Bicuspid mitral bioprosthesis.

A bicuspid mitral bioprosthesis was prepared by mounting glutaraldehyde-processed porcine pericardium onto commercially available Brownlee-Yates stents. The bioprostheses were inserted into 17 dogs. Haemodynamic performance and long-term function of the valve was assessed. Of the 11 animals in the survival group, eight died within the 24-72 hour postoperative period. The clinical picture of these animals revealed progressive left ventricular failure although the bioprostheses were tested and found competent both at insertion and at necropsy. The causes of the late deaths were deterioration of the porcine pericardium in two, and cerebral embolism in another. The acute haemodynamic studies showed a significantly high closing reflux from within the tubular bioprosthesis, and this reflux was found to be inherent in the design. It was concluded that any stented bicuspid valve where the stent assumes the function of the papillary muscles, has to be tailored so that parts of the tissue can assume the function of the chordae tendineae to minimise the closing reflux.

An in vivo study of the total occlusion method for the analysis of forward and backward pressure waves.

The arterial pressure wave, Pm, is composed of a forward travelling wave, Pf, and a backward travelling wave, Pb, resulting from partial reflection of PI at the peripheral beds. The magnitude of Pb depends on the degree of vasoconstriction. Total occlusion of the vessel distal to the pressure measurement site results in a wave which is equal to 2Pf. Subtraction of the derived Pf wave from Pm gives Pb, provided there is no re-reflection of Pb by proximal discontinuities. To test the validity of this total occlusion method for the determination of the components of Pm, measurements were carried out in an anaesthetised dog and the results compared with the method of Westerhof et al., (1972). The measurements were made at three arterial sites: 1. in the aorta just proximal to the junction of the renal artery; 2. at the termination of the aorta; and 3. in the femoral artery. The magnitude of Pb was varied by the infusion of vasoactive drugs. There was good agreement between the two methods at site 1 but at the other two sites the agreement was less good. It is shown that this is the result of re-reflection of the backward travelling waves produced by the occlusion at the aortic termination and the renal artery junction.

Time dependence of [131I]albumin uptake following removal of an experimental aortic stenosis.

The uptake of trypan blue and [131I]human serum albumin (HSA) has been studied in the dog's abdominal aorta between 1 and 42 days after removal of an experimental stenosis (approximately 90%) applied 1 week previously. Previous work has shown that when the stenosis was present during circulation of these markers, their uptake was increased immediately proximal to the stenosis decreasing to normal by the renal artery level. Distal to the stenosis uptake was reduced apart from small areas of high uptake probably due to turbulent jet impacts. Within the stenosed section the uptake was normal. In this present study it was found that, after removal of the stenosis, proximal uptake initially remained elevated, returning to normal after approximately 15 days whilst the distal uptake returned to normal after approximately 10 days. In the previously stenosed section uptake was increased markedly following the release of the stenosis but returned to normal within approximately 20 days. The relationship of these findings to alterations in the local haemodynamic state and to possible changes in endothelial morphology are discussed.

Partial standing wave formation above an abdominal aortic stenosis.

Reflection from an arterial discontinuity, such as a stenosis, would be expected to produce partial standing waves of pressure and flow proximal to the stenosis. This phenomenon is demonstrated by determining the amplitude changes of the harmonic components of pressure and flow waves recorded at three sites at different distances from an experimental stenosis of the abdominal aorta in dogs. To minimise the effect of reflections from other arterial sites, such as the peripheral beds, the animals were vasodilated. Impedance and propagation velocity measurements were made to estimate the contribution of peripheral reflections in the harmonic components of the aortic pressure and flow pulses. In general, the peripheral contribution appeared to be small for harmonics greater than the first. The results indicate that, for the 'closed' type of reflection at the stenosis, a pressure antinode and a flow node occur immediately proximal to the stenosis. As the distance from the stenosis to the measurement site is increased, nodes and antinodes of pressure and flow occur at frequencies which correspond to integer multiples of lambda/4. Similar fluctuations take place in the impedance modulus proximal to the stenosis, such that close to the stenosis the modulus is a maximum, at the lambda/4 distance the modulus is a minimum and at the lambda/2 distance it is again a maximum. The extent of these impedance changes with distance indicates that the attenuation of the reflected, backwardgoing waves is greater than found for forwardgoing waves and that the diagnostic assessment of a vascular obstruction by means of a proximal measurement of pressure or flow may be subject to error if the measurement is not made close to the obstruction.

Influence of experimental stenosis on uptake of albumin by the abdominal aorta.

The influence of abdominal aortic stenosis on the uptake of the protein-binding trypan blue dye and 131I human serum albumin (HSA) has been studied. The major change was a region of high uptake proximal to the stenosis, returning to normal by the level of the renal arteries. There was reduced uptake distal to the stenosis, apart from occasional small areas of high uptake probably due to turbulence. The increase in uptake immediately proximal to the stenosis was dependent on the severity and duration of the stenosis. Removal of the stenosis immediately before the injection of dye and 131I-HSA still resulted in elevated uptake in the proximal region. Some haemodynamic modifications resulting from a stenosis are described. It is suggested that the most satisfactory haemodynamic explanation of the observed uptake change is a proximal increase in oscillatory pressure/strain. The relevance of these findings to atherosclerotic development at and above arterial junctions is discussed.

The dynamic and static elastic response of the abdominal aorta of the dog.

The dynamic and static elastic response of the abdominal aorta was studied in beagle dogs of similar age and weight. The dynamic response was measured in situ by means of an optical transmission wedge resting on the exposed vessel to record the pulsatile changes in wall displacement and the pressure pulse recorded at the same site via a hypodermic needle. The static response was recorded (1) in situ by observing the change in radius which occurred with a change in the mean blood pressure, (2) post mortem using both transmission wedge and radiographic techniques. The change in radius (deltaR) for a change in pressure (deltaP=20 mm Hg[2.67kPa]) was calculated for both the dynamic and static cases. It was found that the aorta was stiffer for the dynamic case, the extent of the stiffening being dependent on the mean pressure. At pressures less than 80 mm Hg (10.64 kPa) the dynamic/static deltaR ratio was 1.8 decreasing to 1.1 at 160 mm Hg (21.28 kPa). This behaviour is interpreted in terms of smooth muscle relaxation and the subsequent transference of circumferential tension to collagen.

Hemodynamic effects of acute experimental aortic coarctation in the dog.

Pressure and flow pulses were recorded immediately proximal and distal to a temporary stenosis of the thoracic aorta of the dog. The severity of the stenosis was varied, and the magnitude of the pulse changes was correlated with the degree of stenosis expressed as the area ratio, As/All, where As is the cross-sectional area of the stenotic section and A-s that of the normal vessel. The harmonic amplitudes of the pressure and flow waves during the stenotic period were obtained by Fourier analysis and normalized to their values when the stenosis was released. It was found that as A-s/All yields 0 the proximal pressure amplitude increased and the distal pressure and the proximal and distal flow amplitudes decreased. These changes were such that the proximal fluid impedance calculated from the corresponding pressure and flow amplitude ratios increased as A-s/A-ll yields 0; the distal impedance remained relatively unaffected. These findings can be interpreted in terms of reflection of the pulse waves at the stenosis. The normalized proximal pressure amplitudes approximate to 1 + R and the distal pressure and both the proximal and the distal flow vary according to 1 minus R, where R is the reflection coefficient. The variation of R with A-s/A-ll can be determined from the proximal fluid impedance changes; we found that R increased relatively slowly for moderate stenoses, varying from 0 at A-s/A-ll 1.0 (no stenosis) to similar to 0.2 at A-s/A-ll equals 0.2. At more severe stenoses, it increased more rapidly approaching 1.0 at A-s/A-ll equals 0. The generation of turbulence was most marked at A-s/A-ll yields 0.2.