Comparing Clinical Outcomes in High-Volume and Low-Volume Off-Pump Coronary Bypass Operation Programs

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1 Comaring Clinical Outcomes in High-Volume and Low-Volume Off-Pum Coronary Byass Oeration Programs Philli P. Brown, MD, Michael J. Mack, MD, Aril W. Simon, MSN, Salvatore L. Battaglia, BS, Lynn G. Tarkington, RN, Steven D. Culler, PhD, and Edmund R. Becker, PhD HCA The Healthcare Comany, Nashville, Tennessee, Cardiac Data Solutions, Inc, and Rollins School of Public Health at Emory University, Atlanta, Georgia Background. A growing body of data suggests that hysicians or medical centers erforming a higher volume of atient services achieve better outcomes. We hyothesized that an imortant dimension of coronary artery byass graft (CABG) volume is the cardiovascular surgical team exerience with erforming off-um coronary artery byass (OPCAB) rocedures, as well as the team s ability to rovide the otimal aroach for the atient s clinical condition. Teams erforming 100 or more OPCAB oerations have imroved clinical decision-making rocesses and technical skills regarding treatment of all CABG atients, regardless of whether cardioulmonary byass is used. We hyothesized that this exerience and choice of aroaches for CABG rocedures translates into better clinical outcomes. Methods. Using data on 16,988 consecutive atients in 72 hositals from the HCA The Healthcare Comany casemix database, we identified high- and low-volume OPCAB sites and then analyzed the atient and hosital characteristics that had an imact on clinical outcomes. Results. The mortality rates for the high- and lowvolume OPCAB facilities both averaged 2.9% ( NS). Patients at the high-volume OPCAB facilities had significantly lower rates of major comlications (shock/ hemorrhage, neurologic, renal, and cardiac) than those at the low-volume OPCAB facilities. Of the seven minor comlications, rates for six were lower in the highvolume OPCAB facilities, but none of the differences reached statistical significance. High-volume OPCAB sites were significantly more likely to discharge their atients directly home than were low-volume OPCAB sites (80% versus 66%; 0.001). Conclusions. The results suggested that surgical team exerience and choice of aroaches to erforming CABG had an imact on atient outcomes. (Ann Thorac Surg ) 2001 by The Society of Thoracic Surgeons Agrowing body of literature suggests that hysicians or medical centers erforming higher volumes of surgical rocedures may achieve imroved atient outcomes [1 10]. In their comrehensive review of coronary artery byass graft (CABG) oeration guidelines, Eagle and associates [11] concluded that survival after CABG is negatively affected when carried out in institutions that erform fewer than a threshold number of cases annually. Although the inherent logic in ractice makes erfect may seem inescaable, imortant caveats can be raised relevant to the introduction and diffusion of new cardiovascular technologies such as off-um coronary artery byass (OPCAB) oerations. One of these caveats relates to surgical team exerience in erforming new OPCAB techniques. Surgical team members, esecially those in anesthesia and nursing, need to be esecially Presented at the Seventh Annual Cardiothoracic Techniques and Technologies Meeting 2001, New Orleans, LA, Jan 24 27, Address rerint requests to Dr Becker, Rollins School of Public Health at Emory University, 1518 Clifton Rd NE, Atlanta, GA 30322; ebeck01@sh.emory.edu. alert regarding the hysiologic management of atients when byass is being erformed off-um. How do these new clinical skills and the volume of cases erformed off-um imact atient outcomes? It has been suggested that the growing use of minimally invasive off-um methods in CABG oerations offers the otential for further reducing CABG mortality and morbidity rates. Patients undergoing OPCAB oerations have exerienced reduced mortality and comlication rates, reduced lengths of hosital stay, and reduced costs at selected sites, subgrous, and centers [12 17]. However, to date, there is a relative aucity of literature describing the relation between hosital and atient volumes and differences in off-um and onum CABG oeration outcomes. We have seculated that as the cardiovascular surgical team exerience with erforming off-um CABG rocedures increases, changes occur in their knowledge base, clinical decision-making rocesses, and technical skills regarding treatment of all CABG atients, regardless of whether or not cardioulmonary byass is used by The Society of Thoracic Surgeons /01/$20.00 Published by Elsevier Science Inc PII S (01)02937-X

2 S1010 CTT SUPPLEMENT BROWN ET AL Ann Thorac Surg That is, as the surgical team becomes more comfortable with the off-um technique and the number of their off-um atients increases, they become better reared with every CABG atient to select either an offum or on-um technique. The decision rests rimarily with the surgeon about which aroach will otimize the atient s outcome. This ability to choose between the two aroaches esecially for surgical teams doing higher volumes of off-um rocedures may equate to better overall clinical outcomes and the ability to treat atients who reviously would not have been candidates for surgical treatment. Based on the literature and our discussion, a number of hyotheses were develoed and analyzed at the atient and facility levels: Higher levels of OPCAB oerations are associated with lower atient and hosital mortality rates than rograms with lower volume levels. Higher levels of OPCAB oerations are associated with lower atient and hosital comlication rates for major outcomes (shock/hemorrhage, neurologic, renal, and cardiac) than rograms with lower volume levels. Higher levels of OPCAB oeration volume are associated with lower atient and hosital minor comlication rates than rograms with lower volume levels. Material and Methods The HCA casemix database contains data on 16,988 consecutive CABG atients in 72 hositals admitted to HCA facilities during the study eriod from January 1, 1999, through December 31, This database was a comrehensive administrative database containing atient, clinical, rocedural, and outcome data. To distinguish between high- and low-volume offum facilities, we used the cut-off of 100 OPCAB oerations with any sites having documented 100 or more rocedures in the high-volume cohort. Whereas the 100-rocedure limit is somewhat arbitrary, given the relative newness of the off-um rocedure and the lack of an external cut-off reference in the literature, we thought this number was a reasonable demarcation. Also, the Deartment of Veteran Affairs and the State of New York both used the category of less than 100 cases annually as a cut-off oint for low-volume institutions [11]. The volume of off-um, on-um, and total rocedures by institution are listed in Table 1. We noted wide ranges among sites in the number and ercentage of off-um CABG rocedures being erformed. As listed in Table 1, staff in our 72 study hositals erformed a mean of 246 CABG oerations in 1999, of which a mean of 34.6 (16%) were OPCAB rocedures and a mean of (84%) were CABG rocedures erformed on-um. The number of OPCAB rocedures erformed ranged from a high of 293 rocedures in one hosital to no OPCAB rocedures at three hositals. Only six hositals erformed more than 100 CABG rocedures off-um and these hositals were identified as high-volume OPCAB hositals, while the 66 other hositals were categorized as low-volume OPCAB hositals. Our investigation reorted on the comarison between these two grous. Patients A total of 16,988 consecutive atients at 72 hositals who underwent CABG rocedures (diagnosis-related grous 106, 107, and 109) were included for calendar year This total included 2,038 atients (12%) who underwent CABG oeration at one of the six high-volume hositals and 14,950 atients (88%) who received CABG oerations at one of the 66 low-volume hositals. Data Collection Patient data from each hosital were collected through the hosital discharge abstract. Patient characteristics, rocedural characteristics, rocedure comlications, and mortality and hosital-rocess 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 atient characteristics and rocedure comlications into categories [18]. A total of 18 atient, 13 outcome, 4 rocedural, and 5 hosital-rocess characteristics were identified, analyzed, and comared. Statistical Analysis The data are resented in two ways. Because the HCA casemix database contains data on a large number (aroximately 5.6%) of all the CABG oerations erformed in the United States using estimates based on American Heart Association data [19], the atient characteristics and outcomes reresented in the database are of interest. Patient rofiles are resented in the tables and include atient means, standard deviations, and values for each of the resective variables in the low- and high-volume OPCAB sites. Second, the atient data were aggregated for each of the 72 hositals and reorted as hosital rofiles. The hosital means, standard deviations, and values for each of these variables are also reorted. The values were calculated for each oulation atient and hosital using Student s t tests. To test the influence of the low- and high-volume OPCAB on mortality and comlications risk, adjusting for otential differences in the two oulations, we also used logistic regression techniques and reorted the odds ratios and values for the high-volume OPCAB coefficient. Twelve logistic regressions were erformed with the deendent variables being atient mortality and each of the 11 comlications: shock/hemorrhage, neurologic, cardiac, renal, mechanical, surgical infection, ostoerative infection, seticemia, resiratory, neumonia, and eriheral vascular. The indeendent variables included in each of the 12 logistic equations were atient age, sex, smoker, history of tobacco use, chronic obstructive ulmonary disease, insulin-deendent diabetes mellitus (IDDM), noninsulin-deendent diabetes mellitus, acute renal failure, chronic renal failure, unsecified renal failure, cardiogenic shock, hyertension, acute

3 Ann Thorac Surg CTT SUPPLEMENT BROWN ET AL S1011 Table 1. Number and Percentage of CABG Cases Off-Pum and On-Pum by Hosital Hosital Off-Pum On-Pum n % n % Total Cases % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % 126 Table 1. Continued Hosital Off-Pum On-Pum n % n % Total Cases % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % 22 Mean % % CABG coronary artery byass grafting. myocardial infarction (MI), old MI, cardiomyoathy, congestive heart failure, eriheral vascular disease, endocarditis, and the surgical volume of the hosital. Results More than 70% of atients in both the low- and highvolume facilities were men (71.1% versus 70.9%; 0.849), and they averaged about 2 years older in the low-volume facilities (66.3 years versus 64.2 years; 0.001). A number of statistically significant differences may have contributed to higher mortality or morbidity between the high- and low-volume OPCAB atients. A significantly higher ercentage of atients in the lowvolume OPCAB oulation, 58%, were 65 years or older. In contrast, only 49% of the atients in the high-volume OPCAB oulation were 65 years or older ( 0.000). Patients in high-volume facilities were significantly more likely to be smokers, but significantly less likely to have hyertension, an acute MI, an old MI, cardiomyoathy, or congestive heart failure. Overall, of the 18 atient characteristic and comorbid conditions studied, eight conditions showed no statistically significant difference between the high- and low-volume atient oulations. More than 89% of atients at the high-volume OPCAB sites received arterial conduits of some tye, comared with only 80% of the atients at the low-volume OPCAB sites ( 0.001). However, there were more vessels byassed for atients at low-volume OPCAB sites than for atients at the high-volume OPCAB sites (3.36 versus 3.11; 0.001). Patients at low-volume OPCAB facilities required nearly double the amount of intraaortic balloon um use as the atients at high-volume OPCAB facilities (7.2 versus 3.7, resectively).

4 S1012 CTT SUPPLEMENT BROWN ET AL Ann Thorac Surg Table 2. Patient, Procedure, and Hosital Characteristics for High- and Low-Volume Off-Pum Patients and Hositals in 1999 HCA Casemix Data Variable Mean SD Mean SD Mean SD Mean SD Number of cases 14, Patient characteristics Male % Patient age (y) Age Smoker COPD Endocrine IDDM Cardiogenic shock Hyertension Acute MI Old MI Cardiomyoathy Congestive heart failure Mitral valve disease Procedure characteristics % venous conduits % artery conduits Number of vessels byassed % IABP use Hosital characteristics Length of stay Hosital surgery volume NA NA NA NA NA COPD chronic obstructive ulmonary disease; IABP intraaortic balloon um; IDDM insulin-deendent diabetes mellitus; MI myocardial infarction; NA not alicable. Patients in high-volume OPCAB facilities had a shorter length of stay than atients in the low-volume facilities (7.7 days versus 8.7 days; 0.001). Partitioning length of stay into two arts (not shown) admission to oeration and oeration to discharge revealed that for both segments of the stay, atients at high-volume OPCAB facilities had significantly shorter mean hosital stays. Table 3 shows mortality and comlication results for Table 3. Patient Mortality and Procedure Comlications for High- and Low-Volume Off-Pum Patients and Hositals in 1999 HCA Casemix Data Variable Mean SD Mean SD Mean SD Mean SD Patient mortality Procedure comlications Shock-hemorrhage Neurological Cardiac Renal Mechanical Imlant infection Postoerative infection Seticemia Resiratory Pneumonia Periheral vascular

5 Ann Thorac Surg CTT SUPPLEMENT BROWN ET AL S1013 Table 4. Discharge Disosition for High- and Low-Volume Off-Pum Patients and Hositals in 1999 HCA Casemix Data Discharge Disosition Mean SD Mean SD Mean SD Mean SD Home Discharged to home health Skilled nursing facility Rehabilitation low- and high-volume OPCAB atients. The mortality rate for the high- and low-volume OPCAB atients was not significantly different (both averaged 2.9%). For the four major comlications (shock/hemorrhage, neurologic, renal, and cardiac), atients at the high-volume OPCAB sites all had significantly lower rates of comlications than atients at low-volume OPCAB facilities. Shock/hemorrhage was more than 1.5% lower in highvolume OPCAB facilities. The rates of neurologic (0.83% versus 1.45%; 0.025), renal (0.34% versus 0.97%; 0.005), and cardiac (3.04% versus 7.47%; 0.001) comlications at high-volume OPCAB facilities were nearly half of the resective rates in the low-volume facilities. Among the seven minor comlications, six were lower in the high-volume OPCAB facilities. Only neumonia aeared to have a higher rate of occurrence in the high-volume OPCAB facility, although this difference was not statistically significant. As shown in Table 4, atients in high-volume OPCAB facilities were significantly more likely to be discharged directly home without home health care than were atients in low-volume OPCAB facilities (81% versus 64%; 0.001). Conversely, nearly 20% of all low-volume OPCAB atients were discharged to home health care, whereas just 6% of high-volume OPCAB atients ( 0.001) were discharged to home health care. Low-volume OPCAB facilities discharged 8% of their atients to skilled nursing facilities, whereas high-volume OPCAB facilities discharged 5% of their atients to skilled nursing facilities. The hosital rofiles show atient data aggregated to the facility level and summarized means. As such, hosital rofiles reflect facility averages and suggest the overall erformance of the facility because, unlike the atient oulation means described above, hosital rofile means are not weighted by the number of CABG rocedures being erformed. As such, the t-test comarisons comared the means of the six hositals in the highvolume OPCAB sites and the 66 hositals in the lowvolume OPCAB sites. As shown in Table 2, aroximately 70% of the oulations in both the low- and high-volume OPCAB facilities were male (69.7% versus 71.0%; 0.727) with an average age of 66 or 65 years old, resectively. Whereas the high-volume OPCAB facilities tended to have lower rates on all but three of the oulation characteristics and comorbid conditions (smoking, IDDM, and revious CABG), none of the values were statistically significant at conventional levels. At the hosital level, the differences in the number of vessels byassed and the ercentage receiving venous conduits was not statistically significant between highand low-volume OPCAB sites. The utilization of arterial conduits remained significantly higher (87%) in highvolume OPCAB sites comared with the low-volume OPCAB sites (79%; 0.027). The usage of intraaortic balloon ums aeared marginally significant ( 0.053), with 8.1% at the low-volume OPCAB facilities comared 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; 0.105). For the admission to oeration and oeration to discharge ortions of the hosital stay (not shown), the atient oulation at high-volume OPCAB facilities had shorter hosital stays, but neither difference aroached statistical significance at conventional levels. The mortality rate for the high- and low-volume OPCAB facilities reorted in Table 3 was 3.0% and 3.1%, resectively ( 0.787). For the four major comlications (shock/hemorrhage, neurologic, renal, and cardiac), the high-volume OPCAB facilities had lower rates of comlications than the low-volume OPCAB facilities, but only shock/hemorrhage (1.18% versus 3.77%; 0.044) was statistically significant. The difference in the renal comlication rate (0.83% versus 0.26%; 0.068) aroached statistical significance. Among the seven minor comlications, six were lower in the high-volume OPCAB facilities, although none of these differences was statistical significant. Although the rate of neumonia 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 atients directly home than were low-volume OPCAB facilities (80% versus 66%; 0.001). Conversely, 16.52% of all low-volume OPCAB site atients were discharged to home health care, comared with only 6.96% of highvolume OPCAB atients ( 0.211). High- and lowvolume OPCAB facilities discharged about the same ercentage of atients to skilled nursing and rehabilitation.

6 S1014 CTT SUPPLEMENT BROWN ET AL Ann Thorac Surg Table 5. Logistic Regression Coefficients for Death and Procedure Comlications by High- and Low-Volume Off- Pum Facilities a Variable Odd-ratio Patient mortality Death Procedure comlications Shock/hemorrhage Neurologic Cardiac Renal Mechanical Surgical infection Postoerative infection Seticemia Resiratory Pneumonia Periheral vascular a Variables included in the logistic regression but whose coefficients are not reorted are age, sex, smoker, history of tobacco use, chronic obstructive ulmonary disease, insulin-deendent diabetes, noninsulindeendent diabetes, acute renal failure, chronic renal failure, unsecified renal failure, cardioshock shock, hyertension, acute myocardial infarction, old myocardial infarction, cardiomyoathy, congestive heart failure, eriheral vascular disease, endocarditis, and hosital surgical volume. Multivariate Analysis To assess the influence of atient characteristics and comorbid conditions on death and comlication rates, we used multivariate regression techniques. Using death and the comlication rates, resectively, as deendent variables we controlled for atients in high-volume OPCAB facilities, atient age, sex, smoker, history of tobacco use, chronic obstructive ulmonary disease, IDDM, noninsulin-deendent diabetes mellitus, acute renal failure, chronic renal failure, unsecified renal failure, hyertension, acute MI, old MI, cardiomyoathy, congestive heart failure, and eriheral vascular disease. Because the deendent variables in the atient oulation were dummy variables, binary logistic regression was used and the odds ratios and values for just the high-volume OPCAB facilities variable are reorted in Table 5. Controlling for atient characteristics and comorbid conditions, 10 of the 12 deendent variables in Table 5 indicated that high-volume OPCAB facilities had lower odds of the comlications than low-volume OPCAB facilities. Thus, for examle, atients in high-volume OP- CAB facilities were 41% less likely to exerience shock and hemorrhage than atients in low-volume OPCAB facilities ( 0.001). Similarly, atients in high-volume OPCAB facilities were 59% and 62% less likely to exerience cardiac and renal failure, resectively, than atients in low-volume OPCAB sites. No statistically significant difference was indicated for mortality rates at the high- and low-volume sites. Comment Our first hyothesis was that facilities where OPCAB oerations were erformed more frequently would be associated with lower atient mortality rates and lower hosital mortality rates than rograms with lower OPCAB volume levels. We found no suort for this hyothesis. 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 hyothesis we ostulated that higher volumes of OPCAB oerations would be associated with lower atient and facility comlication rates for major outcomes (shock/hemorrhage, neurologic, renal, and cardiac) than lower volumes. We found strong suort for this hyothesis. For both the atient and hosital rofiles, high-volume OPCAB facilities had lower rates on all four of these major comlications comared with the low-volume OPCAB facilities. For the atient rofiles all four were statistically significant. Our final hyothesis stated that higher volumes of OPCAB oerations would be associated with lower atient and hosital minor comlication rates (ostoerative infection, resiratory comlications, imlant infection, mechanical comlications, eriheral vascular comlications, neumonia, and seticemia) than rograms with lower OPCAB volume levels. There was some modest suort for this hyothesis. In both the atient and hosital rofiles, six of the seven minor comlication rates were lower for high-volume OPCAB facilities than for low-volume OPCAB facilities. The results suggested that for many comlications, outcomes at high-volume OPCAB oeration facilities are better than outcomes at low-volume OPCAB oeration sites although no difference was found in mortality rates. We recognize that this study has a number of imortant limitations and our results should be interreted cautiously. 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