Pressure Diameter Research Articles

Beat to Beat Modulation of Arterial Pulse Wave Velocity Induced by Vascular Smooth Muscle Tone.

The level of smooth muscle activation or `tone' (VSMT) can be defined as an intrinsic spontaneous level of vasoconstriction that may change in response to biomechanical stimuli such as flow, pressure, hormonal stimuli, neural stimuli, and drugs. Arteries can actively modify arterial pressure pulse wave velocity (PWV) by changing smooth muscle tone and diameter with little change in the mean arterial blood pressure. To evaluate the influence of aortic VSMT in terms of beat to beat PWV variations. Aortic pressure-diameter (P-D) relationships were invasively evaluated in five conscious animals. An `incremental' PWV was directly derived from the P-D loops, both in purely elastic conditions as well as in the presence of VSMT. Beat to beat PWV waveform variations showed different behavior, where mean PWV values were higher when VSMT was included in the P-D evaluation. Changes in incremental beat to beat PWV could be attributed to the influence of VSMT.

Validation of a Pressure Diameter Method for Determining Modulus and Strain of Collagen Engagement for Long Branches of Bovine Pulmonary Arteries

Recent studies have shown that capacitance measurements of large arteries provide better prognosis and diagnosis than tests of resistance alone in pulmonary hypertension (Mahapatra et al., 2006, "Relationship of Pulmonary Arterial Capacitance and Mortality in Idiopathic Pulmonary Arterial Hypertension," J. Am. Coll. Cardiol., 47(4), pp. 799-803; Reuben, 1971, "Compliance of the Human Pulmonary Arterial System in Disease," Circ. Res., 29, pp. 40-50]. Decreased arterial capacitance causes increased load to the heart and is the direct result of increased stiffness and elastic modulus of the arterial wall. Here, we validate a pressure-diameter (PD) method for comparing the elastic modulus and collagen engagement for post-hilar pulmonary arteries with a large range of arterial diameter. The tissue mechanics of the post-hilar arteries are not well-characterized in pulmonary hypertension. It is believed that future studies with this method will provide useful insight into the role of passive tissue mechanics of these arteries in the pathophysiology of pulmonary hypertension, eventually improving clinical diagnosis, prognosis, and treatment. Post-hilar pulmonary arteries, excised from healthy and hypertensive calves and healthy cows, were inflated over a range of 0 [mm Hg] to 110 [mm Hg] in an isolated tissue bath. Internal pressure was recorded with an electric pressure catheter. Artery diameter and longitudinal stretch were recorded photographically. Stress-strain data curves were extracted using Lame's law of thick-walled tubes. Radial strips were removed from each section and tested in a uniaxial (MTS) tester for validation. Both the elastic modulus and collagen engagement strain were similar to results obtained by more traditional means. The average difference between measured values of the two methods for collagen engagement strain was 3.3% of the average value of the engagement strain. The average difference between the measured values of the two methods for modulus of elasticity was 7.4% of the average value of the modulus. The maximum, theoretical, relative error for the stress determined with the PD method was calculated at 20.3%. The PD method proved to be a suitable replacement for uniaxial strain tests in comparing collagen engagement strains. The method allowed faster testing of tissues of multiple diameters, while removing the effect of end conditions. The PD method will be of further utility in continued study of tissue mechanics in pulmonary hypertension studies.

In vivo measurement of proximal pulmonary artery elastic modulus in the neonatal calf model of pulmonary hypertension: development and ex vivo validation

Developing clinical work suggests that vascular stiffening plays a role in the progression of pulmonary hypertension (PH), while recent studies in animal models of hypoxic PH have found significant proximal vascular stiffening in the diseased population. Here, we develop and validate a minimally invasive, clinically realizable method to estimate the local elastic modulus of the proximal pulmonary arteries from pressure-diameter (PD) data. PD measurements were made in the main pulmonary arteries of 16 calves; lumen diameter was assessed using color M-mode tissue Doppler imaging ultrasound, while pressure was measured via catheter. Two methods corresponding to thin-walled pressure vessel theory ("thin") and Lame's equation for a thick-walled cylinder ("thick") were used to approximate the artery elastic modulus from PD measurements. The harvested arteries were tested independently to determine their "true" ex vivo elastic modulus and stiffness. Both approximations displayed excellent correlation with ex vivo elastic modulus of the calf main pulmonary artery (thin r(2) = 0.811; thick r(2) = 0.844; both P < 0.01). Bland-Altman analysis indicated that the thick-walled approximation has better overall agreement with ex vivo modulus. The approximations displayed quantitatively distinct regression slopes that were statistically different (P = 0.02). The elastic modulus of the main pulmonary artery can be reasonably estimated from combined color M-mode tissue Doppler imaging ultrasound and catheter pressure measurements in calves. Such measurements may be a valuable tool in the diagnosis and treatment of human PH.

Anisotropic in vitro vessel model using poly(vinyl alcohol) hydro gel and mesh material

AbstractIn the present study, the authors fabricated straight multilayer hybrid tubular in vitro vessel models (inner diameter Din = 10 mm; thickness T = 4 mm) composed of poly(vinyl alcohol) hydrogel (PVA‐H) and anisotropic mesh materials. The authors performed tensile, stress‐relaxation and cyclic‐tensile tests using axial and circumferential test pieces as well as pressure‐diameter (P‐D) tests using tubular test piece. In the tensile and stress‐relaxation tests, the anisotropic and nonlinear mechanical properties and hysteresis characteristic of the in vitro models were confirmed. The in vitro models also showed behavior qualitatively similar to that of native arteries in cycle‐tensile and P‐D tests. These results demonstrate that the mechanical properties of native vessels can be duplicated in an in vitro model by controlling the components of the mesh material, the orientation of elastic fibers in the mesh material, and the concentration and thickness of PVA‐H layers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Pressure‐diameter behavior of airways measured from tantalum bronchograms in isolated mouse lungs

In a previous study, conscious mice in response to methacholine aerosol reached a limit in airway constriction that occurred near zero airway elastic recoil, based on an analysis using elastic and active properties of large dog airways (J. Appl. Physiol. 105: 933‐941, 2008). We wondered whether mice have airway elastic properties similar to dogs. Balb/c mice (n = 5, 23‐27 g) were anesthetized and tantalum dust (5 micron) was insufflated via a 22‐G catheter into the lungs using a 0.2 ml air injection to produce a well‐defined bronchogram. Postmortem, the chest was opened and the lungs were inflated to 25 cmH2O transpulmonary pressure (Ptp) then deflated in steps to 10, 5, 0, ‐5 and ‐10 cmH2O. Tantalum bronchograms were taken at each Ptp. Airway diameters (8‐10) located between 1 and 8 airway generations were measured from the bronchograms of each mouse. Airway diameter decreased from a maximum (Dmax) at 25 cmH2O Ptp to 87, 79, 60, 50 and 47 %Dmax at 10, 5, 0, ‐5 and ‐10 cmH2O Ptp, respectively. The pressure‐diameter (PD) behavior was similar for both large (Dmax of 0.75 ± 0.11 (SD) mm) and small (Dmax of 0.32 ± 0.026 mm) airways. The measured PD behavior of mice was similar to that reported for dog airways (J. Appl. Physiol. 44: 859‐868, 1978). Supported by NIH grant HL 36597.

The effect of sympathetic stimulation on proximal brachial artery mechanics in humans--differential behaviour within the length of the brachial artery?

The mechanical properties of arteries play a major role in the regulation of blood pressure and cardiac performance. The effect of sympathetic stimulation on the mechanical properties of the proximal brachial artery was analysed in 18 healthy volunteers, nine young (25 +/- 2 years) and nine elderly (69 +/- 2 years). A non-invasive ultrasonic echo-tracking system for measurement of systolic/diastolic variation of the proximal brachial artery diameter in combination with intra-arterial pressure measurements was used to determine wall mechanics. The pressure-diameter (P-D) relationship, distensibility coefficient (DC), compliance coefficient (CC) and stiffness(beta) were obtained at rest and during sympathetic stimulation induced by lower body negative pressure (LBNP). The peripheral vascular resistance increased by 100 and 72%, respectively in the young and elderly during LBNP (P < 0.001). Simultaneously, the mechanical properties of the proximal brachial artery remained unaltered, as estimated from both P-D relationship and stiffness in young (beta-index rest: 5.2 +/- 0.9, LBNP: 5.5 +/- 1.3, NS) as well as elderly (beta-index rest: 13.6 +/- 4.6, LBNP: 16.1 +/- 4.7, NS). LBNP-induced sympathetic activation does not change proximal brachial artery mechanics, in contrast to earlier reports on the muscular distal brachial artery. This may imply that the transition between elastic and muscular artery behaviour is within the length of the brachial artery, where the site of transition from elastic to muscular wall structure needs to be specified in future studies.

Small-diameter compliant arterial graft prosthesis: Design concept of coaxial double tubular graft and its fabrication.

To minimize compliance mismatch between native artery and arterial graft prosthesis over the entire pressure regions, we proposed a coaxial double tubular artificial graft which consists of an enhanced compliant inner tube and a less compliant outer tube, both of which were fabricated using well-controlled multiply micropored segmented polyurethane (SPU) films. Double tubular grafts were coaxially assembled by inserting the inner tube into the outer tube. First, the pressure-diameter (P-D) relationship of canine common carotid arteries, which exhibited a "J" curve, was determined as a targeted artery. Two determinant variables, the pressure-induced distensibility of each tube and the intertubular space distance, were defined and formulated in several models of coaxial double tubular SPU grafts, which had various intertubular space distances, micropore densities, and wall thicknesses. The distensibility of the inner tube determined the distensibility in the low-pressure regions, which was adjusted using wall thickness and microporosity. Thinner films with higher porosities resulted in a high pressure-induced distensibility. On the other hand, a low pressure-induced distensibility in the high-pressure regions was realized using an outer tube with a thicker wall and lower microporosity. The transition point from low- to high-pressure regions was determined by the intertubular distance using the theoretical values. On the basis of these results, we presented a prototype model of a coaxial double tubular graft that exhibited well-matched compliance with canine carotid artery.

Sex difference in the mechanical properties of the abdominal aorta in human beings

Purpose: A previous study has shown age- and sex-related differences in abdominal aortic compliance. In that study blood pressure determined by auscultation in the brachial artery was assumed to be equal to blood pressure in the abdominal aorta. To validate our findings we investigated the pressure-diameter (P-D) relationship of the abdominal aorta.Methods: Diameter and pulsatile diameter change of the abdominal aorta were determined noninvasively by an ultrasound phase-locked echo-tracking system with simultaneous measurement of aortic pressure resulting in P-D curves in 27 healthy male and female volunteers 23 to 72 years of age. The degree of error in aortic compliance as calculated from blood pressure determined by auscultation of the brachial artery rather than from direct measurement of aortic pressure was evaluated. Compliance was defined as the inverse of pressure strain elastic modulus (Ep) or of stiffness (β).Results: There was no significant difference in the systolic pressure at the two sites, but the diastolic pressure was systematically overestimated by approximately 10 mm Hg when determined by the auscultatory method (p < 0.01) leading to a 15% to 20% underestimation of Ep and stiffness (β). The individual P-D curves exhibited hysteresis, were nonlinear, and revealed that the aorta is more distensible at lower than at higher pressures. The steepness of the P-D curve decreased with increasing age and this occurred at an earlier age in men than in women.Conclusion: This investigation demonstrates a decrease in abdominal aortic wall distensibility with age, which occurs at an earlier age in men, and confirms earlier results by use of the indexes Ep and stiffness (β). This implies that the abdominal aorta in men is more prone to degenerative changes, which may be one of the factors responsible for the sex difference in aortic vascular disease.

Preventive effect of chronic converting enzyme inhibition on aortic stiffening induced by renovascular hypertension in conscious dogs

The aim was to assess the influence of the renin-angiotensin system on the geometrical and elastic properties of the aorta in conscious dogs, using a model of renovascular hypertension, and to examine the effects of inhibition of the system by the angiotensin converting enzyme inhibitor spirapril. The aortic elastic behaviour in response to renovascular hypertension was studied in 15 conscious dogs instrumented with a pressure microtransducer and a pair of ultrasonic diameter dimension gauges in the upper descending thoracic aorta. Renovascular hypertension was induced by surgical occlusion of one renal artery and stenosis of the other. One day after renal surgery, dogs were randomly assigned to two groups receiving for two months either the new angiotensin converting enzyme inhibitor spirapril (n = 8) or a placebo capsule (n = 7). The two groups of dogs were compared to a control group of normotensive dogs (n = 7). After two months of treatment the elastic properties of the aorta were studied by computation of the beat to beat pressure-diameter hysteresis loops obtained during transient increase of pressure induced by bolus doses of angiotensin. The aortic pressure-diameter (P-D) relationship, obtained over a wide range, was fitted by an exponential fit (P = alpha.e beta D), where beta is the stiffness index. A decomposition of the P-D curve according to a biphasic model of the parallel arrangement of elastin and collagen enabled two pressure-diameter elastic moduli to be obtained, one representing the resistance to stretch at low pressure levels (elastic fibres and smooth muscle), and the other representing the resistance to stretch at the highest pressures (collagen fibres). The pressure-diameter curve of the placebo group was shifted to the left compared to the curves of the control and spirapril groups, showing that renovascular hypertension was associated with isobaric reduction of aortic diameter. The stiffness index beta was higher (p < 0.05) in the placebo group [0.605(SD 0.304) mm-1] than in either the control group [0.362(0.126) mm-1] or the spirapril group [0.348(0.083) mm-1], suggesting that renovascular hypertension was associated with aortic stiffening. The biphasic analysis showed that the collagen pressure-diameter elastic modulus was unaffected by spirapril, whereas the elastin pressure-diameter elastic modulus was significantly reduced by converting enzyme inhibitor with respect to the placebo (p < 0.05). Chronic converting enzyme inhibition by spirapril prevents the isobaric aortic diameter reduction induced by renovascular hypertension in conscious dogs and decreases aortic stiffness, in particular by changing the elastic behaviour of the elastin fibres rather than of the collagen fibres.

Effect of diltiazem on aortic pressure-diameter relationship in dogs.

To study the effects of diltiazem (Dz) on aortic (Ao) pressure-diameter (P-D) relationships, the external diameter (D) of the thoracic Ao (sonomicrometry) and Ao pressure (AoP, micromanometer) were simultaneously measured in 10 anesthetized dogs. The P-D relationship from 80 ms after the dicrotic notch of AoP (t80) to 20 ms after the peak of the next R wave on the electrocardiogram (ECG) (t20) was plotted at 30-ms intervals. To depict the P-D relationship in a wide range of AoP, Ao occlusion immediately after the release of inferior vena caval (IVC) occlusion was performed both before and after a Dz (0.03 mg.kg-1.min-1, over 10 min) infusion. The P-D relationship so obtained was a sigmoid curve under both conditions and was reasonably described by a cubic curve (P = aD3 + bD2 + cD + d, where a, b, c, and d are constants and r greater than 0.99). The first derivative of cubic regression equation (dP/dD) value of the cubic curve was used as an index of Ao stiffness. In all dogs, the dP/dD-D relationship, which was parabolic, was similar to the delta P/delta D-D relationship measured from the actual P-D relationship (delta P/delta D is the ratio of the change in AoP to that in AoD), suggesting that it corresponded to a cubic curve.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of vessel wall in acute resetting of aortic baroreceptors.

Previously we found that baroreceptor (BR) pressure-frequency (PF) curves rapidly (5 min) reset to changes in mean arterial pressure (MAP) exclusive of sympathetic efferents or circulating hormones. Since BRs are mechanically coupled to vessel wall structures, resetting may be due to changes in the wall. This was examined using an in vitro rat aortic arch-aortic nerve preparation and a photoelectric device to measure arch diameter. Pressure-diameter (PD) curves were constructed at selected MAPs following the resetting protocol used previously [Am. J. Physiol. 244 (Heart Circ. Physiol. 13): H672-H680, 1983]. Overall there was no consistent relationship between the PD curves and MAP, which was in sharp contrast to BR PF resetting. To determine if smooth muscle tone could affect aortic or BR function, the arch was constricted or relaxed with norepinephrine (NE) or sodium nitroprusside (NP), respectively. BR PF and aortic PD curves were constructed before and after drug treatment. NE reduced and NP increased discharge at given pressures. Diameter-frequency (DF) plots were not altered by either drug. PD and PF curves at constant MAP were stable after the initial responses to NP relaxation were completed. Subsequent MAP elevations in NP reset BRs to higher pressures, whereas PD curves shifted to lower pressures. Hence, DF plots shifted to higher diameters. These results show that neither smooth muscle activity nor mechanical changes in the whole wall can account for BR rapid resetting. The process probably occurs in the receptors.

159 LEFT VENTRICULAR PRESSURE-DIAMETER RELATIONS FOLLOWING ANGIOCARDIOGRAPHY IN CHILDREN

The LV pressure-diameter (P-D) relations of 8 children (3 - abnormal LV, 5 - normal LV) were evaluated before and 5 minutes after right heart angiography (preA, postA) using Millar catheter transducer and M-mode echocardiogram. A digitizer interfaced to H-P computer obtained sequential LV diameter, wall thickness and pressure of 4 cardiac cycles. Circumferential wall force (F), meridional wall stress (S), D/P and P/D ratios, dD/dt, dD/dt/D, dF/dt and dS/dt were obtained q 10 msec. Systolic and diastolic segments of P-D, P-S and P-F loops were analyzed. Paired t-test of preA vs postA showed no change in heart rates and endsystolic diameters. However, significant changes were observed postA: LVEDP (+67%, p < .005), LVEDD (+6.6%, p < .01), ejection period (+9.9%, p < .02), peak systolic pressure (+11.5%, p < .01), peak S (+23%, p < .005) and total systolic S (+34%, p < .005). S/unit pressure rise during isovolumic contraction did not change postA. The increase in peak S postA was accompanied by drop in peak dD/dt/D (-31%, p < .05) suggesting decreased pump function despite increased sarcomere length. The diastolic segment of P-F loop characterized early relaxation (E) and later ventricular filling (L). Although diastolic F was higher postA during E and L, the P-D-F relations did not change significantly suggesting unaltered ventricular compliance. The change in LV performance following angiography is related to increased preload plus some reduction of pump function.

Effect of parenchyma and length changes on vessel pressure-diameter behavior in pig lungs.

At several transpulmonary pressures (Ptp), the pressure-diameter (PD) behavior of the largest intraparenchymal arterial segment in the isolated pig lung was compared with the behavior of the segment after its excision from the parenchyma and its extension to lengths equivalent to those in the intact state. For physiological changes in length, as may occur with lung inflation during Ptp changes from 4 to 25 cmH2O, excised-vessel diameters did not change significantly at a constant transmural pressure. The excised-vessel PD behavior was not significantly different from the intact-vessel PD behavior at a Ptp of 4 cmH2O. At any constant arterial pressure, intact-vessel diameters became larger as Ptp increased. Estimates of the perivascular pressure (Px) obtained by directly comparing intact-vessel and excised-vessel PD curves were as follows: 1) Px was equal to pleural pressure at a Ptp approximating the functional residual capacity; 2) Px decreased almost linearly as Ptp increased; and 3) Px decreased with a fall in arterial pressure. These results are consistent with direct measurements of the perivascular interstitial fluid pressure.

A continuum mechanics analysis of pulmonary vascular interdependence in isolated dog lobes

The pressure-diameter (PD) behavior of intact pulmonary vessels was measured roentgenographically at several fixed deflation transpulmonary pressures (Ptp). At any constant vascular pressure (Pv), the intact-vessel diameter was larger for higher Ptp. The behavior of the parenchyma was described by a nonlinear analysis. An excised-vessel PD behavior was computed which was consistent with both nonlinear parenchymal behavior and measured intact-vessel PD behavior. Estimates of Px', the difference between the perivascular pressure and the pleural pressure, as a function of Ptp and Pv were obtained. For physiological values of Pv, mean values of Px' for arteries were -1 cmH2O at Ptp of 4 cmH20 and decreased almost linearly to -15 cmH20 at Ptp of 25 cmH2O. Veins showed a similar behavior, but Px' was more positive. Increasing Pv raised Px' and decreasing Pv lowered Px' at all values of Ptp. These results indicate that the interdependence effect is small at functional residual capacity, increases with lung inflation, and is greater for arteries than veins.

Effect of vagal tone on airway diameters and on lung volume in anesthetized dogs

In 18 open-chest dogs we obtained pressure-diameter (P-D) curves from tantalum bronchograms and pressure-volume (P-V) curves by plethysmography. After vagotomy most of the decrease in diameter with decreasing transpulmonary pressure (Ptp) occured below 10 cmH2O and there was no P-D hysteresis. Smaller airways narrowed more with decreasing Ptp than larger ones. Bronchodilators did not increase diameters after vagotomy (P less than 0.2). With vagi intact, diameters were smaller at all Ptp (P less than 0.01) and exhibited hysteresis, but the lung P-V curve was unchanged. Vagal stimulation narrowed airways further at all Ptp and hysteresis was marked. Smaller airways narrowed more with vagal stimulation than larger ones. Vagal stimulation did not change the deflation limb of the P-V curve but decreased inflation volumes slightly at all Ptp (P less than 0.01). We conclude that in vivo tone is vagal and that it affects the physical properties of airways, but not of lungs, making the airways remarkably independent from lung parenchyma.