Heart rate variability and vascular function in healthy subjects and in patients with insulindependent (Type 1) diabetes mellitus

The incidence of atherosclerosis parallels the decreasing function of the autonomic nervous system. Both atherosclerosis and autonomic dysfunction are common in patients with diabetes mellitus. Is there a link between the two? Vascular dysfunction (early signs of atherosclerosis) was studied by ultrasound methods and autonomic function by analyse of heart rate variability (HRV). Patients with IDDM, at a high risk of atherosclerosis, and healthy 35-year-old subjects randomly selected from a population register were studied. The diabetic patients were from the SDIS (Stockholm Diabetes Intervention Study). These patients were randomised to intensified conventional insulin treatment (ICT) or standard treatment (ST) approximately 12 years before the present studies were made. The ICT group had better blood glucose control as reflected in a lower mean HbA1c value in the 10-year study. In patients with IDDM, signs of early atherosclerosis, such as arterial wall stiffness and endothelial function, were correlated with the mean of the 10 year HbA1c levels and, the patients with better blood glucose control had less stiff arteries and better endothelial function. The ST group had stiffer arteries than the ICT group.

The findings show that atherosclerosis develops more slowly in patients with better blood glucose control. Patients with IDDM and better blood glucose control had a higher HRV. The ICT group had better values in tests measuring parasympathetic function such as the expiration /inspiration ratio. Subclinical autonomic neuropathy thus develops more slowly in patients with better blood glucose control. As the prognosis in patients with IDDM and autonomic dysfunction is poor, the results probably mean that patients with better blood glucose control have a better prognosis in the long run. There were correlations between vasodilator capacity and smoking in apparently healthy 35-year-old men and women. In women, vasodilator capacity also correlated with conventional risk factors for cardiovascular disease. Women with higher fasting blood glucose levels had higher resting blood flows, suggesting that blood glucose levels also within normal ranges affects vascular function. There was already a correlation in healthy 35-year-old women, but not in men, between autonomic function assessed as heart rate variability and conventional risk factors for ischaemic heart disease. In the men, there was a correlation between leucocyte count, an indicator of inflammation, which has been described as an independent risk factor for ischaemic heart disease, and heart rate variability.

In the diabetic patients, HRV was negatively correlated with carotid arterial wall stiffness. Arterial stiffness could be a factor affecting baroreceptor function. If HRV is largely a measure of baroreceptor function, a stiff arterial wall could lead to attenuated baroreceptor function and a decrease in HRV. Low HRV could thus partly be a sign of more advanced atherosclerotic disease or be due to dysfunction of the autonomic nervous system, which affects blood flow and blood-pressure regulation, which, in turn, could affect arterial wall stiffness. In the patients with IDDM there was also a correlation between early atherosclerosis and peripheral autonomic neuropathy. The function of the autonomic nervous system could be an important link between diabetes and vascular disease.