Non-invasive studies of peripheral vascular compliance using a non-occluding photoplethysmographic method.

A non-invasive technique is implemented to measure a peripheral vascular compliance index Cindex, using an infrared photoplethysmographic waveform as an indicator of intravascular volume change and a continuous blood pressure monitor to measure the blood pressure during each heart-beat. The non-linear behaviour of Cindex with pressure and the effect of age on Cindex are studied in 62 males (15-73 years). Repeatability tests and the effect of ice-water exposure of a portion of a limb are studied in 10 and 14 subjects, respectively. For each individual, Cindex measurements are taken at discrete values of local mean arterial pressure (Pmean), and a Cindex against Pmean plot is obtained. There is a statistically significant difference (p < 0.05) in Cindex for the lower values of Pmean (60-100 mmHg) between two age groups formed (15-52 and 58-73 years). The cold-pressor test (CPT) shows a 68% median decrease in Cindex, with an inter-quartile range of 60-77%, in a matter of seconds. The results suggest that Cindex may be a useful noninvasive indicator of peripheral vascular compliance in humans.

A low-hemolysis blood aspirator conserves blood during surgery.

Blood damage caused by traditional vacuum-operated suction tubes, particularly when air is aspirated along with the blood, usually exceeds damage from all other components. In addition to platelet injury, there is a high degree of hemolysis, which leads to high plasma hemoglobin levels and reduces the number of red blood cells available for reinfusion during cases of blood conservation, such as autologous transfusion and cardiac bypass. This work was undertaken to minimize hemolysis, and the accompanying platelet destruction, during aspiration, with the design of a jet-driven aspirator that separates and removes air from blood immediately within the suction tip. The jet-driven aspirator can suction blood at a range of rates from 100 to at least 700 ml/min, separates and removes 80-100% of aspirated air, operates at any orientation, and generates subatmospheric pressures on the order of only 1 inch H2O. In-vitro hemolysis testing showed a significant reduction in average plasma hemoglobin release, from 19.4 mg/dl to 1.8 mg/dl, when air was removed during blood aspiration. In comparative testing with a conventional vacuum suction tube, the jet-driven aspirator showed significantly less hemolysis than the conventional aspirator at comparable rates of air and blood aspiration.

An atraumatic aspirator for use in autologous transfusion and cardiac bypass.

Surgical aspiration has long been seen as the weak link in surgical blood recovery. Aspiration causes a high degree of hemolysis that leads to high plasma hemoglobin levels and reduces the number of erythrocytes available for reinfusion. Aspiration also damages platelets, generates emboli, and increases bleeding time. The major source of the high hemolysis and the other blood damage that occurs during suction is the aspiration of air with the blood. To address this problem, a jet driven aspirator that immediately separates and removes air during suction was designed and tested. The aspirator suctions blood at a range of rates from 100 to at least 700 ml/min, separates and removes 80-100% of the aspirated air, operates at any orientation, and generates subatmospheric pressures on the order of only an inch of water. During in vitro hemolysis testing, the removal of air by the jet driven aspirator showed a significant reduction in hemolysis: plasma hemoglobin levels increased 19.4 mg/dl without air removal and only 1.8 mg/dl with air removal (p < 0.001). In comparative testing with a conventional vacuum operated suction tube, the jet driven aspirator showed significantly less hemolysis (p < 0.001) than did the conventional aspirator at comparable blood and air aspiration rates.

Left ventricular assist using a jet pump.

A simple, effective, cardiac assist device was developed using a jet pump, a device that performs pumping by energy transfer from a high speed jet to low speed surrounding fluids. This jet pump is inserted retrograde through the aorta and placed in the left ventricle transvalvularly. The jet of oxygenated venous blood entrains blood inside the left ventricle and pumps into the aorta through the aortic valve. Jet velocity is kept below the hemolytic threshold of 1000 cm/sec. The device was placed in a mock circulatory system that stimulates the left ventricle and vascular system by generating a pressure wave (120/75 mmHg) with a 4 L/min cardiac output (CO). A bypass loop (from the venous reservoir to aorta using a Biomedicus pump, Biomedicus Inc., Eden Prairie, MN) was set up, and the jet pump was installed. When the jet pump is turned on, bypass flow rate (BF) is 2.5 L/min, entrainment pumping 1.5 L/min, and peak ventricular pressure (VP) falls below aortic pressure (AP), while maintaining the mean AP. Time tension index (TTI) is decreased 31%. This result, when compared with simple bypass at differing BF, shows more than a 20% reduction in TTI. This simple jet pump provided significant unloading of the left ventricle and may be potentially useful as a left ventricular assist device.

Extracorporeal blood pumping during heart-lung bypass.

This article describes three different types of extracorporeal blood pumps and the physiology of blood flow and analyzes the circulatory changes introduced by the pumping during heart-lung bypass. Associated clinical problems are briefly discussed.

A new method for measuring the yield stress in thin layers of sedimenting blood.

A new method is presented to describe the low shear rate behavior of blood. We observed the response of a thin layer of sedimenting blood to a graded shear stress in a wedge-shaped chamber. The method allows quantitation of the degree of phase separation between red cells and plasma, and extracts the yield stress of the cell phase as a function of hematocrit. Our studies showed that the behavior of normal human blood underwent a transition from a solid-like gel to a Casson fluid. This transition began at the Casson predicted yield stress. The viscoelastic properties of blood were examined at shear stresses below the yield stress. The measured Young's elastic moduli were in good agreement with published data. The yield stress of blood showed a linear dependence on hematocrit up to 60%, and increased more rapidly at higher hematocrit.

Calcium-induced erythrocyte rigidity: the roles of cellular metabolism, hydration, and ionic balance.

Previous investigations have shown that incubation of human erythrocytes with the ionophore A23187 and calcium causes accumulation of the cation, losses in potassium, water, and cellular volume, hydrolysis of adenosine triphosphate (ATP), conversion of biconcave discocytes to echinocytes and spheroechinocytes, and marked increases in erythrocyte resistance to micropipette aspiration. Subsequent studies demonstrated that prevention of water and potassium loss blocked the influence of calcium loading on erythrocyte stiffness without affecting calcium uptake by the cells or hydrolysis of ATP. In the present study erythrocytes were exposed to conditions that permitted individual or coordinate manipulation of cellular ATP, water, potassium, and calcium in order to determine which factors developing as a result of calcium loading were responsible for the calcium-induced changes in erythrocyte viscoelastic properties. Results of the study demonstrate that volume loss, ATP hydrolysis, and potassium depletion do not individually or in combination cause increases in erythrocyte stiffness. However, all of these changes are essential and must develop in conjunction with calcium loading in order for erythrocytes to develop diminished deformability and elasticity.

Hydraulic conductivity of the endothelial and outer layers of the rabbit aorta.

Pressure-driven fluid flow across the arterial wall was measured to determine wall hydraulic conductivity (Lp) before and after removal of the endothelium. The thoracic aortas of rabbits, anesthetized with Nembutal, were cannulated, perfused with oxygenated Ringer solution, and removed. With one cannula connected to a capillary manometer and the other closed, the manometer meniscus shift could be used as an indication of fluid loss through the wall plus vessel volume increase (creep). The latter effect, when measured, accounted for about one-fourth of the total volume displacement. The Lp given in cm/(s.cmH2O) +/- SD, was 3.30 +/- 0.96 x 10(-8). Another method employed continuous weighing of a closed aortic segment to obtain fluid loss, and yielded an Lp of 4.07 +/- 1.3 x 10(-8), and after mechanically removing the endothelium, the Lp became 7.73 +/- 2.8 x 10(-8). Using the above data, an Lp could be calculated for aortic endothelium of 8.6 x 10(-8). This suggests that about half the total transmural pressure drop occurs across the endothelium. Scanning electronmicrographs were used to check the condition of the endothelium.

Retention of water and potassium by erythrocytes prevents calcium-induced membrane rigidity.

Modest increases in intracellular calcium concentrations, in association with ATP depletion, cause the appearance of pathologic changes in erthrocyte shape and deformability. The loss of erythrocyte ATP and simultaneous increase in cellular calcium have previously been considered the sole requisites for the appearance of erythrocyte membrane rigidity. We report that red cells suspended in high-potassium buffers may be simultaneously loaded with calcium (through exposure to the divalent cation ionophore A23187) and depleted of ATP without incurring drastic changes in shape or in membrane stiffness. Incubation of erythrocytes under these conditions effectively blocks both water and potassium loss normally caused by calcium accumulation. However, the high external potassium has no influence on either the ionophore-induced accumulation of calcium or on the the concomitant hydrolysis of cellular ATP. These results suggest the involvement of at least one further parameter, ie, changes in cell water and cation content, in the development of calcium-induced erythrocyte rigidity.

Two new concepts that might lead to a wearable artificial kidney.

A wearable artificial kidney involving two novel components is proposed. It consists of a turbulent flow ultrafiltering shunt, which supplies 20 liters of ultrafiltrate per day to a disposable activated charcoal cartridge (where creatinine, uric acid, and other tightly bound solutes are adsorbed) and then to an artificial loop of Henle (where the urea is concentrated into 2 liters of ultrafiltrate per day and discarded) from which 18 liters of cleansed, rewarmed ultrafiltrate containing 87% of the glucose is returned to the patient.

Influence of the ionophore A23187 on the plastic behavior of normal erythrocytes.

Previous studies have demonstrated that A23187, an ionophore which selectively transports divalent cations across cell membranes, has profound effects on human erythrocytes: it causes red cells to take up calcium; lose potassium, water, and ATP; convert from biconcave discs to echinocytes and spheroechinocytes; and become more rigid. The present study has explored the influence of calcium uptake induced by the ionophore on the behavior of individual erythrocyte membranes by the micropipette aspiration technique. Exposure of erythrocytes to calcium and A23187 for intervals of up to 30 minutes resulted in marked changes in membrane viscoelastic properties, including the development of increased resistance to aspiration. The most striking manifestation of altered membrane mechanics was apparent after 10 minutes on incubation. Cells pulled into the pipette for a few seconds and the extruded back into the medium retained the deformity imposed by the pipette for several seconds to a few minutes before regaining the form they manifested prior to initial aspiration. The calcium-induced changes in erythrocyte behavior observed in this study strongly support the concept that extrinsic proteins located inside the membrane provide mechanical support to the cell wall, and that increased levels of calcium cause precipitation or cross-linking of the proteins responsible for the increased resistence to deformation and recoil observed after aspiration into micropipettes.

Formed element deposition onto filtering walls.

Particles are expected to deposit onto filtering surfaces provided the non-dimensional parameter (see article) U (see article) R2S3/2 exceeds a certain value. For erythrocytes this value has a measured value of 0.15, and for platelets the value ranges from 0.01 to 0.15. If deposition is expected, the rate of particle deposition is proportional to the ultrafiltration flow rate. However, the other fluid mechanic mechanisms may transport a greater number of platelets to the surface. We expect these platelets will be fluid mechanically held to the surface even if it is non-sticky for platelets.

Measurement of the elastic modulus for red cell membrane using a fluid mechanical technique.

Red cells which adhere to a surface in a parallel plate flow channel are stretched when acted on by a fluid shear stress. Three types of stretching are studied: whole cell stretching, the stretching of a red cell evagination, and tether (long, thin membrane process) stretching. In addition, the stretching of a large scale model cell attached to a surface is studied in a Couette flow channel. The results indicate that the uniaxial stretching of red cell membrane can be described by a linear stress-strain relationship. Simple theories developed from free body diagrams permit the calculation of a value for the modulus of elasticity of cell membrane in each of the three experiments. In all cases the value for the modulus is on the order of 10(4) dyn/cm(2) for an assumed membrane thickness of 0.01 mum. It was also observed that red cell tethers steadily increase in length when the fluid shear stress is greater than approximately 1.5 dyn/cm(2) and tether lengths in excess of 200 mum have been achieved. Tethers appear to possess both fluid and elastic properties.