Barry S. Gow

Measurement of Viscoelastic Properties of Arteries in the Living Dog

Dynamic elastic moduli (Edyn) and viscous moduli (ηω) of the arterial wall were obtained at a number of sites of the systemic vascular tree of living dogs anesthetized with pentobarbital. Constants were calculated using the first harmonics of pressure and diameter obtained from a Fourier analysis of simultaneously recorded pulse waves. The means and standard errors of Edyn, in dynes cm-2 × 106, obtained at mean blood pressures ranging from 87 to 130 mm Hg and pulse frequencies from 1.1 to 2.8 cps were: 3.0±0.33 (9 midthoracic aortas), 9.8±1.2 (7 midabdominal aortas), 11.0 (4 iliac arteries), and 12.3±1.2 (11 femoral arteries). The viscous modulus was approximately 9% of Edyn in the midthoracic aorta and approximately 12% of Edyn in the abdominal and femoral arteries. Pulse wave velocities calculated from values of Edyn and relative wall thickness by the Moens-Korteweg equation agreed with accepted values. A Fourier analysis of pressure and diameter waves was shown to be unsatisfactory for determining the frequency dependence of viscoelastic constants because of anomalous behavior of the viscoelastic parameters. In the midthoracic aorta this behavior may have been partly attributable to artefact; however, there was strong evidence that elsewhere nonlinear pressure and diameter relationships interfered with the accurate determination of the relatively small, higher-order harmonic components.

Detecting Right Ventricular Volume Changes Using the Conductance Catheter

The purpose of this study was to assess the role of conductance catheter position within the right ventricle in obtaining adequate indications of phasic changes in ventricular volume. Possible applications of this lechnology are in rate responsive pacemakers and implantable defibrillators. The conductance catheter was placed in the right ventricle by cannulation of a jugular or femoral vein or a branch of the pulmonary artery. Position within the ventride was documented from biplane fluoroscopy. Stroke volume was perturbed by: bolus injection of blood, vagal stimulation, venous infusion of methylcholine chloride, or isoprenaline. Four criteria were used to assess the quality of volume signals: (1) volume signal phase relative to the electrocardiogram; (2) magnitude parity of volume change from each electrode pair; (3) freedom from artifact; and (4) indication of stroke volume change during interventions. Greyhound dogs of either sex (n = 33), weight 20–32 kg. A total of 236 recordings from 14 distinct catheter positions were analyzed. Catheter positions originating from a femoral cannulation and one position from the pulmonary artery gave markedly superior volume transduction compared to those from the jugular route. Although right ventricular volume transduction was possible from all catheter trajectories, those resulting from the femoral approach were clearly superior. In the right ventricle, the inability to transduce a sufficient proportion of ventricular volume, in concert with the potential sensitivity of the catheter to atrial volume changes, may seriously limit the potential of the conductance technique in the applications envisaged.

Right ventricular volumetry by catheter measurement of conductance.

The electrical conductivity of blood is sufficiently higher than that of myocardium to make feasible the detection of cardiac volume changes hy measurement of intraventricuiar fluid conductance. An eight‐electrode catheter was used to inject an aiternating current (100 μA or less, at 1500 Hz) via the two electrodes nearest the ventricular base and apex, then the resulting five voltage differences between adjacent pairs of the six intervening electrodes were measured. When current amplitude was held constant, the cross‐sectional area of the ventricuiar cavity slice defined hy planes perpendicular to the catheter through the relevant pair of electrodes was inversely proportional (to the first order) to the voltage difference. Measurement of multiple segments compensated for isovolumic cavity shape changes. The technique had previously been shown to measure left ventricular volume successfully, but the geometry of the right ventricle made this measurement more problematical. Using open‐chested, anesthetized greyhounds, we compared the catheter‐measured right ventricuiar volume change with stroke volume as measured by a pulmonary arterial electromagnetic blood flowmeter. With optimal catheter placement, good correlation between stroke volume and catheter‐measured volume changes was achieved when stroke volume was perturbed on a heat‐to‐heat hasis. In six data records from three dogs, involving two different means of varying stroke volume (rapid injection of blood and sinus node irritation), the correlations yielded r2 values between 0.82 and 0.98. The method detected ineffective (nonejecting) beats associated with normal‐appearing QRS complexes and was thus a more reliable indicator of cardiac mechanical function than an eiectrocardiogram.

An improved microindentation technique to measure changes in properties of arterial intima during atherogenesis

A fully automated instrument was developed that measured the amount and time-course of indentation of arterial intima by a spherical tipped probe 25 micrometers in diameter loaded routinely by forces of 10 microN. The relationship of depth of indentation to time was non-exponential but reached an apparent asymptotic value in about 6 s. Routine indentations were recorded 10 s after loading, the relationship of indentation and force being essentially linear between 0-100 microN. The instrument is an order of magnitude more sensitive than one previously described (Gow and Vaishnav, 1975, J. Appl. Physiol. 38, 344-350) and produces indentations of 1-4 micrometers in the intima of rabbit thoracic aorta with a reproducibility of 1-2%. Preliminary usage of the new microindentor has shown increases in the compliance of the thoracic aorta intima in rabbits during cholesterol feeding.

Scanning electron microscopy of endothelium around an experimental stenosis in the rabbit aorta using a new casting material

Abstract Four New Zealand White rabbits had bands fitted to their thoracic aorta causing a 73 ± 2 (SD) % reduction in cross-sectional area. The resulting stenosis produced a slight pressure drop and a post-stenotic dilatation that was evident at the time of preparation for microscopy 47 ± 2 days after fitting the band. Four other rabbits served as unoperated controls In all animals a portion of the thoracic aorta was glutaraldehyde-fixed and the lumenal surface cast by injecting a silicone rubber impression material. Casts were photographed and measured before being viewed under the scanning electron microscope. Micrographs were taken from predetermined areas and nuclear numerical density measured. No significant dorso-ventral differences in nuclear numerical density appeared in any aorta and no significant differences existed between sites in the 4-cm region of thoracic aorta sampled from control animals. Endothelial cells upstream to stenoses appeared identical to those from control arteries, although there was a greater incidence of spindle cells and cellular adherence than elsewhere. Nuclear density here was significantly higher than in control arteries but not different to any other region in the stenosed aorta. However, cells from within the stenosis and from just downstream were noticeably altered. Within the stenosis the cast reproduced longitudinal folds and the cells were strictly oriented with the long axis of the vessel. Nuclear density here was higher than any other region except just distal to the stenosis where there was a marked loss of normal longitudinal orientation and a change in the shape of some cells from elongated to polygonal. Swirling patterns and other zones of disorientation were evident up to 2 diameters distal to the stenosis, while within 2–4 diameters distal the cells appeared somewhat rounder and less strictly oriented than in control vessels. Beyond 4 diameters downstream, the cells appeared identical with those of control arteries and were of the same density as in mid-dilatation or in controls. These observations relate to expected local flow patterns, and are relevant to the role of haemodynamic factors in atherosclerosis.

Morphometric analyses of rabbit thoracic aorta after poststenotic dilatation.

The aim of this study was to quantify the morphological changes in the arterial wall resulting from poststenotic dilatation (PSD). PSD was produced by placing a split nylon ring around the thoracic aorta of the rabbits at a level of T6-7 during a sterile thoracotomy done under pentobarfoital anesthesia. After period of PSD ranging from 1-51 months these rabbits were anesthetized, as were the control animals, and the descending thoracic aorta from the fourth to the eleventh ribs was removed following perfusion fixation with Karnovskys solution at a constant pressure of 80 mm Hg. The extent of PSD development was variable even though the stenotic ring was the same size in all rabbits. Ultrastructural findings showed degenerative changes of the wall components in the PSD region and were more prominent in the aortas with greater dilatation. Morphometric measurements showed that the PSD was accompanied by a decrease in volume density of both smooth muscle cells (SMCs) and elastin and an increase in collagen and ground substance. These changes were well correlated with degree of dilatation and ratio of internal radius to wall thickness (hence, mean wall stress) but not with duration of PSD. While the number of SMCs per unit volume in the PSD aortas was significantly less than normal (p<0.05), there was no significant change in mean cell volume. Although the reduced muscle mass might be expected to lower the capacity of the vessel to maintain tone, previous results show that this does not occur.

Changes in the Microindentation Properties of Aortic Intimal Surface during Cholesterol Feeding of Rabbits

A new microindentation technique using a small, loaded spherical probe (25 microns diameter) was developed to quantify early mechanical changes in the arterial intima during atherogenesis. Routine use of forces of 10 microN on normal intima for 10 S resulted in indentations of about 2.5 microns which gives an intimal compliance of 0.245 +/- 0.031 m/N. There was a decrease in intimal compliance in the proximal to distal direction in both the rabbit and the dog thoracic aorta. The region immediately distal to an intercostal orifice in the rabbit was often up to 1.6 times more compliant (softer) than surrounding areas while the corresponding region in the dog tended to be less compliant than elsewhere. Feeding a 1% cholesterol diet to rabbits for 8 weeks increased the intimal compliance to 0.368 +/- 0.041 m/N. The regions distal to the intercostal ostium were 32% more compliant after 4 weeks of cholesterol diet than non-orifice-related areas but after 8 weeks of the diet there was no significant difference between these regions. The time course of indentation (viscous component) was characterised by a rapid indentation phase which almost became non-exponentially asymptotic by 6 S and was 66 +/- 3% SD of this value by 1 S. Cholesterol feeding (8 weeks) altered this phase to 76 +/- 4% SD of the asymptotic value after 1 S of indentation, indicating a decrease in initial viscosity. A very slow indentation or creep was also found when the force continued for a period up to 25 min. Cholesterol feeding increased this rate of creep by approximately 15%. The results show that early atheromatous changes induced by cholesterol feeding are accompanied by softening and increased fluidity of the aortic intima.

A computerized scanner to quantify atherosclerosis

A method is described for the quantitation of atherosclerotic lesion after excision and staining of the vessel. Essentially the device consists of a microscope with a computer-controlled stage which moves a photographic negative image over a square beam (o.1 mm sides) of light. At each translation-step of the stage a transmittance reading is taken using a photomultiplier tube for detection. The computer stores topographical values (typically around 6,000) and, by setting a threshold, can determine the fractional area of artery which is diseased.

Analysis of all possible combinations of four measurements determining true propagation in arteries

All possible combinations of four measurements of blood pressure, blood flow and vascular diameter are examined by transmission-line theory. It is found that only nine measurement combinations can give the attenuation coefficient gamma, reflection coefficient R and characteristic impedance Z0 simultaneously. At least one pressure measurement must be included. Their general expressions with arbitrary measuring locations are presented, together with some simplified forms which cover all the previous methods. A greater choice of method is thereby made available for use in practice. The results show that, regardless of the measurement locations, all combinations can be solved in the order gamma first, R second and Z0 last.

A semi-automatic device for the mapping of orifice-related atherosclerotic lesions☆

Abstract A devise is described which allows direct measurement of orifice-related lesions and simultaneous data entry to a computer.