The CACTI cohort is being followed for progression of coronary artery calcification (CAC) for development of clinical CVD and participants have been characterized during the baseline examination (4/00-3/02) and two follow-up re-examinations 3 and 6 years later. We are proposing assess a subset of this population to determine vasodilatory reserve as it relates to early coronary atherosclerosis in T1D. In the CACTI Study sought to determine the relationship between risk factors for CAD and coronary arterial calcification in T1D subjects compared to an age and gender matched control cohort. The trial identified CAC as a high risk marker of CAD and was more extensive in diabetic patients.
T1D patients have more CAC than nondiabetic controls but they also appear to have more impaired coronary vasoreactive function. Impaired response to acetylcholine testing and adenosine vasodilation are reported in diabetic subjects. The relationship between coronary microvascular dysfunction and diabetic cardiomyopathy is supported by observation of similar microvascular abnormalities. Endothelial dysfunction not only precedes and predicts clinical macrovascular disease, but also is an independent prognostic marker of adverse long-term cardiovascular outcomes. Significantly greater CAC in T1D was also noted than in non-DM subjects. There is also a greater rate of increase noted in the CACTI cohort.
Myocardial Flow Reserve by MRI: Myocardial perfusion distribution and myocardial blood flow reserve can be simultaneously assessed using a MRI first-pass Gd contrast injection approach. Rapid MR imaging (MRI) during the first pass injection of Gd compounds is used to assess myocardial perfusion with a spatial resolution of 2-3 mm, and to detect any regional impairments of myocardial blood flow (MBF) that may lead to ischemia. The myocardial perfusion reserve (MPR) is a useful concept for quantifying the vasodilator response. The perfusion reserve can be estimated from the ratio of MBFs during vasodilation and at baseline, similar to that described by labeled microspheres. Assessment of myocardial blood flow and flow reserve by cardiac MR is a robust technique. There is a relationship between CAC and hyperemic flow reserve, although it may be more or less strong depending upon the methodology employed. However, CAC is still a marker of the atherosclerotic process and this process appears to be more aggressive in T1D. The investigation of MBF reserve as a marker of early or more aggressive vasculopathy is important, especially in a disease with significant morbidity and mortality from cardiovascular events.
Regadenoson Vasodilation: Regadenoson is a selective A2A adenosine receptor agonist FDA released as a pharmacologic vasodilator in nuclear stress myocardial perfusion imaging (MPI). It has a higher affinity for A2A receptors than adenosine and is a more potent coronary vasodilator. Adenosine infusion is the gold standard for both invasive and non-invasive assessment of myocardial blood flow reserve, yet regadenoson with potentially greater coronary arterial vasodilation and a simple bolus injection is an ideal agent with which to study differences in CFR between different disease states. Limited data is published on the utility of regadenoson in invasive assessment of CFR and no data are published regarding the utility of this agent in assessing the hyperemic myocardial blood flow response with respect to cardiac MRI.
Research Design and Methods POPULATION CACTI Diabetes Study: We propose to recruit subjects from the CACTI cohort into this sub-study by focusing on individuals completing 6-yr follow-up.
Imaging Studies Non-DM CAC\>100 T1D CAC\< 100 T1D CAC\> 100 MPI 5 10 10 CMR 5 10 10 InvCBF 5 5-10 5-10 Subject will be recruited to complete all three parts of the protocol myocardial perfusion imaging (MPI); cardiac MR assessment of myocardial blood flow reserve (MBF); and only those subject with significant symptoms or clinical indications proceeding to invasive "Catheterization Laboratory" assessment of CBF reserve (Invasive CBF) Aim 1. Determine the relationship between myocardial perfusion index, regional CBF and invasively measured coronary perfusion reserve The first approach will be to assess the differences in myocardial perfusion between T1D and non-DM subjects employing vasodilatory stress with regadenoson. The myocardial perfusion index as described previously will be calculated for each individual. The relative perfusion index will be directly compared to the invasively obtained CBF reserve in each vascular territory assessed.
Stress Myocardial Perfusion Protocols. Subjects will undergo regadenoson stress MPI with 400 mcg administered as a 10 second bolus followed by injection of 30 mCi of Tc-99m-sestamibi. Gated SPECT perfusion imaging is then performed within 30 minutes. A rest study is performed on a separate day with a similar sestamibi dose administered.
Acquisition Protocol. All SPECT studies are performed with parallel hole collimators and gated acquisitions protocol (10% RR window) acquired into 16 time bins. Data Processing of Gated data sets are assessed for mean counts, corrected for dose, decay and time of imaging, are measured in each segment utilizing 4D-MSPECT (INVIA, Ann Arbor, Michigan). MPI is defined as the corrected stress counts divided by corrected rest counts.
Statistical Analysis: Based upon our prior experience approximately a third of the T1D subjects will have a MP index of less that 1.5 to direct compare to the invasive measured flow reserve. This maybe treated as a binary variable or as a continuous variable by combining all T1D subject compared to the non-DM control subjects. Data will be analyzed using unpaired t-test, linear and logistic regression. Continuous variables are expressed as mean ± standard deviation (SD) and are compared across groups by use of unpaired t-tests. All statistical analysis will be performed using two-sided tests with p \< 0.05 considered statistically significant.
Aim 2. Measure coronary blood flow using Regadenoson stress CMR to determine the MBF reserve in type 1 DM subjects compared to non-diabetic controls. Using the method of a two compartment model with K trans function analysis, we will determine the myocardial blood flow reserve by cardiac MR. This approach will use regadenoson bolus as the vasodilator for the hyperemic response of the coronary arterial bed. We will assess the relationship of MBF reserve or lack of reserve to disease state i.e T1D to non-DM subjects. In addition we will regionally compare MBF reserve to the MP index obtained from radionuclide techniques by coronary vascular territories. Similarly CAC values obtained at the 6-yr CACTI follow-up study will be directly compared to blood flow measurement within coronary vascular distributions.
CMR Protocol: An IV injection into the antecubital vein and connected to a power injector will be performed in the MR scanner for functional and perfusion imaging (Symphony 1.5 T, Siemens Medical). All images are acquired with a 4 channel phased array surface coil. Blood flow imaging is performed in three short-axis simultaneous slices (basal, mid and apical) acquired during an initial bolus injection of gadolinium (0.04 mmol/kg at a rate of 5 ml/sec) followed by a 10-ml saline flush administered using the power injector. ProHance is used due to its lower viscosity as the contrast agent. The short-axis slices are acquired using a SSFP imaging sequence with, TR/TE/flip 185ms/1.2ms/50 degrees dynamically acquired and inserted into a 330x 380 matrix). Images are acquired using a field of view ranging from 280 to 400 mm. A first perfusion scan will be performed at rest, followed by a second scan during maximal vasodilation about 15 min later. Hyperemia will be initiated with administration of 400 mcg bolus injection of regadenoson and acquisition started 1 minute post regadenoson administration. Subjects will be monitored using MRI compatible blood pressure monitoring and single lead ECG monitoring.
Data analysis Quantitative analysis of the MR blood flow data will be performed on a Leonardo Workstation using Argus perfusion analysis software. Images were registered manually by adjusting the horizontal and vertical registration of each frame. The left and right ventricular blood pool and six myocardial regions will be analyzed to extract the linear slope during the initial signal increase as well as the starting and peak signal intensities. To extract the slope, base, and peak signals, a line was manually fitted to the upslope signal (typically 4-6 time points), and the base and peak positions of the upslope data were manually identified. Normalized slope (SlopeN) was calculated by division by the left ventricular blood pool slope, and the net signal gain (SG) determined by subtracting the initial from the peak signal intensity. The myocardial flow reserve index was calculated for each myocardial region as the ratio of the normalized hyperemic time-intensity slope to the normalized baseline time-intensity slope.
Statistical Analysis: Based upon prior experience in the literature approximately a third of the T1D subjects will have a MP index of less than 1.5 to direct compare to the invasive measured flow reserve. Data will be analyzed using unpaired t-test, linear and logistic regression.
Aim 3. Determine the relationship between CBF reserve in vascular distributions to the degree of coronary arterial calcification.
Coronary artery calcification. All returning patients have previously undergone EBT scan. These data as described above will be used to compare to the new MPI, CMR myocardial blood flow reserve, and to the invasively measured CFR. Regional change will determined within standard coronary vascular distributions.
Flow Reserve Substudy Protocol: High-risk diabetics with CAC\> 100 (n=10) and stratified random sample high risk non-diabetics (n=10) will be studied. The inclusion of subjects with an abnormal perfusion reserve is important, because this value identified subjects with high risk in the first phase of the study and therefore is used to select individuals with high risk of cardiac events that clinically would undergo invasive assessment of coronary anatomy. Perfusion imaging will then be the test that stratifies subjects into an additional higher risk than the CAC \> 100. Selected patients identified as part of Specific Aim 2 will undergo a clinically indicated cardiac catheterization and baseline CFR study. Coronary angiography is the clinically accepted standard for evaluation and diagnosis of coronary artery disease.
Subsequently, coronary artery vasodilator reserve will be assessed using the regadenoson intravenous challenge technique measured as the coronary flow reserve and fraction flow reserve assess by a Doppler flow guide wire.
Methods for Flow Reserve:Selective coronary angiography is performed using standard technique with 5F diagnostic catheters in the LCA and RCA. The artery or interest determined by the abnormal area of myocardial perfusion by SPECT or MR imaging will be interrogated. The artery of interest will be cannulated with a 6F or 7F Guide catheter. A 0.014 in x 300cm pressure and Doppler-tipped guidewire (CombMap XT, Volcano Corp) will be advanced into the distal bed of the vessel of interest. After optimization of the pressure and Doppler signal, velocity and pressure measurements will be obtained at rest and after induction of maximal hyperemia with an intravenous bolus of regadenoson. Coronary flow reserve (CFR) will be measured in the target vessel of interest after 1-2 minutes following the IV bolus. Absolute CFVR will be calculated as the ratio of hyperemic to baseline average peak flow velocity and the relative CFVR as the ratio of the absolute CFVR in the target vessel to the absolute CFVR in the reference artery. FFR and CFR values will be categorized according to published cut-off values of 0.75 and 2.0, respectively.
