Perivascular Pressure Research Articles

Abstract TP81: Flow Independent Dynamics in Aneurysm (FIDA): Pressure Measurements within an Aneurysm Before and After Partial and Complete Flow Impairment

BACKGROUND: The correlation between different hemodynamic variables and aneurysm growth and rupture has not been fully understood. We aimed to study intravascular and peri-vascular pressure in the aneurysm after partial and complete flow impairment within the aneurysm. MATERIALS AND METHODS: A silicone model of an 8 mm sized aneurysm (neck diameter: 5mm, vessel size: 4 mm) was used. The aneurysm wall was encapsulated and sealed within a 5ml syringe filled with saline and a pressure sensor guidewire (PrimeWire, Volcano Corp.) providing pressure changes in the peri-vascular compartment (outer aneurysm wall). Through a 5 F guiding catheter, a second pressure sensor guidewire was advanced inside the aneurysm sac. The aneurysm connected to a pulsatile fluid pump. Both pressure sensors continuously measured pressure inside and outside the aneurysm under pulsatile flow under the following conditions: 1) baseline (reference); 2) a 16 mm by 3.75 mm flow diverting stent (ev3, Irvine, California) deployed in front of the aneurysm; 3) two flow diverting stents (16 mm by 3.5 mm) were deployed; and 4) a covered stent (4 mm by 16 mm VeriFlex cronory artery stent covered with rubber sheet) was deployed. RESULTS: Mean baseline pressures inside and outside the aneurysm were 55 mmHg (range 120-40 mmHg) and 15 mmHg (range 40-8 mmHg), respectively. There was no change in pressure inside and outside the aneurysm after deploying the first and second flow diverting stents (partial flow impairment) and pressure remained at 55 mmHg and 15 mmHg inside and outside the aneurysm, respectively. The pressure recording from outside the aneurysm dropped from 15mmHg to zero after deploying the covered stent (complete flow impairment). There was no change in pressure inside the aneurysm after deploying the covered stent. Mean pressure within the aneurysm was 55 mmHg and it remained the same after covered stent deployment. CONCLUSION: Our findings suggest a major discordance between the pressures within the aneurysm and partial or complete flow impairment (flow-independent). The outer wall pressure is reduced after covered stent placement. These finding may assist physicians in better understanding of aneurysm hemodynamics and rupture after flow diverting stent deployment.

Increased cerebrovascular resistance after retrograde cerebral perfusion: A Doppler study

Deep hypothermic circulatory arrest (DHCA) with retrograde cerebral perfusion (RCP) under high central venous pressure (CVP) is often used in aortic arch surgery under cardiopulmonary bypass (CPB). We hypothesized that DHCA with RCP under high CVP causes cerebral vascular compression because of increased perivascular pressure due to extravasation of fluid into intracranial tissue. In a retrospective study, we evaluated the pulsatility index (PI) and resistance index (RI) of the internal carotid arteries (ICA) and external carotid arteries (ECA) before and after CPB in 15 patients who underwent DHCA/RCP (group 1) and 17 patients who underwent regular CPB without DHCA/RCP (group 2). Both indices are known to reflect vascular resistance distal to the measurement point. The PI and RI of the ICA increased significantly after the procedure in group 1 but did not change in group 2. The PI and RI of the ECA did not change in either group. In group 1, the rate of increase in PI and RI correlated with the duration of RCP, which was significantly higher in patients who had postoperative delirium than in patients who did not experience postoperative delirium. As increases in PI/RI after DHCA/RCP occurred only in the ICA, we concluded that the changes were due to compression of vessels as a result of increased perivascular pressure. The greater increase in the PI/RI in patients who experienced postoperative delirium indicates that increased perivascular pressure plays a role in the occurrence of postoperative delirium after DHCA/RCP.

Vascular Compromise and Hemodynamics in Pulmonary Arterial Hypertension: Model Predictions

A computer model of the normal pulmonary circulation based on animal (dog) data obtained in the literature was previously developed by Bshouty and Younes (for details see references [S1, S2]). The model is described briefly and only details not included in the original model development will be included here. The original model is a multibranch model that bifurcates sequentially starting from the pulmonary artery (PA) up to eight (four generations) precapillary and capillary channels. The model was later modified to include five generations ending in sixteen precapillary and capillary channels (Figure S1). On the venous side, the vessels converge and reunite sequentially to end in the left atrium (LA). The resistance of a given vessel in the model is dependent on vessel dimensions (length and cross-sectional area). Changes in vessel dimensions occur in response to changes in lung volume and intravascular pressure (P v ). The original multibranch model was developed between 1987 and 1989, when microprocessors operated at 4.77 MHz. At the time, each extra-alveolar vessel was divided into twenty-five segments. This was done in order to shorten the time needed to go through the thousands of calculations and iterations needed to produce a stable solution (which at the time, took several minutes per one set of data). With the development of faster microprocessors, the number of segments of each extra-alveolar vessel was gradually increased initially to one hundred segments and is currently set at a thousand segments. Results obtained with twenty-five segments differed from results obtained with one hundred segments at the second decimal level and results obtained with a thousand segments differed from results obtained with one hundred segments mostly at the third decimal level and are therefore, not clinically significant. Similarly, results obtained with the four-generation model when compared with the five-generation model differed only at the second decimal level and are therefore, also not clinically significant. All simulations presented in this paper were generated with the five-generation model and 1000 segment vessels (See footnote). Cross-sectional area of each arterial (A a ) and venous (A v ) segment is calculated based on the transmural pressure (P tm ) across that segment, P tm being intravascular minus perivascular pressure (P x ). The characteristic behaviour of A a as a function of P tm was derived from data obtained by Smith and Mitzner (S3) and Maloney et al (S4) and described in detail by Bshouty and Younes (S1). It is expressed by the following linear relationship,

Case Scenario: Acute Postoperative Negative Pressure Pulmonary Edema

A 25-yr-old man (weight, 68 kg; height, 183 cm) presentedto the surgery center for excision of back and thigh schwan-nomas on the same day. The patient’s medical history wassignificant only for his history of multiple schwannoma re-sections and a history of smoking one pack of cigarettes perweek for the past 5 yr. He denied previous problems withgeneral anesthesia, and his baseline peripheral oxygen satu-ration was 99% on ambient air.Thepatientwaspremedicatedwith2mgmidazolam,andanesthesia was induced with 250 mg fentanyl, 500 mg thio-pental, and 8 mg vecuronium given for facilitation of tra-cheal intubation. He was atraumatically intubated with a7-mm ID endotracheal tube using a no. 3 Macintosh laryn-goscope (Teleflex Medical, Research Triangle Park, NC) onthe first attempt with direct visualization of the vocal cords.The patient was turned prone, bilateral breath sounds werereconfirmed, and schwannoma excisions were performed ontheleftthighandtheleftflank.Atotalof0.5mghydromor-phone was administered for analgesia. The intraoperativecoursewasunremarkable.Thepatientwashemodynamicallystable with minimal blood loss and was easily ventilated andoxygenated. A total of 500 ml lactated Ringer’s solution wasadministeredduringthe65-minsurgicalprocedure.Thepa-

Surrounding tissues affect the passive mechanics of the vessel wall: theory and experiment

The stress and strain in the vessel wall are important determinants of vascular physiology and pathophysiology. Vessels are constrained radially by the surrounding tissue. The hypothesis in this work is that the surrounding tissue takes up a considerable portion of the intravascular pressure and significantly reduces the wall strain and stress. Ten swine of either sex were used to test this hypothesis. An impedance catheter was inserted into the carotid or femoral artery, and after mechanical preconditioning pressure-cross-sectional area relations were obtained with the surrounding tissue intact and dissected away (untethered), respectively. The radial constraint of the surrounding tissue was quantified as an effective perivascular pressure on the outer surface of the vessel, which was estimated as 50% or more of the intravascular pressure. For carotid arteries at pressure of 100 mmHg, the circumferential wall stretch ratio in the intact state was approximately 20% lower than in the untethered state and the average circumferential stress was reduced by approximately 70%. For femoral arteries, the reductions were approximately 15% and 70%, respectively. These experimental data support the proposed hypothesis and suggest that in vitro and in vivo measurements of the mechanical properties of vessels must be interpreted with consideration of the constraint of the surrounding tissue.

Lung inflation distends small arteries (< 1 mm) in excised dog lungs

We utilized microfocal fluoroscopic angiography to study the influence of lung inflation on small (0.2- to 1.3-mm-diam) pulmonary arteries in isolated left lower lobes from dog lungs during both flow and no-flow conditions. Alveolar pressure, which in this preparation was equal to transpulmonary pressure, was set at 2, 8, or 14 mmHg while vascular pressure was varied from 0 to 24 mmHg. The diameters of these small arterial vessels increased with lung inflation. No differences were observed between the results obtained during flow and no-flow conditions. Thus, arteries in this diameter range can be considered as extra-alveolar, and the effect of lung inflation on these small extra-alveolar arteries was qualitatively similar to that previously described for larger extra-alveolar vessels. Quantitatively, the degree of vessel distension was about the same per unit increase in transpulmonary pressure at constant vascular pressure as for a change in vascular pressure at constant transpulmonary pressure. Accordingly, inflation produced a decrease in perivascular pressure surrounding these small arteries that was approximately equal to the increase in transpulmonary pressure.

Assessment of flow profile of left anterior descending coronary artery in hypertrophic cardiomyopathy by transesophageal pulsed Doppler echocardiography

This study assessed the flow velocity profiles of the left anterior descending coronary artery (LAD) in 7 patients with nonobstructive hypertrophic cardiomyopathy (HC) and in 6 normal subjects by transesophageal pulsed Doppler echocardiography, and evaluated their characteristics and the hemodynamic determinants. Systolic peak flow velocity of the LAD (7 ± 30 cm/sec) was significantly tower in patients with HC than in normal subjects (34 ± 11 cm/s, p < 0.05), and there was a significant inverse correlation between systolic peak flow velocity and the thickness of the ventricular septum (r = 0.81, p < 0.01). In 2 cases of HC with ventricular septal thickness of >20 mm, a remarkable systolic reverse flow was observed in the LAD. However, there was no significant difference in diastolic peak flow velocity between HC and normal subjects. During early diastole, the acceleration time of LAD flow velocity was significantly prolonged (210 ± 67 vs 95 ± 15 ms, p < 0.01) and the acceleration rate was significantly decreased (3.6 ± 2.0 vs 6.6 ± 1.8 m/s 2, p < 0.02) in patients with HC. The time constant of the left ventricular pressure decay was significantly prolonged in patients with HC (55 ± 6 ms) compared with normal subjects (39 ± 2 ms, p < 0.001). In HC, increased intramural perivascular pressure of the thickened ventricular septum during systole may be attributed to systolic LAD flow pattern. However, the early and mid-diastolic LAD flow pattern may be affected by impaired left ventricular relaxation.

Venous Microvasculature of the Pineal Body and Choroid Plexus in the Rat

Rats were perfused from aorta with Ringer's solution and with Karnovsky's fixative, and injected from the right atrium with Mercox resin. Specimens were properly taken, observed under LM, TEM, SEM, and stereo-photographed. Fenestrated endothelial cells of the pineal capillary were observed to contain plenty of microtubules running parallel to the long axis of the vessel. The endothelial basal lamina appeared anchored by microfibrils onto fine collagen fibrils and onto the basal lamina of the perivascular cells. These findings indicate resistance of the capillary endothelium against compression by perivascular hydrostatic pressure, and strongly suggest that the perivascular cerebrospinal fluid is absorbed into the capillary lumen. Resin cast of the pineal venous system looked like a glomerule with two longer collecting veins on the ventral surface and with shorter ones on the dorsal side all emptying into the straight sinus and the confluence of sinuses. Resin cast of the choroid plexus venous system tended to lobulate and looked like a vine with a spiral collecting vein emptying into the great vein. Two veins were found connecting the great vein to the confluence and to the transverse sinuses. These could be effective collaterals in case of occlusion of Galen's vein.

Distensibility and pressure-flow relationship of the pulmonary circulation. I. Single-vessel model

To ascertain the relative contributions of vascular distensibility and nonhomogeneous behavior within the pulmonary circulation to the distinctive nonlinear relationship between inflow pressure (Pin) and flow [pressure-flow (P-F) relationship] and between Pin and outflow pressure (Pout) at constant flow (Pin-Pout relationship), we developed a multibranched model in which the elastic behavior of, and forces acting on, individual branches can be varied independently. The response of the multibranched model is described in the companion article (J. Appl. Physiol. 68: 1514-1527, 1990). Here we describe the methods used and the responses of single components of the larger model. Perivascular pressure is modeled as a function of intravascular and transpulmonary pressures (Pv and Ptp, respectively) and vessel length as a function of lung volume. These and the relationship between vascular area (A) and transmural pressure (Ptm) were modeled primarily from the dog data of Smith and Mitzner (J. Appl. Physiol. 48: 450-467, 1980). Vasomotor tone is modeled as a radial collapsing pressure (Pt) in the same plane as Ptm. In view of lack of information about the relationship between Pt and A for a given active state, different patterns were assumed that span a wide range of possible relationships. The P-F and Pin-Pout relationships of single vessels were very similar to those reported for the entire intact circulation. Of note, the slope of the Pin-Pout relationship in the low Pout range (0-5 Torr) was very low (less than 0.25) and increased gradually with Pout toward unity. Vasomotor tone caused an apparent parallel shift in the P-F relationship in the physiological flow range of the dog (2-8 l/min) regardless of the pattern used to model the Pt vs. A relationship; different patterns affected the P-F relationship only over the low flow range before the parallel shift was established.

Microvascular pressures measured by micropipettes in isolated edematous rabbit lungs.

To study the mechanical effects of lung edema on the pulmonary circulation, we determined the longitudinal distribution of vascular resistance in the arteries, veins, and microvessels, and the distribution of blood flow in isolated blood-perfused rabbit lungs with varying degrees of edema. Active vasomotor changes were eliminated by adding papaverine to the perfusate. In three groups of lungs with either minimal [group I, mean wet-to-dry weight ratio (W/D) = 5.3 +/- 0.6 (SD), n = 7], moderate (group II, W/D = 8.5 +/- 1.2, n = 10), or severe (group III, W/D = 9.9 +/- 1.6, n = 5) edema, we measured by direct micropuncture the pressure in subpleural arterioles and venules (20-60 micron diam) and in the interstitium surrounding these vessels. We also measured pulmonary arterial and left atrial pressures and lung blood flow, and in four additional experiments we used radio-labeled microspheres to determine the distribution of blood flow during mild and severe pulmonary edema. In lungs with little or no edema (group I) we found that 33% of total vascular pressure drop was in arteries, 60% was in microvessels, and 7% was in veins. Moderate edema (group II) had no effect on total vascular resistance or on the vascular pressure profile, but severe edema (group III) did increase vascular resistance without changing the longitudinal distribution of vascular resistance in the subpleural microcirculation. Perivascular interstitial pressure relative to pleural pressure increased from 1 cmH2O in group I to 2 in group II to 4 in group III lungs.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of cerebral blood flow changes by transfontanelle Doppler ultrasound in infantile hydrocephalus.

Doppler ultrasound investigation of cerebral blood flow velocity was performed in hydrocephalic infants through the anterior fontanelle. Systolic (S) and end-diastolic (D) frequency values recorded on the anterior cerebral artery were used to define the pulsatility index (PI) calculated from the equation PI=S-D/S. Comparison between systolic, end-diastolic and pulsatility index values of 50 normal infants and 10 hydrocephalic infants showed a statistically significant difference (P less than 0.05) for systolic and pulsatility index values. However, no significant difference was found for end-diastolic values. The authors believe that the phenomenon could be explained as an increase of the cerebrovascular compliance which counteracts the increase of the perivascular pressure in an attempt to maintain a normal cerebral blood flow. Therefore, the transfontanelle Doppler ultrasound technique may provide a useful and early tool in diagnosing cerebral blood-flow changes in hydrocephalic infants.

Pulmonary vascular tone is a determinant of basal lung perfusion in normal seated subjects

In the human upright lung the downward increase in lung perfusion reverses in the lower third, thus giving rise to a zone of reduced basal perfusion (zone 4). The flow in zone 4 is regulated by the extra-alveolar vessels, the diameter of which is determined by lung volume, perivascular interstitial pressure, and vasomotor tone. To estimate the role of pulmonary vascular tone in the formation of zone 4, we infused nitroprusside (NTP), a potent pulmonary vasodilator, in six normal seated subjects. We measured their regional perfusion distribution using 133Xe in control conditions and at two dose levels of NTP (20.8 and 52.1 micrograms/min). Regional perfusion distribution was measured similarly and according to the same protocol in six subjects receiving only a placebo solution. In four of the six subjects receiving NTP, right-heart catheterization allowed simultaneous estimations of cardiac output and pulmonary arterial pressure to be made. NTP slightly decreased the perfusion of the nondependent parts of the lungs and markedly increased the perfusion of the lung bases, thus reducing the extent of zone 4. No changes were observed in the placebo experiments. Cardiac output and indices of ventilation and gas exchange did not change significantly. Peripheral and pulmonary arterial pressure fell slightly but significantly during NTP infusion. We attribute the observed changes in basal perfusion to the vasodilatory effects of NTP on the extra-alveolar vessels. Our findings thus support the hypothesis that in normal subjects zone 4 is partly created by the pulmonary vascular tone.

Analysis of pulmonary vascular interdependence in excised dog lobes.

The pressure-volume behavior of intraparenchymal extra-alveolar arteries and veins was measured at various lung inflation states by inflating and deflating the vasculatures with air. The vascular volumes at specific vascular pressures (Pv) and transpulmonary pressures (Ptp) were partitioned into components of axial length and cross-sectional area. An analysis of the interrelationships between the perivascular pressure (Px), the vascular pressure vs. cross-sectional area behavior, and the Ptp is presented. For in vivo values of Pv, at functional residual capacity, the vascular-parenchymal mechanical interaction was small and values of arterial and venous Px approximately Ppl. With increasing Ptp to 30 cmH2O, values of both the arterial and venous Px relative to Ppl (Px - Ppl) decreased to approximately -5 cmH2O, indicating that the magnitude of the vascular-parenchymal interaction with increasing Ptp is similar for both arteries and veins in the in vivo state. At any fixed Ptp, values of arterial and venous Px - Ppl decreased nearly linearly with decreasing vascular cross-sectional area. These results were generally consistent with a linear continuum elasticity solution relating stress and deformation in the perivascular parenchyma.

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

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

Perivascular pressure measurements by wick-catheter technique in isolated dog lobes.

Saline-filled wick catheters were inserted intraparenchymally in the loose connective tissue separating the bronchus and pulmonary artery of unperfused isolated dog lobes. Px, the difference between perivascular pressure and pleural surface pressure (which was atmospheric pressure), was measured by the wick for arterial pressures (Pa) of 10 and 25 cmH2O at lobe transpulmonary pressures (Ptp) of 25, 15, 10, 5, and 2 cmH2O during a deflation pressure-volume maneuver. The response time of Px to step changes in Pa was relatively short, a pressure plateau always occurring in less than 3 min. For Pa of 25 cmH2O, mean Px in five lobes was -2 cmH2O at Ptp of 2 cmH2O and decreased almost linearly to -9 cmH2O at Ptp of 25 cmH2O. Reducing Pa from 25 to 10 cmH2O resulted in a mean decrease in Px of 1 cmH2O at Ptp of 2 cmH2O and 2 cmH2O at Ptp of 25 cmH2O. These results are generally consistent with predictions from a continuum-mechanics analysis of pulmonary vascular interdependence and do not support the concept that perivascular interstitial fluid pressure is different from surface pressure.

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

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

Elastic properties of lung parenchyma: The effect of pressure-volume hysteresis on the behavior of large blood vessels

The elastic constants of lung parenchyma were measured for states of uniform expansion. The bulk modulus was determined from small quasistatic pressure-volume ( PV) perturbations, and the shear modulus was determined by indentation tests at fixed transpulmonary pressures. During full inflation-deflation PV maneuvers, both linear elastic constants were unique functions of transpulmonary pressure and were independent of the volume, which differed at the same pressure owing to PV hysteresis. Large-deformation solutions for the symmetric inflation of a cylindrical hole in the parenchyma were computed using a second-order strain-energy function for which the elastic constants were linear functions of the pressure. The solutions were used to explain changes in diameter of large intraparenchymal arteries when transpulmonary and vascular pressures were varied. Estimates of perivascular pressure (the pressure acting on the outside of intraparenchymal arteries) were obtained for both inflation and deflation limbs of the PV curve.

Cerebral hemodynamic effects of aminophylline in the treatment of patients with hypertensive headaches

CEREBRAL HEMODYNAMIC EFFECTS OF AMINOPHYLLINE IN THE TREATMENT OF PATIENTS WITH HYPERTENSIVE HEADACHES. John H. Moyer and (by invitation) Sam I. Miller, Arthur B. Tashnek and Harvey Snyder. (From the Departments of Medicine and Pharmacology, Baylor University College of Medicine and the Jefferson Davis Hospital, Houston, Tex.) During the course of evaluating the cerebral effect of intravenous aminophylline@ it was noted that this drug gave prompt and complete relief of headaches associated with severe hypertension. For this reason a study on the effect of aminophylline (0.5 gm. intravenously) on cerebrovascular hemodynamics, cerebrospinal fluid pressure and cerebral metabolism was undertaken in nine patients exhibiting hypertensive headaches. Similar studies were conducted in seven patients to whom caffeine (0.5 gm.j was administered instead of aminophylline. Aminophylline caused a significant increase in cerebrovascular resistance and a decrease in cerebral blood flow (mean of 53 for the group reduced to 36 cc./100 gm./minute). Headache was relieved in all those patients in whom the cerebral blood flow decreased. This was also associated with a sharp reduction in cerebrospinal fluid pressure (mean of 210 reduced to 132 mm. water). The mean (group) jugular venous pressure decreased 13 mm. water. Cerebral metabolism as noted in oxygen and glucose uptake was not altered. Caffeine showed similar effects on cerebral dynamics but they were less dramatic and less consistent. These observations were interpreted to indicate that the cerebrospinal fluid pressure is decreased due to a reduction in arterial blood flow into the head, thus reducing the intravascular volume. The relief of headaches is probably due to reduced capillary and perivascular tissue pressure surrounding and distal to the constricted arterioles. HOMOGENOUS RENAL TRANSPLANTS AFTER BILATERAL NEPHRECTOMY. E. E. Muirhead. (From the Department of Pathology, Southwestern Medical School of the University of Texas, Dallas, Tex.) Dogs have been maintained by means of peritoneal irrigation after removal of both kidneys for intervals of approximately twelve or fourteen days. At this time a homogenous kidney was transplanted to the neck of the bilaterally nephrectomized animal and its activity appraised. Following bilateral nephrectomy and-the use