The results of this population-based survey of SSI complicating CABG surgery deserve further comment. The overall incidence (7.0%) was higher than expected for “clean” cases and was primarily due to the number of harvest site infections. The incidence of SSI varies depending on the infection definition and type of surgery. In 3 previous studies,5,19,20
for example, strict definitions were used and likely underestimated the true incidence of SSI complicating CABG surgery. Although the CDC criteria used in this survey are mainly considered a surveillance definition, their definition was intended to define clinical syndromes for making therapeutic decisions, and, thus, they may have missed some SSI cases.
Published population-based and multicenter data on SSI incidence after CABG surgery are summarized in . Steingrimsson et al21
conducted a population-based study in Iceland that included only patients with deep SSI with an incidence rate slightly higher (2.5%) than what we observed. In the Netherlands, Manniën et al22
conducted surveillance across multiple centers and found an overall SSI rate of 5.6%, a superficial sternal SSI rate of 1.1%, a deep sternal SSI rate of 1.3%, and an LHSI rate of 3.2%. In this nationwide epidemiologic study, a surveillance period of 42 days instead of 30 days was chosen, and 13% of recorded SSIs were diagnosed 30 to 42 days postoperatively. In addition, they used postdischarge surveillance of SSI after cardiothoracic procedures to minimize the risk of missing cases. Because the surveillance was elective for LHSI and mandatory for sternal SSI, it is conceivable that some LHSI cases were missed. The first Norwegian study of national baseline incidence rates of SSI after CABG surgery performed between 2005 and 2009 was conducted by Berg et al.23
The overall SSI incidence rate was 14%, the sternal SSI rate was 5.1%, and the LHSI rate was 8.9%. However, the surveillance system used was not yet validated, and numbers of participating hospitals varied. In the United States, Fowler et al4
used clinical information obtained from the STS national cardiac database of almost two-thirds of all US bypass procedures (300,000 patients) performed during 2002-2003. They considered only deep sternal SSIs and LHSIs and reported their incidence rates to be 0.9% and 1.1%, respectively. The combined incidence rate was approximately 2.1%. These low rates of deep sternal SSI and LHSI may reflect, in part, the large geographically diverse population in the STS database, which increases generalizability and also uniformity in identification of infectious complications of CABG surgery in the STS database. Moreover, rates of deep sternal SSI may be higher in centers with active infection control surveillance rather than voluntary reporting, as occurs with the STS database. The lower rate of deep sternal SSIs may also reflect that the STS database primarily captures acute events, whereas some infections may become apparent much later after surgery.4
The previously mentioned studies found a large range of SSI incidences after cardiovascular procedures. However, comparisons should be performed carefully because of differences in SSI criteria, durations of follow-up, types of cardiac surgery, and methods of postdischarge surveillance. Owing to these limitations, the focus should be on following SSI incidence time trends within a country instead of on comparing countries with each other.
General Characteristics of Population-Based and Multicenter Cardiac Surgery–Related SSI Studies
In addition, these data showed that incidence trends for overall SSIs were consistently decreasing over time and that this finding could be attributable to multiple factors. Most important, it could be related to improvement in infection control measures that include judicious use of prophylactic antibiotics, with an effective screening and surveillance strategy. Also, several innovations in coronary revascularization surgery, such as minimally invasive CABG and “off-pump” operations, have been adopted widely in the past decade with the promise of improved clinical outcomes compared with older revascularization technologies and techniques. Furthermore, endoscopic vein harvest has been used increasingly as an alternative to the open technique, and this has led to a significant decrease in the incidence of leg incision complications, including SSIs.24,25
Surgical site infection had been diagnosed in 50% of patients in the outpatient setting, with an overall mean time to onset of 13 days; 34% of those who developed an SSI required hospital readmission. This study demonstrated the importance of follow-up to ensure that all SSIs are captured and is consistent with findings from previous investigations that found that 68.4%22
of SSIs were diagnosed after hospital discharge.
Coagulase-negative staphylococci were the most common pathogens (21%) in this study. In contrast, Staphylococcus aureus
was predominant in 2 other investigations.21,22
This difference could be due, in part, to the routine use of mupirocin for methicillin-resistant S aureus
decolonization that was adopted by the Division of Cardiovascular Surgery, Mayo Clinic, in 2002 for each patient undergoing CABG surgery.
This study has several important strengths. To our knowledge, it is the first population-based study to examine temporal trends in all types of SSIs in a geographically defined US population. The essentially complete ascertainment of all SSI cases by active search in an established medical records linkage system for a population of known size and age distribution allowed unbiased estimation of the incidence rate of SSI. Furthermore, to reduce the risk of incomplete SSI case ascertainment, we used the institutional STS adult cardiac surgery database. Data completeness in the STS database is high and extremely accurate.26
Limitations of this study include its retrospective nature, which relied on the accuracy and completeness of clinical records, and the fact that it is subject to several biases, including reviewer bias. Also, the race/ethnicity of the Olmsted County population is 90% white, suggesting that the results of this study may not be applicable to other populations. However, the homogenous nature of the population limits external biases, and, thus, these observations reflect a pure assessment of the temporal trends of SSI in a preserved population.