藤代 健太郎¹, 山下 晃平²

Kentaro FUJISHIRO¹, Kouhei YAMASHITA²

¹日本健康増進財団, ²北久里浜脳神経外科

¹Japan Health Promotion Foundation, ²Kita-Kurihama Neurosurgery Clinic

キーワード : ultrasonic, stiffness parameter β, carotid, aorta, arteriosclerosis

Stiffness parameter β（stiffness β）は血圧に依存しない血管弾性の指標として血管径，拍動幅と血管内圧から算出される．血管径の測定は超音波でRF-tracking法またはM-mode法で行っている．対象血管は総頸動脈，椎骨動脈，上行大動脈および胸部下行大動脈の報告がある．血管機能検査の中でstiffness βは検者内と検者間の変動係数がほぼ10％以内と良好である．計測に用いる血管径を外膜間距離または内膜間距離とするかで異なり，拍動幅も計測装置により若干の差異がでるので，診断や研究では機器と測定法を統一することが望まれる．総頸動脈の内膜間距離よりも外膜間距離の方が年齢との相関が高い．内膜間距離は内中膜複合体が加齢で肥厚するために減少することがあるので，動脈のスティフネスを評価するには外膜間距離を用いる方が適切であると考える．Cardio-Ankle Vascular Indexとの対比では，胸部下行大動脈のstiffness βとの相関は高く，総頸動脈のstiffness βとは相関はあるが大動脈ほどではない．最近の研究には，高血圧前症患者の大動脈のstiffness βが運動と生活習慣の改善で値が小さくなるなど，動脈硬化の初期変化の指標とするものが多い．血管のスティフネスは末梢への送血を良好に保つために重要であるが，加齢とともに亢進し，頸動脈のスティフネスの異常は虚血性脳血管障害のリスクになるので，早期からの動脈硬化診断にstiffness βを役立てることが望まれる．

Stiffness parameter β (stiffness β) is calculated from carotid diameter, distension and local carotid pressure as an index of blood pressure-independent blood vessel elasticity suitable for measuring distensibility of the carotid artery. This value is also measured in the ascending aorta and descending thoracic aorta. Among the indices for measuring distensibility of the carotid artery, stiffness β offers good intra- and inter-observer reproducibility, showing a coefficient of variation within 10%. Measurement values differ depending on whether the vascular pulsation is measured by the ultrasonic RF-tracking method or the M-mode method, and whether vessel diameter is measured as the media-adventitia (outer vessel wall) distance or lumen-intima (inner vessel wall) distance. Also, since slight differences in pulsation width are seen depending on the apparatus used, comparison of the apparatus and measurement method in a unified manner is desirable. Outer vessel wall distance of the common carotid artery correlates more closely with age than does inner wall vessel distance, so measuring the distance to the adventitia would be more appropriate to evaluate arterial stiffness. There is a strong correlation between cardio-ankle vascular index (CAVI) and stiffness β using transesophageal echography of the descending thoracic aorta. The correlating CAVI with stiffness β of the common carotid artery is not as large as that of the aorta. Most recent studies of stiffness β have focused on indicators of early changes in arteriosclerosis, such as stiffness β in the aorta of pre-hypertensive patients decreasing with improvements in exercise and lifestyle habits. Appropriate stiffness is necessary for blood vessels to maintain good blood flow to the periphery. However, arterial stiffness increases with age. Stiffness β, as an indicator of stiffness, is used to measure early changes in local arteriosclerosis, and abnormalities in carotid arteries represent a risk factor for ischemic cerebrovascular disease.