Ann Thorac Surg 2005;79:819-824
© 2005 The Society of Thoracic Surgeons
Original article: Cardiovascular
Effect of Diabetes on Outcome and Changes in Quality of Life After Coronary Artery Bypass Grafting
Otso Järvinen, MD, PhDa,*,
Juhani Julkunen, PhDb,c,
Timo Saarinen, LicA(Psych)b,
Jari Laurikka, MD, PhDa,
Matti R. Tarkka, MD, PhDa
a Heart Center, Department of Cardiac Surgery, Tampere University Hospital, Tampere
b Rehabilitation Foundation, Helsinki
c Department of Psychology, University of Helsinki, Helsinki, Finland
Accepted for publication August 18, 2004.
Abbreviations and Acronyms AMIacute myocardial infarction; BMIbody mass index; CABGcoronary artery bypass grafting; CCUcoronary care unit; CIconfidence interval; CK-MBcreatine phosphokinase isoenzyme MB; COPDchronic obstructive pulmonary disease; CPBcardiopulmonary bypass; ICUintensive care unit; IDDMinsulin dependent diabetes mellitus; LVleft ventricule; MCSmental component summary; NIDDMnoninsulin dependent diabetes mellitus; NYHANew York Heart Association; ORodds ratio; PCSphysical component summary; PTCApercutaneous coronary angioplasty; QOLquality of life; RAND-36RAND-36 Health Survey; SDstandard deviation
* Address reprint requests to Dr Järvinen, Tapulinkatu 20 33400 Tampere, Finland (E-mail: otsojarvinen{at}koti.soon.fi).
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Abstract
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BACKGROUND: An increasing proportion of patients undergoing coronary artery bypass grafting are diabetics who are known to carry a higher mortality and morbidity in association with operation, but data on whether health-related quality of life improves similarly after coronary artery bypass grafting in diabetic and nondiabetic patients are limited. We assessed in detail changes in health-related quality of life (RAND-36 Health Survey) during the first year after coronary artery bypass grafting.
METHODS: Seventy-four of the 508 patients (14.6%) operated on in a single institution had a history of diabetes and were compared to nondiabetics. The RAND-36 Health Survey was used as an indicator of quality of life. Assessments were made preoperatively and repeated 12 months later.
RESULTS: Thirty-day mortality was 2.7% versus 1.6% (p = 0.511) and one-year survival was 94.6% versus 97.0% (p = 0.287) in the diabetics and nondiabetics, respectively. Diabetics improved significantly (p < 0.005) in seven, nondiabetics (p < 0.001) in all eight RAND-36 dimensions. Physical component summary and mental component summary scores on the RAND-36 improved significantly (p < 0.001) in diabetics as well as in nondiabetics. Both groups experienced closely similar freedom from anginal symptoms at one year (86.2% vs 90.5%, p = 0.280).
CONCLUSIONS: Although diabetic patients differ from nondiabetics having slightly inferior quality of life before and one year after coronary artery bypass grafting, they gain similar improvement of quality of life in one year after surgery when compared to nondiabetics.
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Introduction
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Diabetes is a well-established risk factor for atherosclerotic coronary heart disease [1], and coronary heart disease is the leading cause of death among adult diabetics accounting for three times as many deaths among diabetics as among nondiabetics [2]. Coronary artery disease is not only more prevalent in diabetic patients compared with the rest of the population but tends to be more extensive, involving multiple vessels and being rapidly progressive [3]. Diabetes is also a risk factor in association with myocardial revascularization procedures [4, 5] but, according to recent studies, coronary artery bypass grafting (CABG) may be the treatment of choice in this group of patients [6, 7].
During the past two decades, progress in surgical technique, in anesthesia, and in postoperative care has improved the results of coronary artery bypass surgery. At the same time, however, the profile of patients undergoing CABG has altered towards higher age with frequent preoperative comorbid conditions, especially diabetes, and increased postoperative morbidity [8–10]. Their gain in added years of life achieved by the CABG may be limited. Quality of life (QOL) thus becomes a pertinent issue as providers and consumers of health care debate on the benefits to be gained from expensive medical and surgical interventions. However, there are limited data available on whether health-related QOL improves similarly after CABG in diabetic and nondiabetic patients. We therefore assessed the changes in health-related QOL (RAND-36) during the first year after CABG surgery in diabetics. We also compared the preoperative risk profiles, perioperative variables, early postoperative outcome, 1-year symptomatic status, and hospital readmissions between the diabetics and nondiabetics.
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Patients and Methods
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Procedure and Subjects
The data were obtained from Tampere University Hospital between May 2, 1999 and November 30, 2000. The cohort comprised 508 patients who underwent isolated CABG. The study was approved by the institutional review board of Tampere University Hospital (April 15, 1999) and each patient gave written informed consent to participate. There were 420 (82.7%) male patients in the sample. Age range was from 34 to 92 years (median, 63). Three hundred and ninety-eight (78.3%) of the procedures were performed electively and 110 (21.7%) urgently or as emergencies. Four hundred and fifty-three (89.2%) patients underwent bypass grafting through a sternotomy incision with cardiopulmonary bypass (CPB; on-pump) and 55 (10.8%) were operated without CPB (off-pump).
Seventy-four (14.6%) patients had a history of diabetes mellitus on admission that had necessitated active therapy with medication. Thirty-nine (52.7%) of these patients had a noninsulin dependent diabetes mellitus (NIDDM) and 35 (47.3%) patients had an insulin dependent diabetes mellitus (IDDM). In the analysis, these diabetes types were treated together because the number of patients in the subgroups was relatively small for a reasonable statistical handling.
During the primary hospital stay a comprehensive preoperative, perioperative, and postoperative medical data body was collected. Most patients were discharged on the sixth day (median) after the operation to the local district hospital. The data from these eighteen secondary discharge hospitals were collected by referring physicians and sent to the first author (OJ) for analysis. All outcome events, including thirty-day mortality and complications, were recorded for joint analysis with the primary hospital data. Statistics Finland provided causes and dates of death after discharging. Major postoperative complications included mortality, stroke, mediastinitis, sepsis, low output syndrome, prolonged ventilatory support (> 36 hours), acute renal failure requiring dialysis, perioperative myocardial infarction (a new Q-wave in the electrocardiogram or a peak level of CK-MB > 150 µmol/L), pulmonary embolism, and severe cardiac failure or severe ventricular arrhythmia requiring intensive care unit (ICU) or coronary care unit (CCU) stay in the primary or secondary referral hospital. Atrial fibrillation was recorded as a minor complication.
Assessment of Health-Related Quality of Life and Functional Capacity
All assessments were made preoperatively and repeated 12 months later. The baseline self-report questionnaire was given to the patients the day before surgery. The follow-up questionnaire including the same measures was mailed to the participants one year after the bypass operation. Seventeen (3.3%) had died during this postoperative period. Four hundred and sixty-five (94.7%) of the 491 surviving patients returned the follow-up questionnaire, mean time of follow-up being 12.6 (standard deviation 1.2) months. Compared with the 465 patients who completed the form, those 26 patients who did not were younger (median age, 54 vs 63 years, p = 0.006). However, there were no significant differences in the majority of variables, including sex, Euroscore risk sum, priority of operation, or in the New York Heart Association (NYHA) class.
We used the Finnish adaptation of the RAND-36 generic health-related QOL scale, for which there are reference values available for the Finnish population [11]. The RAND-36 is a widely used and validated scale, which yields scores for eight dimensions of health-related QOL: (1) general health, (2) physical functioning, (3) role limitations due to physical problems, (4) bodily pain, (5) emotional well-being, (6) role limitations due to emotional problems, (7) social functioning, and (8) energy [12, 13]. The scores for each domain range from 0 to 100, 0 being the poorest and 100 the best possible health status. To reduce the number of outcome variables two summary scores can also be used: the Physical Component Summary (PCS) equals the mean value of the physical subscales [1–4] while the Mental Component Summary (MCS) equals the mean value of subscales [5–8] reflecting psychosocial functioning [14–16]. Preoperative and postoperative functional capacity was ranked according to the NYHA classification.
Exclusion Criteria
Patients unable or unwilling to complete the baseline survey were excluded from the study. The total number of patients undergoing CABG in our institution during the study period (May 1999 to November 2000) was 1,128. Of these, 508 (45.0%) completed a baseline survey. Compared with the 508 included patients, exluded ones proved to be older (median age, 68 vs 63 years, p < 0.001), less often men (64.8% vs 82.7%, p < 0.001), and had a higher Euroscore risk sum (median, 4 vs 2, p < 0.001); they were more often operated urgently (47.1% vs 21.3%, p < 0.001), and more often had three-vessel disease (68.0% vs 60.4%, p = 0.029).
Statistical Analysis
Patient and outcome variables are expressed mostly as a percentage of the total. Categorical variables between the diabetic and nondiabetic groups were compared using Pearson's 
2 test. Continuous variables were compared by the independent samples t test for variables with normal distributions and the Mann-Whitney test for variables with nonnormal distributions. Baseline and follow-up variables were compared using paired-samples t test and intergroup differences were analyzed by independent samples t tests. The p values of 0.05 or less were considered statistically significant. Putative predictors of decreased QOL scores were taken for multivariate regression analysis to investigate their independency as predictors. The power analysis revealed our data to have 70% power (at 5% level of significance) to detect the improvement in RAND-36 QOL scores between the diabetics and nondiabetics. Statistical analyses were performed using SPSS 9.0 for Windows (SPSS Inc, Chicago, IL).
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Results
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Preoperative Data
The diabetic group was characterized by slightly higher Euroscore risk score mean, and more patients with triple-vessel disease, low ejection fraction, raised serum creatinine concentration, and extracardiac arteriopathy. Diabetics and nondiabetics were closely similar in terms of other preoperative characteristics. Details are given in Table 1.
Intraoperative Data
Most of the procedures were performed with CPB in both groups. The diabetic group was characterized by a higher number of distal anastomoses, and longer aortic cross-clamp and CPB times. Details are given in Table 2. When compared to the initial preoperative coronary angiogram, main target vessel revascularization was achieved in 94.6% of diabetic and 94.5% of nondiabetic patients.
Early Postoperative Outcome
Univariate analysis revealed diabetes as a significant (p = 0.003) risk factor for superficial sternal and limb wound infections (odds ratio [OR] 2.74 with 95% confidence interval [CI] 1.39 to 5.41). The diabetic group had slightly longer mean ICU and hospital stays. In other adverse events the diabetics and nondiabetics were closely equal. Overall results are given in Table 3. A great majority of diabetic (83.8%) and nondiabetic patients (86.9%) were discharged without any major complication (p = 0.474).
Mortality
Thirty-day mortality was 1.7 times higher although not significantly associated with diabetes; two (2.7%) deaths occurred in the diabetic group (one NIDDM patient and one IDDM patient) and seven (1.6%) in the nondiabetic group (p = 0.511). During the later follow-up period up to 12 months, two additional deaths occurred in the diabetic (2.7%) group (both were IDDM patients) as against six (1.4%) in the nondiabetic group. Thus, the 1-year survival rates were 94.6% for diabetic and 97.0% for nondiabetic (p = 0.287) groups, respectively. All deaths in the diabetic group were directly attributable to cardiac-related factors, whereas in the nondiabetic group only three out of seven (42.9%) deaths at thirty-day and three out of six (50%) deaths at one year were for cardiac reasons. Diabetics were an average of four years older than nondiabetics at death.
Subsequent Outcome
Eighty percent and 84.9% of the diabetic and nondiabetic patients (p = 0.621), respectively, had improved by at least 1 NYHA functional class one year after the operation, 13.8% and 9.5% of the patients (p = 0.280) in these groups, however, being in NYHA functional class III or IV. During the first postoperative year, 8.1% of the diabetic and 12.2% of the nondiabetic (p = 0.321) patients were readmitted to a secondary referral or primary hospital for cardiac-related reasons (such as recurrent chest pain, myocardial infarction, arrhythmia, dyspnea, and congestive heart failure). A new coronary angiography was performed in 2 diabetic (2.7%) and 17 nondiabetic patients (3.9%), leading to coronary angioplasty in 4 (0.9%) and to a redo CABG in 3 (0.7%) nondiabetic patients.
Changes in Health-Related Quality of Life
Both diabetic and nondiabetic patients evidenced depressed preoperative health status in all eight dimensions of the RAND-36 as compared to the general Finnish population (Fig 1). In general, the baseline scores between the study groups were closely similar being, however, in the diabetic group slightly lower in six and moderately lower in two (general health and physical functioning) RAND-36 dimensions. All health scores improved significantly (p < 0.001) among the patients without diabetes (Fig 1). In diabetics, significant (p < 0.05) changes were seen in all but one RAND-36 dimension (emotional well-being). Group differences between the diabetics and nondiabetics were statistically nonsignificant in all but one dimension (role limitation due to physical problem, mean changes in diabetics vs nondiabetics being +19.0 vs +36.0, p = 0.005) indicating similar improvement for diabetics and nondiabetics in seven out of eight RAND-36 dimensions.
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Fig 1. Preoperative (grey bars) and follow-up (black bars) Rand-36 QOL-scores for diabetics and nondiabetics. Broken line indicates reference values for the general population. Dimensions are as follows: E = energy; EW = emotional well-being; GH = general health; P = pain; PF = physical functioning; RLE = role limitation due to emotional problems; RLP = role limitation due to physical problems; SF = social functioning.
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A highly significant (p < 0.001) improvement was seen in the RAND-36 MCS and PCS summary scores in both patient groups (Fig 2.). Diabetic patients experienced slightly less benefit than nondiabetics, but these differences were not statistically significant (p values were 0.306 for the MCS and 0.086 for the PCS scores). In all, 67.7% of the diabetics and 74.5% of those patients without diabetes (p = 0.262) had a positive change in their MCS scores, while the corresponding figures for PCS scores were 84.6% and 86.0% (p = 0.772), respectively. On the other hand, 14% of the patients, in fact, showed a small reduction in their RAND-36 PCS scores at follow-up. When altogether 11 preoperative putative predictors of an adverse outcome (diabetes, higher age, female gender, left ventricular ejection fraction under fifty percent, Euroscore risk scores at least six, extracardiac arteriopathy, redo surgery, multivessel disease, unstable angina, high creatinine level [at least 141 µmol/L], and the use of CPB) were taken for multivariate regression analysis, only higher age proved to be an independent predictor (OR 2.54, 95% CI 1.22 to 5.30, p = 0.013) of this adverse QOL change.
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Comment
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An increasing proportion of patients undergoing CABG are diabetic [17]. However, the proportions of CABG patients that are diabetic have ranged from 11.8% to 27.7% within the European community [18]. In the United States, patients with diabetes represent roughly 25% of the nearly 1.5 million surgical and percutaneous coronary revascularization procedures performed annually [19]. In our institution the rate has varied between 12% and 15% annually. It is generally known that diabetic individuals have a higher mortality and morbidity in association with CABG, but less is known about the long-term subjective well being of these patients; especially, there are limited data on the impact of CABG on the QOL in diabetics. Some incident reports show that QOL gain is at maximum at 1 year after the operation and that diabetic patients may gain less QOL from the operation [20, 21]. This motivated us in finding out the subsequent postoperative outcome of our diabetic population, and especially the QOL after the operation.
Our study was limited to a strict time interval and included all individuals entering CABG in a geographically well-defined area where our institution was the only provider of cardiac operations and percutaneous coronary angioplasties (PTCAs). Although all patients were evaluated in terms of medical conditions, for practical reasons not all were able to fill in the RAND-36 score data and therefore we were unable to recruit all of the patients who were planned for surgery. The proportion of recruited was close to half (45%) of all cases in our institution, which alone provides cardiac surgery in the area. Acutely ill, urgently operated, and those with high age were in many instances unable to fill in the comprehensive baseline self-report RAND-36 score data. Although we could partly control the potential effects of selection in the total population by comparing the baseline data between the studied individuals to those in whom QOL data were not available, the selection bias due to this low recruitment rate is a factor which may limit the generalization of our results. The follow-up of the study cohort, on the other hand, was complete in 95% of the alive patients, and except for younger age, no other differences were found between the nonresponders and responders in preoperative clinical characteristics. Therefore, the conclusions well represent our cohort and nonresponse bias is not likely to affect the comparisons much. Moreover, all the QOL-related assessments were made preoperatively and repeated 12 months later, and our data also included events in the secondary referral hospital, which is important, since only 2.4% of the patients were discharged directly home.
Most previous studies have found diabetes to be associated with an increased early postoperative mortality [22–24]. In our study the differences in thirty-day and 1-year mortalities between the diabetics and nondiabetics did not reach statistical significance. However, in agreement with previous studies we found an unfavorable effect of diabetes on outcome, with a 1.7-fold increased risk of thirty-day mortality and a 1.8-fold risk of 1-year mortality compared with nondiabetic patients. As with other reports [25, 26], we identified diabetes as a significant risk factor causing superficial wound infections. Otherwise, the diabetics and nondiabetics were closely equal in terms of postoperative complications, although the diabetics in our study cohort were at higher predicted risk (mean Euroscore, 3.4 in diabetics vs 2.6 in nondiabetics) and had more often three-vessel involvement requiring longer operation with more grafts. Indeed, despite the more widespread coronary plaques,the perioperative myocardial infarction rate was lower in diabetics. Potentially the higher number of bypass grafts could explain a part of the lower rate; the methods of diagnostics were the same for both groups. The diabetic population also used, slightly longer, the ICU services but this difference could have been caused by a lower average ejection fraction and related need of contractile support in diabetics. In all, it is possible that the lack of statistically significant differences in mortality and morbidity between the diabetics and nondiabetics is partly due to the relatively small number of diabetics in the study cohort.
The prevalence of hospital readmissions in the postdischarge period after cardiac operation has not been extensively studied, possibly because readmissions are difficult to track as they take place in other hospitals. However, rehospitalization rates after surgical procedures can be used as an indicator of quality of care, since they affect QOL, and they also have financial implications. In our study, readmissions for cardiac-related reasons during the first postoperative year were even slightly more common among the nondiabetic patients as compared with those with diabetes.
Even though the diabetic individuals had wider spread coronary artery disease, they experienced closely similar freedom from anginal symptoms at one year. This was at the same time when more rerevascularization procedures (CABG or PTCA) were performed in nondiabetics. Diabetic patients are known to suffer from silent myocardial ischemia, and the reexamination threshold might have been higher in them.
A variety of health measurement tools have been developed over the last decade for the purpose of quantifying and differentiating between different health states. We chose the Finnish version of the RAND-36 Health Survey questionnaire, because it has been carefully translated and yields population-based reference values derived from the representative samples of Finnish population [11]. The generic health-related QOL measure has also previously been used to evaluate health status in general population surveys, to determine the effectiveness of medical treatments in patients with angina [27], and also more recently to evaluate the impact of mitral valve [28] and CABG surgery [29] on patients' QOL.
Importantly, although slightly lower in the diabetics, the RAND-36 baseline scores were still well comparable between the diabetics and nondiabetics. Moreover, the mean ages did not differ between the groups, thus simplifying the analysis because we have earlier shown higher age to predict less marked improvement in health-related QOL after CABG [29]. The overall QOL benefit of the operation was considerable in both groups. This could be demonstrated in significantly increased postoperative scores in all eight QOL dimensions in the nondiabetics and in seven out of eight dimensions among the diabetics. A significant improvement was seen also in the RAND-36 MCS and PCS scores in both groups, although a slight and nonsignificant tendency toward less marked improvement among the diabetics was seen. Furthermore, multivariate logistic regression analysis revealed higher age (but not diabetes) to be an independent predictor of lower score at follow-up.
Interestingly, the results of the operation had only a limited effect in the scores related to emotional well being in both groups. The average observed emotional well-being scores in the study groups were very close to the values derived from a (healthy) normal population and, therefore, things related to the social security and trust in the medical services may have stabilized the scores to near normal. We could not find any other conclusive explanation to this observation.
To conclude, our results suggest that diabetic patients differ from nondiabetic patients having slightly inferior QOL both before and 1 year after CABG, but their QOL improves after surgery similarly to nondiabetics. Diabetic patients can undergo CABG with an acceptable 1-year morbidity and mortality risk which is only marginally different from nondiabetic patients.
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Abstract
Introduction
= MCS, nondiabetics; • = MCS, diabetics; 
= PCS, nondiabetics; 
= PCS, diabetics.