Pressure-flow Characteristics Research Articles

Pressure-flow characteristics in the right and left ventricular perfusion territories of the right coronary artery in swine

Regional pressure-flow relationships within the right coronary artery (RCA) circulation of swine were determined. Enflurane-anaesthetized swine (n = 7) were studied during step-wise reductions of RCA perfusion pressure using an extracorporeal circuit. Regional blood flow was measured using microspheres and contractile function of the right ventricle was measured using sonomicrometry. The RCA perfusion territory was divided into its anatomic components: right ventricular free wall (RV), interventricular septum (with further division in transmural thirds; SEP-LV, SEP-MID and SEP-RV) and right atrium (RA). Pressure-flow relations were constructed for each region and autoregulatory capacity assessed through calculation of an autoregulatory index (AI, closed-loop gain). The pressure-flow relationship for the entire RCA exhibited autoregulation down to a pressure of 40 mmHg. The SEP-LV exhibited a similar relationship with loss of autoregulation at approximately 40 mmHg. The pressure-flow relationship of the RV, however, showed autoregulation to a pressure of 30 mmHg with a decrease of blood flow only at a pressure of 20 mmHg. Little autoregulation was observed in the RA. Autoregulatory gain assessed by AI was similar in RV, SEP-LV and SEP-RV as pressure was reduced from 90 to 55 mmHg (RV = 0.54 +/- 0.41; SEP-LV = 0.58 +/- 0.36; SEP-RV = 0.83 +/- 0.36). With further reductions of pressure, AI was highest in the RV, followed by the SEP-RV and then SEP-LV.(ABSTRACT TRUNCATED AT 250 WORDS)

Airflow through modulated louvre systems

Whilst the use of louvres for the control of solar radiation and daylight has been extensively studied, their impact on airflow through window openings seems to be relatively poorly researched. An experimental study has been carried out to investigate the airflow characteristics through a system of modulated louvres. The pressure flow characteristics across a full-scale louvre system were measured. Airflow models were developed for different louvre inclinations using quadratic relationships of the form ΔP = AQ 2 + BQ and compared to the power law expression of the form ΔP = aQ". The changes to airflow at various angles of louvre inclination for a range of pressure differences were also evaluated.

Fluid Delivery Systems: High Flow, Low Pressure, the Key to Safe Resection

The transurethral resection of the prostate (TURP) syndrome has been attributed to the effects of the excess absorption of irrigating fluid. Absorption may be reduced by preventing bladder distension using continuous irrigating resectoscopes or suprapubic trocar drainage. Pressure rises should be limited by the head of pressure in the fluid delivery systems. The flow pressure characteristics of two competing systems, Flowfusor (Kendall-McGaw) and Uromatic (Baxter), were evaluated. We have demonstrated that the Kendall-McGaw Flowfusor V4537 system provides adequate flow at a reduced head of pressure by virtue of the rigid container system and intrinsic negative pressure. The safety of TURPs is increased by reducing the risk of absorption.

A Standardized System for Describing Flow/Pressure Relationships in Vascular Access Devices

Catheters are usually characterized by length and external diameter only. We developed a system to describe flow-pressure characteristics with a single number, M, patterned after a Reynolds number-friction factor correlation. A simple alignment chart was constructed that can be used to determine M for catheters of regular internal diameter and given length. If tapers, side holes, or integral connectors are present, M can be determined in-vitro with water by measuring pressure drop at various flows. The chart describes flow pressure functions for blood (Hct = 0.41) if M is known. Using this system, any catheter, needle, valve seat, extracorporeal tubing, or vascular graft can be assigned a single number that describes hydrodynamic performance characteristics.

A Standardized System for Describing Flow/Pressure Relationships in Vascular Access Devices

Catheters are usually characterized by length and external diameter only. We developed a system to describe flow-pressure characteristics with a single number, M, patterned after a Reynolds number-friction factor correlation. A simple alignment chart was constructed that can be used to determine M for catheters of regular internal diameter and given length. If tapers, side holes, or integral connectors are present, M can be determined in-vitro with water by measuring pressure drop at various flows. The chart describes flow pressure functions for blood (Hct = 0.41) if M is known. Using this system, any catheter, needle, valve seat, extracorporeal tubing, or vascular graft can be assigned a single number that describes hydrodynamic performance characteristics.

Mechanical Complications in Shunts

A retrospective study was conducted on 1,719 hydrocephalic patients, treated between 1974 and 1983 at the Hospital for Sick Children (Toronto) and l'Hôpital des Enfants Malades (Paris), in order to better understand shunt failure. The statistical analysis demonstrates the following: (1) A probability of occurrence of shunt malfunction of 81% at 12 years of follow-up. (2) A high prevalence of shunt obstruction (56.1% of all malfunction) and fracture or disconnection of the tubings (13.6%). (3) A higher risk of proximal occlusion with flanged ventricular catheter. (4) Less malfunction with proximal-non-slit valves as compared to distal-slit valves. (5) The importance of pressure-flow characteristics of the shunt; because of an indirect relation between overdrainage and proximal occlusion. (6) A correlation between connectors and migration or fracture of the shunt. (7) Less distal obstructions when an open-ended distal-catheter was used. These complications were of some clinical, psychological and economical consequences. Their rate could be lowed.

Pressure-flow characteristics of coronary collaterals in dogs

Pressure-flow characteristics of coronary collaterals in dogs

Effects of altered hematocrit on pulmonary artery pressure-flow characteristics in canine pulmonary embolism

We used the relationship between mean pulmonary artery pressure (PAP) and cardiac output (CO) to investigate pulmonary hemodynamic effects of altered hematocrit in an intact canine model of embolic pulmonary hypertension. The slope of the PAP-CO relationship defines the incremental vascular resistance OR) and the extrapolated pressure intercept (P 1) represents the effective vascular outflow pressure. Injection of autologous clot did not affect ventricular filling pressures, but decreased CO (from 3.1 to 1.9 l/min −1, P < .01) and markedly increased PAP, from 15 to 35 mm Hg ( P < .01). In this setting isovolemic exchange transfusion decreased Hct from 42% to 21 % ( P < .01) and markedly improved pulmonary hemodynamics. Since P 1 was unaffected, this improvement was entirely explained by a decrease in the incremental vascular resistance. Hemodilution decreased IR by 25% from 4.4 to 3.3 mm Hg/l −1 /min ( P < .05). We conclude that in this canine model of embolic pulmonary hypertension a decrease in viscosity significantly alters pulmonary hemodynamics by decreasing the slope of the pressure-flow relationship. These findings do not address the clinical implications of hemodilution on systemic oxygen delivery.

Pulmonary vascular pressure-flow characteristics in canine pulmonary embolism

We tested the hypothesis that, in canine embolic pulmonary hypertension, upstream transmission of increased left atrial pressure (LAP) is inversely related to the level of the pressure intercept (PI) obtained by extrapolation from the linear pulmonary vascular pressure-flow (P-Q) plot. P-Q coordinates were obtained by varying Q through systemic fistulas. Seven group 1 dogs were embolized with autologous blood clot to produce marked pulmonary hypertension and mean pulmonary arterial pressure (PAP), and PI increased from 15 to 41 mmHg (P less than 0.001) and from 8.8 to 31 mmHg (P less than 0.001), respectively. Before and after embolization we assessed effects of increased LAP, produced by inflation of a left atrial balloon, on PAP at constant Q. Embolization depressed the mean slope of this relationship from 0.78 to 0.16 (P less than 0.001). Subsequently, six group 2 dogs were embolized to produce moderate pulmonary hypertension with a mean PI of 22 mmHg. This value was significantly less than PI in group 1 (P less than 0.01). After embolization, the slope of the PAP-LAP relationship was greater in group 2 than group 1: 0.47 vs. 0.16 (P less than 0.01). We conclude that the upstream transmission of left atrial pressure is inversely related to PI and that marked embolic pulmonary hypertension produces an effective vascular waterfall.

Finit-element analysis of two-dimensional peristaltic flows. 2nd Report, Pressure-flow characteristics.

A numerical method employing a finite-element method is adopted for an investigation of peristaltic pumping in which both the wall-wave slope and the wave amplitude are large. Various peristaltic flows are calculated at moderate Reynolds number, and the effect of wave amplitude, wall slope, and the Reynolds number on the pumping performance is considered. It is shown that the effect of large wall slope makes the pressure rise at zero time-mean flow increase remarkably. The effect of fluid inertia makes the pressure rise decrease in case of small amplitude, while it makes the pressure rise increase for large amplitude.

Pressure flow characteristics of the valve in spacer devices.

Pressure flow characteristics of the valve in spacer devices.

Effects of temperature on the mussel pump

The nature of the bivalve filter-pump was further elucidated in the n~ussel Mytilus edulis by investigating effects of temperature on pump and system characteristics. Pumping rates increased with temperature, linearly correlated with the temperature-determined decrease in viscosity of the water. The variation in pumping rate with temperature thus corresponded to the varying viscous resistance to water flow in the canal system of the mussel-pump. The increase in beat frequency of the lateral cilia with temperature therefore had no clear effect on the pumping rate. Also, spawning stimulation of beat frequency was without effect on pumping rates. In the back pressure-flow characteristic of fully open, active mussels, pump pressure at zero flow (AH;) was independent of temperature. Predictions based on the pump modelled as a leaky, viscous, constant-force pump were in good agreement with measured and estimated pump parameters, further supporting the concept of suspension-feeding bivalves as filter-pumps that process the ambient water autonomously that is, in the absence of physiological mechanisms for regulating water pumping.

Pressure-Flow Characteristics of Crab Gills: Implications for Regulation of Hemolymph Pressure

Flow conductances (resistance−1) ofsaline-perfused gills of Carcinus maenas and Ovalipes catharus varied with transmural pressure (Ptrans, internal pressure minus bath pressure). In the range Ptrans = + 1 to + 4 cmH2O, at constant gillpressure drop, conductance was at a plateau. Below +0.5 cmH2O, conductance fell sharply, and at Prans = −0.5 cmH2O conductance was 67% (Carcinus) and 27% (Ovalipes) of plateau conductance. At Ptrans = −2 cmH2O, conductances fell to only 29% and 9%, respectively, of plateau. In vivo, small changes in branchial chamber pressure or hemolymph pressure could alter branchial conductance. Changes in Ptrans during unilateral ventilatory pauses or reversals would favor perfusion of the forwardly ventilated gills. However, bilateral ventilation changes probably cause only minor changes in Ptrans because of the rigid exoskeleton and incom pressible body fluids. All circulatory pressures contain a hydrostatic component, the mean circulatory filling pressure (MCFP orstatic pressure). Unlike the case in vertebrates, MCFP in the crab's open circulation depends on total body volume (not circulating volume) and on exoskeletal compliance (not total vascular compliance). The gills and branchial chamber linings appear to be the most compliant surface structures in crabs and thus determine MCFP. It is argued that the mean pressure in the lamellar hemocoel must normally be close to MCFP and largely independent of cardiac output. However, MCFP, Ptrans, gill volume, and gill conductance are expected to be very sensitive to change in total internal volume (e.g., osmosis, feeding, urine release, hemorrhage). Mechanisms for long-and short-term control of internal volume and pressure are essential for regulation of respiratory blood flow. Flow rectification was observed in the gills of both species. This property may be associated with the efferent valves and with extensions of the lamellar septum into the marginal canal. This asymmetry could assist unidirectional gill perfusion during venous pressure oscillations generated by movement.

Pressure flow characteristics of the valve in spacer devices.

The valve system in the mouth piece of two spacer devices was analysed. Pressures required to open and close the non-rebreathing valve were very low (less than 0.1 kPa). Inspiratory flow requirements were within physiological limits for infants' normal tidal breathing. Expiratory flow requirements varied significantly, and the flow required to prevent rebreathing from the chamber may exceed the physiological flow limits for normal tidal breathing.

Collateral influence on pressure-flow characteristics of coronary circulation.

The purpose of this study was to determine the role of the coronary collateral circulation on the shape of the coronary pressure-flow (P-F) relationship and its effects on the pressure at zero flow (PZF) or pressure intercept. We investigated the P-F characteristics of the coronary circulation under two conditions. 1) To minimize the influence of collaterals, we measured coronary flow by timed collections of coronary sinus outflow in 15 dog hearts, as perfusion pressure to all vessels was varied; 2) to maximize the effect of collaterals, we measured circumflex artery flow in six dog hearts, as perfusion pressure to only the circumflex coronary artery was varied and the pressure in the remaining vessels was maintained constant. We used an isolated heart preparation in which ventricular chamber and venous outflow pressures equalled atmospheric pressure and the vessels were maximally dilated with adenosine. In the first condition, the P-F relationship was curvilinear with a PZF of 0 mmHg; in the second condition, the P-F relationship was curvilinear with a PZF of 16 +/- 2 mmHg, and flow was retrograde at pressures below PZF. We conclude that in both conditions the curvilinearity of the coronary P-F relationship was the result of nonlinear elastic properties of blood vessels, not requiring the "waterfall" concept to be invoked, and that in the second condition the influence of collateral vessels produced the positive pressure intercept.

Oscillations in a collapsed-tube analog of the brachial artery under a sphygmomanometer cuff.

To determine whether self-excited oscillations in a Starling resistor are relevant to physiological situations, a collapsible tube conveying an aqueous flow was externally pressurized along only a central segment of its unsupported length. This was achieved by passing the tube through a shorter and wider collapsible sleeve which was mounted in Starling resistor fashion in a pressure chamber. The tube size and material, and all other experimental parameters, were as used in our previous Starling resistor studies. Both low- and high-frequency self-excited oscillations were observed, but the low-frequency oscillations were sensitive to the sleeve type and length relative to unsupported distance. Pressure-flow characteristics showed multiple oscillatory modes, which differed quantitatively from those observed in comparable Starling resistors. Slow variation of driving pressure gave differing behavior according to whether the pressure was rising or falling, in accord with the hysteresis noted on the characteristics and in the tube law. The results are discussed in terms of the various possible mechanisms of collapsible tube instability, and reasons are presented for the absence of the low-frequency mode under most physiological circumstances.

Recombinant tissue-type plasminogen activator in canine embolic pulmonary hypertension. Effects of bolus versus short-term administration on dynamics of thrombolysis and on pulmonary vascular pressure-flow characteristics.

We used a canine model of embolic pulmonary hypertension, induced by injection of autologous radioactive blood clots, to investigate effects of recombinant tissue-type plasminogen activator (rt-PA) on dynamics of thrombolysis and on pulmonary pressure-flow (PQ) characteristics. Over 5 (rt-PA5) or 15 (rt-PA15) minutes, 1 mg/kg rt-PA was infused. Rate and extent of thrombolysis were assessed by counting over both lung fields with a gamma camera. Emboli increased mean pulmonary artery pressure from 14 to 36 mm Hg (p less than 0.005). This change was predominantly due to an increase in the effective outflow pressure (PI) (from 9 to 29 mm Hg, p less than 0.001), obtained by extrapolation from the linear PQ relation. While pulmonary hemodynamics improved with rt-PA5 and rt-PA15, the change was greatest with rt-PA15. For example, the increase in PI that occurred with embolization was abolished with rt-PA15. Also, the decrease in pulmonary artery pressure was greatest with rt-PA15. While not significantly different, extent of total clot lysis tended to be greatest with rt-PA15 (p less than 0.07). Also, while during infusion, the concentration of rt-PA5 was threefold that of rt-PA15, the corresponding rate of thrombolysis was similar with rt-PA5 and rt-PA15. These results indicate that the improvement in pulmonary hemodynamics with rt-PA is primarily explained by a decrease in PI. Furthermore, they suggest an upper limit to the dose-thrombolytic rate relation with rt-PA.

Pressure-flow characteristics of coronary collaterals in dogs

In this study we utilized two methods to investigate the pressure-flow, P-F, relationship of the coronary collateral vessels in a beating, blood-perfused, isolated heart preparation. In the first method (free-flow method), 12 dog hearts were perfused at pressures ranging from 100 to 0 mmHg, whereas the retrograde flow (index of collateral flow) was measured on the circumflex coronary artery, LCA, against atmospheric pressure, first during autoregulation and then after maximum vasodilation. In the second method (back-pressure method), the back pressure to retrograde flow was varied from 0 to 100 mmHg, whereas the perfusion pressure to the remaining vessels was maintained constant at 100 mmHg. This procedure was performed on four hearts with and without embolization of the LCA by 25-microns spheres. The free-flow method demonstrated a linear P-F relationship with an average correlation coefficient, r, of 0.98. The pressure intercept was 1.7 +/- 1.2 mmHg. The back-pressure method yielded a relationship that was more curvilinear with an average pressure intercept of 13 mmHg without embolization and 38 mmHg with embolization. An analog of the coronary and collateral circulation was used to illustrate that, in the back-pressure method, changes in the coronary resistance at low pressures contributed to the nonlinearity of the collateral P-F characteristics and that the network formed between the collateral and coronary resistances was responsible for the higher pressure intercept value.

Experimental study of two-dimensional peristaltic flows. (Pressure-flow characteristics in an asymmetric two-dimensional channel flow).

Peristaltic pumping in an symmetric two-dimensional channel with one fixed wall and one moving wall is investigated experimentally for a wide range of amplitude ratio, and for Reynolds numbers ranging in order from unity to 103. Measurements of the mean pressure rise and of the mean flow are carried out, and the interdependent relations between them describing the primary properties of the pumping performance are discussed. It is found that for zero mean flow, the pressure rise is independent of Re for a small Re, while it increases proportionally to Re1/2 for a large Re. However, the relations between the pressure rise and the mean flow are approximately linear for large Reynolds numbers as well as for small Reynolds numbers.

The effect of positive end-expiratory pressure on the coronary blood flow.

Positive end-expiratory pressure (PEEP) is used liberally whenever a ventilated patient shows signs of increased pulmonary venous shunting. Clinicians using PEEP to improve blood oxygenation may face the cardiovascular side effects which limit utilization of the desired respiratory effects of PEEP. We measured the pressure flow characteristics of the cardiovascular system and the coronary arterial system as a function of PEEP, using closed-chest surgically instrumented dogs, in order to assess its effects on myocardial blood flow with respect to the left ventricular energy demands. The aortic left ventricular blood pressure as well as the aortic blood flow decreased with increasing PEEP values. The coronary blood flow decreased by 5% for PEEP values of 4 cm H2O, and by 25% for 14 cm H2O of PEEP. PEEP values under 10 cm H2O reduced the left ventricular end-diastolic pressure (LVEDP), while higher PEEP values caused an increase in LVEDP. The relation between the alterations of coronary and aortic blood flows changed with PEEP values. Low PEEP values (less than 10 cm H2O) had a tendency for higher relative reduction of aortic blood flow, whereas higher PEEP values (higher than 10 cm H2O) reduced the coronary blood flow more than the reduction occurring in the aortic blood flow. Our results suggest that low PEEP values may have beneficial effects on the relation between aortic blood flow and coronary blood flow, therefore low PEEP application may minimize hypoxic myocardial alterations. Further studies that will measure left ventricular workload or another metabolic index for estimating myocardial perfusion relative to its metabolic demand are essential before clinical conclusions can be drawn from our results.