Undiagnosed diabetes mellitus in coronary artery bypass graft surgery

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Cardiology Review® Online, May 2006, Volume 23, Issue 5

We examined the prevalence and risks associated with undiagnosed diabetes mellitus among more than 7000 patients who had undergone coronary artery bypass graft surgery. At the perioperative and postoperative stages, patients with undiagnosed diabetes showed a significantly higher morbidity and mortality rate.

The prevalence of diabetes mellitus among individuals aged 20 to 34 years in the United States is 2%, but 20% for individuals between the ages of 65 and 74 years, according to the Second National Health and Nutrition Examination Survey (NHANES II). About half of individuals with diabetes of all ages had not been diagnosed when that study was performed.1 Although several studies on the frequency of undiagnosed diabetes have since been done, the prevalence and effects of undiagnosed diabetes in patients at high risk, such as those undergoing coronary artery bypass graft (CABG) surgery, are still relatively unknown. Therefore, we evaluated the frequency and risks associated with patients with undiagnosed diabetes who underwent bypass surgery.

Patients and methods

We assessed 32 preoperative attributes and 8 postoperative progress measurements in CABG surgery patients (n = 7310) from 1996 to 2003, excluding those who underwent re-do surgeries, more than 1 procedure, or emergency procedures. Information was collected from the Data Mart database.2 Patients were divided into 3 groups: those with known diabetes, those without diabetes, and those with undiagnosed diabetes. Any patient with a fasting blood glucose (FBG) level ≥ 126 mg/dL was considered to have diabetes, according to American Diabetes Association recommendations.3 Patients were considered to have known diabetes if they were diagnosed with diabetes prior to hospital admission or if they were taking insulin or oral antidiabetic medication before admission. Patients with FBG levels < 126 mg/dL were considered not to have diabetes, and those with FBG levels ≥ 126 mg/dL on admission were considered to have undiagnosed diabetes.

The percentage of CABG surgery patients with known diabetes was 29.6%. The prevalence of patients with undiagnosed diabetes mellitus was 5.2%. As shown in Table 1, patients without diabetes undergoing CABG surgery were younger than patients with diabetes. Among patients with diabetes, we found a higher proportion of women, patients with arterial hypertension, and patients with a higher body mass index (BMI). Carotid stenosis and peripheral vascular disease occurred less frequently in patients without diabetes than in patients with undiagnosed or known diabetes. The results for pa&shy;tients with undiagnosed diabetes tended to fall somewhere be&shy;tween pa&shy;tients with diabetes and those without diabetes, although their data were not always markedly different from patients without diabetes. Coronary artery bypass graft surgery patients with undiagnosed diabetes were classified as having New York Heart Association (NYHA) functional class IV more often, followed by patients with diabetes and those without diabetes.

As shown in Table 2, resuscitation after CABG surgery was required for 4.2% of patients with undiagnosed diabetes versus 1.5% of patients with diabetes and 1.7% of patients without diabetes (P < .01). Patients with undiagnosed diabetes also had a higher reintubation rate (5.0%) than patients with and without diabetes (3.5% and 2.1%, respectively; P < .01). A larger proportion of patients with undiagnosed diabetes received ventilation for more than 1 day (10.5%), compared with those with and without diabetes (7.4% and 5.6%, respectively; P < .01). The perioperative mortality rate was also higher for patients with undiagnosed diabetes (2.4%), compared with patients with and without diabetes (1.4% and 0.9%, respectively; P < .01).

According to logistic regression analysis, undiagnosed diabetes mellitus was shown to be an independent risk factor for a longer period of ventilation, more frequent reintubation, more frequent resuscitation after sur&shy;gery, and a higher 30-day mortality rate (Table 3).

Discussion

In concurrence with other studies,1,4,5 CABG surgery patients without diabetes in our study had higher ejection fractions than patients with diabetes. In addition, patients with diabetes had NYHA functional class IV on hospital admission more often than patients without diabetes. Indications of diabetic neuropathy commonly occur more frequently in patients who have diabetes for a longer period of time. In these patients, cardiac infarctions may develop uncharacteristically or without pain, or may be demonstrated as ar&shy;rhythmias, cardiac shock, and cardiac insufficiency.6 Diagnosis is harder in these cases, resulting in a possible delay in treatment. In 1 study, ischemic events developed with no symptoms in 90% of patients with diabetes.7 Because of the high rate of ischemic events that are not symptomatic in these patients, they may be later diagnosed as not having diabetes and therefore may have severe coronary artery disease at the time they receive CABG surgery.

Even with adjustment for such risk factors as chronic obstructive pulmonary disease, BMI, and age, the increased number of reintubations and the longer periods on a respirator after surgery were significantly greater for patients with diabetes. This may be explained if one considers that a target organ of diabetic microangiopathy is the lung. Basal lamina thickening of the capillary endothelium and alveolar epithelium has been shown in patients with diabetes.8 Following extracorporeal circulation, pathophysiologic states, such as capillary leaks, could reveal restricted pulmonary action, which would generally be hidden under normal circumstances.

Our results showing that patients with diabetes had an increased hospital mortality rate are consistent with those of other studies.1 The current study also showed that patients with undiagnosed diabetes had the greatest mortality. Although these patients had risks similar to patients with diabetes, they did not receive sufficient therapy. Regarding their vascular status (eg, peripheral vascular disease, carotid disease, past stroke) the patients with undiagnosed diabetes appear to be less handicapped than patients with diagnosed diabetes. With respect to the cardiovascular risk factors (eg, age, BMI, arterial hypertension, etc) these patients are somewhere between pa&shy;tients with known diabetes and non-diabetics. These characteristics indicate that patients with undiagnosed diabetes may have a briefer period of disease, which might shed light on the reasons for the lack of diagnosis, especially because type 2 diabetes is not diagnosed on average until as many as 4-7 years after the start of the disease.9 Coronary artery bypass graft surgery patients with undiagnosed diabetes, however, are at increased risk immediately before and after surgery, according to data from our study. The participants in our study underwent an in-depth selection procedure before hospital admission and therefore do not represent a random sample of the general population. The high prevalence of patients with undiagnosed diabetes in our study was unexpected.

This unexpected finding may be explained by the results of a study by Levetan and colleagues, in which 33% of 1034 patients hospitalized in an urban, tertiary-care teaching hospital were found to have undiagnosed diabetes on admission.10 Only 7.3% of these patients were later diagnosed as having diabetes during hospitalization. This may possibly be ex&shy;plained by the attribution of the high glucose levels to stress hyperglycemia rather than to undiagnosed diabetes by the treating physicians. Contributing to the problem is the fact that there are no definitive standards for the diagnosis of diabetes in stressful clinical settings. In 1 study, only 33% of patients admitted to the hospital because of possible myocardial infarction, whose in&shy;creased glucose levels would have been likely to be considered stress hyperglycemia, were shown to have normal glucose levels after 2 months of follow-up.11 These results were corroborated in a study by Norhammar and colleagues.12 Because diabetes is such a strong risk factor, physicians should strive to confirm a diagnosis of diabetes when patients have increased blood glucose levels.

Conclusions

This study is the first to establish the prevalence of undiagnosed diabetes mellitus in CABG surgery patients. During the perioperative and postoperative periods, patients with undiagnosed diabetes showed a significantly higher morbidity and mortality rate. Based on the results of our study and because of the high risks associated with diabetes, especially for patients undergoing CABG surgery, all patients with high glucose levels should be assumed to have diabetes until proven otherwise.