Six hundred and thirteen (99.5%) and 605 (98.2%) patients had postoperative glucose values less than 200 mg/dl on POD 1 and 2, respectively, and this was not significant between the three groups either on POD 1 (P=0.06) or POD 2 (P=0.18). To analyze whether DSWI rates for patients in the PGC group were a function of postoperative glucose control, models were fitted to the data from the subgroup where A1C was greater than 8.5% that included mean BG values from POD 1 and 2, both on the original scale and after a log transformation. The tests of fit of these variables were not significant. Higher A1C percentage was associated with an increased incidence of DSWI (hazard ratio=1.38, P=0.009) for each unit increase in A1C (Fig. 4).
In this study of patients with known DM, a higher preoperative A1C level was associated with a proportional increase in DSWI rates (a 38% increase in DSWI for each unit increase in A1C). Because the confounding effects of postoperative BG were well controlled, the preoperative BG association to outcomes is especially valid and is this study’s major strength. In the last decade, several trials have well characterized the association between postoperative BG levels and poor outcomes 12,31,32; therefore, the findings of this study add yet another dimension to glucose control and surgical complications.
Insulin resistance during surgery is evaluated in cardiac, colorectal, and gall bladder procedures 36–39. The association between intraoperative insulin resistance, preoperative glycemic control, and adverse events after cardiac surgery was investigated by Sato et al. 37 who found insulin resistance during surgery rather than a known diagnosis of DM, increased the risk of complications. In several studies, a high BMI was associated with DSWI but in our study this relationship did not attain statistical significance probably because of the smaller study size which was a subset of the DM population.
As with other retrospective analyses, our study is limited by multiple factors. A large number of patients did not meet inclusion criteria because of missing preoperative A1C data which could result in selection bias. As A1C testing occurred before surgery, the impact of this exclusion should be minimal. The 616 patients who were included could have preoperative medical care and comorbidities potentially confounding the association of A1C and DSWI; therefore, an expansive list of baseline demographic characteristics were analyzed. There also were a relatively small number of DSWIs which could weaken the statistical power to estimate risks after multivariable correction for confounders. For the same reason, all confounders could not be adjusted for in the same model.
Clinical limitations were a change in the antimicrobial prophylaxis administered following cardiothoracic surgeries from vancomycin to the addition of cefuroxime for postoperative prophylaxis. Although preoperative antibiotic prophylaxis or antibiotic washes administered before and during sternotomy were not evaluated, the practices were stable during the study period. The severity of disease before surgery was not recorded, and was assumed to be similar for the entire population.
The strengths of this study are the use of a well-collected and prospective data for all cardiac surgery procedures. To overcome the statistical limitations, we also evaluated baseline characteristics of the large excluded group that did not have a preoperative A1C recorded. Statistically significant differences were lower BMIs, less hypertension and chronic lung diseases, less use of insulin, and better BG control on POD 1 and 2, all characteristics suggesting a lower risk group. The major strength of this study was that postoperative BG management was stringent and used standardized insulin protocols; therefore, no statistically significant differences between BG control were found between patients with and without DSWI (P=0.07). DSWI cases were identified and classified with the assistance of a hospital infection control team using the CDC criteria which complement those defined by The Society of Thoracic Surgeons 40.
The results of this study suggest that DM patients at highest risk of infections can be identified by preoperative A1C levels. Interventions to improve BG before surgery may impact outcomes, even though postoperative BG is controlled to appropriate goals. Therefore optimizing A1C in the available time before cardiac surgery may help improve patient outcomes.
In patients with DM undergoing cardiac surgery, higher A1C levels demonstrated an increased incidence in DSWI. The association of preoperative BG control and postoperative complications in this, and other surgical cohorts needs to be evaluated by prospective clinical trials.
This work utilized the Biostatistics Core of the Michigan Diabetes Research and Training Center funded by DK020572 from the National Institute of Diabetes and Digestive and Kidney Diseases.
Dr Roma Y. Gianchandani is on the Speaker’s Bureau of Sanofi. For the remaining authors there are no conflicts of interest.
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