THE CEREBRAL VASCULAR-RESPONSE TO A RAPID DECREASE IN BLOOD-GLUCOSE TO VALUES ABOVE NORMAL IN POORLY CONTROLLED TYPE-1 (INSULIN-DEPENDENT) DIABETES-MELLITUS

The effect of rapid lowering of blood glucose on cerebral blood flow ( CBF) was studied in 10 Type 1 (insulin-dependent) diabetic patients (a ge 23.5 +/- 3.8 years; mean +/- S.D.) with longstanding, poor metaboli c control (HbA(1c) 11.2 +/- 1.0%; normal value 4.0-5.3%) using an intr avenous xenon 133 single photon emission computed tomography technique . After a fall in blood glucose, during 81 +/- 11 min (mean +/- S.E.M. ), from 18.2 +/- 1.4 mmol/l to 9.2 +/- 0.9 mmol/l CBF was unchanged, b ut increased from its initial value of 48.8 +/- 2.9 ml/100 g per min t o 57.1 +/- 2.4 ml/100 g per min (P < 0.001) when the blood glucose lev el was restored. The CBF was higher in the right compared to the left hemisphere at all measurements (1.8 +/- 0.5 ml/100 g per min, P < 0.01 ; 1.9 +/- 0.5 ml/100 g per min, P < 0.05; 2.1 +/- 0.7 ml/100 g per min , P < 0.05, respectively). The change in CBF was inversely correlated with time for fall of blood glucose, but there was no correlation with absolute levels of blood glucose. The respiratory end-tidal PCO2 decr eased during the low blood glucose level, but there was no correlation between the PCO2 and CBF. The cerebral volume was unchanged during th e study. The results indicate that in patients with chronic hyperglyce mia a rapid fall in blood glucose may cause a rise in CBF of the same magnitude as previously shown during absolute hypoglycemia in patients with well controlled diabetes mellitus and in normal subjects. Howeve r, the rise occurred when the blood glucose had been restored to its i nitial value, indicating an altered vascular response to lowering of b lood glucose during chronic hyperglycemia.