Abstract
Background
Patients undergoing coronary artery bypass grafting (CABG) are at risk for developing a variety of infections. While investigators have focused on predictors of these adverse sequelae, less attention has been focused on characterizing hospital-level variability in these outcomes.
Methods
365,686 patients in the STS Adult Cardiac Surgery Database underwent isolated CABG across 1084 hospitals between July 2011 and December 2013. Hospital-acquired infections (HAIs) were defined as: pneumonia, sepsis/septicemia, deep sternal wound infection/mediastinitis, vein harvest/cannulation, or thoracotomy. Hospitals were ranked based on their HAI rate: Low (≤10th Percentile) vs. Medium (10th–90th Percentile) and High (>90th Percentile). Differences in peri-operative factors and composite morbidity/mortality endpoints across these groups were determined using the Wilcoxon rank-sum and chi-square tests.
Results
HAIs occurred among 3.97% of patients overall, but rates varied across hospital groups (Low:<0.84%, Medium:0.84–8.41%, High:>8.41%). Pneumonia (2.98%) was the most common HAI, followed by sepsis/septicemia (0.84%). Patients at high rate hospitals more often smoked, had diabetes, chronic lung disease, NYHA Class III-IV, and received blood products, (p<0.001); however, they less often were prescribed the appropriate antibiotics (p<0.001). Major morbidity/mortality occurred among 12.3% of patients, although varied by hospital group (low: 8.6%, medium: 12.3%, high: 17.9%, p<0.001).
Conclusions
Substantial hospital-level variation exists in postoperative HAIs among patients undergoing CABG, driven predominantly by pneumonia. Given the relatively small absolute differences in comorbidities across hospital groups, our findings suggest factors other than case mix may explain the observed variation in HAI rates.
Keywords: infection, surgery, cardiopulmonary bypass
INTRODUCTION
Hospital-acquired infections (HAIs), including sepsis, pneumonia, and sternal wound or harvest site infections, occur in up to 5% of patients undergoing coronary artery bypass grafting (CABG) surgery.1 Patients developing HAIs are at increased risk of subsequent morbidity, mortality and resource utilization.1–5 Fowler and colleagues reported a more than five-fold increased risk of mortality among patients developing major infections following cardiac surgery (17.3% vs. 3.0%).4 LaPar and associates found a greater incremental cost associated with either pneumonia ($50,025) or deep sternal wound infection ($56,003) after isolated CABG, even after adjusting for baseline pre-operative risk.2
Few articles have characterized hospital-level variability in HAI rates. Rogers and colleagues analyzed a cohort of Medicare beneficiaries who underwent CABG surgery between 2003 and 2006.6 Using administrative data, the authors reported wide variability in infection rates across hospitals, especially among women. More recently, Shih and colleagues identified significant variability in the observed rates of HAIs across hospitals participating in the Michigan Society of Thoracic and Cardiovascular Surgeons Quality Collaborative.1 The authors reported an overall HAI rate of 5.1%, which varied between 0.9% and 19.1% across hospitals. While this cohort consisted of 20,896 patients, only 33 hospitals were involved in their study. A more thorough characterization, including a nationally representative cohort, would improve our overall understanding of the epidemiology of HAIs in this setting.
In this descriptive study, we characterized the hospital-level variability in HAI rates across hospitals participating in the Society of Thoracic Surgeons Adult Cardiac Surgery Database (STS ACSD).
METHODS
This study was approved by the Duke University Health System Institutional Review Board, which declared it be research not involving human subjects.7
The STS Adult Cardiac Surgery Database
We included 365,686 patients undergoing isolated CABG surgery between July 2011 and December 2013 at one of 1,084 hospitals participating in the STS ACSD. The STS believes that current STS participating hospitals represent more than 90% of hospitals providing adult cardiac surgery in the US.
Study Variables
The primary outcome for this analysis was the post-operative development of a hopital-acquired infection (HAI), defined as presence of any one of the following: pneumonia, sepsis or septicemia, harvest or cannulation site infection, deep sternal wound infection, or thoracotomy/parasternal site infection.
Rates of overall HAIs were compared across hospitals, and 3 categories of HAI hospitals were defined based on percentile cutoffs: low rate (≤10th percentile), medium (>10th and ≤90th percentile), and high (>90th percentile).
We computed the rates of major in-hospital morbidity (stroke, reoperation, renal failure, and prolonged ventilation) or mortality associated these rates with the 3 categories of HAI hospitals.
Statistical Analysis
Continuous variables are presented as median (interquartile range: 25th – 75th) and categorical variables as counts and percentages. Continuous variables are compared using the Wilcoxon rank-sum test, while categorical variables were compared using Pearson’s chi-square test. Given the influence that pneumonia has on a center’s HAI rate, we conducted a sensitivity analysis using a modified definition of HAI excluding pneumonia, and estimated the degree of concordance, defined as the percentage of hospitals/patients remaining in the same infection category (Low/Medium/High), between definitions.
Statistical analyses were performed using SAS version 9.3 (SAS Institute, Inc. Cary, NC) and R version 2.15.2 (R Foundation for Statistical Computing, Vienna, Austria). The two-tailed tests were considered significant at p<0.05.
RESULTS
The overall rate of HAI was 3.97% and varied significantly across hospitals (Figure 1). The HAI rate was <0.84% in the low rate hospitals, 0.84%–8.41% in the medium rate hospitals, and >8.41% in the high rate hospitals.
Figure 1.
Overall Rate of Hospital-Acquired Infections Among 1084 Participating Hospitals. Hospital-acquired infections (HAIs) were defined as: pneumonia, sepsis/septicemia, deep sternal wound infection/mediastinitis, vein harvest/cannulation, or thoracotomy
Differences in case mix and processes of care among the 365,686 patients from the three categories of HAI hospitals are displayed in Table 1. Overall, the average age was 65 years, 25.8% were female, 22.3% were current smokers, 45.0% had a history of diabetes, 87.5% were hypertensive, 24.3% had a history of chronic lung disease, 2.6% had immunosuppressive treatment, 14.2% had peripheral vascular disease, 1.9% were in cardiogenic shock, 17.0% had congestive heart failure, of which 23.9% were in NYHA Class IV, and the median ejection fraction was 55.0%. Overall, 32.4% received intra-operative blood products, 90.4% of diabetics had a left internal mammary artery (IMA), 3.1% had bilateral IMAs, 13.2% had the saphenous vein removed through an open as opposed to minimally invasive approach, 96.6% had appropriate antibiotic selection, 97.6% had appropriate antibiotic timing prior to incision, and 96.2% had appropriate antibiotic discontinuation. More than 1 in 10 patients (12.3%) developed major morbidity or mortality.
Table 1.
Patient and Process Measures Across Hospital-Acquired Infection Rate Categories
| Hospital-Acquired Infection Rate Categories | |||||
|---|---|---|---|---|---|
| Overall N=365686 |
Low (<0.84%) N=25685 |
Medium (0.84–8.41%) N=317864 |
High (>8.41%) N=22137 |
P-Value | |
| Age | 65.0 (58.0, 73.0) | 66.0 (58.0, 73.0) | 65.0 (58.0, 73.0) | 65.0 (58.0, 73.0) | 0.0562 |
| Gender, Female | 94,255 (25.8%) | 5,087 (24.5%) | 82,720 (25.8%) | 6,448 (27.2%) | <.0001 |
| Body Mass Index (kg/m2) | 29.1 (25.8, 33.1) | 29.1 (25.9, 33.1) | 29.1 (25.8, 33.1) | 28.9 (25.7, 32.9) | 0.0003 |
| History of Smoking | |||||
| Never | 244,821 (66.9%) | 16,519 (64.3%) | 213,658 (67.2%) | 14,644 (66.2%) | <.0001 |
| Past Smoker | 38,787 (10.6%) | 3,449 (13.4%) | 33,242 (10.5%) | 2,096 (9.5%) | . |
| Current Smoker | 81,583 (22.3%) | 5,679 (22.1%) | 70,531 (22.2%) | 5,373 (24.3%) | . |
|
Family History of Coronary Artery Disease |
101,681 (27.8%) | 8,229 (32.0%) | 87,545 (27.5%) | 5,907 (26.7%) | <.0001 |
| History of Diabetes Mellitus | 164,670 (45.0%) | 11,000 (42.8%) | 143,357 (45.1%) | 10,313 (46.6%) | <.0001 |
| History of Dyslipidemia | 316,851 (86.6%) | 21,539 (83.9%) | 276,583 (87.0%) | 18,729 (84.6%) | <.0001 |
| History of Hypertension | 320,079 (87.5%) | 22,050 (85.8%) | 278,713 (87.7%) | 19,316 (87.3%) | <.0001 |
| History of Chronic Lung Disease | |||||
| No | 276,858 (75.7%) | 19,826 (77.2%) | 241,414 (75.9%) | 15,618 (70.6%) | <.0001 |
| Mild | 49,559 (13.6%) | 3,105 (12.1%) | 42,864 (13.5%) | 3,590 (16.2%) | . |
| Moderate | 22,474 (6.1%) | 1,656 (6.4%) | 19,215 (6.0%) | 1,603 (7.2%) | . |
| Severe | 15,906 (4.3%) | 1,019 (4.0%) | 13,605 (4.3%) | 1,282 (5.8%) | . |
| Immunosuppressive Treatment | 9,416 (2.6%) | 459 (1.8%) | 8,267 (2.6%) | 690 (3.1%) | <.0001 |
|
History of Peripheral Vascular Disease |
51,885 (14.2%) | 3,159 (12.3%) | 45,374 (14.3%) | 3,352 (15.1%) | <.0001 |
| Cardiogenic Shock | 6,892 (1.9%) | 389 (1.5%) | 5,948 (1.9%) | 555 (2.5%) | <.0001 |
| Congestive Heart Failure | 62,313 (17.0%) | 4,079 (15.9%) | 54,288 (17.1%) | 3,946 (17.8%) | <.0001 |
| NYHA Class# | |||||
| I | 5,235 (8.4%) | 337 (8.3%) | 4,615 (8.5%) | 283 (7.2%) | <.0001 |
| II | 17,604 (28.3%) | 1,245 (30.5%) | 15,326 (28.2%) | 1,033 (26.2%) | . |
| III | 22,742 (36.5%) | 1,477 (36.2%) | 19,783 (36.4%) | 1,482 (37.6%) | . |
| IV | 14,891 (23.9%) | 868 (21.3%) | 13,037 (24.0%) | 986 (25.0%) | . |
| Ejection Fraction# | 55.0 (45.0, 60.0) | 55.0 (45.0, 60.0) | 55.0 (45.0, 60.0) | 55.0 (43.0, 60.0) | <.0001 |
| Anticoagulants | 147,920 (40.5%) | 8,609 (33.5%) | 129,721 (40.8%) | 9,590 (43.3%) | <.0001 |
|
Blood Products Transfused Intraoperative |
118,535 (32.4%) | 7,756 (30.2%) | 102,095 (32.1%) | 8,684 (39.2%) | <.0001 |
| Packed Red Blood Cell Units | 2.0 (1.0, 2.0) | 2.0 (0.0, 2.0) | 2.0 (1.0, 2.0) | 2.0 (1.0, 3.0) | <.0001 |
| Fresh Frozen Plasma Units | 0.0 (0.0, 2.0) | 0.0 (0.0, 2.0) | 0.0 (0.0, 1.0) | 0.0 (0.0, 2.0) | <.0001 |
| Cryoprecipitate Units | 0.0 (0.0, 0.0) | 0.0 (0.0, 0.0) | 0.0 (0.0, 0.0) | 0.0 (0.0, 0.0) | <.0001 |
| Platelet Units | 0.0 (0.0, 2.0) | 1.0 (0.0, 2.0) | 0.0 (0.0, 1.0) | 1.0 (0.0, 2.0) | <.0001 |
| IMA Use Among Diabetics | |||||
| Left IMA | 148,903 (90.4%) | 9,880 (89.8%) | 129,747 (90.5%) | 9,276 (89.9%) | <.0001 |
| Right IMA | 1,307 (0.8%) | 70 (0.6%) | 1,158 (0.8%) | 79 (0.8%) | . |
| Both IMAs | 5,044 (3.1%) | 291 (2.6%) | 4,457 (3.1%) | 296 (2.9%) | . |
| No IMA | 9,260 (5.6%) | 734 (6.7%) | 7,878 (5.5%) | 648 (6.3%) | . |
| Vein Harvest Technique | |||||
| Endovascular/Endoscopic | 277,599 (75.9%) | 18,085 (70.4%) | 243,720 (76.7%) | 15,794 (71.3%) | <.0001 |
| Direct Vision | 48,159 (13.2%) | 5,045 (19.6%) | 39,499 (12.4%) | 3,615 (16.3%) | . |
| Both | 7,277 (2.0%) | 412 (1.6%) | 6,457 (2.0%) | 408 (1.8%) | . |
| Cryopreserved | 131 (0.0%) | 5 (0.0%) | 120 (0.0%) | 6 (0.0%) | . |
| Appropriate Antibiotic Selection | 353,435 (96.6%) | 24,861 (96.8%) | 307,632 (96.8%) | 20,942 (94.6%) | <.0001 |
|
Appropriate Antibiotic Discontinuation |
351,646 (96.2%) | 23,874 (92.9%) | 307,137 (96.6%) | 20,635 (93.2%) | <.0001 |
| Appropriate Antibiotic Timing | 357,037 (97.6%) | 24,295 (94.6%) | 311,715 (98.1%) | 21,027 (95.0%) | <.0001 |
Where measured/appropriate
Patients at hospitals with higher HAI rates were more often female, had a higher prevalence of current smoking, diabetes, chronic lung disease, peripheral vascular disease, cardiogenic shock, higher NYHA congestive heart failure Class, anticoagulant use, and administration of intra-operative blood products (all p<0.001). In addition, hospitals with higher HAI rates were less likely to operate on patients with a family history of coronary artery disease, and to deliver the appropriate antibiotics (p<0.001); they were also more likely to develop major morbidity or mortality (p<0.001, Figure 2).
Figure 2.
Risk of Composite Endpoint by Strata of Hospital-Acquired Infections Among 1084 Participating Hospitals. Morbidity/Mortality defined as any of the following: operative mortality, reoperation for cardiac reasons, renal failure (in patients without preoperative dialysis or preoperative creatinine >4mg/dl) or prolonged ventilation.
Rates of HAI subtypes varied: pneumonia: 2.98%, sepsis/septicemia: 0.84%, harvest/cannulation site: 0.34%, deep sternal wound infection: 0.32%, and thoracotomy: 0.01% (Table 2). Hospitals with high HAIs rates were more likely to have higher rates of each of the HAI subtypes.
Table 2.
Infection Subtypes Across Hospital-Acquired Infection Rate Categories
| Hospital-Acquired Infection Rate Categories | ||||
|---|---|---|---|---|
| Overall | Low | Medium | High | |
| Pneumonia | 2.98% | <0.38% | 0.38–6.67% | >6.67% |
| Sepsis/Septicemia | 0.84% | 0.00% | 0.00–2.04% | >2.04% |
| Vein Harvest/Cannulation | 0.34% | 0.00% | 0.00–1.06% | >1.06% |
| DSWI/Mediastinitis | 0.32% | 0.00% | 0.00–0.89% | >0.89% |
| Thoracotomy (max 1.61%) | 0.01% | 0.00% | 0.00–0.00% | >0.00% |
| Overall HAI Rate | 3.97% | <0.84% | 0.84–8.41% | >8.41% |
Hospital-acquired infections (HAIs) were defined as: pneumonia, sepsis/septicemia, deep sternal wound infection/mediastinitis, vein harvest/cannulation, or thoracotomy. Thoracotomy incisional infection involving thoracotomy or parasternal site; Vein Harvest/Cannulation infection involving a conduit harvest or cannulation site.
In the sensitivity analysis, we found reasonable concordance at the hospital (76.7%) and patient-level (84.2%) between definitions of HAI when using or excluding pneumonia.
DISCUSSION
We utilized the STS Adult Cardiac Surgery Database to describe institutional variability in HAI rates. These findings are important for at least three reasons. First, our study reflects the largest series to characterize the variability in HAIs in the setting of isolated CABG surgery. Rates varied between 0% and 38.5% across the 1,084 hospitals participating in the STS ACSD. Second, while we have characterized HAIs more broadly than in other series, HAI remained significantly associated with risk of further morbidity and mortality. For instance, an absolute difference of 9.3% existed in rates of major morbidity or mortality across hospital HAI strata. Third, we identified a number of patient and process measures that differ across strata of HAI hospitals. Some of these factors are “low hanging fruit” for targeted quality improvement interventions, including the appropriate selection of pre-operative antibiotics and intra-operative transfusions. These and other modifiable factors may be viewed as opportunities for reducing hospital-level variation in HAIs.
Patients undergoing CABG surgery are at risk for a number of infectious sequelae. Given their association with considerable risk of morbidity and mortality, sternal wound infections have been most extensively studied.8–11 Indeed; mediastinitis is one of several conditions whose treatment will not be reimbursed by Medicare. Other types of infections that are commonly described include: pneumonia, sepsis, thoracotomy, and harvest site location.1, 4 Rogers and colleagues undertook a study of Michigan Medicare beneficiaries, and reported a HAI rate of 12.2%.12 HAIs were broadly defined based on ICD-9 codes, and included an event occurring in any of the following clinical categories: infectious and parasitic diseases, nervous system and sense organs, circulatory system, respiratory system, digestive system, genitourinary system, skin/subcutaneous tissue, musculoskeletal system, bacteremia, and other injuries. Fowler and colleagues, using the STS ACSD, reported a rate of 3.51% for major infections, defined as any one of the following: mediastinitis, thoracotomy or vein harvest site infection, or septicemia.4 Shih and colleagues, using STS ACSD data, reported a HAI rate of 5.1% among patients undergoing CABG surgery at any of 33 hospitals in the state of Michigan.1 Similar to Shih, pneumonia was the most common type of HAI, occurring among 3.1% of patients. Indeed, pneumonia is one of the most commonly reported complications following cardiac valve procedures.13
Prior work has explored hospital-level variability (and risk factors) in HAIs after cardiac surgery. Shih and colleagues1 used patient characteristics (age, body mass index, cardiovascular disease, smoking status, ejection fraction, dyslipidemia, hypertension, chronic lung disease, immunosuppressive therapy, peripheral arterial disease, diabetes mellitus, congestive heart failure, New York Heart Association class, cardiogenic shock, and anticoagulant usage) to estimate each center’s predicted rate of HAI. While the predicted risk of HAIs differed by 2.8% across hospitals, observed rates differed by 18.2%. This finding suggested that factors other than case mix were driving variability in hospital HAI rates. Hospital strata were developed based on the distribution of rates across their statewide collaborative: low (<4.5%), medium (4.5%–6.9%), and high (≥7.0%). Overall infection rates, irrespective of hospital strata, were predominantly driven by pneumonia rates, which varied from 1.5% at low rate hospitals to 8.4% at high rate hospitals. Rogers and colleagues used a cohort of 24,789 Medicare beneficiaries to study the relationship between hospital-level transfusion rates and outcomes in the setting of CABG surgery6. The overall rate of infection was 16.2%. Infection rates were highest among patients receiving allogeneic blood (18% among patients receiving allogeneic blood, 9.7% with autologous blood, and 6.6% receiving no blood, p<0.001). The authors estimated an infection rate of 11.8% if hospital transfusion rates were lowered to 50% from their rather high rate of 83.9%. While we were not able to identify the sources (autologous vs. allogeneic) of blood products in our current analysis, packed red blood cells were used more frequently at high vs. low-rate hospitals (p<0.001).
Considerable advances have occurred over the quarter century in reducing center-level variability in clinical outcomes. The Center for Medicare and Medicaid Services (formerly the Healthcare Financing Administration) found wide variability in center-level mortality rates after CABG surgery.14 These data were publicly reported in 1986, resulting in a number of responses including the development of the first regional quality collaborative: Northern New England Cardiovascular Disease Study Group (NNE) and subsequent creation of the STS ACSD, among others. Using data from their regional registry, O’Connor and colleagues found a more than twofold variability in in-hospital mortality rates15, and used targeted quality improvement interventions to achieve a 24% reduction in mortality.16 Similar approaches have been undertaken by other statewide collaborates to reduce transfusion rates17, 18 and to improve internal mammary artery utilization19, among others. These and other quality improvement initiatives are a result of using clinical data to engage frontline clinical care providers. Our current study suggests that blood product utilization and antibiotic selection, both supported by clinical practice guidelines20, 21, are apt areas for targeted quality improvement. Similar approaches have been used outside of cardiac surgery with similar success through changes in physician behavior.22
We recognize some limitations to the present study. First, we cannot rule out any systematic bias in the reporting of HAI rates across STS ACSD participants. We imagine that any under-reporting of HAIs should be consistent across hospitals; we would expect that if the STS had perfect reporting, the HAI rates would be slightly higher overall, but each individual hospital would fall in similar rankings, and thus, our fundamental results would be unchanged. Systemic under-reporting is unlikely in this database due to the data audits currently in place across STS-participating hospitals. Recent data accuracy through such audits have been excellent (i.e. 96.9%). Furthermore, the reported rate of HAIs in this manuscript is quite similar to what has been reported in other publications, including a recent analysis from the Michigan Society of Thoracic and Cardiovascular Surgeons Quality Collaborative.1 Second, while we acknowledge that pneumonia was the biggest driver of HAIs in this setting, our sensitivity analysis suggested that our findings would not fundamentally change if we were to define HAIs exclusive of pneumonia. Third, our study was observational in nature, and is subject to the effects of unmeasured confounding.
Conclusion
In this large, nationally-representative descriptive study, we report large-scale variation in HAI among 1,084 hospitals participating in the STS ACSD. HAI rates occurred among 3.97% of patients undergoing isolated CABG surgery, mostly driven by pneumonia. While some patientlevel factors likely contributed to the observed rates, other factors including antibiotic selection and blood product utilization were contributory. We hypothesize that HAI rates may be reduced through targeted quality improvement efforts aimed at modifiable processes of clinical care.
Acknowledgments
The authors wish to acknowledge the editorial review provided by Katie Wopinsky.
Funding Sources: This project was supported by grant numbers R01HS022535 and R03HS022909 from the Agency for Healthcare Research and Quality. The content is solely the responsibility of the authors and does not necessarily represent the official views of the Agency for Healthcare Research and Quality.
Footnotes
Presented at the 51st Annual Meeting of the Society of Thoracic Surgeons, San Diego, CA, January 24–28, 2015.
Disclosures: The authors have no disclosures.
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