Carotid morphology and stiffness measurements by vascular ultrasound
These methods has been described before . All women underwent the right common carotid artery (RCCA) measurements with MylabTwice ultrasound instrument (Esaote, Italy) equipped with Automatic QIMT (Quality Intima-Media Thickness) and QAS (Quality Arterial Stiffness) packages. Vascular probe LA523 with a frequency of 12 MHz was employed. All the measurements and calculations were done by two investigators (Drs. Xue &Yuan). Six consecutive measurements were performed and only when these consecutive 6 times measurements met the quality standard (Quality Control shown in green number on the screen during the scanning), was the average taken as the final result for this patient. These measurements and average calculations were automatically done and displayed on the left side of the sonogram.
QIMT and carotid diameter measurements were performed in the longitudinal way, strictly perpendicular to the ultrasound beam, with both walls clearly visualized. High-quality image was acquired along a minimum of 1.5 cm length of RCCA segment for reproducible measurements. The automatic QIMT calculation was activated and the thickness between the intima and the media on the standard B-mode image in real time was automatically measured using the radio-frequency reception signal. QAS Measurements automatically measured the modification of the arterial diameter between the systolic and diastolic phases on the same RCCA segments as in the measurement of IMT. Theoretically, carotid diameter waveforms were assessed by means of ultrasound and converted to carotid pressure waveforms using an empirically derived exponential relationship between pressure and arterial cross-section. The derived carotid pressure waveform is calibrated to brachial end diastolic and mean arterial pressure by iteratively changing the wall rigidity coefficient. This allows the calculation of the arterial stiffness . Carotid stiffness indices: pulse wave velocity (PWV, m/s), distensibility coefficient (DC, 1/KPa), compliance coefficient (CC, mm2/kPa), α, β and augmentation index (AIx, %) were obtained.
Specifically, Pulse wave velocity (PWV) was calculated from the following equation:
, where, D: Diastolic diameter; ΔD: Change of diameter in systole; DC: Distensibility coefficient; Δp: Local pulse pressure; ρ: Blood density. PWV is a functional parameter directly affected by arterial wall stiffness. Due to the extensive data linking PWV to the risk of incident cardiovascular events, PWV is regarded as the in vivo “gold standard” index of arterial stiffness.
Distensibility Coefficient (DC) is the fractional change in cross-sectional area relative to the change in arterial pressure. DC was calculated as:
, where A: Diastolic area; ΔA: Change of area in systole; D: Diastolic diameter; ΔD: Change of diameter in systole; Δp: Local pulse pressure; Compliance Coefficient (CC) is calculated as: CC = ΔDπDs/2ΔP, where DS:diameter in systole; Stiffness index β was expressed as ln (SP/DP) × D/ΔD, where SP and DP are carotid systolic and diastolic pressure respectively. PT1 might not be an independent hemodynamic variable, though it would be augmented due to increased PWV and the return of reflected waves to the heart from the periphery.
Augmentation index (AIx) is another parameter that could be obtained from pressure measurements at a single site. It is calculated as the difference between the second and the first systolic peaks observed on the arterial waveforms, and it is expressed as a percentage of the pulse pressure (AIx = [AP/(Loc Psys – Loc Pdia)] × 100, where, Loc Psys: Local pressure – systolic; Loc Pdia: Local pressure – diastolic. AP: augmented pressure), and is regarded as an indirect measure of arterial stiffness and wave reflection.
In addition to these morphological and stiffness parameters, the resistance index (RI) and the pulsatility index (PI) of the right carotid artery were also acquired by traditional vascular ultrasound. RI = (maximal flow velocity-minimal flow velocity)/maximal flow velocity and PI = (maximal flow velocity-minimal flow velocity)/mean flow velocity.
All values are expressed as mean ± SD for continuous variables. As gestational age, body mass index, heart rate (HR) and blood pressure could have affected the measurements, the results for carotid arterial diameter and arterial stiffness parameters of PWV, DC, CC, α and β were adjusted for these covariates, using a general linear model. Group profiles were compared with paired or unpaired Student’s t-test. Correlations between arterial parameters and carotid pressures were analyzed with Pearson’s correlation coefficient (r). A p value < 0.05 was considered a statistically significant difference. The statistical software package SPSS 12.0 (SPSS Inc., Chicago, IL) was used for all data analyses. Reproducibility and variability have been performed before and good agreement and correlation were observed between measurements taken by the same observer and the two different observers .