Effect of dialysis and ultrafiltration on osmolality, colloid osmotic pressure, and vascular refilling rate

Kidney International, Vol. 28 (1985), pp. 808—813

Effect of dialysis and ultrafiltration on osmolality, colloid osmotic pressure, and vascular refilling rate MARIANO RODRIGUEZ, JAMES A. PEDERSON, and F1tNcIsco LLACH Department of Medicine, Nephrology Section, University of Oklahoma Health Sciences Center, and Veterans Administration Medical Center, Oklahoma City, Oklahoma, USA

Effect of dialysis and ultrafiltration on osmolality, colloid osmotic pressure, and vascular refilling rate. The effect of regular dialysis and isolated ultrafiltration on plasma osmolality, plasma colloid osmotic pressure (COP), plasma volume, and vascular refilling rate was evaluated in maintenance dialysis patients. Nineteen patients underwent regular dialysis and 11 isolated ultrafiltration. Blood samples from these subjects were obtained from arterial or venous dialysis system ports and peripheral veins. For any decrease in plasma volume, there was an increment in COP with each procedure in both venous and arterial ports and the presence of a progressive decrease of plasma osmolality was observed only during regular dialysis. Second, five additional patients

underwent 2 hrs of regular dialysis and isolated ultrafiltration in separate sessions removing comparable amounts of fluid (— 2.5 liter); after 2 hrs, there were no differences in the changes of plasma volume and COP, but again plasma osmolality decreased only during regular dialysis. These studies demonstrate that moderate changes in plasma osmolality do not affect COP. Furthermore, the ability of the plasma to recruit fluid and generate vascular refilling (as assessed by COP) is similar during regular dialysis and isolated ultrafiltration, provided the rates of ultrafiltration are the same.

Effet de Ia dialyse et de l'ultrafiltration sur l'osmolaiité, Ia pression osmotique colloidale (COP) et Ia vitesse de remplissage vasculaire. Ces mesures ont été évaluées chez des malades en hémodialyse périodique; dix-neuf malades ont subi une dialyse chronique, et 11 une ultrafiltration isolée. Des échantillons plasmatiques de ces sujets ont été obtenus

par les lignes de dialyse artérielie ou veineuse et par les veines périphériques. A chaque diminution du volume plasmatique, il existait

une augmentation de COP pour chaque méthode, dans les lignes veineuses et atrérielles, avec une diminution progressive de l'osmolalité

tration, the degree of hypovolemia is determined by the rapidity

with which the plasma space refills, that is, the magnitude of hypovolemia is dependent on the ratio between the rates of fluid removal and vascular refilling [1]. The vascular refilling rate is

related to the value of colloid osmotic pressure (COP), which has been shown to be an important parameter for estimating changes in plasma volume [5—7]. During regular dialysis a decrease in plasma osmolality is

observed, while isolated ultrafiltration does not change plasma

osmolality. It is known that changes in plasma osmolality determine shifts of fluid between the intra and extracellular compartments. In addition, COP is a major factor in the fluid movement between interstitial and vascular compartments. Thus, it is important to establish if changes in plasma osmolality may affect COP, which in turn is the main force in vascular refilling rate. The present study evaluates the effect of regular dialysis and isolated ultrafiltration on plasma osmolality, plasma colloid osmotic pressure, and plasma volume. Methods

This study includes three sections. The first is an in vitro study, and the other two sections are studies performed in hemodialysis patients.

The rationale of the first section is to establish whether

plasmatique observée uniquement au cours de Ia dialyse chronique. Cinq malades supplementaires ont subi des dialyses régulières de deux hr et une ultrafiltration isolée lors de seances separCes, enlevant des quantités comparables de liquides (-•- 2,5 liter); aprés deux hr, ii n'y avait pas de differences dans les modifications de volume plasmatique et de COP, mais, une fois encore, l'osmolalitC plasmatique n'a diminuC

manipulation of osmolality in various solutions in vitro results in changes in COP. This in-vitro study was performed in four solutions of protein at a concentration of 4, 6, 8 and 10 gIdl, respectively. In each que pendant La dialyse chronique. Ces etudes démontrent que des solution the osmolality was changed by ading NaCL to obtain modifications modérées de l'osmolalitd plasmatique n'affectent pas osmolalities ranging from 200 to 400 mOsm/liter. COP. De plus, Ia capacitC du plasma a récupCrer du liquide et a générer The second section evaluates the ability of plasma proteins to un remplissage vasculaire (mesurC par COP) est identique au cours de recruit fluid as determined in blood samples of patients before la dialyse chronique et de l'ultrafiltration isolée, a condition que les vitesses d'ultrafiltration soient les mêmes. and after passing the dialyzer membrane during regular dialysis

or isolated ultrafiltration. This part included thirty dialysis patients, age 40 to 62 yrs. Diabetic patients were excluded

Ultrafiltration with or without diffusion may produce hypovolemia and hypotension [1—5]. At a given rate of ultrafil-

Received for publication December 12, 1984 and in revised form March 4, 1985

© 1985 by the International Society of Nephrology

because of the possibility of an abnormal vascular permeability and autonomic neuropathy. Regular dialysis, using a dextrose-

free dialysate with a sodium concentration of 135 mEq/liter (Diasol 34, Travenol Labs, Morton Grove, Illinois, USA) and a 1.3 m2 hollow fiber (16/s) was performed in 19 patients for 3.5 hrs. Isolated ultrafiltration was performed in 11 patients for 2

hrs using the same dialyzer. Samples were obtained from arterial lines at 0, 15, 30, 60, and 120 mm in all patients. Patients

808

809

Effects of dialysis and ultrafiltration on COP and osmolality

receiving regular dialysis had additional samples obtained at 210

mm. Seven of the patients receiving regular dialysis and six receiving isolated ultrafiltration had samples obtained from the venous port of the dialyzer and a remote peripheral vein at the same time that the arterial samples were collected. The third section of the study evaluates the magnitude of volume depletion, changes of plasma osmolality, and COP observed in patients treated at different sessions with regular

gravity of total blood and plasma was measured using standard copper sulfate solutions with a sensitivity of 0.0001 and error of 0.00005 [15]. If a decrease in plasma osmolality resulted in an

increase in water content of the red blood cells, the specific gravity of total bood relative to plasma should decrease. The results (mean + SE) of this evaluation showed that hematocrit increased from 25.7 3.3 to 30.5 3.4%, an increment of 19.1% (P < 0.001). Hemoglobin concentration increased from dialysis and isolated ultrafiltration; similar amounts of fluid (2.5 8.5 Ito 10.1 1 g (P <0.001), an increment of 18.9%, which liter) were removed during both dialytic treatments. is similar to that observed in hematocrit. Plasma osmolality This third section included five patients. Each patient re- decreased from 308 4.5 to 290 3.1 mOsmlkg (P < 0.002). ceived 2 hrs of regular dialysis and isolated ultrafiltration; each modality of treatment was separated by at least 2 weeks. There were similar interdialytic weight gains (2.9 0.8 vs. 3.0 0.7 kg) before each treatment, and a similar amount of fluid removal

Total blood and plasma specific gravity increased from 1.0438

23 x i0 to 1.0497 17 x io- (P < 0.002) and from 1.0253 4 x iO to 1.0290 6 x i04 (P < 0.002), respectively. Thus, the ratio of total blood to plasma specific gravity does not

was targeted for each procedure. The dialyzer and dialysate decrease before (1.0191 1.5 x l0—) vs. after dialysis (1.0198 used during the third section of the study were the same as in 13 x 10-i). These results suggest strongly that changes in red the second. Arterial blood samples were obtained at 0, 30, 60, blood cell volume probably do not occur during regular and 120 mm of treatment. hemodialysis. Determinations in the second and third sections of the study Samples from the arterial port and remote peripheral vein had included: colloid osmotic pressure as described previously [5], the same hematocrit, total protein concentration, osmolality,

error of measurement, 0.8%; coefficient of variation, 2.3%; and COP; thus, only the determinations from arterial port plasma osmolality by freezing point depression; hematocrit by samples are reported. Statistical analysis of the data was capillary centrifugation (triplicate), and plasma protein concen- performed using the Student's t test when variance analysis tration by colorimetry. Blood pressure was determined with a demonstrated statistical significance (P < 0.05). Linear regressphygmomanometer from which mean arterial pressures were sion analysis was used to compare COP with total protein and calculated, and body wts using constant weighing electronic changes in hematocrit. The slopes were compared using analybed scales were recorded also. In patients receiving isolated sis of covariance [16]. All results are expressed as the mean ultrafiltration, the entire volume of filtrate was collected in a value SE. graduated flask and was found to be equal to the recorded change in body wt. Among those receiving hemodialysis, ultra-

Results

filtrate volumes were assumed to equal changes in body wt. Weight loss is, therefore, used as the index of ultrafiltration

Section 1

The in vitro effect of osmolality changes on COP in the four volume for both groups of patients. The percentage of change in plasma volumes from baseline values through 15, 30, 60, and solutions at different concentrations is displayed in Figure 1. 120 mm was estimated from hematocrit values using the follow- When osmolality is maintained within the physiologic range (275 to 285 mOsm/liter), there is no effect on COP in any of the ing formula [81: solutions; furthermore, it can be appreciated that only at low 100 100 (Hct1 — Hct2) osmolality (200 mOsmlliter) there is a significant increase in X Plasma volume mcrement = (100 — Hct1) COP, which is more pronounced at high protein concentrations. Hct2 where Hct1 is the hematocrit baseline value and Hct2 is the Section 2 value at 15, 30, 60, or 120 mm. The validity of this method for The ability of plasma to recruit fluid (COP) for a given protein calculating plasma volume changes has been reported by variconcentration was evaluated during regular dialysis and isolated ous investigators [8—14]. Since regular dialysis produces a decrease in plasma osmolality, changes in hematocrit as the ultrafiltration. The relationship of these parameters in samples result of fluid removal may not only reflect hemoconcentration, obtained from the arterial dialysis port is shown in Figure 2. The but also a change in red blood cell volume. However, changes slope of this relationship is not different during both procein hemoglobin concentration, which also reflect hemoconcen- dures. A similar relationship is demonstrated in samples obtained from the venous port during regular dialysis and isolated tration, should not be affected by changes in plasma osmolality. (Fig. 3). To evaluate the reliability of hematocrit as a marker of ultrafiltration The ability of the blood to recruit fluid before (COP in arterial changes in plasma volume, we studied an additional six patients port) and after passing the dialyzer (COP in venous port) in

in whom blood samples were obtained at regular intervals

during a session of regular hemodialysis with hypertonic fluid

removal of 3.5 liter in 3 hrs. Hematocrit was measured in triplicate by standard capillary method and by automatic Coulter counter. Results obtained by both methods were the same. Hemoglobin was determined manually by cyanide method and also by automatic Coulter counter. Results obtained by both methods were the same. In addition, specific

relation to the loss of water was evaluated by correlating percentage changes in COP with percentage changes in hematocrit (Table 1 and Fig. 4); a progressive and similar increase in

COP is observed in the arterial and venous ports throughout both regular dialysis and isolated ultrafiltration. As can be appreciated in Figure 5, the increase in COP occurred despite a

significant decrease in plasma osmolality observed in both venous and arterial ports, and this was observed only during

810

Rodriguez et al 60 Isolated ultrafiltration

Regular dialysis

80.

60

0 40.

20

10g%

8g% 6g% .—.-----—

•4g%

50

0 40 E

0 C-)

30 ..1

20

5

6

7

8

9

10 5

6

7

8

9

10

TP, g % Fig.

2. Relationship between colloid osmotic pressure (COP) and

plasma protein concentration (TP) in samples obtained from arterial port of the dialyzer during regular dialysis and isolated ultrafiltration. Each point represents an individual measurement. The regression line for regular dialysis is: Y = —11.2 + 6.5 X (P < 0.0001) and for isolated ultrafiltration: Y = —14.7 + 6.7 X (P < 0.0001). 60 Isolated ultrafiltration

Regular dialysis

0' 200

250

300

50

400

Osmolality, mOsm/kg Fig. 1. Relationship between colloid osmotic pressure (COP) and osmolality (OSM) in solutions with d(fferent albumin concentrations: 4,

6, 8 and 10 g/dl, respectively.

0 40

00 30

regular dialysis. The decrease in osmolality was more pronounced in samples obtained from the venous port. Section 3

20 5

6

7

8

9

10

5

6

7

8

9 10

The mean values for COP, hematocrit, plasma protein conTP, g % centration, and plasma osmolality for the five patients treated Fig. 3. Relationship between colloid osmotic pressure (COP) and with regular dialysis and isolated ultrafiltration at different plasma protein concentration (TP) in samples obtained from the venous sessions are shown in Table 2. The baseline values for all these port of the dialyzer during regular dialysis and isolated ultrafiltration. Each point represents an individual measurement. The regression line parameters are similar prior to either therapy.

Figure 6 displays the mean percentage changes (%) of plasma osmolality, total protein concentration, COP, and estimated plasma volume during both regular dialysis and isolated ultrafiltration. The changes in body wt are expressed in absolute values (kg). The percentage increase in COP, total protein concentration,

and the decreases in estimated plasma volume are similar during both procedures, although there is a trend for all these parameters to be higher during isolated ultraffitration, possibly reflecting the moderately greater amounts of fluid removed 0.4 liter. A during this procedure; 2.6 0.4 liter vs. 2.4 significant decrease in plasma osmolality is observed only during regular dialysis; at 120 mm, plasma osmolality is significantly lower than that observed during isolated ultrafiltration (P < 0.01).

for regular dialysis is: Y = —14.5 + 6.9 X (P < 0.0001) and for isolated ultrafiltration: Y = —22.8 + 7.8 X (P < 0.0001).

Discussion

This study demonstrates that in vitro changes in osmolality within physiological range do not affect COP. Second, during

regular dialysis, hypoosmolality does not alter the value of COP. Finally, for the same rate of fluid removal, the decrement

in plasma volume is similar after 2 hrs of regular dialysis or isolated ultrafiltration, suggesting that the vascular refilling rate is comparable during both treatments. Colloid osmotic pressure is defined by the osmotic pressure

of the proteins plus that exerted by all univalent ions that are distributed passively across the capillary membrane (Donnan

811

Effects of dialysis and ultrafiltration on COP and osmolalizy

Table 1. Percentage changes in COP and hematocrit from both arterial and venous ports during regular dialysis and ultrafiltration Regular dialysis

Venous Port

Arterial Port

% Hct

i% COP

Time, mm

0 15 30 60 120

0 1.5 1.2 1.7

17.0

4.2

m = 176, b =

1.73,

0

0

0

0.7 2.2 7.4

—1.1

1.4

21.0

3.6

0.8 3.0 8.6

1.6

24.3

1.8

29.2 45.2

4.7 5.4 7.5

2.7

r = 0.989

% Hct

% COP

m = 2.20, b =

1.52, r

8.0 9.2

2.8 2.9

14.5 19.2

2.7 4.0

= 0.988

Ultrafiltration

Venous Port

Arterial Port

% Hct

% COP

Time, mm

0

0

15

0.5

30 60

4.7 12.0 28.2

120

% COP

0 0.3 3.4

1.1 1.0

2.4

7.1

5.1

14.2

0.7 0.8

—0.39,

r = 0.994

0

0

1.0 1.4

m = 1.89, b =

z% Hct

32,0 43.6 52.5 67.7

5.4 6.0 5.7 4.3

m = 2.23, b = 0.02, r =

14.5

3.1

20.2 24.5 29.5

2.6 3.7 3.9

0.998

Comparison of slopes shows no statistical difference (P > 0.05). Abbreviations are: m, slope; b, y-intercept; r, regression coefficient; i.% COP, percentage change of COP; i% Hct, percentage change of

hematocrit; % COP, y; % Hct, x.

Isolated

Regular

Isolated ultrafiltration

Regular dialysis

ultrafiltration

dialysis

320

40—

300

2'

00 C)

0

E 280

0(0

r'4 0

20 —

60

120

210 0

60

120

Time, mm

Fig. 5. Mean changes (± SE) of plasma osmolality in relation to the

& I 0

time of regular dialysis and isolated ultrafiltration. 0 represent samples obtained from the arterial port of the dialyzer and •, samples from the venous port.

0 I

I 20

% Hct

I

I

I

0

20

I

% Hct

Fig. 4. Relationship between mean increment (± SE) of both colloid osmotic pressure (% Hct) during regular dialysis and isolated ultrafiltration. , 0, , and A denote values (± SE) at 15, 30, 60 and 120

range, as defined by Landis and Pappenheimer [18]. This implies that the potential for recruitment of fluids at a given

plasma protein concentration is similar during both procedures and is not different from that of normal subjects [181. mm, respectively. Arterial port samples are represented by open It can be appreciated that for a given increment in hematocrit symbols and venous port by closed symbols. The regression line for regular dialysis is: Y = 2 + 2.1 X (P < 0.0001) and for isolated in the venous port, the increase in COP is similar for both ultrafiltration: Y = —2.46 + 2.3 X (P < 0.0001). regular dialysis and isolated ultrafiltration (Fig. 4). This suggests that fluid recruitment capacity from the interstitial space Effect) [17]. Thus, the direct measurement of COP reflects the of the blood leaving the dialyzer is similar during both procedures. Furthermore, because the slope of the relationship ability of a given plasma protein sample to recruit fluid. The relationship between COP and plasma protein concen- between z% COP and % Hct is similar for both arterial and tration in samples from the arterial and venous ports is similar venous port samples, the process of diffusion during regular during both regular dialysis and isolated ultrafiltration (Figs. 2 dialysis probably does not affect the increment in COP proand 3); furthermore, this relationship is within the physiological duced by hemoconcentration.

812

Rodriguez et a! Table 2. Mean values of the parameters evaluated in patients undergoing regular dialysis (D) and isolated ultrafiltration (U) 0

Time, miii

D U

COP, cm H20

30

6.4 6.4

U D U

OSM, mOsmlkg

1.3 2 .3

28

D

TP, gidi

30 30 29

310

6

305

7

1.5

1.7 .3

6.5 6.6

.4

306 306

120

60

•4

31

30

6.6 6.8 303

7

305

1.6

2b

34 33

.3

6.9

5b

71

5b 7

299 307

l.5a 35b .3 5b

4b

7

a P < 0.05; b P < 0.01; as compared with 0 mm time. Abbreviations are: COP, colloid osmotic pressure; TP, plasma protein concentration; OSM, plasma osmolality. 0

tration. During the course of fluid removal, plasma proteins are concentrated, and this will be reflected in a change in COP [5] and, as discussed, in the ability of the plasma proteins to recruit fluid.

11

In this study, for a similar rate of fluid removal during regular

dialysis and isolated ultrafiltration, the decrease in plasma (I)

0

volume and increase in COP is not significantly different during

—5

greater vascular refilling rate during isolated ultrafiltration [30].

either procedure (Fig. 6). This contrasts with previous observations from van Stone, Bauer, and Carey who suggested a

0 3

0o 0

/

10

0 60

Time, mm

A hypothetical explanation for these apparent discrepancies may be that, in the present study, dialytic treatment was given

—10

treatment [30]. In this situation, it is theoretically possible that with regular dialysis, since there is a decrease in osmolality in

for 2 hrs while van Stone and co-workers used 4 hrs of

> <1

0

0

120

both the vascular and interstitial space (Fig. 7), water may 0

60

120

Time, mm

Fig. 6. Mean change (± SE) for increasing colloid osmotic pressure

(LCOP, %) and plasma protein concentration (ATP, %), and decreasing weight loss (kg), plasma volume (A.PV, %), and osmolality (iWsm, ) and ultrafiltration (—). %) through 120 mm of regular dialysis (

As reported previously [19—21], a decrease in plasma

osmolality is observed only during regular dialysis in samples from both arterial and venous ports (Fig. 5). However, the relationship between COP and total protein concentration was similar for both procedures. Thus, the decrement in plasma osmolality occurring during regular dialysis does not seem to significantly affect this relationship. Changes in serum sodium concentrations may alter COP. Kesselman has observed that a decrease in sodium concentration, in vitro, produces a minor rise in COP [22], which has been

move to the intracellular space and simultaneously, water will also move to the vascular space driven by COP; the end result will be a significant decrease in interstitial hydrostatic pressure, which may decrease the vascular refilling rate. Our observation of a similar vascular refilling rate during regular dialysis and isolated ultrafiltration is compatible with

Bergstrom's data showing no significant changes in blood volume during regular dialysis without ultrafiltration despite a significant decrease in plasma osmolality [2]. Likewise, Henrich

et al have observed similar changes in hematocrit and total protein concentration among patients receiving regular dialysis

and simultaneous isovolemic infusion of either isotonic or hypertonic mannitol [21]. With a pronounced decrease in plasma osmolality, as evaluated by van Stone, Bauer, and Carey, the vascular refilling rate seems to be affected [30]. A hypothetical explanation for the similar vascular refilling

rate observed during both procedures may be that during hemoconcentration, there is an increase of plasma protein

interpreted to be a consequence of the Donnan effect [18, (negatively charged) and, in order to maintain electroneutrality, 22—25]. Indeed, in vitro, the effect of changes in sodium cations from the interstitial space must cross the capillary concentration on COP is minimal, as shown in Figure 1. This is membrane into the vascular space (Donnan effect). The end in agreement with previous published observations [25, 26]. result is an ion excess in the vascular compartment of higher This relationship is similar to that observed by Fogh-Andersen, protein concentration that establishes an osmolal gradient fa-

concentration observed during regular dialysis do not alter

voring the passage of fluid into the vascular compartment. In conclusion, the ability of the plasma to recruit fluid (COP) is similar during regular dialysis and isolated ultrafiltration, provided that the same rate of ultrafiltration is present.

COP. The vascular refilling rate has been defined as the difference

Acknowledgments

Thode, and Siggard-Andersen, who studied, in vitro, the Donnan ratio as a function of several sodium concentrations [27]. Therefore, it is likely that the magnitude of changes in sodium

between total fluid loss and plasma volume loss per unit time This study was supported in part by a Small Grant Award from the [28, 29]. Thus, for a given rate of ultrafiltration, the vascular University of Oklahoma Health Sciences Center, Oklahoma City, refilling rate is related inversely to the degree of hemoconcen- Oklahoma USA.

813

Effects of dialysis and ultrafiltration on COP and osmolality

kreislaufstabilitat bei chronischer haemodialyse and tion in press

Regular hemodialysis

Interstitial

Cellular

H20

Vascular

tOsm

8. VAN BEAUMONT W: Evaluation of hemoconcentration from hematocrit measurements. J AppI Physiol 32:712—713, 1972 9. VAN BEAUMONT W, GREENLEAF JE, JUH05 L: Disproportional changes in hematocrit, plasma volume and proteins during exercise and bedrest. J AppI Physiol 33:55—61, 1972

Osm

H20-Solutes

Hydrostatic pressure

haemofiltra-

Fluid removal (hypertonic)

t COP

10. DILL DB, COSTILL DL: Calculations of percentage changes in volumes of blood, plasma, and red cells in dehydration. J Appl Physiol 37:247—248, 1974 11. DAUGIRDAS JT, SHEN WT, ING TS, HUMAYAN HM, RAZMA PE:

Measurement of plasma volume changes during extracorporeal ultrafiltration. Abstracts, 27th Ann Mtg Am Soc Artf Intern Or-

H20—Solutes

gans, Anaheim, California, 1981, p 41 12. HAGAN RD, DIAz FJ, HORVATH SM: Plasma volume changes with

movement to supine and standing positions. J Appl Physiol 45:414—418, 1978 13. TANAKA Y, M01UMOT0 T, MIKI K, NOSE H, MIYAZAK M: On-line

control of circulating blood volume. Jpn J

Interstitial

Cellular

H2O

31:427—431, 1981

during stress in man: Osmolality and red cell volume. J Physiol 47: 1031—1038,

Vascular 15.

H20

Physiol

14. GREENLEAF JI, CONVERTINO VA, MANGSETH GR: Plasma volume

Isolated ultrafiltration

Appl

1979

PHILLIPs RA, VAN SLYKE D, HAMILTON PB, DOLE VP, EMERSON

K, ARCHIBALD RM: Measurement of specific gravities of whole blood and plasma by standard copper sulfate solutions. JBiol Chem Osm

Osm

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183:304—330, 1950 16. ZERBE GO, ARCHER PG, BANCHERO N, LECHNER AJ: On comparV

Fluid removal (isotonic)

ing regression lines with unequal slopes. Am

J Physiol 242:R178—

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17. HAYS RM: Dynamics of body water and electrolytes in: Clinical Disorders of Fluid and Electrolyte Metabolism, edited by MAXWELL H, KLEEMAN CR. New York, McGraw-Hill, 1980, pp. 1—36 18. LANDIS EM, PAPPENHEIMER JR: Exchange of substances through

the capillary walls in Handbook of Physiology, edited by HAMILTON WR, Washington, D.C., The American Physiologic

Fig. 7. Schematic representation of movement of fluid between compartments that probably occurs during regular hemodialysis and isolated ultrafiltration. COP is colloid osmotic pressure; Osm is osmolality. Wide arrow represents greater fluid movement than narrow arrow. Note that a decrease in osmolarity occurs only during regular hemodialysis and results ultimately in a decrease in interstitial hydro-

static pressure, which, in turn, will favor a shift of fluid from the

Society, (vol. 2)1963, pp 961—1034 19. BERGSTROM J: Ultrafiltration without dialysis for removal of fluid

and solutes in uremia. Clin Nephrol 9:156—164, 1978 20. RODRIGO F, SHIDEMAN J, MCHUGH R, BUSELMEIER T, KJELL5TRAND C: Osmolarity changes during hemodialysis. Ann Intern Med 86:554—561, 1977 21. HENRICH W, WOODARD TM, BLACHLEY J, GOMEZ-SANCHEZ C,

PETTINGER W, CRONIN R: Role of osmolality in blood pressure stability after dialysis and ultrafiltration. Kidney mt 18:480—488,

vascular into the interstitial compartment.

The authors thank Mrs. W. Savage and Mrs. C. Clay for secretaral assistance in the preparation of this manuscript. Reprint requests to Dr. F. Liach, Chief, Nephrology Section (luG), Veterans Administration Medical Center, 921 N.E. 13th Street, Oklahoma City, Oklahoma 73104 USA

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