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Objective: To determine which measurements and test conditions on posturography are most useful for identifying balance problems in older people., Subjects: Two samples of 70 community-dwelling older subjects (> 75 years). One group (controls) considered their balance normal for their age, and the other (patients) complained of imbalance., Measurements: Velocity of sway on static (with and without foam) and dynamic posturography, Tinetti gait and balance score, self-reported fear of falling, and number and circumstances of falls., Results: Mean sway velocity was significantly increased in patients compared with controls. The greatest difference between patients and controls occurred with measures of anterior-posterior sway velocity during angular tilt of the platform. Sway velocity was not significantly increased in patients or controls who reported falls compared with those who did not report falls. Even when comparing those who fell as a result of loss of balance with those who fell because of trips or slips, there was no significant difference in sway velocity. By contrast, those who reported fear of falling (patients and controls) had significantly increased sway velocity compared with those who did not report fear of falling., Conclusion: On average, velocity of sway (particularly in the anterior-posterior direction) is higher in older subjects who complain of imbalance compared with age-matched controls, and the difference is greater with dynamic posturography than with static posturography. However, the posturography data provided little information about the cause of the imbalance and did not correlate with the frequency of reported falls.

It has been shown in Poiseuille flow, that the ADP-induced aggregation of human platelets in citrated plasma from female donors is significantly greater than from male donors over a range of mean tube shear rate, G, from 41.9 s-1 to 1920 s-1 and mean transit time, t, from 0.2 to 86 s. The present work verifies the sex difference at G = 335 s-1 and t = 43 s and deals with the effect of free Ca2+ on it. An inverse correlation between the extent of single platelet aggregation and donor hematocrit, and between hematocrit and the plasma ionized calcium concentration, [Ca2+], as well as a positive correlation between the extent of single platelet aggregation and [Ca2+] was found. This indicated that the sex difference is due to hematocrit-dependent differences in the [Ca2+] that result when a fixed volume of the chelating agent citrate is used to anticoagulate blood. When the initial citrate concentration was adjusted to compensate for the variable volume dilution of citrate in plasma among donors and the [Ca2+] of males raised above that of females, the sex difference was reversed. Again, aggregation correlated with [Ca2+]. At the physiological [Ca2+] in both heparinized PRP and hirudinized PRP, the rate of aggregation and aggregate size were much greater than in citrated plasma but no sex difference was detected.

The effect of red blood cells, rbc, and shear rate on the ADP-induced aggregation of platelets in whole blood, WB, flowing through polyethylene tubing was studied using a previously described technique (1). Effluent WB was collected into 0.5% glutaraldehyde and the red blood cells removed by centrifugation through Percoll. At 23 degrees C the rate of single platelet aggregation was upt to 9 x greater in WB than previously found in platelet-rich plasma (2) at mean tube shear rates G = 41.9, 335, and 1,920 s-1, and at both 0.2 and 1.0 microM ADP. At 0.2 microM ADP, the rate of aggregation was greatest at G = 41.9 s-1 over the first 1.7 s mean transit time through the flow tube, t, but decreased steadily with time. At G greater than or equal to 335 s-1 the rate of aggregation increased between t = 1.7 and 8.6 s; however, aggregate size decreased with increasing shear rate. At 1.0 microM ADP, the initial rate of single platelet aggregation was still highest at G = 41.9 s-1 where large aggregates up to several millimeters in diameter containing rbc formed by t = 43 s. At this ADP concentration, aggregate size was still limited at G greater than or equal to 335 s-1 but the rate of single platelet aggregation was markedly greater than at 0.2 microM ADP. By t = 43 s, no single platelets remained and rbc were not incorporated into aggregates. Although aggregate size increased slowly, large aggregates eventually formed. White blood cells were not significantly incorporated into aggregates at any shear rate or ADP concentration. Since the present technique did not induce platelet thromboxane A2 formation or cause cell lysis, these experiments provide evidence for a purely mechanical effect of rbc in augmenting platelet aggregation in WB.

Leukocyte margination in the vessels of the microcirculation has been attributed to a flow-dependent interaction with red cells. To determine the extent of this effect, experiments with human blood were done in 100- to 180-micron tubes to detect changes in cell distribution as a function of hematocrit and flow rate. Using a flow visualization technique, the leukocyte concentration distribution was determined in 45% ghost cell suspensions. Migration of cells toward the wall was observed at centerline velocities greater than 1 mm sec-1 and increased with increasing flow rate. The effect was probably due to a more rapid inward migration of ghosts than leukocytes because of fluid inertia and cell density differences. Experiments were therefore carried out in whole blood at hematocrits from 20 to 60%, measuring the number concentration of leukocytes and erythrocytes within the tube, nt, and comparing it to that in the infusing reservoir, no, (Fahraeus effect). At mean tube shear rates G less than 100 sec-1, nt/no less than 1 for both leukocytes and erythrocytes showing net migration of cells away from the wall, although at nearly all hematocrits there was an enrichment of leukocytes relative to erythrocytes in the tubes. At G less than 50 sec-1, nt/no remained less than 1 for erythrocytes but increased to greater than 1 for leukocytes showing migration toward the wall, the increase being greatest at 20% hematocrit in the 100-micron tubes. The nature of the effect was revealed by cine films which showed that, as the flow rate decreased, erythrocytes formed rouleaux which migrated inward creating a core and displacing leukocytes to the periphery. In control experiments using washed blood cells in phosphate buffer-albumin, nt/no less than 1 for both leukocytes and erythrocytes at all G and hematocrits, and leukocytes were now distributed. Cine films of washed blood confirmed that, in the absence of rouleaux, no significant inward migration of erythrocytes occurred.