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Interact CardioVasc Thorac Surg 2007;6:753-758. doi:10.1510/icvts.2007.158709 © 2007 European Association of Cardio-Thoracic Surgery
Diabetes is not a risk factor for hospital mortality following contemporary coronary artery bypass grafting
a Department of Cardiothoracic Surgery, Mount Sinai School of Medicine, 1190 Fifth Avenue, New York, NY 10029-1028, USA Received 1 May 2007; received in revised form 24 August 2007; accepted 27 August 2007
*Corresponding author. Tel.: +1 212 659 6820; fax: +1 212 659 6818.
The outcome of coronary artery bypass grafting (CABG) in diabetic patients has traditionally been worse than in non-diabetic patients. Recent studies have suggested an improvement in outcome in diabetic patients undergoing contemporary CABG. However, the direct impact of diabetes on mortality and morbidities following CABG remains unclear. We retrospectively analyzed prospectively collected data of 2725 CABG patients from January 1998 to December 2005: one thousand and eighty-five (40%) diabetics and 1640 (60%) non-diabetics [mean age 65±11 years, 1882 (69%) male]. Subgroup analysis was performed for two study periods (1998–2002 vs. 2003–2005). The overall hospital mortality was 1.8% [n=50; diabetics: 2.4%, non-diabetics: 1.5% (P=0.07)]. The mortality rate among diabetics decreased from 3.1% in 1998–2002 to 1.0% in 2003–2005 (P=0.021). Diabetes was not an independent predictor of hospital mortality but predicted the occurrence of deep sternal wound infection (OR=3.77). Diabetes significantly decreased long-term survival [1-year and 5-year survival 94.7±0.7% and 81.9±1.4% for diabetic vs. 95.4±0.5% and 85.9±1.0% for non-diabetic patients (P=0.01)]. Excellent results following contemporary CABG can be expected in diabetics with a similar mortality compared to non-diabetics. Therefore, our data suggest that diabetes may, in fact, not be a risk factor for adverse outcome following CABG. However, long-term survival in diabetics remains significantly inferior compared to non-diabetics.
Key Words: Coronary artery bypass grafting; Diabetes mellitus; Risk factors; Outcome
The outcome of diabetic patients following coronary revascularization has traditionally been worse compared to the non-diabetic population [1]. In a study reported by Thourani et al., mortality in diabetic patients was significantly higher compared to non-diabetic patients with a direct impact of diabetes on early postoperative outcome [2]. More recently, two studies have suggested an improved outcome in diabetic patients and have raised questions regarding the potential influence of this disease on early operative results [3, 4]. A variety of factors, such as advances in operative techniques and improved peri-operative management have contributed to better clinical outcomes. Specifically, since the recently described and generally accepted practice of metabolic control of inflammation and hypercatabolism has been implemented [5]. In this study, we sought to determine the outcome of CABG in diabetic patients in terms of early mortality, morbidities, and late survival in a contemporary series.
We retrospectively analyzed 2725 patients undergoing CABG between January 1998 and December 2005. The protocol was approved by our local institutional review board. Clinical variables were prospectively entered into the New York State Department of Health (NYSDH) data registry and used for this retrospective analysis. According to the NYSDH, ‘diabetes’ is defined by the need for oral medication or insulin. This definition does not include patients managed by nutritional therapy alone. Table 1 summarizes pre-, intra-, and postoperative variables and their definition. In addition, the logistic EuroSCORE was used for risk stratification [6].
Follow-up survival information was obtained by cross matching the patient's social security number with the web-based social security death index [http://ssdi.rootsweb.com]. Through a median sternotomy, 15% (n=409) of procedures were performed without the use of cardiopulmonary bypass (CPB). The remaining 85% (n=2316) procedures were performed utilizing CPB. Distal anastomoses were performed first. The proximal anastomoses were constructed using the single clamp technique [7] in on-pump patients and using a site-bite clamp in the off-pump group. Since January 2003, two important measurements were implemented in our practice. First, epi-aortic ultrasonography was performed systematically to detect any atherosclerotic lesions prior to manipulation or cannulation of the ascending aorta [8]. Second, we routinely used an intensive insulin therapy with a target glucose level of 80–110 mg/dl for both diabetic and non-diabetic patients. An intravenous insulin infusion was started intraoperatively and continued postoperatively. The intravenous insulin therapy was transitioned to subcutaneous insulin therapy, oral anti-diabetic therapy or discontinued altogether after 48 h. Normally distributed continuous variables are represented as mean±standard deviation (S.D.) or as the percentage of the sample. The 2-test and Fisher's exact test were used to determine differences in patient characteristics in univariate analysis. Stepwise multivariate logistic regression was then performed to assess the influence of diabetes as an independent risk factor for hospital mortality and postoperative morbidities. A P-value <0.05 was considered significant for all tests. Long-term survival was analyzed using Kaplan–Meier survival curves. Differences in patient characteristics were controlled by Cox proportional hazard analysis.
A total of 2725 adult patients were included in this study. Diabetes was present in 1085 (40%) patients. There were significant differences when comparing diabetic and non-diabetic patients (Table 2).
The characteristic of diabetic patients changed when comparing the two study periods. In 2003–2005, diabetics were more likely to present with hypertension (P=0.017), previous cerebrovascular accident (P=0.044), and have undergone previous percutaneous coronary intervention (PCI) (P=0.01) and less likely to present with previous myocardial infarction (P<0.001). Table 3 reports the operative details for diabetic and non-diabetic patients.
3.1. Mortality The overall hospital mortality was 1.8% (n=50). The mortality among diabetics was 2.4% compared to 1.5% in non-diabetics (P=0.07) (Table 4). The mortality rate among diabetics significantly decreased from 3.1% in 1998–2002 to 1.0% in 2003–2005 (P=0.021). Table 5 shows risk factors for hospital mortality in univariate and multivariate analysis. The presence of diabetes was not found to be an independent predictor of hospital mortality [odds ratio (OR) =1.1, 95% confidence interval (CI)=0.6–2.1, P=0.7].
3.2. Morbidities Major postoperative morbidities according to the study group are displayed in Table 4. Diabetic patients had a significantly higher rate of deep sternal infections (DSWI) and renal failure (Table 4). However, in multivariate logistic regression analysis, diabetes had only an independent influence on the development of DSWI (OR=3.8, CI=1.8–7.8, P<0.001) whereas the association with renal failure disappeared (OR=1.9, CI=0.8–4.9, P=0.16). Table 6 shows the influence of diabetes on mortality and postoperative morbidities after adjustment for potential confounders.
The postoperative length of stay was significantly different when diabetics were compared to non-diabetics (11±17 vs. 9±14 days, P=0.009). The mean follow-up time was 4.2±2.5 years and 98% complete. Long-term survival of diabetics was significantly decreased compared to non-diabetics. One-year and five-year survival rates were 94.7±0.7% and 81.9±1.4% for diabetic patients vs. 95.4±0.5% and 85.9±1.0% for non-diabetic patients (P=0.01). Fig. 1 shows unadjusted and adjusted Kaplan–Meier survival curves.
4.1. Incidence and demographic factors In previous studies, the reported prevalence of patients with diabetes referred for CABG has been in the range of 15–20% [1–4, 9]. Data analysis from the Society of Thoracic Surgeons National Adult Cardiac Database (STS Database) showed an increase in the prevalence of diabetes in patients referred for CABG (21% in 1990, 33% in 1999) [10]. In our contemporary series, we show a markedly higher number of diabetic patients referred for coronary revascularization with an overall prevalence of 40% (n=1085). In addition, we demonstrated a significant increase in diabetic patients from 38% (1998–2002) to 45% (2003–2005) (P<0.001). Despite the increase in diabetic patients, the demographic profile is similar to that reported by other investigators [3]. Accordingly, in our series, patients with diabetes were younger, more likely to be female, and presented with more atherosclerosis-related risk factors. The differences in the incidence of diabetes as reported by previous authors are partly related to varying inclusion criteria. Szabo et al. defined diabetes as the need for diet, oral medication or insulin and reported a rate of 19.4% [3]. We used the NYSDH data base, which includes diabetic patients requiring oral medication or insulin whereas diabetic patients managed with nutritional therapy alone were not included. Our study, therefore, confirmed the trend of an increasing prevalence of diabetes and its impact on the development of cardiovascular diseases requiring therapeutic intervention [11–13]. During the last decade, the broader application of PCI has modified the profile of patients referred for CABG. Because of the excellent results of PCI in non-diabetic patients, the number of these patients referred for coronary surgery has steadily decreased, causing a relative increase in the percentage of diabetics in the cohort of CABG patients. The second effect, as demonstrated by our study, is that a significant number of diabetic patients have undergone previous PCI before being referred for CABG. Interestingly, our study shows that the proportion of patients with previous PCI has increased when comparing the two study periods. This is probably due to the recent implementation of drug eluting stents (DES) in the armamentarium of interventional cardiologists. These new generation DES are currently under investigation by the National Institution of Health. Results from ongoing randomized trials comparing CABG and DES, such as the ‘Future Revascularization Evaluation in Patients With Diabetes Mellitus: Optimal Management of Multivessel Disease’ (FREEDOM) trial, are necessary to further determine the optimal treatment strategy in the diabetic patient population (www.clinicaltrials.gov/ct/show/NCT00086450). The negative impact of diabetes on the outcome of patients with coronary artery disease (CAD) is well established and related to its atherosclerotic, pro-inflammatory and pro-thrombotic effects [14]. Until recently, patients with diabetes and multi-vessel CAD undergoing revascularization were considered high-risk due to increased morbidity and mortality [15]. In the study reported by Thourani et al., mortality was significantly higher among the diabetic population (3.9%) compared to the non-diabetic population (1.6%) [2]. Similarly in the STS database, mortality was 3.7% in diabetic patients compared to 2.7% in non-diabetic patients [1]. More recent studies have challenged these findings by reporting more favorable results in diabetic patients undergoing CABG. Szabo et al. and Rajakaruna et al. reported a mortality rate of 2.6% and 2.2% in diabetic patients compared to 1.6% and 1%, respectively, in non-diabetics undergoing CABG [3, 4]. However, these two studies did not identify diabetes as an independent predictor of mortality in multivariate analysis. Similar to these previous studies, our clinical series confirmed the improvement of outcome in diabetic patients with a decrease from 3.1% in the period from 1998–2002 to a rate as low as 1.0% in the period from 2003–2005. To the best of our knowledge, this is the lowest reported mortality in diabetic patients undergoing CABG and approaches the outcome of non-diabetic patients.The remarkable decrease of mortality in diabetic patients is most likely multifactorial, owing mainly to improvements in peri-operative management of cardiac surgery patients. Our study was not designed to quantify the potential impact of each peri-operative measurement. However, we believe that our approach for a better peri-operative glucose control has contributed to improving surgical outcome. Previous studies have demonstrated the positive impact of intensive insulin therapy in diabetic patients undergoing CABG [5]. Furnary et al. have reported that maintaining glucose level in the range of 100–150 mg/dl decreased mortality in their study group (2.5% vs. 5.3%, P<0.001). These authors were able to show that this treatment strategy was an independent predictor of improved outcome following CABG [5]. Our favorable results further confirm the importance of systematic application of the current recommendation for tight glycemic control (BG in the 80–110 mg/dl range) in the early postoperative period in all patients undergoing CABG [5]. Previous studies have associated diabetes in patients undergoing CABG with several postoperative complications including infection, renal failure, cerebrovascular events and gastrointestinal complications [2–4]. Rajakaruna et al. and Szabo et al. reported an OR of 2.0 (CI=1.4–3.0, P<0.001) and 2.6 (CI=1.6–4.5, P<0.001) describing the association of diabetes with stroke after CABG [3, 4]. Both authors also identified diabetes as independent predictor of renal failure (OR=1.6 and 1.8, respectively). In our series, we observed an increased rate of DSWI and renal failure in patients with diabetes after CABG. However, multivariate analysis revealed an independent influence of diabetes only on the development of DSWI (OR=3.8). Long-term survival of diabetic patients following surgery has been described as lower than that of non-diabetic patients. Several previous studies have reported a 5-year survival in the range of 80–85% [2, 3]. Our data are in accordance with these results with a 1- and 5-year survival of 94.7±0.7% and 81.9±1.4% in diabetic and 95.4±0.5% and 85.9±1.0% in non-diabetic patients, respectively. The increased all-cause mortality among diabetic patients is predominantly related to the occurrence of new cardiac events that suggest the development of new arteriosclerotic lesions [2]. This circumstance justifies close follow-up by primary care physicians and cardiologists in order to detect worsening of cardiac function as well as other organ dysfunction. It underscores the importance of tertiary prevention to avoid the negative impact of concomitant risk factors for the progression of atherosclerotic disease. Our study has certain limitations. First, this is a retrospective observational study; therefore any conclusions should be limited in their implications. Second and perhaps more importantly, this study was not designed to correlate specific types of diabetes, the occurrence of stress hyperglycemia in non-diabetics, or parameters of diabetic management, like A1c levels, individual blood glucose (BG) level, or units of insulin used per day, with specific outcome events. Finally, the outcome analysis is limited to hospital mortality, morbidities and late survival. We were not able to determine any late complications or causes of death during follow-up.
Excellent results following CABG can be expected in diabetic patients with a similar mortality compared to non-diabetic patients. In addition, the rate of major postoperative morbidities was also not significantly increased with the exception of DSWI, which occurred with a higher rate in diabetic patients. Our data suggest that the implementation of insulin therapy with a goal BG of 80–110 mg/dl may have played an important role in the improvement of outcome among diabetic patients undergoing CABG in the recent period. We therefore conclude that diabetes should not be considered as a risk factor in the outcome of patients undergoing CABG. However, long-term survival in diabetic patients remains significantly poorer than that of non-diabetic patients, reflecting the burden of this disease. Thus, close follow-up of these patients, as well as tertiary prevention of additional risk factors, remains essential in diabetic patients to improve long-term survival following cardiac surgery.
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Abstract
1. Introduction
