One advantage of risk attribution by a single measure at any point in time such as CIMTAR would be to select subjects earlier in life without having to wait for the time-based increase in wall thickness to pass a threshold beyond normal. Such a single measure might also select subjects at risk prior to the development of risk factors. The similar values of CIMTAR from trials of differing patient populations (older, low risk, or young with familial hypercholesterolemia) suggest a linear accretion of wall thickness independent of baseline LDL-C levels and support the validity of the concept of a snapshot determination of risk. The larger mean CIMTAR in our population may be a result of the selection of the maximum CIMT for each subject and increased baseline risk in the referral population. A CIMTAR of ≤0.016 mm/year may prove to be a useful cutpoint for populations such as ours.
At every level of baseline LDL-C, half of the population was variably distributed above the median CIMTAR, while the other half was densely grouped below. Between these two groups, traditional risk factors did not account for their separation, and baseline LDL was not associated with elevated maximum wall thickness or CIMTAR. The increase in carotid wall thickness was not determined by the concentration gradient of LDL between serum and the subendothelial space. In previous studies (), similar CIMTAR values were noted despite varying mean LDL levels suggesting that factor(s) other than the LDL gradient determine maximum wall thickness. Potential contributors to an excess wall thickness might be trafficking of lipoproteins in the arterial wall. However, in our patient population, high-density lipoprotein (HDL) and triglyceride (TG) levels were also comparable between the two groups. Another possibility may be that variability in vascular endothelium barrier properties contributed to the excess in wall thickness and apparent accretion rate.
The single baseline variable associated with an elevated CIMTAR for both the overall population and those without lipid-lowering therapy at baseline one was systolic blood pressure. In meta-analyses of hypertensive trials, elevations in systolic blood pressure were associated with an increase in risk of vascular outcomes with a 40% increment for every 10-mmHg increase (Chobanian et al. 2009
). There are multiple mechanisms by which hypertension may increase maximum wall thickness: increased lipid entry into the subendothelial layer, loss of smooth muscle architecture with hyperplasia/dedifferentiation/lipid ingestion, and increases in lipid oxidation, inflammation, and peptidergic signaling among others (Mulvany and Aalkjaer 1990
; Ross 1999
). Alternatively, the increase in systolic blood pressure in the above median CIMTAR group could be from cytoarchitectural change in the distal arterioles.
Limitations of our study include the operator dependence of ultrasound measures. Although automated means of maximum wall thickness measure may help reduce operator error, we used short-axis examination to confirm the longitudinal measurements. The addition of morphologic measures of the content of the carotid wall such as grayscale median (GSM) might enhance the accuracy of risk stratification (Wohlin et al. 2009
; Graebe et al. 2010
). Serial follow-up of a primary prevention population would be necessary to establish the clinical utility of CIMTAR. However, recent results from the Framingham Offspring Study support the use of maximum wall thickness to enhance risk stratification (Polak et al. 2011
Earlier identification of vascular risk by a single imaging measure such as CIMTAR may enable earlier treatment and expanded benefit from a longer duration of care. Enhanced communication of such risk may increase adherence to risk reduction programs, which is critical for long-term or lifetime treatment strategies. There is abundant need for more efficient treatments of larger patient populations to reduce vascular outcomes such as acute coronary syndrome, stroke, and sudden death.