hypotheses were developed and analyzed at the patient and facility levels:

Higher levels of OPCAB operations are associated with lower patient and hospital mortality rates than programs with lower volume levels.

Higher levels of OPCAB operations are associated with lower patient and hospital complication rates for major outcomes (shock/hemorrhage, neurologic, renal, and cardiac) than programs with lower volume levels.

Higher levels of OPCAB operation volume are associated with lower patient and hospital minor complication rates than programs with lower volume levels.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
The HCA casemix database contains data on 16,988 consecutive CABG patients in 72 hospitals admitted to HCA facilities during the study period from January 1, 1999, through December 31, 1999. This database was a comprehensive administrative database containing patient, clinical, procedural, and outcome data.

To distinguish between high- and low-volume off-pump facilities, we used the cut-off of 100 OPCAB operations with any sites having documented 100 or more procedures in the high-volume cohort. Whereas the 100-procedure limit is somewhat arbitrary, given the relative newness of the off-pump procedure and the lack of an external cut-off reference in the literature, we thought this number was a reasonable demarcation. Also, the Department of Veteran Affairs and the State of New York both used the category of less than 100 cases annually as a cut-off point for low-volume institutions [11].

The volume of off-pump, on-pump, and total procedures by institution are listed in Table 1. We noted wide ranges among sites in the number and percentage of off-pump CABG procedures being performed. As listed in Table 1, staff in our 72 study hospitals performed a mean of 246 CABG operations in 1999, of which a mean of 34.6 (16%) were OPCAB procedures and a mean of 211.3 (84%) were CABG procedures performed on-pump. The number of OPCAB procedures performed ranged from a high of 293 procedures in one hospital to no OPCAB procedures at three hospitals. Only six hospitals performed more than 100 CABG procedures off-pump and these hospitals were identified as high-volume OPCAB hospitals, while the 66 other hospitals were categorized as low-volume OPCAB hospitals. Our investigation reported on the comparison between these two groups.

Data collection
Patient data from each hospital were collected through the hospital discharge abstract. Patient characteristics, procedural characteristics, procedure complications, and mortality and hospital-process characteristics were coded directly from the discharge abstract (Table 2). The International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) was used to aggregate patient characteristics and procedure complications into categories [18]. A total of 18 patient, 13 outcome, 4 procedural, and 5 hospital-process characteristics were identified, analyzed, and compared.

To test the influence of the low- and high-volume OPCAB on mortality and complications risk, adjusting for potential differences in the two populations, we also used logistic regression techniques and reported the odds ratios and p values for the high-volume OPCAB coefficient. Twelve logistic regressions were performed with the dependent variables being patient mortality and each of the 11 complications: shock/hemorrhage, neurologic, cardiac, renal, mechanical, surgical infection, postoperative infection, septicemia, respiratory, pneumonia, and peripheral vascular. The independent variables included in each of the 12 logistic equations were patient age, sex, smoker, history of tobacco use, chronic obstructive pulmonary disease, insulin-dependent diabetes mellitus (IDDM), noninsulin-dependent diabetes mellitus, acute renal failure, chronic renal failure, unspecified renal failure, cardiogenic shock, hypertension, acute myocardial infarction (MI), old MI, cardiomyopathy, congestive heart failure, peripheral vascular disease, endocarditis, and the surgical volume of the hospital.

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Patient profile
More than 70% of patients in both the low- and high-volume facilities were men (71.1% versus 70.9%; p = 0.849), and they averaged about 2 years older in the low-volume facilities (66.3 years versus 64.2 years; p 0.001). A number of statistically significant differences may have contributed to higher mortality or morbidity between the high- and low-volume OPCAB patients. A significantly higher percentage of patients in the low-volume OPCAB population, 58%, were 65 years or older. In contrast, only 49% of the patients in the high-volume OPCAB population were 65 years or older (p = 0.000). Patients in high-volume facilities were significantly more likely to be smokers, but significantly less likely to have hypertension, an acute MI, an old MI, cardiomyopathy, or congestive heart failure. Overall, of the 18 patient characteristic and comorbid conditions studied, eight conditions showed no statistically significant difference between the high- and low-volume patient populations.

More than 89% of patients at the high-volume OPCAB sites received arterial conduits of some type, compared with only 80% of the patients at the low-volume OPCAB sites (p 0.001). However, there were more vessels bypassed for patients at low-volume OPCAB sites than for patients at the high-volume OPCAB sites (3.36 versus 3.11; p 0.001). Patients at low-volume OPCAB facilities required nearly double the amount of intraaortic balloon pump use as the patients at high-volume OPCAB facilities (7.2 versus 3.7, respectively).

Patients in high-volume OPCAB facilities had a shorter length of stay than patients in the low-volume facilities (7.7 days versus 8.7 days; p 0.001). Partitioning length of stay into two parts (not shown)—admission to operation and operation to discharge—revealed that for both segments of the stay, patients at high-volume OPCAB facilities had significantly shorter mean hospital stays.

Table 3 shows mortality and complication results for low- and high-volume OPCAB patients. The mortality rate for the high- and low-volume OPCAB patients was not significantly different (both averaged 2.9%). For the four major complications (shock/hemorrhage, neurologic, renal, and cardiac), patients at the high-volume OPCAB sites all had significantly lower rates of complications than patients at low-volume OPCAB facilities. Shock/hemorrhage was more than 1.5% lower in high-volume OPCAB facilities. The rates of neurologic (0.83% versus 1.45%; p = 0.025), renal (0.34% versus 0.97%; p = 0.005), and cardiac (3.04% versus 7.47%; p 0.001) complications at high-volume OPCAB facilities were nearly half of the respective rates in the low-volume facilities.

As shown in Table 4, patients in high-volume OPCAB facilities were significantly more likely to be discharged directly home without home health care than were patients in low-volume OPCAB facilities (81% versus 64%; p = 0.001). Conversely, nearly 20% of all low-volume OPCAB patients were discharged to home health care, whereas just 6% of high-volume OPCAB patients (p 0.001) were discharged to home health care. Low-volume OPCAB facilities discharged 8% of their patients to skilled nursing facilities, whereas high-volume OPCAB facilities discharged 5% of their patients to skilled nursing facilities.

As shown in Table 2, approximately 70% of the populations in both the low- and high-volume OPCAB facilities were male (69.7% versus 71.0%; p = 0.727) with an average age of 66 or 65 years old, respectively. Whereas the high-volume OPCAB facilities tended to have lower rates on all but three of the population characteristics and comorbid conditions (smoking, IDDM, and previous CABG), none of the p values were statistically significant at conventional levels.

At the hospital level, the differences in the number of vessels bypassed and the percentage receiving venous conduits was not statistically significant between high- and low-volume OPCAB sites. The utilization of arterial conduits remained significantly higher (87%) in high-volume OPCAB sites compared with the low-volume OPCAB sites (79%; p = 0.027). The usage of intraaortic balloon pumps appeared marginally significant (p = 0.053), with 8.1% at the low-volume OPCAB facilities compared with 3.9% at the high-volume OPCAB facilities.

The total length of stay was essentially the same for both high- and low-volume OPCAB facilities (8.81 versus 8.83 days; p 0.105). For the admission to operation and operation to discharge portions of the hospital stay (not shown), the patient population at high-volume OPCAB facilities had shorter hospital stays, but neither difference approached statistical significance at conventional levels.

The mortality rate for the high- and low-volume OPCAB facilities reported in Table 3 was 3.0% and 3.1%, respectively (p = 0.787). For the four major complications (shock/hemorrhage, neurologic, renal, and cardiac), the high-volume OPCAB facilities had lower rates of complications than the low-volume OPCAB facilities, but only shock/hemorrhage (1.18% versus 3.77%; p = 0.044) was statistically significant. The difference in the renal complication rate (0.83% versus 0.26%; p = 0.068) approached statistical significance. Among the seven minor complications, six were lower in the high-volume OPCAB facilities, although none of these differences was statistical significant. Although the rate of pneumonia was higher in the high-volume OPCAB facility, this difference was not statistically significant.

As indicated in Table 4, high-volume OPCAB facilities were significantly more likely to discharge patients directly home than were low-volume OPCAB facilities (80% versus 66%; p = 0.001). Conversely, 16.52% of all low-volume OPCAB site patients were discharged to home health care, compared with only 6.96% of high-volume OPCAB patients (p = 0.211). High- and low-volume OPCAB facilities discharged about the same percentage of patients to skilled nursing and rehabilitation.

Multivariate analysis
To assess the influence of patient characteristics and comorbid conditions on death and complication rates, we used multivariate regression techniques. Using death and the complication rates, respectively, as dependent variables we controlled for patients in high-volume OPCAB facilities, patient age, sex, smoker, history of tobacco use, chronic obstructive pulmonary disease, IDDM, noninsulin-dependent diabetes mellitus, acute renal failure, chronic renal failure, unspecified renal failure, hypertension, acute MI, old MI, cardiomyopathy, congestive heart failure, and peripheral vascular disease. Because the dependent variables in the patient population were dummy variables, binary logistic regression was used and the odds ratios and p values for just the high-volume OPCAB facilities variable are reported in Table 5.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Our first hypothesis was that facilities where OPCAB operations were performed more frequently would be associated with lower patient mortality rates and lower hospital mortality rates than programs with lower OPCAB volume levels. We found no support for this hypothesis. Although high-volume OPCAB facilities did have lower mortality rates, they were only marginally lower than low-volume OPCAB facilities and the difference was not statistically significant.

In our second hypothesis we postulated that higher volumes of OPCAB operations would be associated with lower patient and facility complication rates for major outcomes (shock/hemorrhage, neurologic, renal, and cardiac) than lower volumes. We found strong support for this hypothesis. For both the patient and hospital profiles, high-volume OPCAB facilities had lower rates on all four of these major complications compared with the low-volume OPCAB facilities. For the patient profiles all four were statistically significant.

Our final hypothesis stated that higher volumes of OPCAB operations would be associated with lower patient and hospital minor complication rates (postoperative infection, respiratory complications, implant infection, mechanical complications, peripheral vascular complications, pneumonia, and septicemia) than programs with lower OPCAB volume levels. There was some modest support for this hypothesis. In both the patient and hospital profiles, six of the seven minor complication rates were lower for high-volume OPCAB facilities than for low-volume OPCAB facilities.

The results suggested that for many complications, outcomes at high-volume OPCAB operation facilities are better than outcomes at low-volume OPCAB operation sites although no difference was found in mortality rates. We recognize that this study has a number of important limitations and our results should be interpreted cautiously. First, the HCA casemix database is an administrative database and lacks clinical details that would be useful in segmenting patients and clinical characteristics. Second, we do not know the timing of events (preoperatively, intraoperatively, or postoperatively). Third, we know only whether a pump was used in the CABG operation. The physician’s intention to treat could not be identified. Lastly, we used the facility’s number of off-pump procedures, not the individual surgeon’s off-pump experience. It is possible that the individual surgeon’s experience could be of more importance than that of the overall surgical team.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This research was supported in part by a grant from HCA Healthcare Inc

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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