Measurement of arterial compliance in vivo

Measurement of arterial compliance is of interest in evaluating patients with atherosclerosis and other diseases which affect the vessels. Arterial compliance is the relation between changes in transmural pressure and volume of an arterial segment, where a high compliance signifies large changes in volume per change in transmural pressure. The relation between changes in transmural pressure and volume is far from linear as compliance increases progressively with decreases in blood pressure. A change in compliance could indicate static changes in arterial wall composition, i.e. the relation between elastic and collagen fibres and accumulation of disease related deposits or dynamic changes caused by alterations in muscular tone. The most used method reflecting arterial compliance is the measurement of pulse wave velocity. However, the pulse wave velocity method measures compliance at ambient transmural pressures and is affected both by the actual blood pressure and the rate of pressure change. Another commonly used method employs the echo-tracking technique to measure the arterial diameter simultaneously with continuous blood pressure monitoring. By this method it is possible to calculate arterial compliance for continuous pressure values between the diastole and the systole. The volume-oscillometry method is based on the fact that the artery can be made to collapse at the end of the diastole by an occlusive cuff while it remains open in a pressure dependent manner during the rest of the cardiac cycle. Changes in the arterial volume is transmitted to the cuff, where it induces a measurable change in pressure, and hence the volume of the artery can be calculated at different values of transmural pressures. Using this method on normal subjects has shown that the arterial compliance decreases with increasing age and that females have lower compliance than males primarily due to a smaller diameter of their arteries. It has also been shown that patients with essential (diastolic) hypertension have compliances which are higher or equal to those of normal subjects, and that patients with systolic hypertension have lower arterial compliances than normal subjects. The former finding is in contrast with pulse wave velocity measurements, where diastolic hypertension was associated with low arterial compliance.