Particle Spallation Induced by Blood Pumps in Hemodialysis Tubing Sets

ArtiJicicil Organs 10(3):226-235, Raven Press, New York 0 1986 International Society for Artificial Organs

Particle Spallation Induced by Blood Pumps in Hemodialysis Tubing Sets

David Barron, Sarah Harbottle, Nicholas A. Hoenich, *Adrian R. Morley, ?David Appleton, and $John F. McCabe

Departments of Medicine, *Pathology, ?Medical Statistics, and $Dental Materials, University of Newcas'I? upon Tyne, Newcastle upon Tyne, Engiand

Abstract: The repeated flexion and compression of pump segments by the rollers of peristaltic pumps results in cracking and abrasion of the inner surfaces of the pump segment, leading to shedding of particles into the extracorporeal circuit. A series of studies to assess the rate of particle release from silicone rubber, polyvinyl chloride (PVC), and Pivipol, a coextruded polyurethane-coated PVC tubing, when these materials were used with blood pumps of the type found in hemodialysis units, was un-

dertaken. The studies show that with all tubing/pump combinations there is an overall increase in the total number of particles released, but an analysis of the par ticle size distribution indicates that the majority of the particles are <16 pm in diameter. The rate of increase may be reduced, however, by decreasing the occlusion pressure. Key Words: Hemodialysis-Spallat ion-Sili- cone.

Peristaltic roller pumps are widely used in extracorporeal circulation, and although a number of different designs are available, their mode of opera- tion is similar. A segment of compressible tubing, normally of silicone or polyvinyl chloride (PVC), is inserted into a curved roller track and held in position by clips. The rollers of the pump rotate within this track and compress the tubing. This compression generates the flow in the circuit since it creates a pressure gradient across the insert and forces the fluid along and out of the pump segment. After each passage of the rollers the tubing segment re- sumes its original shape, fresh fluid is drawn into it to refill it, and the process is repeated.

Peristaltic roller pumps in clinical use have two or three rollers on the pump shaft, and their speed of rotation is potentiometrically controlled. The position of the rollers relative to the tubing may be fixed or adjustable by means of a Vernier scale whereas in some newer designs the rollers are

Received August 1985; revised February 1986. Address correspondence and reprint requests to N. A.

Hoenich at Department of Medicine, University of Newcastle upon Tyne, Floor 4, Clinical Block, Framlington Place, New- castle upon Tyne NE2 4HH, U.K.

226

spring-loaded to enable optimum occlusion to be achieved even in the presence of dimensional varia- tions of the pump segment that may exist between different batches of tubing.

The repeated flexion and compression of the segment by the rollers and its passage over the pump insert result in cracking and abrasion of the internal surfaces of the pump segment. This leads to shedding of particles into the extracorporeal circuit. Spallation of particles in extracorporeal circuits used in cardiac surgery is a well-recognised compli- cation of the procedure (1 -3). Hemodialysis, unlike cardiac surgery, is performed on a routine, three- times-a-week basis, with each treatment lasting between 4 and 6 h depending on the patient's size and clinical requirements.

Postmortem specimens from patients receiving long-term dialysis have demonstrated the presence of refractile particles. These particles have been identified as silicone originating from the extracorporeal circuit used in the treatment (4-7). As a consequence of these findings, we have undertaken a series of studies to assess the rate of particle release, and to analyse the factors influencing particle release in blood tubing sets used for haernodialysis.

PARTICLE SPALLATION IN HEMODIALYSIS TUBING

Fine bore tubing r;

Sampling point

r > I 'r

, 9 ! I

22 7

-100

-

Occlusion - setting device

0 Sampling beaker of Coulter Counter

MATERIALS AND METHODS mouth, U.K.), a three-roller pump in which the roller position is fixed relative to a curved roller track, but which may be moved to alter the occlusion; the Bellco B L 705N (Bellco SPA, Mirandola, Italy), a twin-roller pump in which the position of the rollers is adjusted relative to the roller track by

Blood pumps Three commonly used blood pumps of the type

found in hernodialysis uni ts were studied: the Watson Marlow MHRE (Watson Marlow Ltd., Fal-

I ( j f

pressure gaugl

Isolator - Expansion chamber

\ + /

51

Hours

FIG. 2. Totd particle count based on the mean of observations during four runs for Watson Marlow MHRE pump during 6 h. Total number of particles in the 2-80 pm range are shown. Silastic, silicone rubber.

ArtifOrgaris, Vol. 10, No. 3 , 1986

228

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D. BARRON ET AL.

Si lastic

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means of Vernier screws; and the Fresenius blood pump (Fresenius AG, OberurseVTaunus, F.R.G.), a twin-roller pump in which the rollers are spring- loaded. This last pump was studied with two spring tensions: 96N (Fresenius A ) and l l 5 N (Fre- senius B).

Blood tubing materials Three blood tubing materials were selected for

use with the above pumps: medical grade silicone rubber (Extracorporeal SA, Waremrne, Belgium, and Fresenius AG, Bad Homburg, Germany); PVC (Bellco SPA, Mirandola, Italy, and Fresenius AG) and a co-extruded polyurethane-coated PVC tubing (Pivipol, Bellco SpA). All tubings used in the studies came their internal

E

E !i Q '1

from single-production batches, and diameters ranged from 6.3 to 8 rnm

while their wall thicknesses varied from 1.7 to 2.1 mm.

Experimental technique The experimental circuit shown in Fig. 1 was

used. In order that the degree of occlusion in each of the studies was comparable, the pump roller oc- clusions were set by the use of a technique in which the working occlusion was defined as the point where a 100-cm column of water supported by the pressure of the occlusion fell at a constant rate of 2.5 cm/min. To obtain the working occlusion with the Bellco pump, the Vernier screw was adjusted for each experiment, whereas with the Watson Marlow pump, the position of the race track on its angled base plate was moved relative to the rollers. No such adjustments were possible in the :Fresenius pumps, which worked on a fixed occlusion.

Silastic

PVC

- - -

A B

- w lsoton

I I I I I 1 2 1 2 3 4 5 6

0 particle count .- w

Hours FIG. 4. Total particle count based on the mean of observations during four runs for Fresenius blood pump durinlg 6 h. Total number of particles in the 2-80 p,m range are shown. Silastic, silicone rubber; PVC, polyvinyl chloride.

Artif Orgnns, Vol. 10, No. 3 , 1986

PARTICLE SPALLATION I N HEMODIAL YSIS TUBING 22 9

TABLE 1. Total iiuniher of parrides between 2 and 80 ,urn in diameter (countiL x I @ ) generated during ti ibii ig/pimp studies

Material Pump 0

Silicone rubber Watson Marlow Bellco Fresenius A Fresenius B

Fresenius A Fresenius B

PVC Bellco

Pivipol Bellco

1,587 937 469 518

1,188 563 450

1,250

0.5

1.362 1,050

701 743

1,375 602 507

1,462

1

I SO0 1,313

706 796

1,387 556 503

1,450

1.5

1.812 1.475

722 816

1,462 578 498

1,513

Time (h)

2 2.5

1,338 2,238 1,338 1.638

789 767 928 904

1,425 1,425 590 603 533 474

1,450 1,550

3

2,575 1,975

846 945

1,638

498

1,850

-

3.5

2,832 2,200

932

1,788 523

2.038

-

-

4 4.5 5

- 3,113 3,450 - 2,525 2,763 712 751 791 960 982 1,085 - 1,600 1,662 529 545 512 466 517 472

- 2,287 2,375

5.5

4,000 3,000

798 1,047

1,963 540 455

2,512

Average coefficient of variation for the four observations was 27%.

The extent of spallation observed with the tubing pump combinations over a 6-h period was assessed. Silicone rubber tubing was used with all three pumps, the use of PVC was confined to the Bellco and Fresenius pumps, and Pivipol tubing was used only with the Bellco pump. Owing to the low torque generated by the Watson Marlow MHRE pump, it was not possible to use the pump with ei- ther PVC or Pivipol tubing.

Assessment of the number of particles with diameters between 2 and 80 Frn released during the study period was by the use of a Coulter Tall particles counter (Coulter Electronics, Luton, Bed- fordshire, U.K.). The instrument was connected to the flow circuit by fine-bore tubing, as shown in Fig. 1 . The tubes were arranged in such a way that

Diameter Ipm)

X (hounl

- 64

V ~ L U M E WM SILASTIC 1983

FIG. 5. Isometric plot of total volume of particles observed using the Watson Marlow MHRE pump. Mean of observations during four runs is shown. Silastic, silicone rubber.

the pressure gradient across the pump insert could be utilized to deliver and return fluid from the circuit to the sample chamber. The flow through these tubes was 28 ml/min at the time of sampling, and the sampling volume used on each occasion was 2 ml. Sedimentation in the samples drawn for analysis was prevented by a mechanical stirrer in the sampling chamber.

The volume of the extracorporeal circuit was 300 ml, and a flow of 200 ml was maintained throughout the studies to simulate clinically used flow rates. Withdrawal and return of the fluid to the reservoir was a t two different levels to ensure thorough mixing in the reservoir, particularly of larger particles during the experimental study. In addition, a pressure of 100 mm Hg in the venous drip chamber was maintained.

Before each series of measurements, the circuit was flushed with 2 L Isoton 11, a buffered filtered electrolyte solution, which was also used as the re- circulated fluid.

A series of four studies for each of the pump insert material and pump combinations was performed after the technique has been established; each study lasted 6 h , and samples were taken for particle analysis every 30 min during the study. The volume lost by sampling was not returned to the circuit, and results were corrected to account for the changes in concentration resulting from volume changes induced by the sampling. Calibration of the Coulter counter was performed prior to each run by the use of a latex suspension in distilled water con- taining particles of known diameter.

Each sample taken was recorded in terms of the number of particles of a given size per liter, and hence the total and percentage volume of particles of a given size.

A supplementary series of studies was performed to determine the particle counts of fresh electrolyte

Artf Organs, Vol. 10, No. 3 , I986

230 D. BARRON ET AL.

V ~ L U M E 0 SILASTIC 1983

4

VOLUME PIVIPOL 1983

solution (200,000-250,000 particles/L). Airborne dust contamination from the surroundings was assessed by exposure of Isoton 11 to the laboratory environment for a 5-h period, with samples being removed every hour. No discernible contamination was present, but a small number of particles in the 20-50-pm range was observed. In order to mini- mise the influence of these particles on the overall results, the experimental circuit and sampling chamber were isolated from the surrounding laboratory environment.

Since the samples drawn for analysis were me- chanically stirred, the possibility of spurious counts from microbubbles introduced by the stirring process could not be excluded. To minirnise this effect, a series of studies in which the influence of stirrer

VOLUME 0 PVC 1983

FIG. 6. Isometric plot of total volume of particles observed using the Bellco blood pump. Mean of oDservations during four runs is shown. Silastic, silicone rubber; PVC, polyvinyl chloride.

speed on particle count was undertaken. These studies demonstrated that a t maximum stirrer speed setting there was a significant co'ntribution from this source (2,000,000 particledl); however, at the speeds used for the experiments, the contr ibution f rom this source was small1 (20,000 particles/L).

At the termination of each experiment , samples of tubing were removed from the pump insert for scanning electron microscopy.

RESULTS

The particle counts observed for each of the pumphaterial combinations, based on the mean of observations during four runs, are shown in Figs. 2-4 and summarized in Table 1.

Avtif Orgnns, Vol. 10, No. 3 , 1986

PARTICLE SPALLATIO N IN HEMODIAL YSIS TUBING 231

Figures 5-7 show the isometric plots of the total volume of particles for each blood-pumphubing combination based on the mean of observations during four runs. The x axis in these plots represents the diameter of the particles on a logarithmic scale, the y axis represents time, and the z axis represents the volume per milliliter of fluid, made up by particles of a given diameter a t a given time. Thus, the frequency polygons for total volume as a function of time at a given diameter and as a function of diameter at a given time are shown.

Clidical interest is focused on particles >I6 pm

50

Volume IX 10' mi)

64

Y

VOLUME 0 SILASTIC 1984 B

~ 5 0

VOLUME O7

PVC 1984 B

diameter, and a further analysis of the data ob- tained for such particles was performed. Since the series of studies performed in 1983 (Watson Marlow and Bellco pumps) exhibit different features from the 1984 series (Fresenius A and B), the results have been analysed separately.

Analysis of covariance was performed to estab- lish the relationship of number of particles (>16- pm diameter) with elapsed time of pumping and types of pump and material. For the 1983 series, number of particles increased linearly with time, at a rate not significantly affected by pump or mate-

A 50

30 Volume l x 10 ml)

4

X lhourrl

VOLUME SILASTIC 1984 A

VOLUME PVC 1984 A

FIG. 7. Isometric plot of total volume of particles observed using the Fresenius blood pump. Mean of observations during four runs is shown. Silastic, silicone rubber; PVC, polyvinyl chloride.

ArrifOrgrrns, Vol. 10, No. 3, 1986


TABLE 2. Relationship between number of particles (>I6 krn) released per rnl and elapsed time

of pumping (h)

Pump Material Particles releasedhl

Watson Marlow" Silicone rubber 18.9 + 2.7 x time Bellcoa Silicone rubber 22.5 + 2.7 x time

] Fresenius Ab Silicone rubber Fresenius B Silicone rubber 3.8 - 0.35 x time

Bellco" PVC 14.4 + 2.7 x time Fresenius Ab PVC Fresenius Bb PVC ] 6.7 - 0.35 x time

Bellco" Pivipol 13.4 + 2.7 x time

1983 experimental series. 1984 experimental series.

rial. However, there were highly significant (p < 0.001) differences between materials, there being more particles after a given time with1 silicone rubber than with PVC or Pivipol. Results are sum- marised in Table 2.

A similar analysis using the 1984 data for the Fre- senius pumps showed a decrease with time for particles >I6 ,um diameter. The materials here significantly different with PVC worse than' silicone rubber. There was, however, no difference between different tensions, a finding contrary to that of Bommer e t al. (8), who demonstrated that reduction of occlusion pressure effectively reduced spallation in silicone rubber.

Macroscopic examination of the pump inserts after 6 h of pumping showed a permanent flattening

FIG. 8. Montage of lumenal surface of si l icone rubber examined by scanning electron microscopy. a: unused material; b: after 6 h of pump ing w i t h Watson Marlow MHRE, showing multiple parallel grooves and ridges; c: after 6 h of pumping with Bellco pump, showing mult ip le cracks; d: after 6 h of pumping with Fresenius A pump, showing adherent fragments; e: after 6 h of pumping with Fresenius B pump, showing lateral defects and adherent particles.

Arrf Organs. Vo/ . 10, NO. 3 . 1986

PARTICLE SPALLATION IN HEMODIAL YSIS TUBING 233

of the tube to an oval form with a loss of translu- cency for PVC and Pivipol and an increased ca- pacity of the normally opaque silicone rubber. At the point of maximum flexure, external ridges were present.

Scanning electron microscopy was performed on the materials before and after use. Figure 8 shows the montage of lumenal surface of silicone rubber, and Figs. 9 and 10 show similar montages for PVC and Pivipol.

Unused tubing was comparable in appearance with occasional superficial imperfections. In con- trast, used tubing demonstrated lateral cracks at the point of maximum compression together with superficial fractures. Furthermore, adherent particles were also noted on the surface of the material originating from the fractures.

DISCUSSION AND CONCLUSION

Our studies have demonstrated an overall increase in the total number of particles with time for the Watson Marlow and Bellco pumps when used with PVC, silicone rubber, and Pivipol. The mean patterns illustrated were observed consistently within each set of experiments, and the average coefficient of variation was 27%. The largest number of particles released using silicone rubber with these two pumps probably occurred early in the experiment. Use of PVC and Pivipol with the Bellco pump resulted in a reduction in the total number of particles observed, but we failed to distinguish between PVC and Pivipol, which suggests that the polyurethane lining of the latter offers only a lim- ited protection from structural damage induced by the action of the pump rollers. The second series of studies using the Fresenius pumps demonstrated no overall increase in the number of particles with time.

Initial counts in all experimental studies were found to be higher than can be explained by the particle count in the Isoton solution, this difference being attributable to the shedding of particles during the setting up and equilibriating of the experimental circuit.

Analysis of the data in terms of particle size distribution shows that the majority of the particles are <16 km in diameter. Particles greater than this show an increase with time, except in the case of

~~

FIG. 9. Montage of lumenal surface of polyvinyl chloride (PVC) examined by scanning electron microscopy. a: unused material: b: after 6 h of pumping with Bellco pump, showing adherent fragments; c: after 6 h of pumping with Fresenius A pump, showing multiple cracks.

ArlifOvgciiis, V d . 10, No. 3. 1986


.. m

Arrif Organs, Voi. 10, No. 3 . 1986

PARTICLE SPALLATION IN HEMODIAL YSIS TUBING 235

the Fresenius pump, where a paradoxical decrease was observed. Two possible explanations are of- fered. First, clumping of the particles may have occurred, causing their size to fall outside the limit of measurement; or second, since the rotational speed of this pump is higher than for the others studied, the large particles may have been broken up into smaller particles, the size of which lay outside the detection limits.

Scanning electron microscopy has demonstrated the presence of cracks in the material near the line of flexure formed by the repeated compression and relaxation of the tubing caused by the passage of the rollers. These cracks are the subject of complex forces during pumping, which result from the compression perpendicular to the direction of roller movement, as well as compression ahead of the roller and tensile stress after it. These forces cause the cracks to open and close while the walls may also slide against each other-actions that lead to the release of particles. In addition, the inner surfaces of the pump insert are subject to frictional drag forces occurring as a result of the internal surfaces of the insert being brought together between a moving roller and a fixed pump housing.

The greatest damage in the pump insert appears to be at the edges of the pump insert rather than between the opposed surfaces at the midpoint of the tubing, suggested that frictional drag forces are of secondary importance in the release of particles but may contribute to the breakdown of larger particles and account for the observed decrease in large particles noted with the Fresenius pump.

Boretos and Wagner (3) demonstrated that spallation may be reduced if the tubing wall thickness is decreased, whereas Bommer et al. (8) investigated

the role of occlusion pressure and similarly showed that decreased occlusion pressure resulted in decreased spallation.

Optimal occlusion of the pump insert is highly desirable, since excessive occlusion causes increased damage to the pump insert material, it being necessary for the tubing to be completely compressed during pumping to enable flow to occur. Improvements in roller design and alteration of tubing shape may lead to a reduction in damage to the lateral margins of the pump insert and hence the minimisation of spallation.

Acknowledgment: We should like to express our grati- t ude to Bellco SpA and Fresenius AG for financial and practical support of this study.

REFERENCES 1 . Hubbard LC, Kletschka HD, Olsen DA, Rafferty EH,

Clausen EW, Robinson AR. Spallation using roller pumps and its clinical implications. AmSECT Proc 1975;27-32.

2. Kurusz M, Christman EW, Williams EH, Tyers GFO. Roller pump induced tubing wear: another argument in favor of ar- terial line filtration. J Extracorporeal Techno/ 1980; 12: 49-59.

3. Boretos JW, Wagner FR. Particle fragmentation generated in pump sets [Letter]. J Biomed Marev Res 1971;5:411-2.

4. Leong ASY, Disney APS, Gove DW. Spallation and migra- tion of silicone from blood-pump tubing in patients on hemodialysis. N Engl J Med 1982;306:135-40.

5 . Laohapand T, Osman EM, Morley AR, Ward MK, Kerr DNS. Accumulation of silicone elastomer in regular dialysis. Proc EDTA 1982;19:143-52.

6 . Bommer J , Waldherr R, Ritz E. Silicone storage disease in long-term hemodialysis pa t ien ts . Contr ib Nephrol 1983;36: 1 15-26.

7. Bomrner J , Ritz E. Spallation of dialysis materials- problems and perspectives. Nephron 1985;39:285-9.

8. Bommer J, Pernicka E, Kessler J , Ritz E. Reduction of silicone particle release during haemodialysis. Proc EDTA- ERA 1984;21:287-90.

ArrifOi-gnns, Vul. 10, Nu. 3 , 1986

Documents

Particle Spallation Induced by Blood Pumps in Hemodialysis Tubing Sets