Distribution of Marcaine in an in vitro model of the subarachnoid space conforming to actual spinal column geometries.

To study factors influencing the distribution of local anaesthetics in the subarachnoid space, an in vitro model is constructed which takes into account the natural curvature of the spinal column and the volume occupation of spinal cord and nerve fibres to resemble the in vivo situation. Three Marcaine solutions of different baricity (1003, 1008, 1030 kg/m3) are injected with a 22 G, a 27 G Quincke point needle and a 18 G multiport catheter into three models of non-pathological spinal columns with injection flow speeds of 0.6, 0.2 or 0.1 ml/s. Methylene blue is added for visual and qualitative assessment of drug distribution. Baricity is the main actor in the spreading of the drug solution. For all other variables, no significant difference is found after ten minutes, though the initial distribution may differ according to the geometry used. A hypobaric solution yields a remarked difference between fast and slower injections. The position of the catheter should be controlled.

Red cell injury assessed in a numeric model of a peripheral dialysis needle.

The highest shear stresses in a dialysis system are expected to be found in the needle, where the largest velocity-diameter ratio appears. Shear is a known source of hemolysis and related patients' discomfort. To assess the magnitude of blood cell injury and the location of its sources, a finite element model is used to calculate three-dimensional velocities and shear stresses in peripheral dialysis needles, concentrically placed in a rigid wall fistula. The boundary conditions consist of time dependent in vivo measured pressures. Cell damage is computed for different cell tracks into the needle by means of Wurzinger's empirical formula, which expresses the hemoglobin (Hb) release as a function of shear stress and shearing time. Near the needle wall, velocities are low and shear stresses high, resulting in a significantly higher level of cell damage: 0.1% vs 0.001% in bulk flow for a mean flow of 91 ml/min into a 14G needle with a peak velocity of 220 cm/sec. The deviation from the classic Poiseuille velocity profile is shown. Less than 5% of the flow passes through this high damage path. A vortex at the inner side of the needle has a cumulative damage of 0.007% per 0.23 sec trip around the vortex.

(A)typical symptoms during single needle dialysis.

In 5 elderly patients, an abnormally high occurrence of some symptoms was noted during dialysis. All patients were dialyzed with biocompatible membranes, bicarbonate dialysate, and a blood flow of 250 to 300 ml/min by a single needle system, on a fistula 14 Gauge catheter-needle. These symptoms were: 1) "angina," resistant to O2 and nitrates, with biochemical stigmata of infarction, but without electrocardiogram (ECG) localization; 2) intractable persistent hypotension, not hypovolemic, lasting 1 or 2 days; 3) esophagal spasms, with inability to swallow solid food. Because we knew that these symptoms were compatible with hemolysis (biochemically proven by the increase in serum LDH during dialysis and by a fall in haptoglobin) due to red cell fragmentation (RCF), we switched these patients from fistula dialysis (A) to central catheter dialysis (B), with the same apperture, blood flow, etc. The total number of sessions of A versus B were 512 and 891; the mean LDH ratios (serum LDH postdialysis divided by predialysis) were 1.8 and 1.0 (= no RCF); angina events were 132 (26%) for A, and 25 (3%) for B; persistent hypotension was seen 37 (7%) times in A and 5 (0.6%) times in B; esophagal spasms were noted 65 (13%) times for A, and 0 times for B. This clinical improvement was so overwhelming that 3 patients refused to be dialyzed again using their well functioning fistulae. This study also proved the need for a better designed and manufactured peripheral dialysis catheter-needle.