Risks associated with control of blood sugar in the ICU

Efforts by hospital intensive-care unit teams to reduce glucose readings of patients with diabetes might do more harm than good, according to an analysis published today in Diabetes Care.

Dr. Michael Schwartz, lead author and a UW Medicine endocrinologist, said he decided to study the phenomenon after talking with Dr. Irl Hirsch, a colleague who had witnessed problems emerge among his patients in the ICU. 

Schwartz and co-authors found that, among patients with diabetes, efforts to reduce blood glucose levels to what is considered normal in a non-diabetic person may actually harm the patients by triggering a dangerous reaction.

The article noted that relative hypoglycemia – or a decrease in glucose greater than or equal to 30% below prehospital admission levels – “has emerged as a major clinical concern because the standard glycemic target recommended for patients in the intensive care unit is associated with an increased mortality risk among some of the critically ill patients with diabetes.”

Low blood glucose, or hypoglycemia, can be dangerous because the brain depends on a steady supply of glucose to function. When someone’s blood glucose levels fall below a level of approximately 40 mg/dL to 60 mg/dL, the sympathetic nervous system triggers the release of hormones and other chemical signals to drive blood glucose back up. This phenomenon is known as a counterregulatory response. 

While this response can help return the blood glucose level to normal, in parallel it also increases the heart rate and blood pressure, and perhaps activates the immune system. Schwartz and others in this review suspect this counterregulatory response may be the cause of higher death rates among ICU patients with diabetes who are treated for a high glucose level.

Patients with diabetes generally have a higher blood sugar level (100 to 200 mg/dL) than patients without diabetes, the study noted. For a patient without diabetes, normal levels are 70 to100 mg/dL

“The target range that is established in in the ICUs doesn’t differentiate between a patient with diabetes and a patient without diabetes,” Schwartz said. To establish the best blood sugar range, he said, a randomized clinical trial would need to determine the ideal glycemic level for ICU patients with, and without, diabetes.

People with diabetes usually have higher than normal blood sugar levels. Over time their bodies get used to these high blood sugar levels. As a consequence, when their blood sugars levels are brought into the normal range with treatment, their bodies incorrectly perceive the levels to be dangrously low, thereby triggering the counter regulatory response. Schwartz and his colleagues are studying how the body monitors and regulates blood sugar levels to try to understand how this response might be prevented or corrected.

While the brain can clearly sense when blood sugar is too low, exactly how this occurs is not well understood.For many years, it was thought that cells responsible for monitoring and regulating blood glucose levels resided in the brain. But work by the UW Medicine researchers now indicate that blood glucose sensing neurons reside outside of the brain, located in places like the liver and along blood vessels. These sensors monitor glucose concentrations in the blood and other tissues and send signals to brain centers that then respond to changes in levels, the study noted.

“We anticipate that future strategies aimed at reversing the underlying defect can ameliorate or even eliminate the problem of relative hypoglycemia in patients with diabetes,” the authors conclude, “To achieve this goal will require an improved understanding of how brain glucose sensing works in normal individuals and how it becomes impaired with patients with diabetes.”