The effect of storage time and repeated measurements on the elastic properties of isolated porcine aortas using high resolution x-ray CT.

A Laboratory CT scanner with a resolution of (0.1 mm)3 was used to determine if storage up to 7 days in saline at 4 degrees C and (or) repeated measurements would alter the compliance, C, and incremental elastic modulus, Einc, of isolated porcine aortas. All specimens were obtained fresh, made pressure-tight, and then mounted in the scanner, with humidified air used to produce adequate x-ray contrast. The specimens were imaged at pressures of 4, 8, 12, 16, 20, and 24 kPa, and vessel measurements were then obtained with a computerized technique and analyzed. Seven thoracic aortas were studied on days 0, 3, 5, and 7, with a significant change (p < 0.05) in compliance first occurring after three imaging studies (i.e., day 5). Compliance of the fresh thoracic aortas (mean +/- SD) was 0.90 +/- 0.28 mm/kPa at 14.4 kPa and 0.85 +/- 0.31 mm/kPa at 22.5 kPa. Six thoracic aortas were studied only on days 0 and 6 with no intermediate measurements. They showed no change in either compliance (0.88 +/- 0.07 mm/kPa at 14.4 kPa and 0.64 +/- 0.09 mm/kPa at 22.5 kPa) or Einc (0.46 +/- 0.05 MPa at 14.4 kPa and 0.88 +/- 0.15 MPa at 22.5 kPa) from day 0 to day 6. Thus, number of measurements rather than time appears to be the important factor. Six abdominal aortas were studied similarly but on days 0, 3, and 6. No significant change occurred in compliance (0.15 +/- 0.06 mm/kPa at 14.4 kPa and 0.032 +/- 0.026 mm/kPa at 22.5 kPa) but Einc showed a change, possibly due to their viscoelastic properties. We conclude that this nondestructive CT measurement method is suitable for repeated studies on porcine thoracic aortas, but not abdominal aortas, if the measurement involves two consecutive imaging sessions separated by no more than 6 days.

Elastic response of human iliac arteries in-vitro to balloon angioplasty using high-resolution CT.

Previous angioplasty studies have used angiography and intravascular ultrasound to obtain vascular dimensions. These imaging methods do not always provide reliable measurements due to limitations in image orientation and resolution. In this study, high-resolution (0.1 x 0.1 x 0.5 mm) transverse CT slices were obtained from human common-iliac arteries in vitro to study their elastic response pre- and post-angioplasty. Seven iliacs from five patients were imaged over the physiological pressure range both pre- and post-angioplasty. Contrast was obtained with humidified air surrounding the artery. Angioplasty was done with 10 or 12 mm diameter Medi-Tech balloon catheters with a balloon pressure of 300 kPa held for 30 s. Lumen circumference, c, measured from the images, was plotted against pressure, P, and curve fitting showed c = A(1 - e(-KP)) + B where A, K, and B are fitting parameters. Six lesions appeared soft and were compressed, while one was calcified and partially lifted off the wall. Normalized changes in parameters B and K were much higher post-angioplasty in the calcified lesion, and were over 3 standard deviations from the means of the normalized changes in the six compressed lesions. Balloon/stenosed lumen diameter ratios greater than 1.2 produced a lumen area increase of 38.6 +/- 4.1%(S.D.)(n = 3); ratios less than 1.2 produced an increase of 4.4 +/- 5.1%(S.D.)(n = 4). There was no correlation between area increase and balloon/normal lumen diameter ratio (the value used clinically). Arteries with lesions containing stiffer plaques that tear from the artery wall during angioplasty appear more distensible over the physiological pressure range post-angioplasty.

Determination of appropriate recording force for non-invasive measurement of arterial pressure pulses.

1. Non-invasive recording techniques of the arterial pressure pulse will distort the arterial wall and may alter pulse wave measurements. We hypothesized that intersubject variability of these measurements would be reduced if recording forces were normalized to reflect individualized arterial occlusion forces. 2. In 10 normal male subjects (age 24 +/- 1 years), brachial, radial and finger arterial pressure pulses were recorded simultaneously using volume displacement pulse transducers (Fukuda TY-303) and a finger pressure monitoring system (Finapres, Ohmeda 2300) and were made at 2, 5 and 10-100% (10% increments) of the brachial arterial force associated with marked distortion of finger pulsations. Forces were applied at the brachial site in a randomized order while a constant 1.8 N force was applied at the radial artery site. Pressure pulses were analysed using the discrete fast Fourier transform. 3. Pulse amplitude, contour, wave velocity and relative transmission ratios remained relatively constant until the branchial artery recording force exceeded 59.9 +/- 0.3% of the largest recording force used in each subject (7.14 +/- 0.75 N). The finger pulse pressures (P < 0.0001), radial pulse amplitudes (P < 0.0001) and contours (harmonics 2-6, P < 0.003), pulse wave velocity (P < 0.021) and relative transmission ratios (harmonics 3-7, P < 0.01) then decreased with higher recording forces. 4. To avoid distortion, non-invasive recordings of arterial pressure pulse amplitude, contour, pressure wave velocity and relative transmission ratios along a peripheral arterial segment should use recording forces of less than 60% of the force associated with marked distortion of finger pulsations.

Influence of applied brachial recording forces on pulse wave velocity and transmission in the brachio-radial arterial segment.

Arterial stiffness in hypertension and heart failure may increase afterload on the left ventricle. Pulse wave velocity and transmission ratio measurements are noninvasive methods to assess arterial stiffness. Since noninvasive pressure pulse recording requires sufficient applied force to distort the vessel wall, we hypothesized that the pulse wave velocity, transmission ratios, and distal pulse amplitudes and contours may be altered by the recording technique. Brachial and radial arterial pressure pulses were recorded simultaneously using a piezoelectric pulse transducer in 14 young, normal, male subjects using 10 brachial artery recording forces (0.35-3.58N, approximately equal to 0.36N increments) applied in a random order and a constant radial force (2.35N). Pulses were Fourier analyzed. One subject was excluded from analysis because of improper transducer positioning over the brachial artery. In 8 subjects, no significant changes occurred in any variable over all brachial recording forces. In the remaining 5 subjects, the measured variables remained constant until brachial artery recording forces exceeded 2.42 +/- 0.03N. The pulse wave velocity (p < 0.04), transmission ratios (harmonics 2-5, p < 0.0001), radial pulse amplitude (p < 0.0003), and relative powers (harmonics 2-5, p < 0.02) then decreased. In these subjects, brachial artery depths were less than the other subjects (5.9 +/- 0.4 vs. 7.7 +/- 0.4 mm, p < 0.05). The brachial and radial artery recording forces normally used during clinical measurements by 2 investigators were 1.43 +/- 0.01N (95% confidence intervals (CI) = 1.23N, 1.62N) and 1.88 +/- 0.11N (95% CI = 1.65N, 2.10N), respectively. Therefore, at forces normally used by clinical investigators, the pulse wave velocity, harmonic transmission ratios, and pulse amplitudes and contours obtained at the brachial and radial artery are not significantly influenced by forces applied at the brachial artery. However, these variables may be decreased in subjects with more superficial arteries when higher recording forces are used.

The effects of ultrasound treatment on flexor tendon healing in the chicken limb.

We report the effects of early and late ultrasound treatment protocols on healing of surgically lacerated zone 2 flexor tendons in a chicken model. Ultrasound was administered directly using a coupling gel. Treatment was shown to increase range of movement, to advance scar maturation and to decrease the amount of inflammatory infiltrate around the repair site. No adverse effects on tensile strength were noted in either group. Early (starting 7 days post-operatively) administration was more effective than late (starting 42 days post-operatively) administration in achieving these effects. These results suggest that ultrasound therapy may be of benefit in the early healing process of flexor tendon lacerations.

Applied recording force and noninvasive arterial pulses.

Arterial pressure pulses are often measured noninvasively, but the influence of external recording forces has not been well documented. Accordingly, the main goal was to delineate pulse contour and amplitude distortion with progressively compressive forces applied to an external transducer. Ten young normal male subjects were studied while supine. Pulses were recorded at the end of a normal expiration over a range of forces (0.45-4.29, 0.42N increments) applied in a randomized order over first the brachial and then the radial artery. Pulse contours were Fourier analyzed and harmonic powers were normalized to the peak power at the fundamental frequency. Brachial artery pulse amplitudes progressively decreased (p = 0.013), whereas, those at the radial artery peaked at a recording force of 1.79 +/- 0.01N (p < 0.001) and subsequently decreased parallel to the brachial data with larger forces. No significant pulse contour distortions occurred at either the brachial or radial artery with applied forces of up to 2.18 +/- 0.02 and 2.59 +/- 0.02N, respectively, as indicated by the similarity of the relative power for harmonics 2-7. Radial artery pulses were distorted at and beyond a force of 2.99 +/- 0.01N as indicated by the increased relative power of harmonics 3-7 (p < 0.05). Therefore, despite significant alterations in pulse amplitude, the relative shape of pulses remains similar over a large range of lower recording forces.

Calcium mass in human aortas from autopsy.

We used a single-energy x-ray method and an image processing system to measure the amount of calcium salts deposited in 60 human aortas (41 male and 19 female) of ages 15-88 y. The aortas were removed at autopsy, slit lengthwise and preserved in formalin. The aortas were x-rayed while flat, together with a calibration block containing known amounts of CaHPO4. We determined the amount of calcium salt, referenced to CaHPO4, from the x-ray film with a personal computer using a frame-grabbing board. There was less calcium in the thoracic than the abdominal aorta both in males and females and those of ages less than 50 y. The coefficient of correlation for abdominal aortic calcium mass versus age in 19 females was 0.557 and in 41 males was 0.321. The higher correlation of mass with age for females may be related to differences in calcium metabolism between sexes.

Frequency analysis of the peripheral pulse wave detected in the finger with a photoplethysmograph.

A photoelectric plethysmograph is described that performs a frequency analysis of the peripheral volume pulse wave using a portable computer. It was used to determine how the pulse shape varied with age using 54 subjects in three age groups, 10-29, 30-59, and 60-89 years. The youngest group had a larger power in the second harmonic, (normalized to the fundamental), with p less than 0.05 than the older two groups. The decrease of power in the harmonics of the peripheral pulse wave with age may be a useful noninvasive measure of aging and vascular disease.

Effects of age on the anisotropy of the descending human thoracic aorta determined by uniaxial tensile testing and digestion by NaOH under load.

Strips of human thoracic aortic wall taken at autopsy from 23 individuals aged 15-81 years have been tested in two ways: in uniaxial loading and by digestion in 0.1 M NaOH at 75 degrees C under a load of 50 g. The circumferentially oriented strips were more extensible in loading and took a longer time to fail while being digested under load than the longitudinal strips from the same location. The stress versus strain curve was fitted to an exponential equation of the form, stress = A[exp(B strain)]. For circumferentially oriented strips from 19 subjects, parameter A was independent of age. Parameter B increased by a factor of 2.75 from 19 to 81 years. For longitudinally oriented strips for 14 subjects, A was also independent of age and B increased by 2.4 times from 25 to 81. With digestion under load for 11 matched pairs of strips aged from 32 to 75 the circumferential strips failed in 117 +/- 23 min (standard error), while the longitudinal ones failed in 20.3 +/- 3.2 min (standard error) (p less than 0.003). The results have important implications for vessel attachments to the aorta in heart transplantation and in vascular surgery.

Extensibility changes of calcified soft tissue strips from human aorta.

Some degree of calcification was noted in more than half of the 59 aortas of individuals aged from 15 to 88 we have examined at autopsy. The calcification, which is determined by x-raying the opened and flat aorta, is in patches. We have studied the influence of calcification on stress versus strain, breaking strength, and modulus of elasticity of strips of aorta to determine its importance in vascular disease. Strips of aortic wall 5 x 30 mm were cut with orientation parallel or perpendicular to the vessel axis. Elongation versus load was measured with an Instron tensile testing machine. The true stress and true strain were calculated for both calcified and uncalcified strips from the thoracic and abdominal regions in both orientations. From the stress-strain curve the following values were selected: strain, stress, and slope at 80 mmHg equivalent pressure (1 mmHg = 133.3 Pa); maximum stress, strain, and slope; and breaking stress, strain, and slope if the sample broke. There were statistically significant differences in 13 of the 36 categories between calcified and uncalcified strips. The breaking strength and strain is lower in the calcified strips. The stress-strain curve for the uncalcified strip was mathematically transformed by reducing the amount of elongation so that the curve coincided with that of the calcified strip for eight matched pairs from the same individuals. The calcification appears to immobilize part of the strip, probably causing the boundary of the calcified tissue to be a region of high stress where tissue breakdown can occur.

The elastic properties of canine abdominal aorta at its branches.

The elastic properties of the abdominal aorta at regions of junctions were studied using strips from 17 dogs. Strips of tissue cut longitudinally and circumferentially at the celiac, mesenteric, and renal branches were used to compare the properties of the proximal and distal junctions, as well as the aorta and artery regions adjoining. The tissues were stored for at least 24 h to ensure that no active component of the smooth muscle remained. The elastic properties measured here are due to elastin and collagen, with a small contribution from dead smooth muscle cells. The tissue strips were tested at 20 degrees C while immersed in saline using an Instron tensile testing machine. Elongation of the three regions was measured from photographs taken as the tissue was stretched. Stress values went to 200 kN/m2 as the strain increased to approximately 0.8. (The physiological range for the dog was calculated as 40-85 kN/m2.) The distal junctional region was found to be the most extensible for both longitudinally and circumferentially oriented strips. These results have important implications for flow models as they imply that the shape of the junctional region probably changes between diastole and systole.

Mechanical anisotropy of purified elastin from the thoracic aorta of dog and sheep.

Mechanical anisotropy was found in elastin from the wall of the thoracic aorta by testing digested vessels from seven mature dogs and five pregnant ewes. The elastin was purified by removing the muscle and collagen with 0.1 N NaOH at 75 degrees C for 5 h. Strips oriented circumferentially and longitudinally with respect to the vessel axis were cycled at increasing loads until failure. Combining the results for four dogs and three sheep the ultimate tensile strength was 405 X 10(3) +/- 155 X 10(3) SD N/m2 circumferentially and 189 X 10(3) +/- 85 X 10(3) SD N/m2 longitudinally. The difference is significant at p less than 0.01. The ultimate strain was not significantly different, 1.19 +/- 0.29 SD circumferentially and 1.01 +/- 0.26 SD longitudinally. At 0.4 strain, the Young's modulus for four samples from four dogs was 404 X 10(3) +/- 64 X 10(3) SD N/m2 circumferentially; the modulus for five samples from five dogs was 245 X 10(3) +/- 60 X 10(3) SD N/m2 longitudinally. The difference is significant at p less than 0.05. At 0.7 strain, the Young's modulus for four dogs was 486 X 10(3) +/- 82 X 10(3) SD N/m2 circumferentially and 343 X 10(3) +/- 62 X 10(3) SD N/m2 longitudinally. The difference is significant at p less than 0.05. The reasons for the anisotropy in terms of the structure are not obvious from preliminary studies using the scanning electron microscope with stretched samples.

Action potential fatigue in nonmyelinated nerve fibers: garfish olfactory and rabbit vagus nerve.

A decrease in amplitude and conduction speed in the compound action potential is observed with time in gar olfactory and rabbit vagus nerve when it is stimulated between 4 and 15 Hz at 26 degrees C in vitro. The amplitude decays exponentially for 1-3 min before reaching a steady state. Recs 15 s for gar olfactory nerve and 50 s for rabbit vagus nerve. The steady state values are 14% and 36% of the original amplitude, respectively, and conduction speeds are reduced by 25 % in both nerves. The effect results from completion between ion flow during the action potential and active transport. The accumulation of K+ions and depletion of Na+ ions in the restricted extracellular space contributes to the amplitude fatigue while the depletion of Na+ ions alone causes the decrease in conduction speed. Ouabain increases the fatigue rate in both preparations, but more so for gar. The fatigue and recovery measurements may provide a useful method to investigate active pumping, including both the total pumping rate and the electrogenic component.

Dischroism of the infrared spectrum of nerve.

Infrared spectra were obtained from the sciatic and optic nerves of the frog (Rana sp.) and the trigeminal and olfactory nerves of the garfish (Lepisosteus osseus). The myelinated nerves showed dichroism at several absorption peaks, particularly 1,220-1,230 cm(-1), but the nonmyelinated nerves showed little or no dichroism. The dichroic peaks indicate that in myelinated nerve, there is an ordered arrangement of protein and lipid molecules which was not found in nonmyelinated nerve.

Electric-field-induced shifts in the infrared spectrum of conducting nerve axons.

Difference spectra between resting and excited nerve in the infrared region between 2000 and 1000 cm(-1) have been examined with a resolution of 0.5 cm(-1). Spectra were obtained with a modified Perkin-Elmer model 521 grating infrared spectrophotometer (Perkin-Elmer Corp., Instrument Div., Norwalk, Conn.), and the signal-to-noise (S/N) ratio was improved by time averaging and digital smoothing. Peaks occurring in the regions around 1030 and 1066 cm(-1) are identified as P-O-C stretch, at 1410-1414 cm(-1) as C-H deformation, and at 1750 cm(-1) as carbonyl stretch. The difference peaks appear to be due to a shift of about 1 cm(-1) in the absorption band to lower frequencies for the three lower frequency bands and to a higher frequency for 1750 cm(-1) band. Since the difference peaks appear when the nerve is modulated by a propagated action potential it is concluded that the changing electrical field across the nerve membrane is perturbing the absorption spectrum. From evidence presented it appears likely that these difference peaks are due to phospholipids in the nerve membrane and that they may be related to conformational changes associated with membrane permeability.