Article Text

Original article
Bilateral internal mammary artery grafts, mortality and morbidity: an analysis of 1 526 360 coronary bypass operations
1. Shinobu Itagaki1,
2. Paul Cavallaro2,
4. Joanna Chikwe1
1. 1Department of Cardiothoracic Surgery, The Mount Sinai Medical Center, New York, New York, USA
2. 2Department of Cardiothoracic Surgery, The Mount Sinai School of Medicine, New York, New York, USA
1. Correspondence to Dr Joanna Chikwe, Department of Cardiothoracic Surgery, Mount Sinai Medical Center, 1190 Fifth Avenue, New York, NY 10029, USA; Joanna.chikwe{at}mountsinai.org

## Abstract

Objectives The objective of this study was to investigate the impact of bilateral internal mammary artery (BIMA) on early outcomes after coronary artery bypass grafting.

Design Retrospective database analysis.

Setting US hospitals.

Patients 1 526 360 patients (mean age 65 years, 73% male) from the Nationwide Inpatient Sample from 2002–2008 who underwent isolated coronary artery bypass grafting with at least one internal mammary artery.

Interventions Single versus BIMA bypass grafting.

Main outcome measures Inhospital mortality, deep sternal wound infection (DSWI).

Results The rate of BIMA use was 3.9%. Use of BIMA was independently associated with slightly lower inhospital mortality (unadjusted rate 1.1% vs 1.7%, adjusted OR 0.86, 95% CI 0.79 to 0.93). The DSWI rate was 1.4%. The independent predictors of DSWI were female gender (OR 1.06), congestive heart failure (OR 6.22), chronic pulmonary disease (OR 1.57), obesity (OR 1.17), diabetes mellitus (OR 1.04; OR 1.51 with chronic complication) and chronic renal failure (OR 2.13; OR 2.63 with dialysis). The use of BIMA was not an independent predictor of DSWI (OR 1.03, 95% CI 0.96 to 1.10). BIMA was associated with higher incidence of DSWI in patients with chronic complications of diabetes mellitus (OR 1.90, 95% CI 1.51 to 2.41).

Conclusions BIMA grafting is associated with increased risk of DSWI only in patients with severe, chronic diabetes. The incremental morbidity and mortality of DSWI does not justify denial of BIMA in the majority of patients.

• CARDIAC SURGERY

## Introduction

The left internal mammary artery is the conduit of choice to bypass the left anterior descending coronary artery in coronary artery bypass graft surgery (CABG).1 The best conduit to bypass the circumflex or right coronary artery remains controversial. Since its first clinical application in the 1970s,2 observational data suggest that the right internal mammary artery may have superior patency and associated survival benefits compared with vein and radial artery grafts.3–14 In the 2011 American College of Cardiology/American Heart Association guidelines, bilateral internal mammary arteries (BIMA) use was a Class IIa recommendation.1 BIMA is used relatively infrequently in the USA (<5% of isolated coronary bypass operations),15–17 and one of the main reasons for this is evidence suggesting increased risk of deep sternal wound infection (DSWI) with BIMA use, particularly in high-risk patients with diabetes mellitus18–20 but data is conflicting.6 ,7 ,18–22 The present study was therefore designed to investigate the effect of BIMA grafting on the rate of DSWI after CABG using national outcome data from the Nationwide Inpatient Sample (NIS).

## Methods

### Data source and population

The NIS was used to identify patients who underwent isolated CABG between 2002 and 2008. The NIS is the largest all-payer inpatient care database and contains discharge data from over eight million hospitalisations per year, which is approximately 20% of all hospitalisations in the USA. International Classification of Disease, the ninth edition (ICD-9) diagnosis codes were used to identify all patients in NIS who underwent CABG (36.10, 36.11, 36.12, 36.13, 36.14, 36.15, 36.16). Patients who underwent concomitant valve procedures (35.20, 35.21, 35.22, 35.23, 35.24, 35.25, 35.26, 35.27, 35.28, 35.11, 35.12, 35.13, 35.14) were excluded. Patients were stratified into two groups based on whether only single internal mammary artery (SIMA) was used or BIMA was used, using ICD-9 procedure codes of SIMA use (36.15) and BIMA use (36.16). Patients who did not receive SIMA or BIMA were excluded. A total of 1 526 360 patients were included in the analysis. Patient demographics, comorbidities and provider characteristics are listed in table 1. High-risk subgroups of diabetes mellitus (diabetes mellitus with chronic complications) and renal failure (end-stage renal failure requiring dialysis) were also identified. The ICD-9 diagnosis codes used to code for these comorbidities, based on Agency for Healthcare Research and Quality comorbid disease categories23 are provided in online supplementary table S1.

Table 1

Patient and provider characteristics

To include the effect of surgical volume on outcomes, annual surgeon CABG volume was also obtained. Eleven states specifically reported unique physician identifiers for the study time period: Florida, Iowa, Kentucky, Maryland, New Hampshire, Nevada, New York, Pennsylvania, Texas, Virginia and West Virginia. Annual surgeon CABG volume was determined by calculating the total number of CABGs performed by an individual surgeon divided by the number of years that the surgeon was surveyed. Annual surgeon CABG volume was categorised into quartiles, such that a similar number of patients would fall into each quartile (very low <50 cases: 29.6%, low 50–75 cases: 20.8%, high 75–100 cases: 18.1% and very high >100 cases: 31.5%).

The study protocol was reviewed by the Mount Sinai School of Medicine Institutional Review Board and was compliant with a waiver of informed consent, and the Health insurance Portability and Accountability Act regulations.

### Clinical outcomes of interest

The primary endpoint was deep sternal infection, which was identified using ICD-9 diagnosis codes (998.59, 998.32, 998.31, 519.2).24 The other endpoints were inhospital mortality, length of hospital stay and total costs of admission.

### Statistical analysis

Continuous variables are expressed as means with SDs. Categorical variables are expressed as proportions. Differences between groups were detected using the χ2 test for categorical variables, Student's t test for normally distributed continuous variables and the Mann-Whitney U test for non-normally distributed continuous variables. Binary logistic regression analysis was performed to determine independent predictors of BIMA use and DSWI. Logistic regression for BIMA use was performed using variables related to patient demographics and hospital characteristics. The effect of BIMA use for the rate of DSWI was investigated using logistic regression accounting for age, gender, elective status and clinically relevant patient comorbidities. The same analysis was repeated in the subgroup analysis of each high-risk group which was found to be significant in the first logistic regression model with the whole study population. Annual surgeon CABG volume was also separately included into the model only with patients with this information available. Results are demonstrated as OR and 95% CI. All tests were two tailed. A p value of <0.05 was considered statistically significant. The statistical analysis was performed using IBM SPSS Statistics for Windows, V.20.0 (SPSS, IBM Corporation, Armonk, New York, USA).

## Results

### National trends in BIMA use

The overall BIMA use rate was 3.9% over the study period of 2002–2008, with a gradual increase from 3.9% in 2002 to 4.5% in 2008. The patient demographics, hospital characteristics and clinical comorbidities for the overall population and according to SIMA or BIMA use are listed in table 1. Generally, BIMA patients were younger (60.1 years vs 64.8 years, p<0.001), more likely to be male (81.8 vs 72.9%, p<0.001) and more predominantly Caucasian (83.6 vs 80.8%, p<0.001) with fewer comorbidities, and higher socioeconomic status. The strongest independent predictors of BIMA use were male gender, Caucasian race and higher household incomes (table 2). Hospitals in Northeast urban area with small bed size also tended to use BIMA more frequently. Higher annual surgeon CABG volume had no significant impact on the rate of the use of BIMA.

Table 2

Predictors of bilateral internal mammary artery use based on patient demographics, hospital characteristics and annual surgeon CABG volume

### Operative outcomes

Operative outcomes are shown in table 3. In logistic regression analysis after adjusting for age, gender, elective status and comorbidities, the use of BIMA was independently associated with slightly lower inhospital mortality (unadjusted rate 1.1% vs 1.7%, adjusted OR 0.86, 95% CI 0.79 to 0.93, p<0.001). The overall rate of DSWI was 1.4%: 1.4% in SIMA patients and 1.3% in BIMA patients (p<0.001).

Table 3

Operative outcomes

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