Abstract
Early studies in the 1970s revealed the now well-established link between diabetes and cardiovascular disease, the main cause of death and disability among this cohort. It is believed that biochemical changes to the proteins of the myocardium impair left ventricular function and contribute to diabetic heart disease. Reduced aerobic capacity and attenuated increases in left ventricular stroke volume are early manifestations of diabetes-specific changes. Exercise training improves aerobic capacity and ‘normalises’ some clinical indices of resting left ventricular function in people with type 2 diabetes, but it is unclear whether training improves the performance of the left ventricle during exercise. This thesis aimed to 1) compare the left ventricular performance during sub-maximal upright exercise in people with and without type 2 diabetes and 2) to address whether a period of high intensity interval training (HIIT) can augment the left ventricular response during sub-maximal upright exercise in people with type 2 diabetes.
To investigate myocardial performance during exercise, aerobic capacity and total blood volume were measured in 17 people with diabetes and 16 non-diabetic controls with no evidence of cardiovascular disease. Left ventricular volumes and functional parameters were measured with echocardiography at rest and during semi-recumbent cycle ergometry at 40% and 60% of aerobic capacity. Aerobic capacity was 10.5% lower and peak workload and heart rate reserve were reduced in people with diabetes. Cardiac output, stroke volume and ejection fraction were not different at rest, but increased less in people with diabetes during exercise (100% vs 171%, 24% vs 39%, 15% vs 25% vs in T2D vs CON respectively). Left ventricular end systolic volume was not different between groups in any condition but left ventricular end diastolic volume was smaller in people with diabetes during exercise. Total blood volume was not different between the groups, and was only moderately associated with left ventricular volumes.
To investigate the influence of aerobic training on myocardial performance in people with type 2 diabetes during exercise, 16 people with type 2 diabetes were randomised in a 2:1 ratio to 3-months of supervised HIIT or to a control group (no training). Aerobic capacity, total blood volume and left ventricular volumes at rest and during semi-recumbent cycle ergometry at 40% and 60% of aerobic capacity were collected at baseline and repeated following the 3-month intervention period. HIIT resulted in a 15% increase in aerobic capacity and an 11% increase in stroke volume at rest and during exercise (16% and 13% at 40% and 60% of aerobic capacity respectively). The increase in stroke volume was explained by an increase in left ventricular end diastolic volume (5% and 4% at 40% and 60% of aerobic capacity respectively) and an augmented reduction in end-systolic volume during exercise (decrease of 11% at baseline vs 26% 3-months following the intervention from semi-recumbent rest to 60% of aerobic capacity). There were no differences in blood volume following the intervention period.
These studies confirm that people with uncomplicated type 2 diabetes have a blunted stroke volume response during exercise, and this response is caused by attenuated left ventricular end diastolic volume at higher heart rates. HIIT increases stroke volume by increasing left ventricular end diastolic volume and augmenting the reduction in left ventricular end systolic volume during exercise independent of changes in total blood volume. This suggests that changes in the diabetic heart can be affected by HIIT.