Methods: Between August 2007 and March 2011, a total of 1,673 consecutive patients (1,209 males, 464 females; mean age 58.6±9.9 years; range 21 to 87 years) who underwent isolated CABG at Shariati Hospital, Tehran, Iran were retrospectively analyzed. The patients were classified into four groups based on the BMI values: underweight (BMI ≤19 kg/m2, n=55), normal/overweight (BMI 20-29 kg/m2, n=523), obese (BMI 30-39 kg/m2, n=739) and morbidly obese (BMI ≥40 kg/m2, n=356). Preoperative characteristics were compared among the four groups. Effects of BMI on postoperative mortality and morbidity were evaluated using the stepwise multivariate logistic regression analysis.
Results: There was no significant difference in the mortality rate among the groups. Intra-aortic balloon pump (IABP) was the only independent predictor of postoperative mortality in the multivariate logistic regression analysis (OR=20.5, 95% CI=8.6-50.5; p<0.001). Body mass index was independently associated with postoperative pulmonary complications (OR=1.6, 95% CI=1.2-2.1; p=0.001).
Conclusion: Our study results suggest that BMI is an independent predictor of post-CABG pulmonary complications, but not of post-CABG mortality.
Obesity has emerged as an endemic concern in developing countries like Iran. Simultaneously the proportion of CAD patients who ultimately need CABG has increased. The outcomes of CABG are affected by multiple variables such as the personal characteristics of the patients and operative data, but having the ability to determine the postoperative prognosis of patients who undergo CABG would be beneficial. In this study, we conducted multivariate logistic regression analyses using an extensive database containing more than 200 different variables to investigate the effects of body mass index (BMI) on early surgical outcomes after CABG.
The surgical team was composed of four cardiac surgeons, all of whom used a similar technique. Blood cardioplegia was utilized for on-pump CABG and for single or two-vessel (2VD) CAD; however, off-pump CABG was preferred if the patient’s anatomy was suitable. The four groups were compared with regard to various preoperative and postoperative characteristics such as age, gender, DM, HT, hypertriglyceridemia, hypercholesterolemia, a family history of CAD, cerebrovascular accidents (CVAs), renal function, intraoperative situations, postoperative mortality, and morbidities. Furthermore, the study protocol was approved by the ethics committee of the Tehran University of Medical Sciences (TUMS).
The preoperative data included age, gender, DM, HT, hypertriglyceridemia, hypercholesterolemia, a family history of CAD, cerebrovascular disease (CVD), and renal dysfunction. The primary outcome was defined as an early mortality rate in the first 30 days after surgery, whereas the secondary outcomes were morbidities that occurred during hospitalization.
We used the Cockcroft-Gault equation to estimate creatinine clearance.[15] Opium addiction was defined as inhaling opium smoke and/or eating opium in its crude form at least three times a week. In addition, infectious complications were defined as a deep or superficial sternal infection, leg infection, septicemia, or a urinary tract infection. The patients were considered to have neurological complications if they had suffered a stroke in the first 72 hours, had a transient ischemic attack, or been in a coma. Furthermore, those with a pulmonary embolism, pneumonia, pleural effusion, or a pneumothorax were classified as having pulmonary complications. The patients with ventricular arrhythmia had either ventricular tachycardia, fibrillation, premature ventricular contractions (PVCs), or atrioventricular junctional rhythm (AVJR), whereas those with supraventricular arrhythmia had atrial fibrillation (AF), flutter, or paroxysmal atrial tachycardia (PAT).
Additionally, our study included the following intraoperative findings: average units of transferred packed red blood cells (PRBCs), fresh frozen plasma (FFP) and platelet (PLT) transfusion, perfusion time, aortic cross-clamp time (ACCT), mechanical ventilation time, inotropic support, the need for an intra-aortic balloon pump (IABP) and a proportion of those who underwent on-pump cardiopulmonary bypass (CPB).
Statistical analysis
All of the statistical analyses were performed
using the SPSS version 16.0 for Windows software program (SPSS Inc., Chicago, IL, USA). The results
were reported using mean ± standard deviation
(SD) for quantitative variables and percentages for
categorical variables, and the groups were compared
using a chi-square test (or Fisher’s exact test,
if needed) for categorical variables. A one-way
analysis of variance (ANOVA) was also conducted
to compare continuous variables between the four
groups. Statistical significance was based on the
evaluation of two-sided, design-based tests in which
0.05 represented the level of significance.
In the primary analyses, mortality and pulmonary complication rates differed significantly across the groups; thus, they were selected as dependent variables in order to compute a multivariate logistic regression model. First, the potential effects of the risk factors on surgical outcomes were tested using univariate analyses for postoperative mortality and pulmonary complications. When p<0.2, the variables were entered into the stepwise multivariable logistic regression analyses to construct the final model, and the body mass indices (BMIs) of the patients were also entered in this manner. Other covariates were female gender, age >65 years, DM, HT, smoking, hypertriglyceridemia, hypercholesterolemia, previous myocardial infarction (MI) >1, inotropic support, the use of an IABP, and off-pump CPB.
Table 1: Baseline characteristics of the patients
A comparison between the four groups with regard to operative data is shown in Table 2, and there were significant differences in the average RBC units that were transfused as well as perfusion time. In addition, the mortality rate varied between 1.4% in the morbidly obese patients to 5.5% in the underweight patients, but the differences were not statistically significant between the four groups (p=0.229). We also compared the postoperative complications, and the results are given in Table 3. The between-group differences with respect to post-CABG pulmonary complications were statistically significant (p=0.001), but this was not the case for the surgical infection and neurological, renal, gastrointestinal, and vascular complications.
Table 2: Operative clinical characteristics of the patients
In the univariate analysis, we determined that postoperative mortality was significantly associated with female gender (OR=2.2; 95% CI=1.1-4.4; p=0.021), age >65 years (OR=2.5; 95% CI=1.3-5.2; p=0.007), HT (OR=2.6; 95% CI=1.2-5.6; p=0.011), IABP (OR=19.1; 95% CI=9.2-39.5; p<0.001), and offpump CPB (OR=0.36; 95% CI=0.14-0.93; p=0.028). To offset the confounding variables, the effects of BMI on early surgical outcomes were measured using a multivariable logistic regression in the presence of other covariates, and we found that IABP continued to be an independent predictor of postoperative mortality (OR=20.5; 95% CI=8.6-50.5; p<0.001).
A higher BMI was independently associated with postoperative pulmonary complications (OR=1.6; 95% CI=1.2-2.1; p=0.001). Furthermore, age >65 years (OR=1.6; 95% CI=1.04-2.5; p=0.031), inotropic support (OR=1.9; 95% CI=1.2-2.9; p=0.007), IABP (OR=6.1; 95% CI=3.6-10.4; p<0.001), and coronary distal anastomosis >2 (OR=1.6; 95% CI=1.03-2.5; p=0.034) were also identified as independent predictors of postoperative pulmonary complications in the multivariate analyses.
In both the univariate and multivariate analyses in our study, extreme BMIs were associated with postoperative pulmonary complications; thus, obesity seems to be an independent predictor for pulmonary complications after CABG. In addition, Jenkins and Moxham[29] also reported that obesity has adverse effects on pulmonary function through increased functional residual capacity, decreased vital capacity, and maximum voluntary ventilation; our findings are in consistence with previous reports.[11,13] Likewise, other authors have reported greater respiratory problems and subsequent longer mechanical ventilation times after CABG among underweight patients.[4,9] Based on our findings, it seems that the association between post- CABG pulmonary complications and preoperative BMI also follows a U-shaped pattern, with both very high and very low ranges. Additionally, the CPB times were longer in the underweight group in our study, but the pulmonary complications were higher in the underweight and morbidly obese patients. In the underweight group, the longer CPB time might have been responsible for the higher pulmonary complications, but in the morbidly obese group, obesity may have played a greater role.
Older age has been reported to be an independent factor of pulmonary dysfunction after CABG.[30] Some severe, restrictive pulmonary changes may occur after CABG, and, lung function seems to be more severely affected by CABG in elderly patients compared with those who are younger.[30] Therefore, older patients might be at a greater risk for post-CABG pulmonary complications than would be expected from their preoperative lung function.[31] As Goyal et al.[32] reported, venous grafting is a ssociated with less severe pulmonary complications than arterial grafting. Contrary to our expectations, our results showed that the increased number of venous grafts was correlated with a higher incidence of post-CABG pulmonary complications. The main underlying reason for this is still unclear; therefore, further studies are needed to evaluate this finding. However, the study results of Faritous et al.[33] a greed with our findings which determined that preoperative inotropic agents were predictive factors of pulmonary involvement after CABG. We also found that IABP was another independent factor that affected the post-CABG pulmonary complications. However, the reason for this is also not certain, so further research is needed on this topic as well.
Similar to our findings, Kuduvalli et al.[13] surveyed 4,713 patients who underwent isolated CABG and confirmed that obese patients were more likely to have prolonged mechanical ventilation. However, they found no association between BMI and mortality, stroke, MI, bleeding, or re-exploration, but they did find a significant association between obesity and sternal infection, which was inconsistent with our findings. Several studies have also reported that obese patients can develop sternal wound infections due to poor healing of the adipose chest wall tissue.[9,27,34] Furthermore, the operation time might also play a role in wound healing and subsequent infections.
It was interesting that 21.3% of our patients were classified as morbidly obese based on their BMI. As expected, one reason for this might be that many of the patients with CAD were treated medically, with a considerable number undergoing percutaneous coronary intervention (PCI). In contrast, the patients with a more comorbid disease and risk factors such as obesity were referred for surgery.
The retrospective nature of our study limits the interpretation of our findings; however we applied a multivariable logistic regression analysis to offset any confounding effects. It is possible that BMI might not be an appropriate tool to measure the distribution of body fat, which could have limited our outcomes. However, a systematic review by Coutinho et al.[35] showed that for CAD patients, including those with normal and high BMI, central obesity was related to mortality, which was not true for BMI.
Acknowledgements
The authors would like to thank the Farzan Clinical
Research Institute for their technical support.
Declaration of conflicting interests
The authors declared no conflicts of interest with
respect to the authorship and/or publication of this
article.
Funding
The authors received no financial support for the
research and/or authorship of this article.