Flow through a collapsible tube. Experimental analysis and mathematical model

THE DISTENSIBILITY OF THE HUMAN VENA CAVA AND ITS IMPORTANCE TO IN VITRO STUDIES OF VENOUS COMPRESSION SYNDROMES: A SEARCH FOR A SUITABLE POLYMER FOR 3D PRINTING

BACKGROUND

Venous compression syndromes are clinical conditions in which the large veins are compressed by other anatomical structures. Laboratory simulations may help us better understand the hemodynamics in venous compressions by creating situations similar to those seen in vivo. The aim of this study is to produce a model of the caval bifurcation using a polymer with distensibility similar to the human vena cava.

METHODS

Fragments of the inferior vena cava were collected from 13 deceased kidney donors (aged 15-37 years) and tested for deformation (strain) when subjected to distension at 50 N/cm². Strips of five different polymers - thermic polyurethane and Agilus30 with Vero Magenta (AV) (in three different hardnesses) and silicone - were subjected to the same biomechanical tests and compared with the vena cava. A model of the caval bifurcation was produced with 3-D printing.

RESULTS

The deformation (strain) of the vena cava wall was 0.16±0.9 when submitted to stress close to 50 N/cm2. Silicone showed a strain higher than the standard deviation of venous fragments. The strain of AV resin 95 Shore was lower than the standard deviation of the venous fragments. AV Resins 70 and 85 Shore showed strains within the standard deviation of the venous specimen, with 70 Shore being closest to the mean venous strain. Therefore, this material was selected for modeling the caval bifurcation. The computed tomography scan image generated a computer model of the caval bifurcation and was printed in three dimensions. In addition, segments of two adjacent vertebrae were also printed to reference the compression site.

CONCLUSION

The 3D printing of large veins can produce models with anatomy and biomechanics similar to those of human veins and opens a field of investigation into the hemodynamics of venous compression syndromes. Polymers with Shore A70 appear to have biomechanical properties similar to those of the Vena Cava wall. The model obtained in this study can be used in several in vitro studies of May-Thurner Syndrome.

Modelling patient specific cardiopulmonary interactions

Mechanical ventilation is well known for having detrimental effects on the cardiovascular system, particularly when using high positive end-expiratory pressure. High positive end-expiratory pressure levels cause a decrease in stroke volume, which, under normal conditions, usually bring about a decrease in stressed blood volume. Stressed blood volume, defined as the total pressure generating volume of the cardiovascular system, has been shown to be a potential index of fluid responsiveness, making it a potentially important diagnostic tool. Generally, respiratory and haemodynamic care are provided independently of one another. However, that positive end-expiratory pressure alters both stroke volume and stressed blood volume suggests both the pulmonary and cardiovascular state should be conjointly optimised and used to guide positive end-expiratory pressure. However, the complex and patient-specific nature of cardiopulmonary interactions which occur during mechanical ventilation presents a challenge for accurate modelling of respiratory and cardiovascular interactions required to better optimise care. Previous models attempting to incorporate cardiopulmonary interactions have suffered from poor reliability at higher PEEP levels, largely due to an exaggerated effect of intrathoracic pressure on the cardiovascular system. A new parameter, alpha, is added to a previously validated cardiopulmonary model, to modulate the percentage of intrathoracic pressure applied to the vena cava and left ventricle. The new parameter aims to increase reliability under high PEEP conditions as well as provide a patient specific solution to modelling cardiopulmonary interactions. The results from the identified optimal alpha are compared to the original model to investigate how this new parameter may be used to create a more patient-specific cardiopulmonary model, which would be better suited for guidance of care in the ICU.

Closed cranial window rodent model for investigating hemodynamic response to elevated intracranial pressure

Background

Elevated intracranial pressure (ICP) occurs in several physiological and pathological conditions, yet long-term sequellae are not common, which implies that blood flow is preserved above ischemic thresholds.

Methods

This pilot study sought to confirm this hypothesis using a closed cranial window model in a rat in which ICP was elevated to 120 mmHg for 12 min, and superficial cortical perfusion was measured by laser Doppler flowmetry and laser speckle flowmetry.

Results

Following a transient increase, cortical blood flow decreased to between 25% and 75% of baseline. These levels correspond to disrupted metabolism and decreased protein synthesis but did not exceed thresholds for electrical signaling or membrane integrity. This may partially explain how some episodes of elevated ICP remain benign.

Conclusion

The closed cranial window model provides a platform for prospective study of physiologic responses to artificially elevated ICP during neurosurgery to promote hemostasis.

Hydrodynamic Approach for Revealing Venous Anastomotic Stenosis Formation Within a Dialysis Arteriovenous Graft

A conventional arteriovenous graft in patients on dialysis often leads to anastomotic stenosis, which decreases the blood flow rate and increases the risk of complications. In this study, based on hydrodynamics, the pulsatile pressure at the blood vessel graft-vein junction was investigated experimentally and numerically for revealing the causes of stenosis formation and inward remodeling. In the experiments, the pulsatile pressure and displacement at the anastomotic connection were measured at a branched collapsible tube. It was revealed that the pressure becomes negative between pressure peaks of the pulsatile flow; furthermore, tube diameter changes in accordance with the pressure pulsation. Subsequently, numerical simulations revealed that a relatively large pressure difference occurs at the anastomotic connection because of flow collision and separation as compared with the other part, and the pulsatile pressure. Therefore, it is possible that vein at an anastomotic connection may change its shape under pulsating flow. Furthermore, it was found that the pressure difference slightly increased with the anastomosis angle, but the anastomosis angle did not affect the flow rate. Clinical trials in the next step are required to reveal the causal relationship between stenosis and the pulsatile pressure, but the pulsatile flow and its pressure are likely to be one factor in stenosis and inward remodeling.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Effect of body mass index on initial presentation and outcomes after stenting for quality of life-impairing chronic iliofemoral venous obstruction

OBJECTIVE

The incidence of obesity has been increasing, with recent data indicating that the age-adjusted mean body mass index (BMI) is close to 30 kg/m² in the United States. Prior studies have raised concerns for an increased incidence of chronic venous insufficiency in the obese population. We aimed to build on current knowledge by assessing the effects of BMI on the initial presentation and outcomes after intravascular ultrasound (IVUS) luminal area-guided stenting in patients presenting with quality of life (QOL)-impairing chronic iliofemoral venous obstruction (CIVO).

METHODS

A retrospective analysis of contemporaneously entered electronic medical record data on 464 continuous patients (464 limbs) with initial iliofemoral stents (2014-2017) for QOL-impairing CIVO was performed. The characteristics evaluated and compared included the degree of iliofemoral compression, CEAP (clinical, etiologic, anatomic, pathophysiologic) clinical class, venous clinical severity score (VCSS), grade of swelling (GOS), visual analog scale (VAS) for pain score, ulcer healing, reflux (venous segmental disease score; venous filling index-90), calf pump function (ejection fraction; residual volume fraction), and quality of life (CIVIQ-20 [chronic lower limb venous insufficiency 20-item questionnaire]) for those with a BMI <30 kg/m² (group I) and a BMI ≥30 kg/m² (group II). Paired and unpaired t tests were used for comparisons of the clinical variables and a Kaplan-Meier analysis was used to evaluate stent patency.

RESULTS

Of the 464 limbs in the study cohort, 122 were in group I and 342 in group II. The median BMI was 26.3 kg/m² (interquartile range, 19.6-29.9 kg/m²) in group I and 38.9 kg/m² (interquartile range, 30.0-66.9 kg/m²) in group II. The IVUS luminal area-determined degree of compression was higher in group I than in group II across the common iliac, external iliac, and common femoral segments (P < .01). The supine foot venous and femoral venous pressures were higher in group II than in group I (P < .001). The ejection fraction was higher (57.4% vs 45.6%; P = .0008) and residual volume fraction was lower (27.5% vs 40.5%; P = .0008) in group II than in group I. Although the baseline VCSS and GOS were lower in group I than in group II (P < .05), no differences were found in the VAS for pain scores or ulcer prevalence. The median follow-up was 22 months. At 24 months after stenting, improvement was found in the VCSS, GOS, and VAS for pain score in both groups. The CIVIQ-20 QOL score had improved from 58.1 to 18.8 in group I (P = .0002) and from 60 to 37.5 in group II (P < .0001). At 5 years, primary patency was 70% in group I and 73% in group II (P = .6) and primary assisted patency was 100% in both groups (P = .99) without a significant difference in the reintervention rate (P = .5).

CONCLUSIONS

Obese patients with CIVO-impairing QOL have a lesser degree of iliofemoral venous stenosis, more severe venous hypertension, and better calf pump function than their nonobese counterparts. After stenting, no differences were found in the clinical, stent patency, or QOL-related outcomes between the two groups.

Analysis of patent, unstented lower extremity vein segment diameters in 266 patients with venous disease

OBJECTIVE

The objective of this study was to characterize the average maximum diameters of widely patent lower extremity vein segments in patients with underlying venous disease and the demographic factors that affect these diameters.

METHODS

Maximum axial diameters of each deep vein segment from the diaphragm to the knee were measured from computed tomography venography studies for all patients who underwent venous stent placement during a 20-year period at a single quaternary venous referral institution. Limbs containing only widely patent, unstented vein segments without variant anatomy were identified for inclusion. The final analysis involved diameter measurements from 870 imaging studies of 266 patients. Multivariate linear regression was used to identify factors associated with vein segment diameters.

RESULTS

Average vein segment diameters ranged from 7.8 mm for the left and right femoral veins to 27.9 mm for the long axis of the suprarenal inferior vena cava. Multivariate linear regression demonstrated that women had larger IVC, common iliac vein, and external iliac vein diameters, whereas men had larger common femoral veins. Laterality, height, weight, and sex also had statistically significant associations with the diameters of select vein segments.

CONCLUSIONS

This study provides an estimate of the average diameters of widely patent deep vein segments in the lower extremities from the diaphragm to the knees in patients with underlying venous disease and characterizes covariates that significantly affect vein diameter. These findings may help interventionalists better select devices for endovascular intervention.

Gravity force is not a sole explanation of reflux flow in incompetent great saphenous vein

OBJECTIVE

This study aimed to evaluate the impact of gravity, reservoir size, and competence of the ostial valve on venous reflux in different body positions.

METHODS

Our study included 61 lower limbs with primary incompetence of the great saphenous vein (GSV). The diameter of the GSV and its cross-sectional area, time-averaged mean velocity (TAMEAN), and reflux time (RT) were measured with duplex ultrasound with pulsed wave Doppler. Reflux volume (RV) and reflux volume flow rate (Q) were calculated. The measurements were carried out in three body positions: horizontal, A; seated upright with stretched legs, B; and vertical, C. Distal automatic cuff compression-decompression (120 mm Hg) was used as a provocation maneuver.

RESULTS

There was 100% occurrence of reflux in the patient positions B and C. Reflux was observed in 91.8% of cases in position A. All reflux parameters (TAMEAN, RT, Q, RV) and the size of the vein were significantly different in the three studied positions. The patient's height did not influence the magnitude of change in reflux parameters. All reflux parameters increased more significantly when the position changed from A to B than from B to C (TAMEAN, +103% and +37%; GSV diameter, +33% and +5%; RV, +408% and +65%, respectively).

CONCLUSIONS

Observed positional changes in reflux parameters suggest that gravitational forces are not a sole explanation for reflux flow in incompetent GSV. It is likely that the gravitational effect on venous flow is mediated by the changes in vein diameter and the total volume of the venous reservoir of the leg.

Pelvic cardiovascular magnetic resonance venography: venous changes with patient position and hydration status

BACKGROUND

To determine the effect of hydration as well as prone versus supine positioning on the pelvic veins during cardiovascular magnetic resonance (CMR) venography.

METHODS

Under institutional review board approval, 8 healthy subjects were imaged with balanced steady state free precession, non-contrast CMR venography to measure common and external iliac vein volumes and common femoral vein cross-sectional area in the supine, prone and decubitus positions after dehydration and again following re-hydration. CMR venography from 23 patients imaged both supine and prone were retrospectively reviewed and measurements of common femoral and iliac veins areas were compared using Wilcoxon test.

RESULTS

Common femoral vein area on CMR venography increased with prone positioning (83 ± 35 mm2) compared to supine positioning (59 ± 21 mm2) (p = 0.02) and further increased with hydration to 123 ± 44 mm2 (p < 0.01). With right and left side down decubitus positioning, the common femoral vein area on dehydration increased from 29 ± 17 mm2 in the ante-dependent position to 134 ± 36 mm2 in the dependent position (p < 0. 001). Similarly, common and external iliac veins increased in volume with prone, 5.4 ± 1.9 cm3 and 5.8 ± 1.9 cm3 compared to supine positioning 4.6 ± 1.8 cm3 and 4.5 ± 1.9 cm3 (p = 0.01) and further increase with hydration to 6.7 ± 2.1 cm3 and 6.3 ± 1.9 cm3 (p = 0.01). CMR venography on patients also demonstrated an increase in mean common femoral vein luminal area from 103 ± 44 mm2 in supine position to 151 ± 52 mm2 with prone positioning (p < 0.001) as well as increases in common and external iliac vein volumes from 6.5 ± 2.6 cm3 and 8.0 ± 3.4 cm3 in the supine position to 7.5 ± 2.5 cm3 and 9.3 ± 3.6 cm3 with prone positioning (p < 0.01).

CONCLUSIONS

Common femoral and common/external iliac vein size on CMR venography may be affected by position and hydration status. Routine clinical CMR venography of the pelvis could include prone positioning and avoiding dehydration to maximize pelvic vein distension.

The caval sphincter in cetaceans and its predicted role in controlling venous flow during a dive

A sphincter on the inferior vena cava can protect the heart of a diving mammal from overload when elevated abdominal pressures increase venous return, yet sphincters are reported incompetent or absent in some cetacean species. We previously hypothesized that abdominal pressures are elevated and pulsatile in fluking cetaceans, and that collagen is deposited on the diaphragm according to pressure levels to resist deformation. Here, we tested the hypothesis that cetaceans generating high abdominal pressures need a more robust sphincter than those generating low pressures. We examined diaphragm morphology in seven cetacean and five pinniped species. All odontocetes had morphologically similar sphincters despite large differences in collagen content, and mysticetes had muscle that could modulate caval flow. These findings do not support the hypothesis that sphincter structure correlates with abdominal pressures. To understand why a sphincter is needed, we simulated the impact of oscillating abdominal pressures on caval flow. Under low abdominal pressures, simulated flow oscillated with each downstroke. Under elevated pressures, a vascular waterfall formed, greatly smoothing flow. We hypothesize that cetaceans maintain high abdominal pressures to moderate venous return and protect the heart while fluking, and use their sphincters only during low-fluking periods when abdominal pressures are low. We suggest that pinnipeds, which do not fluke, maintain low abdominal pressures. Simulations also showed that retrograde oscillations could be transmitted upstream from the cetacean abdomen and into the extradural veins, with potentially adverse repercussions for the cerebral circulation. We propose that locomotion-generated pressures have influenced multiple aspects of the cetacean vascular system.

Venography versus intravascular ultrasound for diagnosing and treating iliofemoral vein obstruction

OBJECTIVE

The Venogram vs IVUS for Diagnosing Iliac vein Obstruction (VIDIO) trial was designed to compare the diagnostic efficacy of intravascular ultrasound (IVUS) with multiplanar venography for iliofemoral vein obstruction.

METHODS

During a 14-month period beginning July 2014, 100 patients with chronic Clinical, Etiologic, Anatomic, and Pathophysiologic clinical class C4 to C6 venous disease and suspected iliofemoral vein obstruction were enrolled at 11 U.S. and 3 European sites. The inferior vena cava and common iliac, external iliac, and common femoral veins were imaged. Venograms were measured for vein diameter; IVUS provided diameter and area measurements. Multiplanar venograms included three views: anteroposterior and 30-degree right and left anterior oblique views. A core laboratory evaluated the deidentified images, determining stenosis severity as the ratio between minimum luminal diameter and reference vessel diameter, minimal luminal area, and reference vessel area. A 50% diameter stenosis by venography and a 50% cross-sectional area reduction by IVUS were considered significant. Analyses assessed change in procedures performed on the basis of imaging method and concordance of measurements between each imaging method.

RESULTS

Venography identified stenotic lesions in 51 of 100 subjects, whereas IVUS identified lesions in 81 of 100 subjects. Compared with IVUS, the diameter reduction was on average 11% less for venography (P < .001). The intraclass correlation coefficient was 0.505 for vein diameter stenosis calculated with the two methods. IVUS identified significant lesions not detected with three-view venography in 26.3% of patients. Investigators revised the treatment plan in 57 of 100 cases after IVUS, most often because of failure of venography to detect a significant lesion (41/57 [72%]). IVUS led to an increased number of stents in 13 of 57 subjects (23%) and the avoidance of an endovascular procedure in 3 of 57 subjects (5%). Overall, IVUS imaging changed the treatment plan in 57 patients; 54 patients had stents placed on the basis of IVUS detection of significant iliofemoral vein obstructive lesions not appreciated with venography, whereas 3 patients with significant lesions on venography had no stent placed on the basis of IVUS.

CONCLUSIONS

IVUS is more sensitive for assessing treatable iliofemoral vein stenosis compared with multiplanar venography and frequently leads to revised treatment plans and the potential for improved clinical outcome.

Human jugular vein collapse in the upright posture: implications for postural intracranial pressure regulation

Background

Intracranial pressure (ICP) is directly related to cranial dural venous pressure (P_dural). In the upright posture, P_dural is affected by the collapse of the internal jugular veins (IJVs) but this regulation of the venous pressure has not been fully understood. A potential biomechanical description of this regulation involves a transmission of surrounding atmospheric pressure to the internal venous pressure of the collapsed IJVs. This can be accomplished if hydrostatic effects are cancelled by the viscous losses in these collapsed veins, resulting in specific IJV cross-sectional areas that can be predicted from flow velocity and vessel inclination.

Methods

We evaluated this potential mechanism in vivo by comparing predicted area to measured IJV area in healthy subjects. Seventeen healthy volunteers (age 45 ± 9 years) were examined using ultrasound to assess IJV area and flow velocity. Ultrasound measurements were performed in supine and sitting positions.

Results

IJV area was 94.5 mm² in supine and decreased to 6.5 ± 5.1 mm² in sitting position, which agreed with the predicted IJV area of 8.7 ± 5.2 mm² (equivalence limit ±5 mm², one-sided t tests, p = 0.03, 33 IJVs).

Conclusions

The agreement between predicted and measured IJV area in sitting supports the occurrence of a hydrostatic-viscous pressure balance in the IJVs, which would result in a constant pressure segment in these collapsed veins, corresponding to a zero transmural pressure. This balance could thus serve as the mechanism by which collapse of the IJVs regulates P_dural and consequently ICP in the upright posture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-017-0065-2) contains supplementary material, which is available to authorized users.

Experimental investigation of the structural behavior of equine urethra

BACKGROUND AND OBJECTIVE

An integrated experimental and computational investigation was developed aiming to provide a methodology for characterizing the structural response of the urethral duct. The investigation provides information that are suitable for the actual comprehension of lower urinary tract mechanical functionality and the optimal design of prosthetic devices.

METHODS

Experimental activity entailed the execution of inflation tests performed on segments of horse penile urethras from both proximal and distal regions. Inflation tests were developed imposing different volumes. Each test was performed according to a two-step procedure. The tubular segment was inflated almost instantaneously during the first step, while volume was held constant for about 300s to allow the development of relaxation processes during the second step. Tests performed on the same specimen were interspersed by 600s of rest to allow the recovery of the specimen mechanical condition. Results from experimental activities were statistically analyzed and processed by means of a specific mechanical model. Such computational model was developed with the purpose of interpreting the general pressure-volume-time response of biologic tubular structures. The model includes parameters that interpret the elastic and viscous behavior of hollow structures, directly correlated with the results from the experimental activities.

RESULTS

Post-processing of experimental data provided information about the non-linear elastic and time-dependent behavior of the urethral duct. In detail, statistically representative pressure-volume and pressure relaxation curves were identified, and summarized by structural parameters. Considering elastic properties, initial stiffness ranged between 0.677 ± 0.026kPa and 0.262 ± 0.006kPa moving from proximal to distal region of penile urethra. Viscous parameters showed typical values of soft biological tissues, as τ₁=0.153±0.018s, τ₂=17.458 ± 1.644s and τ₁=0.201 ± 0.085, τ₂= 8.514 ± 1.379s for proximal and distal regions respectively.

DISCUSSION

A general procedure for the mechanical characterization of the urethral duct has been provided. The proposed methodology allows identifying mechanical parameters that properly express the mechanical behavior of the biological tube. The approach is especially suitable for evaluating the influence of degenerative phenomena on the lower urinary tract mechanical functionality. The information are mandatory for the optimal design of potential surgical procedures and devices.

Utility of cone-beam computed tomography in the assessment of the porto-spleno-mesenteric venous system

The common diagnostic tools available to evaluate the porto-spleno-mesenteric venous (PSMV) system provide either good hemodynamic information with limited morphological details [e.g., ultrasonography (US)] or excellent tomographic display of the anatomy with limited information about flow patterns [e.g., multidetector computed tomography (MDCT) and magnetic resonance imaging]. Although catheter-directed selective digital subtraction angiography (DSA) can provide excellent information about flow at a high temporal resolution and can generate images at a high spatial resolution, this technique is often limited by a lack of cross-sectional detail. In the assessment of the PSMV system, DSA is also limited by dilution of contrast and motion artefacts. Combining venous phase cone-beam computed tomography (CBCT) with DSA can generate high-quality tomographic data, which allows detailed evaluation of venous tributaries and flow patterns within the splenic, superior mesenteric, and inferior mesenteric venous systems individually. This enables clinicians to better understand the impact of nonobstructive resistance to flow (e.g., as in patients with cirrhosis) and obstructive resistance to flow (e.g., as in patients with thrombosis) within each system and plan treatment accordingly. In this review, we discuss the limitations of common diagnostic methods and the role venous CBCT in combination with DSA can play in assessing the PSMV system.

Variability in the Cross-Sectional Area and Narrowing of the Internal Jugular Vein in Patients Without Multiple Sclerosis

OBJECTIVE

Chronic cerebrospinal vascular insufficiency is a proposed condition of intraluminal stenosis of the internal jugular vein (IJV) that impedes venous flow from the brain. Calculations of IJV stenosis are vague and described in veins with at least a 50% reduction in IJV caliber at a specific level. The purpose of this study was to assess normal changes in anatomic caliber of the course of the IJV in a generalized population.

MATERIALS AND METHODS

Images from 500 consecutive contrast-enhanced neck CT studies performed in January-July 2011 were reviewed. Cross-sectional areas of the IJV were calculated at the jugular foramen, C1-C7 levels, and jugular angles bilaterally. Patients were excluded if they had severely motion-limited studies; limited clinical data; a history of multiple sclerosis, neck mass, or neck dissection; or known IJV occlusion. Normalized IJV cross-sectional areas at each level were compared with an averaged normalized cross-sectional area from all patients at each level.

RESULTS

Greater than 50% narrowing compared with a normalized average was found in 133 of 363 patients (36.6%) and was seen at all IJV levels. In 36.1% of patients this narrowing occurred at the C1 level. Most of the areas of narrowing greater than 50% occurred above the C4 level.

CONCLUSION

The IJV has marked variability in its course in the neck. Areas of narrowing greater than 50% occur most commonly in the upper cervical and skull base regions. Given the normal anatomic variations in IJV caliber, caution should be used when making the diagnosis of and treating IJV stenosis.

Hummingbird tongues are elastic micropumps

Pumping is a vital natural process, imitated by humans for thousands of years. We demonstrate that a hitherto undocumented mechanism of fluid transport pumps nectar onto the hummingbird tongue. Using high-speed cameras, we filmed the tongue-fluid interaction in 18 hummingbird species, from seven of the nine main hummingbird clades. During the offloading of the nectar inside the bill, hummingbirds compress their tongues upon extrusion; the compressed tongue remains flattened until it contacts the nectar. After contact with the nectar surface, the tongue reshapes filling entirely with nectar; we did not observe the formation of menisci required for the operation of capillarity during this process. We show that the tongue works as an elastic micropump; fluid at the tip is driven into the tongue's grooves by forces resulting from re-expansion of a collapsed section. This work falsifies the long-standing idea that capillarity is an important force filling hummingbird tongue grooves during nectar feeding. The expansive filling mechanism we report in this paper recruits elastic recovery properties of the groove walls to load nectar into the tongue an order of magnitude faster than capillarity could. Such fast filling allows hummingbirds to extract nectar at higher rates than predicted by capillarity-based foraging models, in agreement with their fast licking rates.

Venous hemodynamics in neurological disorders: an analytical review with hydrodynamic analysis

Venous abnormalities contribute to the pathophysiology of several neurological conditions. This paper reviews the literature regarding venous abnormalities in multiple sclerosis (MS), leukoaraiosis, and normal-pressure hydrocephalus (NPH). The review is supplemented with hydrodynamic analysis to assess the effects on cerebrospinal fluid (CSF) dynamics and cerebral blood flow (CBF) of venous hypertension in general, and chronic cerebrospinal venous insufficiency (CCSVI) in particular.CCSVI-like venous anomalies seem unlikely to account for reduced CBF in patients with MS, thus other mechanisms must be at work, which increase the hydraulic resistance of the cerebral vascular bed in MS. Similarly, hydrodynamic changes appear to be responsible for reduced CBF in leukoaraiosis. The hydrodynamic properties of the periventricular veins make these vessels particularly vulnerable to ischemia and plaque formation.Venous hypertension in the dural sinuses can alter intracranial compliance. Consequently, venous hypertension may change the CSF dynamics, affecting the intracranial windkessel mechanism. MS and NPH appear to share some similar characteristics, with both conditions exhibiting increased CSF pulsatility in the aqueduct of Sylvius.CCSVI appears to be a real phenomenon associated with MS, which causes venous hypertension in the dural sinuses. However, the role of CCSVI in the pathophysiology of MS remains unclear.

Cerebrovascular mechanical properties and slow waves of intracranial pressure in TBI patients

Myogenic autoregulation of cerebral blood flow is one of the mechanisms affecting cerebral hemodynamics. Short or long-lasting changes in intracranial pressure (ICP) are believed to reveal the responses of the cerebral system to myogenic stimuli. Through the incorporation of a theoretical model into the experimental measurements of cerebrovascular distensibility and compliance in patients with traumatic brain injury (TBI), the current study is an attempt to explain ICP dynamics in either presence or absence of cerebral autoregulation. The pulse wave velocity and transfer function between arterial blood pressure and ICP were utilized as the major tools to reflect variations in the mechanical properties of distant cerebral artries/arteriols. The results imply that different states of cerebral autoregulation and associated regimes within the cerebrovascular system can lead to different types of interrelationship between the slow variations of ICP, cerebral arterial distensibility, and compliance. Consequently, each of these classes may require different types of treatment on patients with TBI.

Mechanism of and threshold biomechanical conditions for falsetto voice onset

The sound source of a voice is produced by the self-excited oscillation of the vocal folds. In modal voice production, a drastic increase in transglottal pressure after vocal fold closure works as a driving force that develops self-excitation. Another type of vocal fold oscillation with less pronounced glottal closure observed in falsetto voice production has been accounted for by the mucosal wave theory. The classical theory assumes a quasi-steady flow, and the expected driving force onto the vocal folds under wavelike motion is derived from the Bernoulli effect. However, wavelike motion is not always observed during falsetto voice production. More importantly, the application of the quasi-steady assumption to a falsetto voice with a fundamental frequency of several hundred hertz is unsupported by experiments. These considerations suggested that the mechanism of falsetto voice onset may be essentially different from that explained by the mucosal wave theory. In this paper, an alternative mechanism is submitted that explains how self-excitation reminiscent of the falsetto voice could be produced independent of the glottal closure and wavelike motion. This new explanation is derived through analytical procedures by employing only general unsteady equations of motion for flow and solids. The analysis demonstrated that a convective acceleration of a flow induced by rapid wall movement functions as a negative damping force, leading to the self-excitation of the vocal folds. The critical subglottal pressure and volume flow are expressed as functions of vocal fold biomechanical properties, geometry, and voice fundamental frequency. The analytically derived conditions are qualitatively and quantitatively reasonable in view of reported measurement data of the thresholds required for falsetto voice onset. Understanding of the voice onset mechanism and the explicit mathematical descriptions of thresholds would be beneficial for the diagnosis and treatment of voice diseases and the development of artificial vocal folds.

Terrestrial locomotion does not constrain venous return in the American alligator,Alligator mississippiensis

The effects of treadmill exercise on components of the cardiovascular(heart rate, mean arterial blood pressure, central venous pressure, venous return) and respiratory (minute ventilation, tidal volume, breathing frequency, rate of oxygen consumption, rate of carbon dioxide production)systems and on intra-abdominal pressure were measured in the American alligator, Alligator mississippiensis, at 30°C. Alligators show speed-dependent increases in tidal volume and minute ventilation,demonstrating that the inhibition of ventilation during locomotion that is present in some varanid and iguanid lizards was not present in alligators. Exercise significantly increases intra-abdominal pressure; however,concomitant elevations in central venous pressure acted to increase the transmural pressure of the post caval vein and thus increased venous return. Therefore, despite elevated intra-abdominal pressure, venous return was not limited during exercise in alligators, as was the case in Varanus exanthematicus and Iguana iguana. Respiratory cycle variations in intra-abdominal pressure, central venous pressure and venous return indicate that, at high tidal volumes, inspiration causes a net reduction in venous return during active ventilation and thus may act to limit venous return during exercise. These results suggest that, while tonically elevated intra-abdominal pressure induced by exercise does not inhibit venous return,phasic fluctuations during each breath cycle may contribute to venous flow limitation during exercise.

Systemic venous circulation. Waves propagating on a windkessel: relation of arterial and venous windkessels to systemic vascular resistance

Compared with arterial hemodynamics, there has been relatively little study of venous hemodynamics. We propose that the venous system behaves just like the arterial system: waves propagate on a time-varying reservoir, the windkessel, which functions as the reverse of the arterial windkessel. During later diastole, pressure increases exponentially to approach an asymptotic value as inflow continues in the absence of outflow. Our study in eight open-chest dogs showed that windkessel-related arterial resistance was approximately 62% of total systemic vascular resistance, whereas windkessel-related venous resistance was only approximately 7%. Total venous compliance was found to be 21 times larger than arterial compliance (n = 3). Inferior vena caval compliance (0.32 +/- 0.015 ml x mmHg(-1) x kg(-1); mean +/- SE) was approximately 14 times the aortic compliance (0.023 +/- 0.002 ml x mmHg(-1) x kg(-1); n = 8). Despite greater venous compliance, the variation in venous windkessel volume (i.e., compliance x windkessel pulse pressure; 7.8 +/- 1.1 ml) was only approximately 32% of the variation in aortic windkessel volume (24.3 +/- 2.9 ml) because of the larger arterial pressure variation. In addition, and contrary to previous understanding, waves generated by the right heart propagated upstream as far as the femoral vein, but excellent proportionality between the excess pressure and venous outflow suggests that no reflected waves returned to the right atrium. Thus the venous windkessel model not only successfully accounts for variations in the venous pressure and flow waveforms but also, in combination with the arterial windkessel, provides a coherent view of the systemic circulation.

Elevated intra-abdominal pressure limits venous return during exercise inVaranus exanthematicus

The effects of treadmill exercise on components of the cardiovascular(venous return, heart rate, arterial blood pressure) and respiratory systems(minute ventilation, tidal volume, breathing frequency, oxygen consumption,carbon dioxide production) and intra-abdominal pressure were investigated in the Savannah monitor lizard, Varanus exanthematicus B., at 35°C. Compared with resting conditions, treadmill exercise significantly increased lung ventilation, gular pumping, intra-abdominal pressure, mean arterial blood pressure and venous return (blood flow in the post caval vein). However,venous return declines at high levels of activity, and mean arterial pressure and venous return did not attain peak values until the recovery period,immediately following activity. Elevating intra-abdominal pressure in resting lizards (via saline infusion) resulted in significant reductions in venous return when the transmural pressure of the post caval vein became negative (i.e. when intra-abdominal pressure exceeded central venous pressure). Together these results suggest that increments in intra-abdominal pressure compress the large abdominal veins and inhibit venous return. During locomotion, the physical compression of the large abdominal veins may represent a significant limitation to cardiac output and maximal oxygen consumption in lizards.

Cerebral venous blood outflow: a theoretical model based on laboratory simulation

OBJECTIVE

The cerebrovascular bed and cerebrospinal fluid circulation have been modeled extensively except for the cerebral venous outflow, which is the object of this study.

METHODS

A hydraulic experiment was designed for perfusion of a collapsible tube in a pressurized chamber to simulate the venous outflow from the cranial cavity.

CONCEPT

The laboratory measurements demonstrate that the majority of change in venous flow can be attributed to either inflow pressure when the outflow is open, or the upstream transmural pressure when outflow is collapsed. On this basis, we propose a mathematical model for pressure distribution along the venous outflow pathway depending on cerebral blood flow and intracranial pressure. The model explains the physiological strong coupling between intracranial pressure and venous pressure in the bridging veins, and we discuss the limits of applicability of the Starling resistor formula to the venous flow rates. The model provides a complementary explanation for ventricular collapse and origin of subdural hematomas resulting from overshunting in hydrocephalus. The noncontinuous pressure flow characteristic of the venous outflow is pinpointed as a possible source of the spontaneous generation of intracranial slow waves.

CONCLUSION

A new conceptual mathematical model can be used to explain the relationship between pressures and flow at the venous outflow from the cranium.

Numerical analysis for stability and self-excited oscillation in collapsible tube flow

This paper describes numerical analysis of collapsible tube flow based on the one-dimensional distributed parameter model of Hayashi. In the present model the effect of flow separation at the collapsed part is replaced with simple viscous friction along the tube, so no ad-hoc modeling for flow separation in former studies is required. A stable semi-implicit numerical procedure based on the SIMPLE method is developed for the problem of flow and tube interaction. The numerical result for a characteristic self-excited oscillation agrees qualitatively with the experimental result. Nonlinear stability of the steady state dependent on the amplitude of the disturbance is numerically investigated and the result is compared with the linear stability analysis based on the former lumped parameter model. Finally, initiation of the self-excited oscillation is examined by applying the initial disturbance at the upstream end of the tube. The disturbance propagates in the downstream direction and is amplified to the self-excited oscillation.

Is the flow in the giraffe's jugular vein a "free" fall?

There is controversy as to whether or not the heart works against gravity in the arteries to the head in the upright position. One view is that the gravitational effects in the neck arteries are counterbalanced by the gravitational effects in the veins of the neck and the heart does not do extra pressure work. This concept has been challenged by others who claim that the heart works against gravity based on the notion that the jugular vein is collapsed and gravitational effects on jugular blood are inoperative, similar to the "free" fall of liquids. The present study supports the view that blood flow in the collapsible jugular vein of the giraffe is not a "free" fall and that the heart does not spend extra energy to raise the blood to the head.

Modeling the circulation with three-terminal electrical networks containing special nonlinear capacitors

Development, first of analog and later of digital computers, as well as algorithms for analysis of electrical circuits, stimulated the use of electrical circuits for modeling the circulation. The networks used as building blocks for electrical models can provide accurate representation of the hydrodynamic equations relating the inflow and outflow of individual segments of the circulation. These networks, however, can contain connections in which voltages and currents have no analogues in the circulation. Problems arise because (a) electrical current must flow in closed loops, whereas no such constraints exist for hydraulic models; and (b) electrical capacitors have a number of characteristics that are not analogous to those of hydraulic compliant chambers. Disregarding these differences can lead to erroneous results and misinterpretation of phenomena. To ensure against these errors, we introduce an imaginary electrical element, the nonlinear residual-charge capacitor (NRCC), with characteristics equivalent to those of a compliant chamber. If one uses appropriate circuit connections and incorporates the residual-charge capacitor, then all voltages and currents in the model are proper analogues of pressures and flows in the circulation. It is shown that the capacitive current represents the rate of change of volume of blood inside the vessel, as well as the rate of the corresponding displacement of volume of the surrounding tissue.

Hemodynamics of vascular 'waterfall': is the analogy justified?

The concept of 'vascular waterfall' has been used for collapsible vessels in different hemodynamic states which have little similarity to each other from a dynamic standpoint. Examples include (a) flow through large systemic veins entering the thorax, (b) flow through microvessels, such as pulmonary, cardiac, hepatic, cerebral, and (c) flow through the jugular vein of the giraffe. The dynamics of freely falling liquids (waterfall) as compared with flow through collapsible blood vessels (in vivo and in vitro) and in collapsible tubes are dissimilar in too many respects to justify analogy. The flow through collapsible tubes and blood vessels can be explained satisfactorily on the basis of elementary principles of fluid mechanics (Bernoulli-Poiseuille). Hence, the term waterfall as a metaphor is misleading and unjustified. We suggest that the use of the term be discontinued for describing vascular dynamics.

Venous waterfalls in coronary circulation

Several studies of flow through collapsible tubing deformed by external pressures have led to a concept known as the "vascular waterfall". One hallmark of this state is a positive zero-flow pressure intercept (Pe) in flow-pressure curves. This intercept is commonly observed in the coronary circulation, but in blood-perfused beating hearts a vascular waterfall is not the only putative cause. To restrict the possibilities, we have measured flow-pressure curves in excised non-beating rabbit hearts in which the coronary arteries were perfused in a non-pulsatile way with a newtonian fluid (Ringers solution) containing potent vasodilator drugs. Under these circumstances, vascular waterfalls are believed to be the only tenable explanation for Pe. In physical terms the waterfall is a region where the vessel is in a state of partial collapse with a stabilized intraluminal fluid pressure (Pw). It is argued that the most probable site of this collapse was the intramural veins just before they reached the epicardial surface. In accord with the waterfall hypothesis, Pe increased as the heart became more edematous, but flow-pressure curves also became flatter, implying multiple waterfalls with differing Pws, leading to complete collapse of some of the venous channels. The principal compressive force is believed to have been the interstitial fluid pressure as registered through a needle (Pn) implanted in the left ventricular wall, but a small additional force (Ps) was probably due to swelling of interstitial gels. A method is presented for estimating Ps and Pw. Unlike rubber tubing, blood vessels are both collapsible and porous. Apparently because of increased capillary filtration, Pn was found to increase linearly with the perfusion pressure. Thus, Pw was not the same at all points on the flow-pressure curve. This finding has interesting implications with respect to the concept of coronary resistance.

Effect of sympathetic tone on pressure-diameter relation of rabbit mesenteric veins in situ

Although venous capacitance has been studied in the neurally isolated tissue or in the in vitro vein segment, this is the first study of sympathetic regulation of the pressure-diameter relation in mesenteric veins in situ, where innervation is kept intact. In 25 alpha-chloralose-anesthetized rabbits, mesenteric vein diameter (679 +/- 27 microns, ranges of 380-1,050 microns at initial state) and intravenous pressure were measured continuously at the same site by using videomicrometer and micropressure systems. Intravenous pressure was increased in a stepwise fashion from the baseline of 6-9 mm Hg to approximately 10, approximately 13, approximately 16, approximately 19, and occasionally to approximately 22 or approximately 26 mm Hg by occluding the portal vein with a pneumatic occluder. Each intravenous pressure was maintained for 90-120 seconds or 4-5 minutes until the diameter increase reached a plateau. Pressure-diameter curves were generated for the control state, during celiac ganglion stimulation, and during local tetrodotoxin or intravenous hexamethonium administration. Diameter was plotted as a function of pressure, and the curves were nonlinear or sigmoid. These results are different from the linear or curvilinear characteristics of the pressure-diameter or pressure-volume relation observed in the pharmacologically or chemically denervated preparation. Tetrodotoxin and hexamethonium attenuated the sigmoid shape of the pressure-diameter curve and shifted it toward the diameter axis of the curve. On the other hand, celiac ganglion stimulation did not change the sigmoid nature of the curve but shifted the curve toward the volume axis.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional characteristics of intramyocardial capacitance vessels during diastole in the dog

In order to evaluate the functional characteristics of the intramyocardial capacitance vessels during prolonged diastole, we analyzed the response of coronary vein flow after stepwise changes of coronary artery pressure in anesthetized open-chest dogs by using our newly developed laser Doppler velocimeter with an optical fiber. The peripheral portion of the great cardiac vein was isolated and the optical fiber tip was inserted into the vessel. The left anterior descending coronary artery was cannulated and connected to a reservoir to regulate coronary perfusion pressure. Intracoronary adenosine administration was carried out to avoid any change in coronary vasomotor tone. After 15 seconds of occlusion of the perfusion route, the heart was arrested by pacing-off. Two seconds later, coronary perfusion pressure was increased stepwise to a preset target pressure. This procedure was repeated by changing target pressure at 4 (or 5) different pressure levels (31-105 mm Hg). The great cardiac vein flow became zero due to the cardiac arrest and remained at zero for a moment (dead time) after the initiation of reperfusion. Then the flow reappeared and increased with first order time delay. The presence of dead time indicates the existence of unstressed volume, and the first order time delay represents the product of resistance and capacitance. The unstressed volume with a minimal vasomotor tone for perfusion pressure of 60-90 mm Hg was 5.2 +/- 2.2 ml per 100 g left ventricle, which is comparable to coronary blood flow for several beats. The capacitance at perfusion pressure of 60-90 mm Hg was 0.08 +/- 0.04 ml/mm Hg per 100 g left ventricle, while that at low perfusion pressure (30-50 mm Hg) was 0.14 +/- 0.09 ml/mm Hg per 100 g left ventricle. These results indicate that the intramyocardial capacitance vessels have two functional components, and that the phasic nature of coronary vein flow is solely the result of the myocardial squeezing of the blood in the capacitance vessels.

The dynamics of the coronary venous system in the dog

The coronary vascular system is characterised by a small blood-volume, high resistance arterial system, in which flow is primarily diastolic, a capillary-venule exchange system and a large blood volume, low resistance venous system with predominantly systolic flow. In order to understand the venous component we measured intramyocardial pressure (IMP), peripheral (PVP) and central (CVP) coronary venous pressure, as well as phasic coronary sinus outflow. Based on the experimental data, a model is proposed to describe the hemodynamics of the coronary venous system. This model consists of an intramyocardial vascular storage region into which the arterial blood flows during diastole. During systole, the pressure in the storage vessels increases, pushing the peripheral and epicardial venous system. Blocking the inflow tends to empty the 'storage' and the peripheral regions, reducing venous pressures. Occlusion of the outflow increases the blood volume in these regions, elevating venous pressures. The observations of peak (IMP-PVP) vs peak venous flow during vagal, right abd left stellate stimulation conform satisfactorily with the model.

A mathematical model of unsteady collapsible tube behaviour

A simple, third-order lumped parameter model is presented to describe unsteady flow in a short segment of collapsible tube held between two rigid segments and contained in a pressurised chamber. Equilibrium states and their stability are analysed in detail, as is fully non-linear time dependent behaviour, including in particular the excitation and sustenance of limit--cycle oscillations. The model explicitly neglects both wave propagation (and hence the possibility of choking) and the influence on the elastic properties of the tube of longitudinal tension, but it is otherwise firmly based on fluid mechanical principles. The results emphasise the profound importance of (a) the unsteady head loss (but with some pressure recovery) in the separated flow at the oscillating throat, and (b) the mechanical properties of the parts of the system both downstream and upstream of the collapsible segment. The nature of the upstream segment in particular determines whether it is an upstream pressure head or the inflow to the collapsible segment that is held constant during oscillations. The results are discussed in the context both of other models and of experiment.

Flow through collapsible tubes at high Reynolds numbers

The pressure-flow relationships of collapsible tubes were studied utilizing the Starling resistor model. Reynolds numbers much higher than previously reported were used to simulate high cardiac output states. Alterations which occur in vivo, including longitudinal tension, stretch, tubing diameter, length, and outflow resistance were also simulated and systematically investigated. The pressure-flow curves showed an initial rising phase, a plateau phase, as well as a late-rising phase which has not been reported previously. Self-induced oscillations occurred during the plateau phase and persisted throughout the late-rising phase. These perturbations were markedly increased by longitudinal tension and stretch, but were attenuated by increased diameter, length, and outflow pressure. These instabilities may prove to be an explanation for the "venous hum."

Flow through collapsible tubes at low Reynolds numbers. Applicability of the waterfall model

The applicability of the waterfall model was tested using the Starling resistor and different viscosities of fluids to vary the Reynolds number. The waterfall model proved adequate to describe flow in the Starling resistor model only at very low Reynolds numbers (Reynolds number less than 1). Blood flow characterized by such low Reynolds numbers occurs only in the microvasculature. Thus, it is inappropriate to apply the waterfall model indiscriminately to flow through large collapsible veins.

Influence of intratubular pressure on proximal tubular compliance and capillary diameter in the rat kidney

Tubular compliance is the response of tubular diameter to changes in intratubular pressure [7]. Proximal tubular compliance was determined directly by measurements of tubular diameter and pressure and indirectly using a mathematical model of tubular fluid flow based on measurements of the hydraulic pressure gradients along the tubule under free flow conditions and during an induced pressure reduction at the end of the proximal tubule. The two independent methods yielded similar values for compliance. Proximal tubular compliance was found to depend upon the intratubular pressure: tubular compliance was significantly higher (P less than 0.001) when the intratubular pressure was reduced below normal (1.0 micron cm H2O-1) than when the pressure was increased above the control value (0.4 micron cm H2O-1). Almost identical compliance values were measured in sodium pentobarbital and inactin anaesthetized rats (P greater than 0.8). Intratubularpressure changes resulted in inverse changes in the diameters of the adjacent capillaries, suggesting that the peritubular capillaries are distensible structures.