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Eur J Cardiothorac Surg 2002;22:787-793
© 2002 Elsevier Science NL
a Department of Cardiothoracic Surgery, The Cardiothoracic Centre, Liverpool, L14 3PE, UK
b Department of Research and Development, The Cardiothoracic Centre, Liverpool, L14 3PE, UK
Received 9 June 2002; received in revised form 12 July 2002; accepted 24 July 2002.
* Corresponding author. Tel.: +44-151-293-2412; fax: +44-151-220-8573
e-mail: arashid{at}ccl-tr.nwest.nhs.uk
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Abstract |
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 Top Abstract 1. Introduction2. Methods3. Results4. DiscussionReferences |
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35). Associations between obesity and in-hospital outcomes were assessed by use of logistic regression to adjust for differences in patient characteristics. Results: A total of 3429 patients were defined as non-obese, compared to 1041 obese and 243 severely obese. There was no association between obesity and in-hospital mortality, stroke, myocardial infarction, re-exploration for bleeding and renal failure. Obesity was significantly associated with atrial arrhythmia (adjusted odds ratio (OR) 1.19, P=0.037 for the obese; adjusted OR 1.52, P=0.008 for the severely obese) and sternal wound infections (adjusted OR 1.82, P=0.002 for the obese; adjusted OR 2.10, P=0.038 for the severely obese). The severely obese patients were 4.17 (P<0.001) times more likely to develop harvest site infections. Severely obese patients were also more likely to have prolonged mechanical ventilation and post-operative stays, compared to non-obese patients. Conclusions: Obese patients are not associated with an increased risk of in-hospital mortality following coronary artery bypass surgery. In contrast, there is a significant increased risk of morbidities and post-operative length of stay in obese patients compared to non-obese patients.
Key Words: Obesity • Coronary artery bypass surgery • Mortality • Morbidity • Risk adjustment
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1. Introduction |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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In recent years, there has been data published from several institutions suggesting that the risks of undergoing CABG in obese patients may be no different than in the non-obese, especially with regards to mortality, although an increased risk of wound infections and atrial fibrillation have been highlighted [3–11]. However, the relationship of obesity and the risk of adverse outcomes is still unclear due to the small number of patients in several of these studies, for example Brandt and colleagues [7–11].
We therefore aimed to study the effect of obesity on in-hospital morbidity and mortality in patients undergoing isolated CABG at our own institution.
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2. Methods |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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Pre-operative data were collected during the patient's admission as part of routine clinical practice on the following variables: age, sex, BMI, urgency of operation, prior cardiac surgery, New York Heart Association (NYHA) functional class, Canadian Cardiovascular Society (CCS) angina class, history of myocardial infarction, smoking, diabetes, hypercholesterolaemia, hypertension, peripheral vascular disease, cerebrovascular disease, respiratory disease, renal dysfunction, intravenous nitroglycerin therapy, cardiogenic shock, and intra-aortic balloon pump support as well as the extent of coronary disease, and left ventricular ejection fraction. Data on the use of cardiopulmonary bypass (CPB) and the number and type of grafts were also collected.
The outcome measures for this study were in-hospital mortality, stroke, myocardial infarction, atrial arrhythmia, re-exploration for bleeding, renal failure, sternal wound infection, saphenous vein harvest site infection, duration of mechanical ventilation and post-operative length of stay.
In-hospital mortality was defined as death within the same hospital admission regardless of cause. All patients transferred from the base hospital to another hospital were followed up to confirm their status at discharge. Post-operative stroke was defined as a new focal neurological deficit and/or comatose states occurring post-operatively that persisted for >24 h after its onset. We excluded confused states, transient events and intellectual impairment from our study to avoid any subjective bias. Post-operative myocardial infarction was defined as a new Q-wave post-operatively in two or more contiguous leads on an electrocardiogram or a significant rise in post-operative cardiac enzymes combined with haemodynamic and echocardiographic signs of myocardial infarction. Post-operative atrial arrhythmia (atrial fibrillation or flutter) was defined as the occurrence of new atrial arrhythmia in the absence of pre-operative persistent or paroxysmal atrial arrhythmias. Post-operative bleeding was defined as bleeding that required surgical re-exploration after initial departure from the operating theatre. Renal failure was defined as patients with a post-operative creatinine level greater than 200 mol/l or patients requiring dialysis. Criteria for defining sternal wound infections and saphenous vein harvest site infections were in accordance with the published evidence-based guidelines by the Centres for Disease Control and Prevention [13].
The patient's BMI assessed the extent of obesity in our study. BMI, derived from Quetelet's formula, is calculated by the weight in kilograms divided by the square of the height in meters [14,15]. The National Heart Lung and Blood Institute has divided obesity into three classes on the basis of BMI: Obesity Class 1, BMI 30–34.9 kg/m2; Obesity Class 2, BMI 35–39.9 kg/m2; and Obesity Class 3, BMI 40.0 kg/m2 [16]. We divided the patients for our study into three BMI categories. We classified anyone with a BMI <30 kg/m2 as non-obese, BMI 30–34.9 kg/m2 (Obesity Class 1 from the above classification) as obese, and BMI 35 kg/m2 (Obesity Classes 2 and 3 from the above classification) as severely obese.
Different techniques of coronary revascularization were used in our study. These included operations done with and without CPB. The technique of myocardial protection for patients done on CPB again varied according to operator preference. Although blood cardioplegia was the favoured choice, cold crystalloid cardioplegia and intermittent cross-clamp and fibrillation were also used by some surgeons. Mammary arteries were harvested as a pedicle.
2.2. Statistical methods
Continuous variables are shown as median with 25th and 75th centiles and categorical variables are shown as a percentage with 95% confidence intervals (CI). Comparisons were made with Kruskal–Wallis tests and 
2-tests as appropriate. Standard statistical tests were used to calculate odds ratios (OR) and 95% CI. Logistic regression was used to examine the effect of BMI category on in-hospital mortality and morbidity, while adjusting for differences in patient characteristics [17]. These characteristics were the variables (age, sex, previous cardiac surgery, left ventricular ejection fraction, left main stem stenosis, number of major coronary arteries with stenosis >70%, priority of surgery, peripheral vascular disease, diabetes, renal dysfunction, and respiratory disease) suggested for risk adjustment of in-hospital CABG outcomes by Jones et al. [18] and the American College of Cardiology/American Heart Association practice guidelines [1]. This method was preferred over identification of those variables driving the outcome of interest and using these so-called risk factors for adjustment as a consequence of the low event rate and concerns regarding model over-specification. In all cases a P value of <0.05 was considered significant. All statistical analysis was performed retrospectively with SAS for Windows Version 8.
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3. Results |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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Table 1 lists patient and disease characteristics based on obesity category. There were no differences between patients characteristics with respect to severity of angina and dyspnoea, previous myocardial infarctions, current smokers, peripheral vascular disease, cerebrovascular disease, renal dysfunction, respiratory disease, left ventricular ejection fraction, number of diseased coronary vessels, intravenous nitroglycerin therapy, cardiogenic shock, intra-aortic balloon pumps, prior cardiac surgery, and emergency surgery. Obese patients were however younger (P<0.001) and less likely to have left main stem stenosis greater than 50% (P=0.018). Additionally, obese patients were more likely to be female (P<0.001), diabetic (P<0.001), hypertensive (P<0.001), and hypercholesterolaemic (P=0.004).
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The crude and adjusted OR for in-hospital outcomes following CABG are shown in Table 3. There was no association between obesity and in-hospital mortality, re-exploration for bleeding, peri-operative myocardial infarction, post-operative stroke and renal failure in either univariate or multivariate analyses.
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Although obese patients were not associated with harvest site infections, severely obese patients were significantly associated in both univariate and multivariate analyses. The severely obese patients were 4.17 (P<0.001) times more likely to develop harvest site infections compared to non-obese patients.
With respect to the duration of mechanical ventilation, obese patients were not associated with the need for ventilation greater than 48 h. However, in both univariate and multivariate analyses, severely obese patients were 2.74 times more likely than non-obese patients to require mechanical ventilation over 48 h (P<0.001).
Obesity was not associated with a post-operative length of stay greater than 14 days in the univariate analysis. However, after adjusting for differences in patient case-mix, severely obese patients were 1.74 (P=0.009) times more likely to stay for more than 14 days following CABG.
The adjusted rates for in-hospital outcomes following CABG are shown in Figs. 1 and 2 . After adjustment, the incidence of post-operative atrial arrhythmia following CABG surgery was 23.4% (95% CI 22.0–24.8) in the non-obese patients, 26.6% (95% CI 23.9–29.5) in the obese patients, and 31.3% (95% CI 24.9–38.6) in the severely obese patients (P=0.001).
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4. Discussion |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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This study has shown that obesity is not a risk factor for in-hospital mortality, re-exploration for bleeding, post-operative cerebrovascular accidents and renal failure. This is in concurrence with previously published studies [3,5–8,11]. Interestingly, Birkmeyer and colleagues in a study involving 11,101 patients undergoing CABG, showed a protective effect of obesity on post-operative bleeding. However, they were unable to explain the reasons for this [3].
We also could not find an association between obesity and perioperative myocardial infarction, in contrast to the studies by Fasol [8] and Prasad [9]. Both of these studies found that obese patients were more likely to have a myocardial infarction during the post-operative course [8,9].
We identified a significant increase in the duration of post-operative mechanical ventilation for the severely obese patients, who were 2.74 times more likely to be ventilated for more than 48 h. This could be due to impaired respiratory function as a result of relatively decreased vital capacity and a prolonged depression of respiratory drive due to slow release of anaesthetic agents stored in fatty tissues into the blood stream [19]. Schwann et al. [5], in a study of 3560 patients, have demonstrated an increase in the duration of mechanical ventilation in patients with higher BMI. Prasad et al. [9], in their study of 250 obese patients compared with 250 age- and sex-matched controls, have mentioned increased respiratory complications in the obese patients, while Rannuci et al. [10] have shown an increase in minor lung complications. Koshal et al. [11], in a small study involving 200 patients, have described increased use of bronchodilators. None of the studies by Prasad, Rannuci, and Koshal have examined the duration of mechanical ventilation [9–11].
This is in contrast to the findings by Brandt et al. [7], who in their study of 500 consecutive patients undergoing CABG, showed no difference in ventilation times. The study by Brandt and colleagues also found no significant association between obesity and mortality and other major adverse events. However, the findings of Brandt's study might be affected by the relatively small number of patients, with only 100 being classified as obese (BMI >30 kg/m2), and no risk adjustment for differences in case-mix [7].
There was a significant increase in the incidence of post-operative atrial arrhythmia in both the obese categories in our study, the association being more prominent in the severely obese group. This finding is similar to those of Moulton et al. in their study involving 2299 patients [6]. The studies by Fasol [8] and Prasad [9] also showed obesity as a risk factor for atrial arrhythmia.
As with other reports [3–6,20,21], obese patients were more likely to develop sternal wound infections in our study, and this association again was more prominent with the severely obese group. This finding concurs with the observations by Birkmeyer et al. [3], which showed that the risks of sternal wound infection were substantially increased in the obese and severely obese. Moulton et al. [6] have differentiated between superficial and deep sternal wound infections, and have shown an association between obesity and superficial sternal wound infections only. There was no increase in the incidence of deep sternal wound infections in obese patients in their study of 2299 patients undergoing procedures requiring CPB. However, Ridderstolpe and colleagues found that patients with a BMI >30 kg/m2 had a significantly increased risk of developing both superficial and deep sternal wound infections [20]. We did not differentiate between superficial and deep sternal wound infections in our own study because of the low event rate (only 32 deep sternal wound infections), which would not have been sufficient for an accurate risk adjustment [22].
The incidence of conduit harvest site infections was seen to be significantly increased only in the severely obese group, being more than four times as likely compared to the non-obese patients. The obese patients did not have a higher likelihood of developing conduit harvest site infections. This finding is similar to the results of other studies [4,6,9,21]. Engelman and colleagues, in a study of 5168 patients undergoing cardiac surgery, showed that a BMI >30 kg/m2 was associated with increased saphenous vein harvest site infections [4]. Spelman et al. also found that increased obesity was an independent risk factor for the development of harvest site infections [21].
There was a significant increase in the duration of hospital stay in the severely obese group compared to the non-obese group. This could be a direct result of the increased incidence of wound infections, both sternal and harvest site, in this group of patients. All patients with wound infections would have stayed longer in hospital since it is the general policy in our hospital not to discharge the patients until the wounds show signs of clinical and bacteriological improvement and are manageable by home visits by nurses in the community.
Our study represents a relatively recent population of patients undergoing isolated coronary artery bypass surgery with a large sample size. However, there are several limitations, which need to be considered. The use of BMI as the measure of obesity has some limitations because of substantial variations in lean body mass among people of the same height [14]. However, BMI was chosen because it correlates least with height and more with percentage of body fat [14]. Although the risk adjustment may not have taken into account certain variables that may affect the outcomes of interest, for this to have a dramatic effect on our conclusions, the variables used in the risk adjustment would have to be uncorrelated with the variables not adjusted for. Schwann and colleagues [5] have shown an increase in complications for small patients with a BMI <24 kg/m2. In our experience there were no significant differences in complications between patients with a BMI <24 kg/m2 and patients with a BMI between 24 and 30 kg/m2. We therefore combined these patients so we could focus on the effects of obesity. This study included 611 (12.9%) patients who underwent off-pump CABG. However, each BMI group had the same proportion of off-pump cases (Table 1), and therefore, this factor is unlikely to affect our conclusions. Data on surgical site infections were collected on all patients prior to hospital discharge. Surgical site infections, by definition, could occur any time within a month of surgery. Although all patients were followed up between 4 and 6 weeks after discharge as a matter of routine, we unfortunately do not have any data on infections that might have manifested post-discharge and within a month of surgery. In this study we also did not take into account the long-term outcomes of obese patients. A further limitation of our study is the non-availability of data regarding post-operative respiratory complications, which have been shown by some studies to be increased in obese patients [9–11].
In summary, there was no association between obesity and in-hospital mortality, stroke, myocardial infarction, re-exploration for bleeding and renal failure in patients undergoing CABG. Obesity was significantly associated with atrial arrhythmia and sternal wound infections. The severely obese patients were more likely to develop harvest site infections and have prolonged mechanical ventilation and post-operative stays, compared to non-obese patients.
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Acknowledgments |
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References |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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