Phase-contrast pulsatility measurements: preliminary results in normal and arteriosclerotic iliofemoral arteries. Work in progress

Magnetic resonance (MR) flow measurements were obtained in six healthy volunteers and 30 patients with arteriosclerotic disease with a 1.5-T imager and a pulse sequence for flow quantification based on flow-induced phase shifts. The iliac arteries were investigated in eight and the femoral arteries in 28 subjects. A trigger pulse, followed by the acquisition of 30 evenly distributed data sets, was applied every second heartbeat, thus eliminating any acquisition gap in a full heart cycle. For quantitative analysis, flow velocity was plotted as a function of time. Systolic acceleration, postsystolic deceleration, and pulsatility of flow were calculated and compared with stenosis grades determined from recent intraarterial digital subtraction angiograms. The flattening of the temporal flow patterns correlated with local severity of vascular occlusive disease.

Electromagnetic flow measurement using a magnetic resonance imaging static field

RATIONALE AND OBJECTIVES

Electromagnetic flowmeters have been used for many years as a standard method to determine blood flow in animal models. The use of a modified probe to measure electromagnetic induction in response to an external, large direct current (DC) field magnetic resonance (MR) imaging system was investigated.

METHODS

Extracted sheep iliac vessels were inserted into a pumped saline circuit with a modified probe and placed into a 2.0-T MR research imaging system. Voltage readings were collected at various flow rates ranging from 100 to 400 ml/min. Actual flows were measured with a graduated cylinder.

RESULTS

A correlation of .937 (p < .001) was observed between the measured voltage changes and the actual flows. Baseline drift was also linear and within specified limits.

CONCLUSION

The results indicate that electromagnetic induction in a conductive fluid can be accurately measured using electrodes and an MR imaging system and that this technique provides possible opportunities for in situ flow measurements in humans.

Vascular resistance and arterial pressure low-frequency oscillations in the anesthetized dog

The spontaneous variability of heart rate and arterial blood pressure was investigated in chloralose-anesthetized dogs with the left iliac vascular bed mechanically uncoupled from the central circulation. Electrocardiogram, mean arterial pressure (ABP), iliac perfusion and venous pressures, and flow (FLOW) were recorded for 5 min in steady state. Autoregressive spectral and cross-spectral analyses and digital filtering were performed. The variation coefficient (VC%), calculated from the overall variance of each signal, was 5-7%, with the exception of perfusion pressure (VC% = 1%). The frequency-related percentage of total variance was distributed among three frequency bands: two were < 0.20 Hz [lower (F1) and higher (F2; low-frequency range = F1 + F2)], and one was > 0.20 Hz (respiratory, F3). F3 was not always present in RR, which, however, oscillated also in F1 and F2, although with limited amplitude; ABP showed large respiratory and low-frequency oscillations; the FLOW oscillations were in the low-frequency range. Cross-spectral analysis showed high squared coherence in the relevant frequency bands between variables in the three couples: RR-ABP, RR-FLOW, and ABP-FLOW. Changes in RR preceded changes in ABP and in FLOW by > or = 3 s, whereas FLOW was approximately in phase opposition to ABP. It was concluded that, in the chloralose-anesthetized dog, 1) arterial pressure and heart rate oscillate with frequencies corresponding to those described in conscious humans, 2) low-frequency arterial pressure oscillations are due to changes in peripheral vascular resistance, and 3) peripheral vascular resistance does not display respiratory oscillations. Furthermore it was suggested that oscillations of vasomotor tone are generated by a rhythm of central origin and that F1 and F2 oscillations may recognize a common mechanism.

Perinatal accumulation of arterial wall constituents: relation to hemodynamic changes at birth

We compared arterial growth to hemodynamic changes in the perinatal period in lambs. Blood pressure did not change significantly from 120 days gestation to 3 days postpartum, when it was 45.4 +/- 1.9 mmHg; however, pressure rose to 64.8 +/- 2.5 mmHg at 21 days postpartum. Thoracic and abdominal aortic and iliac and carotid arterial blood flows fell > 50% after birth but returned to fetal levels except in the abdominal aorta by 21 days postpartum. Blood flows in mesenteric (BFm) and renal (BFr) arteries increased between 120 days gestation (BFr = 13.4 +/- 1.4; BFm = 41.8 +/- 3.5 ml/min) and 140 days gestation (BFr = 25.9 +/- 1.8; BFm = 189 +/- 18 ml/min) and between 3 and 21 days postpartum (to BFr = 71.1 +/- 14.3; BFm = 334 +/- 59 ml/min). Elastin accumulation accelerated at 140 days gestation in all arteries except the thoracic aorta, in which elastin accumulation was always rapid. Collagen but not DNA accumulation also accelerated in most arteries. Postpartum dexamethasone (0.1 mg/kg twice a day) did not affect abdominal aortic elastin by 10 days of age (23.9 +/- 2.7 vs. 26.4 +/- 4.1 mg for controls); however, dexamethasone upregulated tropoelastin mRNA in fetuses. We hypothesize that cortisol stimulates elastin accumulation in late gestation. Postnatal elastin but neither collagen nor DNA correlated with blood flow changes at birth (r = 0.855, P < 0.05). We infer that accumulation of elastin is sensitive to blood flow rates during perinatal development.

Acute compensation to abrupt occlusion of rat femoral artery is prevented by NO synthase inhibitors

The hemodynamic significance of endothelium-derived relaxing factor (EDRF)-mediated mechanisms in vascular responses to abrupt rat femoral artery occlusion was investigated. Temporary arterial occlusion was produced before and after inhibition of nitric oxide synthase by N omega-nitro-L-arginine methyl ester (L-NAME) or NG-monomethyl-L-arginine (L-NMMA). Iliac artery blood flow and arterial pressures proximal and distal to the occlusion were measured. Normal vascular compensation included a return of resistance to preocclusion levels and a rise in distal pressure to a plateau within 5 min postocclusion. After treatment with L-NAME and L-NMMA, postocclusion resistance remained elevated by 53 and 36%, respectively. Collateral dilation after occlusion, as indicated by the rise in distal pressure, was prevented by L-NAME but not L-NMMA. Increases in adrenergic tone and mean arterial pressure by phenylephrine did not prevent compensation, suggesting the effects of L-NAME and L-NMMA did not result from elevated sympathetic activation or pressure. The results are consistent with the hypothesis that the stimulated release of endothelium-derived relaxing factor mediates the acute vascular compensation to abrupt arterial occlusion.

Influence of heart rate and vasoactive drugs on blood flow patterns at the canine ilio-femoral bifurcation

OBJECTIVE

The aim was to study the effects of altered heart rate and vasoactive drugs on the blood velocity patterns in the region of an arterial bifurcation.

METHODS

Blood velocity profiles were measured in an exposed iliofemoral bifurcation of paced dogs using a pulsed Doppler ultrasound velocimeter with high temporal and spatial resolution.

RESULTS

Decrease of the heart rate from 120 beats.min-1 (2 Hz) to 60 beats.min-1 (1 Hz) increased the peak forward velocity (30%), the peak reverse velocity (20%), and the duration of reverse flow (25%). Each drug caused qualitatively similar changes in velocity patterns at both heart rates. The systemic administration of angiotensin II reduced peak forward velocity (-26% at 2 Hz and -33% at 1 Hz) and forward flow duration (-15% at 1 Hz), the peak reverse velocity (-30% at 1 Hz), and reverse flow duration (-20% at 2 Hz and -28% at 1 Hz). Glyceryl trinitrate also reduced the peak forward velocity (-19% at both 2 and 1 Hz) but prolonged forward flow duration (28% at 2 Hz and 17% at 1 Hz) and that of reverse flow (45% at 2 Hz and 24% at 1 Hz), and also decreased the degree of oscillation (-16% at 2 Hz). Barnidipine hydrochloride (a calcium channel antagonist) also increased the duration of forward flow (48% at 1 Hz) and of reverse flow (31% at 2 Hz) but reduced the peak reverse velocity (-29% at 1 Hz) and flow oscillation (-22% at 2 Hz and 20% at 1 Hz).

CONCLUSIONS

These dramatic changes in the pattern of blood flow, including alterations in the amplitudes and durations of the different phases of the flow cycle, are expected to have important consequences on the shear dependent responses of endothelial cells in the region of the bifurcation.

Physiological assessment of augmented vascularity induced by VEGF in ischemic rabbit hindlimb

This study was designed to assess the physiological consequences of augmented vascularity induced by administration of vascular endothelial growth factor (VEGF), an endothelial cell-specific mitogen, in a rabbit model of hindlimb ischemia. Ten days after excision of the common and superficial femoral arteries from one hindlimb of 24 New Zealand White rabbits, VEGF (n = 15) or saline (control; n = 9) was selectively injected into the ipsilateral internal iliac artery. Limb perfusion was evaluated immediately pre-VEGF (baseline) and again at days 10 and 30. A Doppler guide wire was advanced to the internal iliac artery to record flow velocity at rest and at maximum flow velocity provoked by intra-arterial injection of papaverine. At baseline and at day 10, no differences in flow parameters were observed between the control and the VEGF-treated animals. By day 30, however, flow at rest (P < 0.05), maximum flow velocity (P < 0.001), and maximum blood flow (P < 0.001) were all significantly higher in the VEGF-treated group. These physiological findings complement previous-anatomic studies by providing evidence that a single intra-arterial bolus of VEGF augments flow, particularly maximum flow, in the rabbit ischemic hindlimb. These data thus support the notion that VEGF administration represents a potential treatment strategy for certain patients with lower extremity ischemia.

Role of thrombin in endothelial cell monolayer repair in vitro

PURPOSE

We examined the effect of thrombin on human iliac artery endothelial cell monolayer repair and proliferation after denuding vascular injury.

METHODS

Human iliac artery endothelial cell monolayer repair was determined by scrape wounding confluent monolayers and measuring the advancement of the cells into the wounded area for 3 days. Proliferation studies involved plating human iliac artery endothelial cells at one tenth confluence and counting the increase in cell number every 2 days for a 2-week period. Proliferation during monolayer repair was examined by determining bromodeoxyuridine uptake in cells located at the leading edge of a scrape-wounded monolayer.

RESULTS

Thrombin (1 to 8 U/ml) inhibited human iliac artery endothelial cell monolayer repair in a concentration-related, reversible manner. The effect was augmented by decreasing serum concentration and was independent of the presence of endothelial cell growth supplement. Inactivation of thrombin's proteolytic site with diisopropylfluorophosphate eliminated its effect on monolayer repair. Thrombin (0.5 to 8 U/ml) inhibited human iliac artery endothelial cell proliferation in a dose-related manner. This effect was augmented by decreasing serum concentration. Finally, thrombin (4 U/ml) inhibited the proliferative response of cells located at the leading edge of wounded monolayers compared with control groups.

CONCLUSION

Thrombin inhibits human arterial endothelial cell monolayer repair and proliferation after denuding vascular injury.

Surgical diathermy is not suitable for vascular tissue welding

The possibility of using radiofrequency energy (RFE) to produce a sutureless anastomosis is attractive both to the military surgeon and on the grounds of cost and simplicity. To investigate this, fresh rabbit aorta was divided lengthways into 4 mm strips and apposed, intima-to-itima. A Valleylab Force 30 Electrosurgery machine supplied RFE at 500 kHz. The energy was applied using standard bipolar forceps and 121 attempts were made to weld the tissue. Seventy seven welds resulted of which 36 were able to withstand a distraction force of greater than 0.1N: the strongest weld was disrupted at 0.35N. Intact tissue withstood a distraction force of greater than 2N, whilst sutured joints disrupted at 0.97 N. It is concluded therefore that radiofrequency energy at 500 kHz does not satisfactorily weld vascular tissue.

Evaluation of annexin V as a platelet-directed thrombus targeting agent

Annexin V is a human phospholipid binding protein (M(r) 36,000) that binds with high affinity to activated platelets in vitro. We studied the biodistribution and thrombus binding of annexin V in rabbit and swine models of fully occlusive arterial thrombi formed 1-2 h prior to injection of annexin V. Iodinated annexin V was cleared from blood in a rapid early phase (t1/2 = 6.4 min, 76% of radioactivity) and a slower late phase (t1/2 = 71 min, 24% of radioactivity). Organ uptake was highest in the kidney and spleen and lowest in heart and skeletal muscle. Thrombus/blood uptake ratios were (mean +/- SEM): 6.39 +/- 1.80 for rabbit iliac artery, 6.97 +/- 1.45 for swine carotid artery, and 7.68 +/- 1.70 for swine femoral artery (all p values < 0.01 versus control artery); a control protein, ovalbumin, showed an uptake ratio of 0.59 +/- 0.08 in swine femoral artery thrombi. These results indicate that annexin V is useful as an agent for selective targeting of platelet-containing thrombi.

The relationship between blood flow and diameter in the iliac artery of the anaesthetized dog: the role of endothelium-derived relaxing factor and shear stress

The quantitative relationship between, increase in blood flow and arterial diameter was determined in an anaesthetized dog preparation (pentobarbitone, induction 30 mg kg-1 i.v., maintenance 3 mg kg-1 i.v. every 30 min). Changes in external iliac artery diameter were measured using piezoelectric ultrasound transducers capable of measuring diameters within the range of 2-20 mm with a resolution of +/- 0.005 mm. The diameter of the artery was measured at two sites, at one of which the endothelium was damaged using a balloon angioplasty catheter. Increases in blood flow were brought about by a combination of vasodilatation and cardiac stimulation (intra-arterial administration of acetylcholine, downstream to the sites of diameter measurement, and electrical stimulation of the left ansa subclavia), thereby preventing large changes in blood pressure. The effects of both transient and maintained increases in blood flow on mean arterial diameter in the section of artery with intact endothelium were measured. Transient increases in mean flow from 147 +/- 0.21 to 611 +/- 80.0 ml min-1 caused increases in diameter of 0.12 +/- 0.02 mm from a control of 5.42 +/- 0.19 mm. The mean delay between maximum flow and maximum diameter was 24.51 +/- 1.1 s and the half-time for the return to control diameter was 82.0 +/- 9.6 s, compared with 12.1 +/- 1.5 s for the return to control flow. Maintained (3-4 min) increases in mean blood flow (from 104.7 +/- 15.1 to 694.7 +/- 135.1 ml min-1) produced larger increases in diameter of 0.48 +/- 0.30 mm from a control diameter of 4.89 +/- 0.12 mm. These changes in diameter were abolished by N omega-nitro-L-arginine methyl ester (L-NAME. 10-100 mg kg-1 i.v.). In the section of artery with damaged endothelium, changes in diameter were relatively small and associated with small changes in blood pressure. This effect of a nearly 7-fold increase in flow on arterial diameter is dependent upon the integrity of the endothelium and the release of endothelium-derived relaxing factor and causes a 29% reduction in calculated boundary wall shear stress.

Discordant iliac and femoral artery flow velocity waveforms in fetuses with single umbilical artery

OBJECTIVE

The aim of this study was to investigate pelvic and femoral arterial flow velocity waveforms in fetuses with a single umbilical artery.

STUDY DESIGN

Seven fetuses with single umbilical artery and no other ultrasonographically detected anomalies were studied with color Doppler ultrasonography at a median gestational age of 24 weeks (range 18 to 34 weeks). Flow velocity waveforms from the relevant vessels were obtained as follows: (1) umbilical artery from a free loop of cord, (2) common iliac artery from both sides just below the aortic bifurcation, (3) internal iliac artery and intraabdominal portion of the umbilical artery from the vessel visualized alongside the fetal bladder, and (4) femoral artery on both sides from the upper third of the fetal thigh. The pulsatility index was measured, and comparisons were made with the paired t test. A p value < 0.05 was considered significant.

RESULTS

The pulsatility index in the umbilical artery was normal in all cases. There were highly significant differences between the common iliac arteries in each side (difference in pulsatility index 2.7, 95% confidence interval 2.0 to 3.5, p < 0.001). Significant differences between both femoral arteries were also noted (difference in pulsatility index 1.0, 95% confidence interval 0.3 to 1.7, p < 0.001). In both vessels the pulsatility index was always higher in the side that did not participate in the placental circuit.

CONCLUSION

This study demonstrates that fetuses with single umbilical artery have asymmetric arterial blood flow patterns in the pelvic and lower extremities.

The effect of angle and flow rate upon hemodynamics in distal vascular graft anastomoses: a numerical model study

Flow in distal end-to-side anastomoses of iliofemoral artery bypass grafts was simulated using a steady flow, three-dimensional numerical model. With the proximal artery occluded, anastomotic angles were varied over 20, 30, 40, 45, 50, 60 and 70 deg while the inlet Reynolds numbers were 100 and 205. Fully developed flow in the graft became somewhat skewed toward the inner wall with increasing angle for both Reynolds numbers. Separated flow regions were seen along the inner arterial wall (toe region) for angles > or = 60 deg at Re = 100 and for angles > or = 45 deg at Re = 205 while a stagnation point existed along the outer arterial wall (floor region) for all cases which moved downstream relative to the toe of the anastomosis with decreasing angles. Normalized shear rates (NSR) along the arterial wall varied widely throughout the anastomotic region with negative values seen in the separation zones and upstream of the stagnation points which increased in magnitude with angle. The NSR increased with distance downstream of the stagnation point and with magnitudes which increased with the angle. Compared with observations from chronic in vivo studies, these results appear to support the hypothesis of greater intimal hyperplasia occurring in regions of low fluid shear.

Derivation of shear rates from near-wall LDA measurements under steady and pulsatile flow conditions

Atherosclerosis, thrombosis, and intimal hyperplasia are major forms of cardiovascular diseases in the United States. Previous studies indicate a significant correlation between hemodynamics, in particular, wall shear rate, and pathology of the arterial walls. While results of these studies implicate morphologic and functional changes related to wall shear rate magnitude, a standard technique for wall shear rate measurement has not been established. In this study, theoretical and in-vitro experimental fully developed steady and physiologic pulsatile flow waveforms have been used to obtain velocity profiles in the near-wall region. The estimated wall shear rates from these results are compared to the theoretical value to assess the accuracy of the approximating technique. Experimentally obtained results from LDA suggest that in order to minimize the error in velocity data, and subsequently, the wall shear rate, the first measured velocity has to be 500 microns away from the wall. While a linear approximation did not produce errors larger than 16.4 percent at peak systole, these errors substantially increased as the velocity magnitudes decreased during late systole and diastole. Overall, a third degree polynomial curve fit using four points produced the most accurate estimation of wall shear rate through out the cardiac cycle. Results of higher degree curve-fitting functions can be unpredictable due to potential oscillations of the function near the wall. Hence, based on the results of this study, use of a linear approximation is not recommended; a third degree curve-fitting polynomial, using four points provided the most accurate approximation for these flow waveforms.

Simple and accurate way for estimating total and segmental arterial compliance: the pulse pressure method

We derived and tested a new, simple, and accurate method to estimate the compliance of the entire arterial tree and parts thereof. The method requires the measurements of pressure and flow and is based on fitting the pulse pressure (systolic minus diastolic pressure) predicted by the two-element windkessel model to the measured pulse pressure. We show that the two-element windkessel model accurately describes the modulus of the input impedance at low harmonics (0-4th) of the heart rate so that the gross features of the arterial pressure wave, including pulse pressure, are accounted for. The method was tested using a distributed nonlinear model of the human systemic arterial tree. Pressure and flow were calculated in the ascending aorta, thoracic aorta, common carotid, and iliac artery. In a linear version of the systemic model the estimated compliance was within 1% of the compliance at the first three locations. In the iliac artery an error of 7% was found. In a nonlinear version, we compared the estimates of compliance with the average compliance over the cardiac cycle and the compliance at the mean working pressure. At the first three locations we found the estimated and "actual" compliance to be within 12% of each other. In the iliac artery the error was larger. We also investigated an increase and decrease in heart rate, a decrease in wall elasticity and exercise conditions. In all cases the estimated total arterial compliance was within 10% of mean compliance. Thus, the errors result mainly from the nonlinearity of the arterial system. Segmental compliance can be obtained by subtraction of compliance determined at two locations.

Effect of low-grade conductive heating on vascular compliance during in vitro balloon angioplasty

Radiofrequency-powered, thermal balloon angioplasty is a new technique that enhances luminal dilatation with less dissection than conventional angioplasty. The purpose of this study was to assess the effect of radiofrequency heating of balloon fluid on the pressure-volume mechanics of in vitro balloon angioplasty and to determine the histologic basis for thermal-induced compliance changes. In vitro, radiofrequency-powered, thermal balloon angioplasty was performed on 46 paired iliac segments freshly harvested from 23 nonatherosclerotic pigs. Balloon inflations at 60 degrees C were compared to room temperature inflations in paired arterial segments. Intraballoon pressure and volume were recorded during each inflation as volume infusion increased pressure over a 0 to 10 atm range. Pressure-volume compliance curves were plotted for all dilatations. Six segments were stained to assess the histologic abnormalities associated with thermal compliance changes. Radiofrequency heating acutely shifted the pressure-volume curves rightward in 20 of 23 iliac segments compared to nonheated controls. This increase in compliance persisted after heating and exceeded the maximum compliance shift caused by multiple nonheated inflations in a subset of arterial segments. Histologically, heated segments showed increased thinning and compression of the arterial wall, increased medial cell necrosis and altered elastic tissue fibers compared to nonheated specimens. In conclusion, radiofrequency heating of intraballoon fluid to 60 degrees C acutely increases vascular compliance during in vitro balloon angioplasty of nonatherosclerotic iliac arteries. The increased compliance persists after heating and can be greater than the compliance shifts induced by multiple conventional dilatations. Arterial wall thinning and irreversible alteration of elastic tissue fibers probably account for thermal compliance changes.

Changes in arterial wall compliance after endovascular stenting

PURPOSE

The response of arterial wall to endovascular stenting after angioplasty is not well understood. Additionally, changes in the elastic properties of stented vessels are unknown in situ. Vascular compliance was measured in normal canine iliac arteries (n = 11) before and after intravascular ultrasound-guided deployment of self-expandable metallic stents.

METHODS

Nine animals were restudied and killed 1, 2, and 4 weeks after initial deployment, and two dogs were studied at deployment only. An absolute induction angiometer was used to make in situ measurements of vessel compliance via catheter-based delivery. The angiometer consists of a wire loop probe, which conforms to the diameter of the vessel in which it is placed. Systolic/diastolic changes in loop diameter are translated into measureable changes of induced voltage.

RESULTS

Mean compliance of the artery before and immediately after stenting was 4.4 +/- 2.1 and 1.9 +/- 2.0 (x 10(-2) diameter %/mm Hg), respectively. As early as 1 week after deployment, stented arteries began to lose expansile properties, and some were noncompliant. At explantation diminished compliance was accompanied by a periadventitial fibrous reaction around stented vessels. A thin, unobstructing layer of neointimal hyperplasia covered the iliac stents at all intervals, and all vessels remained patent and free of thrombus.

CONCLUSIONS

The potential advantages provided by a flexible, radially compliant stent are lost within a relatively short time after implantation in nonatherosclerotic canine arteries.

Infusion methods for determination of peripheral resistance: influence of infused medium and back pressure

It has been suggested that peripheral vascular resistance (PR), measured intraoperatively, can predict the outcome of infrainguinal reconstructions. There is, however, a great deal of variability in design and predictive value with this method, and it is prone to technical difficulties and possible errors. The present study evaluated the influence of the choice of infusion medium and the back pressure on PR as measured by the infusion technique. In a porcine model of experimentally induced stenosis, standard PR (calculated by the pressure difference across the stenosis and the Doppler volume flow) was compared with PR based on infusions of blood or saline solution. With blood as the infusion medium there was a significant correlation between the PR and the standard values (r = 0.795, p = 0.0005), whereas there was no correlation when saline solution was infused (r = 0.067, p = 0.345). Subtracting the back pressure resulted in a slight improvement in the correlations between standard PR and PR measured after infusion of blood but not saline solution. In summary, blood should be used as an infusion medium in methods of PR determination, but the influence of back pressure remains uncertain.

An experimental hemodynamic study of the pelvic collateral circulation

Iliac arteries were occluded in adult mongrel dogs to investigate pelvic hemodynamics. When the unilateral common iliac artery was occluded, the blood flow making a "stopover" within the pelvis was found to be significantly less than that of anatomical hemodynamics even under a resting condition. The blood flow decreased more significantly under exercise loading than under a resting condition, which demonstrates the presence of the "steal" phenomenon. This only occurs in the collateral circulation in the pelvis formed by two arterial systems which are related in a series. In deciding the appropriacy of reconstruction for the internal iliac artery in patients with aorto-iliac occlusive disease, this "steal" phenomenon should be kept in mind. In most cases, ischemic symptoms in pelvic organs may be due to a simple decrease of the blood flow supplied to the pelvis, or due to the "steal" phenomenon. If the pelvic region is in the state of ischemia owing to the "steal" phenomenon, reconstruction of the blood vessels flowing into the pelvis is not required.

MR angiography of the pelvis with variable velocity encoding and a phased-array coil

PURPOSE

To evaluate the feasibility of an automated variable velocity-encoding sequence and improve the signal-to-noise ratio (S/N) on magnetic resonance angiograms with use of phase contrast and a pelvic phased-array coil.

MATERIALS AND METHODS

Three cardiac-gated, two-dimensional (2D), phase-contrast (PC) sequences were evaluated in 10 healthy subjects. A 2D gated PC sequence with variable velocity encoding (velocity-optimized phase contrast [VOPC]) was compared with gated 2D PC sequences performed with high or low constant velocity encoding. S/Ns in VOPC images obtained with a pelvic phased-array coil were compared with those in VOPC images obtained with a body coil.

RESULTS

Two blinded readers preferred VOPC for simultaneous display of large and small blood vessels in one acquisition compared with constant low (P = .0105) and high (P = .0067) velocity encoding and for overall image interpretation. VOPC images obtained with the pelvic coil had a 68%-100% better S/N compared with those obtained with the body coil.

CONCLUSION

Use of a phased-array coil and variable velocity encoding improves depiction of segmental vascular anatomic structures of the pelvis.

Blood flow velocity in the uterine and external iliac arteries before and after termination of pregnancy

In normal pregnancy, there is a marked increase in the uteroplacental blood flow, which is accomplished by a decrease in the peripheral resistance. The purpose of the present study was to investigate whether changes occur in the uterine and external iliac arteries during early pregnancy and after termination of pregnancy. Twenty pregnant women between the 7th and the 23rd weeks of pregnancy were investigated. They were examined before termination of pregnancy and followed up after the termination. The examinations were done using a pulsed duplex scanner (Acuson model 128XP/10 computed sonography system). In the right and left uterine arteries, there is a close correlation between both the peak flow velocity (r = 0.84; P < 0.01) and the end diastolic flow velocity (r = 0.71; P < 0.01) and the duration of pregnancy. The Vmax and Vmin in the external iliac arteries did not significantly change. The peak flow velocity (Vmax) in the right uterine artery was gradually reduced after termination of pregnancy. It was 0.81 (0.22) m/s before termination of pregnancy, reduced to 0.72 (0.22) m/s in the 1st post-operative day, and to 0.68 (0.21) m/s in the 2nd post-operative day. There was also a significant decrease of the end-diastolic flow velocity (Vmin), from 0.24 (0.10) m/s before termination of pregnancy to 0.15 (0.08) m/s in the 1st post-operative day. The left uterine artery showed similar changes. There is a strong correlation between Vmax and Vmin in both uterine arteries (in the right r = 0.71; in the left r = 0.57) indicating an influence on resistance indices.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of 5-hydroxytryptamine receptors in rabbit isolated iliac artery

To characterize the 5-hydroxytryptamine (5-HT) receptors, the contractile effects of both 5-HT and the 5-HT1-like receptor agonist sumatriptan were investigated in isolated open ring preparations of the rabbit common iliac artery. 5-HT induced concentration-dependent contractions. Sumatriptan did not induce any contraction of unstimulated preparations, whereas it elicited concentration-dependent contractions in preparations given a moderate tone with a threshold concentration of prostaglandin F2 alpha. In vessel segments precontracted with prostaglandin F2 alpha, Emax values for 5-HT and sumatriptan reached about 85% and 30% of the phenylephrine maximal effect, respectively. The mean EC50 values for sumatriptan and 5-HT were 3.34 microM and 1.5 microM, respectively. Pargyline, cocaine or normetanephrine were without significant effect on the contractions induced by 5-HT or sumatriptan. The 5-HT3 receptor antagonist tropisetron (1 microM) had no effect on 5-HT- and sumatriptan-induced contractions. The 5-HT2 receptor antagonist ketanserin (0.1-1 microM) produced parallel displacements to the right of the 5-HT and phenylephrine concentration-effect curves, without significant reduction in the maximum responses. The pA2 values were 7.85 +/- 0.19 and 7.9 +/- 0.16, respectively. Ketanserin had no effect on the sumatriptan concentration-effect curves. The nonselective 5-HT receptor antagonists methysergide (0.3 microM) and methiothepin (0.01 microM) shifted the concentration-response curve to sumatriptan to the right (mean pKB values of 6.91 and 8.68, respectively). The pA2 value for prazosin against 5-HT (9.98 +/- 0.43) was not significantly different from the value against phenylephrine (9.27 +/- 0.20). These results suggest that the sumatriptan-induced contraction is mediated by a 5-HT1-like receptor, whereas an additional mechanism, probably an alpha 1-adrenoceptor stimulation, plays a role in the contraction induced by 5-HT in the rabbit iliac artery.

Two-dimensional velocity measurements in a pulsatile flow model of the normal abdominal aorta simulating different hemodynamic conditions

The infrarenal abdominal aorta and aortic bifurcation are frequent sites of atherosclerosis. The local hemodynamics are considered to be an atherogenetic factor, and a detailed description of the flow fields in this region of the arterial tree is therefore essential. The aim of this study was to provide quantitative two-dimensional data on the velocity fields in the abdominal aorta, using a realistic flow model of the abdominal aorta and its main branches, under various physiologic flow conditions (i.e. rest and exercise). Velocities in the suprarenal abdominal aorta were antegrade, with very little retrograde and radial velocity components present. In the infrarenal abdominal aorta, velocity profiles were not fully developed, and large-scale retrograde flow was present during part of diastole for the rest condition. For the exercise conditions small-scale retrograde velocities were present during diastole, especially at the distal posterior vessel wall, but not at the distal anterior vessel wall. For the rest and medium exercise conditions, secondary flows were created in the distal abdominal aorta during diastole, most prominent near the posterior wall. Calculated wall shear stress directions revealed the presence of both oscillatory and multidirectional wall shear stresses mainly in parts of the infrarenal abdominal aorta, and were found to correlate well with the published data on the distribution of early atherosclerotic lesions. This quantitative study demonstrates the necessity of carefully modeling both the anatomy and the physiology in order to understand the complex hemodynamics present in the abdominal aorta.

Acute exercise enhances nitric oxide modulation of vascular response to phenylephrine

The influence of the release of endothelium-derived nitric oxide (NO) on the vasoconstrictor response to phenylephrine (PE) was evaluated before and after a single bout of dynamic exercise. Each rat ran on a motor-driven treadmill at 12-18 m/min, 10-18% grade until exhaustion (avg time 45 min). Sprague-Dawley rats (n = 6) were instrumented with a Doppler ultrasonic flow probe around the right common iliac artery. Just distal to the flow probe, a catheter was placed into the right iliac artery for local infusions. A Teflon catheter was placed in the descending aorta to measure mean arterial blood pressure (MAP) and heart rate (HR). PE (0.005-0.075 microgram/kg) and NO inhibitor N omega-nitro-L-arginine methyl ester hydrochloride (L-NAME, 0.2-0.25 mg/kg) were injected into the functionally isolated hindlimb. HR and MAP were not altered by any of the injections because we selected doses below those which elicited systemic responses. Dose-response curves to PE were generated in the control and postexercise condition, with and without the NO synthase inhibitor L-NAME. Exercise significantly attenuated the maximal vasoconstrictor response to PE (45.6 +/- 1.6%). L-NAME enhanced the maximal vasoconstrictor response to PE 49.8 +/- 4.5% in the control condition and 121.4 +/- 5.9% in the postexercise conditions. Thus, although NO inhibition enhanced the vasoconstrictor response to PE in the control and postexercise conditions, the enhanced vasoconstrictor response to PE after L-NAME was significantly greater in the postexercise condition. Results suggest that NO contributes to the exercise induced attenuation of alpha 1-adrenergic receptor stimulation.

A computer simulation of the blood flow at the aortic bifurcation with flexible walls

To understand the role of fluid dynamics in atherogenesis, especially the effect of the flexibility of arteries, a two-dimensional numerical model for blood flow at the aortic bifurcation with linear viscoelastic walls is developed. The arbitrary Lagrangian-Eulerian method is adopted to deal with the moving boundary problem. The wall expansion induces flow reversals or eddies during the decelerating systole while the wall contraction restricts them during the diastole. A flexible bifurcation experiences the shear stresses about 10 percent lower than those of a rigid one.