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Ann Thorac Surg 2002;74:2113-2119
© 2002 The Society of Thoracic Surgeons

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Original article: cardiovascular

Outcomes experience with off-pump coronary artery bypass surgery in women

^a HCA, Inc, Nashville, Tennesee, USA
^b Rollins School of Public Health at Emory University, Atlanta, Georgia and Cardiac Data Solutions, Inc, Zionsville, Indiana, USA

Accepted for publication July 2, 2002.

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

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
BACKGROUND: It has been well documented that women have higher morbidity and mortality rates than men following coronary artery bypass graft (CABG) surgery. In view of this evidence, we investigated the following question: compared with on-pump CABG surgery, is there benefit to off-pump CABG surgery in women?

METHODS: Our investigation analyzes patient mortality and 13 procedure complications controlling for 35 variables representing patient characteristics and comorbid conditions, and for procedure characteristics for a population of 16,871 consecutive women undergoing off-pump and on-pump CABG surgery at 78 hospitals for the period January 1998 to June 2001.

RESULTS: Mean comparisons reveal that the mortality rate for women undergoing off-pump CABG surgery is nearly a percentage point lower than for women undergoing on-pump surgery (3.12 vs 3.90; p = 0.052). The complication rates for all complications analyzed (shock/hemorrhage, neurologic, cardiac, respiratory, renal, acute renal failure, adult respiratory distress syndrome, implant infection, postoperative infection, septicemia, pneumonia, and peripheral vascular) were lower for women off-pump than women on-pump with the exception of mechanical complications. Logistic regression results reveal, after controlling for 35 relevant patient characteristics, comorbid conditions and procedure characteristics, that women undergoing on-pump CABG surgery experience a 42% higher mortality rate (p = 0.0239) than women undergoing off-pump CABG surgery.

CONCLUSIONS: Evidence suggests that off-pump CABG surgery may be better for women than on-pump CABG surgery because it appears to reduce mortality and respiratory complications, shorten lengths-of-stay, and increases discharges directly home. None of the 12 other complications investigated demonstrated an advantage for women undergoing on-pump surgery relative to those receiving off-pump surgery.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
With an aim of minimizng mortality and morbidity surgeons have progressed towards less invasive procedures. In the area of cardiac surgery, techniques of cardiopulmonary bypass (CPB) have been refined for decades, but problems with CPB have been well documented [1]. It has also been well established that women, when compared with men, have higher morbidity and mortality rates following coronary artery bypass graft (CABG) surgery [2, 3]. In view of these higher rates in women, this study examines if women who undergo off-pump surgery receive any benefits compared with women undergoing on-pump surgery.

Although off-pump CABG surgery is increasing, little comprehensive data are available on how women fare off-pump versus on-pump. A number of earlier reports suggested that the female sex was an independent risk factor for higher mortality and morbidity after CABGsurgery, but that long-term survival and functional recovery were similar to those in men undergoing CABG surgery [4–8]. More recent studies have suggested that on average, women have a disadvantageous preoperative clinical profile that may account for much of this perceived difference. These findings are not universal, as significant differences exist in clinical practice between institutions [9].

The potential benefits of off-pump CABG surgery are significant. CPB has been associated with a 1% to 5% incidence of stroke and other complications including postoperative low cardiac output syndrome, adult respiratory distress syndrome, bleeding, and renal insufficiency [10]. Off-pump CABG surgery patients have experienced reduced mortality and complication rates, reduced lengths of hospital stay, and cost reduction at selected centers in selected subgroups [11]. Although it is essential that short-term and long-term safety, benefits, and efficacy of the off-pump approach needs to be evaluated, this study comprehensively compared women undergoing off-pump CABG surgery with on-pump CABG surgery.

Controlling for relevant patient, procedure, and process characteristics, we tested four hypotheses:

• Women undergoing off-pump CABG surgery will have lower mortality rates than women undergoing on-pump CABG surgery;
  
• Women undergoing off-pump CABG will have fewer major complications (shock/hemorrhage, cardiac, neurologic, and acute renal failure) than women undergoing on-pump CABG surgery;
  
• Women undergoing off-pump CABG surgery will have lower rates of minor complications (respiratory complications, renal complications, adult respiratory distress syndrome, mechanical complications, implant infections, postoperative infections, septicemia, pneumonia, and peripheral vascular complications) than women undergoing on-pump CABG surgery; and
  
• Women having off-pump surgery will have shorter lengths of hospital stay than women with on-pump CABG surgery.
  

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Patients
To test these hypotheses, data were collected through The Healthcare Company (HCA) Casemix Database. The HCA Casemix database is a comprehensive, administrative database containing patient, clinical, and outcome data on all cardiovascular patients at HCA hospitals during the study period from January 1, 1998 through June 30, 2001. A total of 16,871 consecutive female patients at 78 hospitals that underwent a CABG surgery procedure and were discharged in one of the three primary CABG diagnostic-related groups (DRGs): 106, 107, or 109. These DRGs are, respectively, CABG procedure with a percutaneous coronary intervention, CABG procedure with a cardiac catherization, and CABG procedure only. There were 2,631 women (15.6%) who underwent off-pump CABG surgery and 14,240 women (84.4%) who underwent on-pump CABG surgery.

Data collection
Patient characteristics, procedure complications, mortality, and hospital-process characteristics were coded directly from the hospital discharge abstract, which contains the principal diagnoses and procedures received by the patient. The definitions in the International Classification of Diseases-9^th Edition (ICD-9) were used to aggregate patient characteristics and procedure complications into categories [12]. A total of 33 patient characteristics and risk factors, 14 patient outcomes, 3 hospital-process characteristics, and 3 length-of-stay characteristics were identified and compared using standard ICD-9 definitions.

We report the volume of women off-pump, on-pump, and total procedures by institutions in the study in Table 1. There are wide ranges among sites in the number and percentage of off-pump CABG procedures being performed. The differences for the full population of men and women were reported in an earlier investigation [13]. The typical hospital averaged 246 CABG surgeries of which 34.6 (16%) were OPCAB procedures while an average of 211.3 (84%) were CABG procedures performed on-pump. In this investigation, focusing on only females undergoing CABG procedures, many of the off-pump CABG surgeries are performed in sites that do a large number of CABG cases but 24 of 78 hospitals (307%) did more than the hospital average of 16.6% cases off-pump. Moreover, more than 50% of the hospitals had 10% or more of their CABG surgeries on female patients performed off-pump. We could discern no consistent bias among the different sites relative to their use of off-pump or on-pump approaches. Among the 78 hospitals the variation in mortality rates ranged from 0% to 17% with a hospital mean of 4.2% and a standard deviation of 2.9%. The hospital with a 17% mortality rate treated only 41 cases.

Other patient characteristics and comorbid conditions are clearly important and are expected to have a mitigating impact on the relationship between the use of a heart pump and procedure outcomes. To account for these confounding variables, we included a number of independent variables in our regression model. The model controls for the following factors: patient characteristics (age and gender); risk factors and comorbid conditions (smoker, history of tobacco use, chronic obstructive pulmonary disease [COPD], S/p CABG, S/p percutaneous transluminal coronary angioplasty (PTCA), insulin-dependent diabetes, noninsulin-dependent diabetes, conduction disorders, intracranial hemorrhage, chronic renal failure, unspecified renal failure, cardiogenic shock, hypertension, acute MI, old MI, cardiomyopathy, congestive heart failure, peripheral vascular disease, unstable angina, acute liver necrosis, endocarditis, mitral valve disease, aortic valve disease, mitral and aortic valve disease, and chronic hepatitis); procedure characteristics (number of vessels bypassed, diagnostic cath, intraaortic balloon pump, hypothermia use, and heart pump); medications (GP2B3A and thrombolytics); time period (14 dummy variables representing each quarter since 1998 with second quarter 2001 the excluded category); and site characteristics (dummy variables for each of the 78 sites performing CABG surgery with one site excluded). Many of these variables have been described elsewhere [13] and reflect conventional aspects of CABG surgery.

Because the dependent variables are bimodal, logistic regression (odds ratios) was used. To conserve space, only the coefficient for the odds ratios for the use of cardiopulmonary bypass and the significance level for each outcome measure is reported in Table 3.

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

Mortality and complications
The unadjusted mortality rate was 0.78 of a percentage point lower for women undergoing off-pump CABG surgery (3.12%) than for women undergoing on-pump CABG surgery (3.90%; p = 0.052). Although these differences may appear small, if we translate the difference to the number of patients dying, a total of 555 patients expired in the female on-pump population. Applying the off-pump population mortality of 3.12% to the on-pump CABG population, this would translate into only 444 female deaths. Thus, the small mortality difference of 0.08% still represents a difference of 111 fewer deaths in the female off-pump CABG population than the female on-pump CABG population.

Four major complications are illustrated in Table 2, shock/hemorrhage, neurologic, cardiac, acute renal failure, all four had lower rates for women undergoing off-pump CABG surgery than for women undergoing on-pump CABG surgery. However, only one complication, shock/hemorrhage (p = 0.001), was statistically significant whereas cardiac complications (p = 0.064) approached statistical significance.

Of the other nine minor complications, all these complications were lower for women undergoing off-pump surgery than for women undergoing on-pump CABG surgery except for mechanical complications. However, only one respiratory complication was statistically significant (p = 0.0285).

Procedure variables and hospital characteristics
On average, women undergoing OPCAB had significantly fewer vessels bypassed with 2.4 vessels per patient, whereas women on-pump had 3.3 vessels bypassed (p < 0.001). Additionally, women undergoing off-pump CABG surgery were more likely to have at least one arterial graft used (83% vs 76%; p < 0.001).

Overall, the unadjusted means indicated that inpatient length-of-stay for women having off-pump CABG surgery averaged 8.8 days, whereas women on-pump averaged 9.6 days (p < 0.001). The length-of-stay from surgery to discharge for women undergoing OPCAB averaged 6.5 days compared with 7.3 days (p < 0.001) for women on-pump. For women undergoing off-pump surgery, the length-of-stay from admission to surgery is actually slightly longer than for women receiving an on-pump procedure (2.36 vs 2.32; p = 0.478).

As illustrated in Table 1, the mean number of procedures performed on women on-pump and off-pump still varies dramatically within hospitals. The typical hospital averages five times more on-pump than off-pump CABG surgeries with just 35 off-pump CABG surgeries and 183 on-pump CABG surgeries (p < 0.001).

Discharge disposition
Of the women undergoing off-pump CABG surgery, 65% were discharged home more than 10% higher than the women discharged home who had undergone on-pump surgery (64.9% vs 53.8%; p < 0.001). Another 15% of the women treated with off-pump CABG surgery were discharged home with home health whereas 23% of the women undergoing on-pump surgery were discharged (p < 0.001) with home health care. Nine percent of the women undergoing off-pump CABG surgery were discharged to skilled nursing facilities, although for women undergoing on-pump surgery 13% were discharged to these facilities (p < 0.001).

Multivariate regression
After controlling for patient, procedure, medication, time period, and site characteristics, the estimated odds ratio (Table 3) indicates that women undergoing on-pump CABG surgery experienced a 42% higher mortality rate than women undergoing OPCAB (p = 0.0239).

Of the 13 complications analyzed, the estimated odds ratios for the variable, as reported in Table 3, indicates the use of CBP was statistically significant in only the respiratory complication regression. Women undergoing on-pump CABG surgery have a 42% higher likelihood of respiratory complications than women undergoing OPCAB (p = 0.0285).

The estimated odds ratio was greater than 1.0 for five other complications in Table 3, favoring off-pump CABG surgery over on-pump CABG surgery in women and trended towards statistical significance. These complications were shock/hemorrhage (p = 0.0919), implant infection (p = 0.0759), cardiac complications (p = 0.1502), adult respiratory distress syndrome (p = 0.1607), and peripheral vascular complications (p = 0.1276) although none were statistically significant at conventional levels.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Our multivariate comparison analyzed 14 outcomes for 14,240 consecutive women who underwent CABG surgery on-pump with 2,631 consecutive women who underwent off-pump CABG surgery in 78 hospitals over a 3.5-year period, controlling for numerous aspects of the patient, procedure, and CABG surgery process. Four hypotheses were tested.

First, we hypothesized that women undergoing off-pump CABG surgery would have lower mortality rates than women undergoing on-pump CABG surgery. We found support for this hypothesis. Women undergoing off-pump CABG surgery experienced a 42% lower mortality rate than women undergoing on-pump CABG surgery.

Second, we premised, that women having off-pump CABG surgery would have fewer major complications (shock/hemorrhage, cardiac, neurologic, acute renal failure) than women having on-pump CABG surgery. Two of the four variables trended toward statistical significance, shock/hemorrhage and cardiac complications. However, they were not statistically significant at conventional levels. The other two complications, neurologic and acute renal failure, did not approach statistical significance.

The third hypothesis we developed speculated that women undergoing off-pump CABG surgery would have lower rates than the nine other complications (respiratory complications, renal complications, adult respiratory distress syndrome, mechanical complications, implant infection, postoperative infection, septicemia, pneumonia, and peripheral vascular complications) than women undergoing on-pump CABG surgery. Three of eight complications reflected a trend revealing better outcomes for women undergoing off-pump CABG surgery than for women undergoing on-pump CABG surgery but only one complication—respiratory—was statistically significant. Respiratory complications were 42% higher for women undergoing CABG surgery than women undergoing off-pump CABG surgery (p = 0.0285).

Finally, we hypothesized that women having off-pump CABG surgery would have shorter lengths of hospital stay than women having on-pump CABG surgery. This hypothesis was confirmed with some interesting caveats. The total length-of-stay for women undergoing procedures off-pump was shorter than that for women on-pump (8.8 days vs 9.6 days; p < 0.001). The difference was mostly in the period from surgery to discharge (6.5 days vs 7.3 days, p < 0.001). For the admission to surgery period, women undergoing off-pump CABG surgery had slightly higher length-of-stay than women undergoing on-pump surgery (2.4 days vs 2.3). These results were not statistically significant.

Overall, the logistic regression results revealed 7 of 14 outcome measures favored women who underwent off-pump CABG surgery over women who received on-pump CABG surgery, with two reaching statistical significance. We conclude that based on the results of the analyses there is some support for the premise that off-pump CABG surgery may be better for women than on-pump CABG surgery because it appears to improve outcomes, shortens lengths-of-stay, and increases discharges directly home.

Limitations
We recognize there are a number of important limitations with our data and our results should be interpreted cautiously. First, the HCA Casemix Database is an administrative database and lacks particular clinical details that might be useful in segmenting female patients into severity levels of coronary heart disease. Second, the data are not risk adjusted for specific clinical characteristics and it is possible that a physician’s choice of procedure is based on clinical differences that are not captured by our data. For example, the differences in the off-pump and on-pump cohorts regarding the number of vessels treated (2.4 vs 3.3, respectively) and the percentage of acute myocardial infarction (21% vs 27%, respectively) could suggest that the female on-pump CABG surgery population had a greater level of clinical severity-of-illness than the female off-pump population. Third, we only know whether or not a pump was used in the CABG surgery. The physician’s intention to treat cannot be identified.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This research was supported, in part, by a grant from the HCA Research Foundation.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Phillip P. Brown Michael J. Mack Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Brown, P. P. Articles by Becker, E. R. Search for Related Content PubMed PubMed Citation Articles by Brown, P. P. Articles by Becker, E. R. Related Collections Coronary disease