抄録
A novel apparatus for noninvasive measurement of pressure-cross-sectional area relationship, i. e., tube law, of brachial artery was developed. It is based on an oscillometric method in which a cuff is wrapped around the upper arm and pressurized to measure the oscillation in the cuff pressure. The oscillation of the cuff pressure is caused by the volume change in the brachial artery due to heartbeats. The volume change was obtained by multiplying the amplitude of the cuff pressure with cuff compliance that had been measured from the increase in the cuff pressure just after loading a precise amount of air into the cuff. Oscillation of the cuff pressure disappears when the artery is collapsed. By using this principle, an occluding pressure OP when the artery begins to collapse was measured. A modified transmural pressure mPt was introduced whose origin was taken at the point where the external pressure of the artery, i. e., the cuff pressure equals to OP. Relationship between mPt and cross-sectional area of the artery A was obtained as the tube law:The volumetric change of the brachial artery was obtained at each mPt and was fitted to the function $\displaystyle A=A_b \left\{ \frac{1}{b} \ln \left( \frac{ {}_mP_t - P_b } {a} +1 \right) +1 \right\}$ where Ab, Pb, a, and b are constants while assuming constant pulse pressure. Volume elastic modulus VE and in vivo cross-sectional area eA of the brachial artery were obtained from the tube law. Estimated tube law was very similar to that obtained by measuring the diameter of the brachial artery with ultrasound while changing the transmural pressure by pressurizing and depressurizing an airtight chamber that had been attached around the upper arm. Estimated eA correlated well with eA measured by ultrasound (R2=0.930, P<0.001, n=20) with maximum error of 9.1%. Difference in VE obtained in the two methods was less than±3% (n=5). The present method is capable of obtaining pressure-cross sectional relationship of the brachial artery correctly with a simple process comparable to blood pressure measurement. Since it has been reported that in vivo brachial artery diameter correlates well with the severity of atherosclerosis, eA obtained in the present method might become a promising index for atherosclerosis.
