Abstract-Several recent results obtained in hypertensive animals and subjects under in vivo isobaric conditions do not confirm the classic view of stiffer arteries in hypertensive subjects. We compared the mechanical behavior of in situ isolated common carotid arteries from normotensive Wistar-Kyoto rats (WKY) and age-matched spontaneously hypertensive rats (SHR) under both static and dynamic conditions for transmural pressure ranging from 50 to 200 mm Hg. Because of the nonlinear pressure-diameter relationship of large arteries, it is difficult to compare the mechanical properties of the large arterial wall in patients or animals with different arterial blood pressure. Furthermore, most of the experiments to assess the mechanical properties of the arterial wall are performed under static conditions, ie, with the pressure being varied by successive steps.2,3 However, under in vivo conditions with pulsatile pressure generated by the cardiac pump, the large arterial wall must be considered in terms of dynamic behavior, ie, taking into account the viscous, frequency-dependent properties of the vascular wall.New ultrasonic technologies have been developed that allow the precise measurement of both in vitro and in vivo instantaneous arterial diameter. 4 The application of these ultrasonic techniques to the calculation of mechanical properties of large arteries in human and experimental hypertension has led to conflicting results. In contrast to the classically reported stiffer arterial wall in hypertensives, similar or even increased arterial compliance and distensibility were calculated in hypertensive subjects and animals. 5,6 However, because of in vivo measurements in the operating arterial pressure range, it was impossible to compare the dynamic mechanical properties of the arterial wall in hypertensives and normotensives under isobaric conditions without an unverified hypothesis and theoretical models of the mechanical behavior of the arterial wall. In fact, to compare the arterial wall properties in vivo under isobaric conditions in normotensives and hypertensives, one must use calculations performed from pressure-diameter values recorded during the systole in the former and during the late diastole in the latter, ie, under markedly different dynamic conditions. It is also possible to modify the operating pressure in 1 group to reduce the arterial pressure in hypertensives and/or to increase it in normotensives, and therefore to compare wall properties in normotensive and hypertensive animals at comparable blood pressures in vivo. 7 However, the calculated parameters do not represent their actual operating values; furthermore, a direct effect of the drugs used to modify the arterial pressure on arterial smooth muscle tone and mechanical properties cannot be excluded.Therefore, the purpose of the present study was to (1) describe an experimental setup that would allow us to simultaneously evaluate in situ the static and dynamic properties of the CCA in normotensive WKY and SHR and (2) differentiate from the alt...

A rational methodology is developed for optimal design of biaxial stretch tests intended for estimating material parameters of flat tissues. It is applied to a structural model with a variety of constitutive equations and test protocols, and for a wide range of parameter levels. The results show nearly identical optimal designs under all circumstances. Optimality is obtained with two uniaxial stretch tests at mutually normal directions inclined by 22.5 deg to the axes of material symmetry. Protocols which include additional equibiaxial tests provide superior estimation with lower variance of estimates. Tests performed at angles 0, 45, and 90 deg to the axes of material symmetry provide unreliable estimates. The optimal sampling is variable and depends on the protocols and model parameters. In conclusion, the results indicate that biaxial tests can be improved over presently common procedures and show that this conclusion applies for a variety of circumstances.

Hypertension is known to decrease arterial distensibility and volumetric compliance, as reported from pressure-volume experiments and ring and strip studies of human and animal large arteries. However, recent data using noninvasive in vivo recording of vessel diameter suggest that the cross-sectional compliance of large arteries can be unchanged or even increased in hypertensive subjects. The present study was performed to test the hypothesis that differences between volumetric and cross-sectional compliance could be related to differences in biaxial mechanical properties in normotensive and hypertensive rats. In normotensive (Wistar-Kyoto [WKY]) rats and spontaneously hypertensive rats (SHR) we measured the simultaneous changes in length and diameter of in situ isolated carotid arteries submitted to static pressures (50 to 200 mm Hg by steps of 25 mm Hg each). Carotid artery diameters and lengths were determined by video microscopy and computer-assisted image analysis. At low transmural pressure (50 mm Hg), carotid artery diameter was 710 +/- 41 microns in WKY rats and 980 +/- 31 microns in SHR (P < .01). In response to pressure increases, the carotid diameter increased by 91 +/- 6% in WKY rats and by 41 +/- 4% in SHR (P < .01). In parallel, the percent increase in carotid length was much larger in WKY rats than in SHR (31 +/- 2% versus 7 +/- 1%, respectively; P < .01). In WKY rats, longitudinal distensibility causes significantly larger volumetric values than cross-sectional compliance values; in contrast, because of the very small longitudinal distensibility, volumetric and cross-sectional compliances are almost identical in SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel physiological cardiac assist device (PCAD), otherwise known as the LEVRAM assist device, which is synchronized with the heartbeat, was developed to assist the left ventricle (LV) in chronic heart failure (CHF). The PCAD utilizes a single cannula, which is inserted in less than 15 s through the apex of the beating LV by means of a specially designed device. Blood is withdrawn from the LV into the PCAD in diastole and is injected back to the LV, through the same cannula, during the systolic ejection phase, thereby augmenting stroke volume (SV) and stroke work (SW). CHF with dilated LV was induced in sheep by successive intracoronary injections of 100-microm beads. The sheep (92.2 +/- 25.9 kg, n = 5) developed stable CHF with increased LV end-diastolic diameter (69.4 +/- 3.3 mm) and end-diastolic volume (LVEDV = 239 +/- 32 mL), with severely reduced ejection fraction (23.8 +/- 7.6%), as well as mild-to-moderate mitral regurgitation. The sheep were anesthetized, and the heart was exposed by left thoracotomy. Pressure was measured in the LV and aorta (Millar). The SV was measured by flow meters and the LV volume by sonocrystals. Assist was provided every 10 regular beats, and the assisted beats were compared with the preceding unassisted beats, at the same LVEDV. The PCAD displaced 13.6 +/- 3.4 mL, less than 8% of LVEDV. Added SW was calculated from the assisted and control pressure-volume loops. The efficiency, defined as an increase in SW divided by the mechanical work of the PCAD, was 85.4 +/- 16.9%. We conclude that the PCAD, working with a small displaced blood volume in synchrony with the heartbeat, efficiently augments the SW of the dilated failing LV. The PCAD is suggested for use as a permanent implantable device in CHF.