Interstitial colloid osmotic and hydrostatic pressures in subcutaneous tissue of human thorax

MICROVASCULAR

RESEARCH

Interstitial

24, 104-113 (1982)

Colloid Osmotic and Hydrostatic Pressures Subcutaneous Tissue of Human Thorax

HARALD NODDELAND,’

ALAN

in

R. HARGENS,* ROLF K. REED, AND KNUT AUKLAND’

Institute of Physiology, University of Bergen, N-5000 Bergen, Norway, Surgery (V-151), University of California and Veterans Administration San Diego, California 92161 Received December

and *Department of Medical Centers,

29, 1981

Interstitial colloid osmotic pressure (P,) of human subcutaneous tissue was measured simultaneously by three different techniques: in fluid collected by implanted nylon wicks or empty wick catheters, and by implantable colloid osmometers. Hydrostatic pressure in interstitial fluid (P,) was measured by wick catheters and by wick-in-needle technique. The implantations were done at heart level on the side of the thorax in 15 healthy male subjects. They were divided in two groups, one for a direct comparison of the methods and one for a further investigation on the effect of suction in empty wick catheters. In nylon wicks implanted for 60 mitt, r, averaged about 16 mm Hg in both groups. In fluid sampled with empty wick catheters, a suction pressure of - 10 cm H,O, and 60-min implantation time, the mean r, was 10.5 and 12.9 mm Hg in the first and second group, respectively. Two factors contributed to lowering of n, in empty wick catheters: dilution of the samples by saline contained in the wick catheter before implantation, and a m,lowering effect of suction. Measurements with implantable colloid osmometers averaged 9.8 mm Hg. Mean implantation time was 30 min. However , the r, value was positively correlated to implantation time and linear extrapolation to 60 min gives T, values similar to implanted nylon wicks. We therefore conclude that the three methods give the same n, in the same area of human subcutaneous tissue provided equal implantation time and little or no suction in empty wick catheters. P, was -0.2 mm Hg with both wick catheter and wick-in-needle technique.

INTRODUCTION The determination of normal levels of interstitial fluid hydrostatic pressure (Pi and interstitial colloid osmotic pressure (vi) in man has been a challenge to investigators ever since the principles of transcapillary fluid balance were outlined (Starling, 1896). Edema fluid for measurement of colloid osmotic pressure is easily sampled through an indwelling catheter without suction (Bramkamp, 1935; Crockett, 1956). In contrast, normally hydrated subcutaneous tissue contains only small amounts of freely flowing interstitial fluid, and interstitial fluid samples are hard to obtain through open cannulas inserted in the tissue. Several methods have therefore been developed for measurement of vi in normally hydrated tissue ’ Recipient of a Research Fellowship from the Norwegian Research Council for Science and the Humanities. ’ To whom reprint requests should be addressed.

M)26-2862/82/040104-10$02.00/0 Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved Printed in U.S.A.

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of experimental animals (Aukland and Nicolaysen, 1981). Only a few of these methods are suitable for measurements in human subjects because of the size and duration of the trauma and because of tissue stabilization problems. Hargens et al. (1981) sampled interstitial fluid in legs of healthy volunteers by using empty wick catheters. They found a ni of about 9 mm Hg in subcutaneous tissue and 8 mm Hg in skeletal muscle. Subcutaneous interstitial colloid osmotic pressure measured in fluid sampled by implanted wicks averaged about 16 mm Hg on the thorax and 11 mm Hg on the leg (Noddeland, 1982a,b). A third approach is direct measurement of ri by a small colloid osmomcter implanted in the tissue (Reed, 1979). In rats the latter method gives essentially the same results as implanted wicks, while only a few pilot measurements have been done in man so far. Interstitial fluid hydrostatic pressure (Pi) is measured as the “fluid equilibration pressure,” i.e., by interposing a fluid column between the tissue and a pressure transducer (Guyton et al., 1971). In presence of edema, P, can be measured by simply connecting an open cannula in the tissue with a fluid column (Wells et al., 1938). Principally the same technique can be used also in normally hydrated tissue, but several authors recommend that the contact between interstitial fluid and the measuring system is improved by wick fibers (Scholander et al., 1968; Fadnes et al., 1977). In the previous studies mentioned above, both the site of the measurements and the experimental conditions were different. Thus, the different results might reflect methodological differences as well as variations in pi within the body. We therefore measured ri simultaneously in the same area by implantable colloid osmometers and in fluid collected by implanted nylon wicks or empty wick catheters. Subcutaneous tissue of thorax at heart level was chosen because unexpectedly high rr, values had been measured here by implanted wicks and also because this area is relatively indifferent to changes in body posture (Noddeland, 1982b). Simultaneously we compared two slightly different methods for Pi measurement: the wick-in-needle technique (Fadnes et al., 1977) and the wick catheter technique (Hargens et al., 1977). MATERIALS

AND METHODS

Fifteen healthy male students aged 20-25 years served as volunteers for this study. Protocol I was followed in eight subjects (Group I), and the other seven subjects (Group II) were examined according to Protocol II (see below). The subjects were reclined comfortably in a dental chair throughout the study. For every point of insertion the skin was anesthetized by an intracutaneous injection of 0.1-0.2 ml lidocaine (Xylocain, 20 mg/ml, Astra, Sweden) a few minutes in advance. On both sides of thorax at heart level the skin was disinfected with 0.5% chlorhexidine (Hibitane, ICI, England) dissolved in 70% ethanol. The surrounding skin was draped by sterile clothing leaving open two areas of about 10 x 15 cm. All equipment used in this field was sterilized. Prior to the measurements every subject was fully informed about the experimental procedure. Determination of Plasma Colloid Osmotic Pressures (rp) Blood was collected by venipuncture during light and short-lasting stasis, and the sample was centrifuged immediately after coagulation.

venous Colloid

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ET AL.

osmotic pressure was measured directly on a colloid osmometer designed for 5~1 samples (Aukland and Johnsen, 1974). The membrane used was impermeable to molecules larger than 30,000 daltons (Diaflo PM30, Amicon Inc., Lexington, Mass.). In all specimens, including tissue fluid, colloid osmotic pressure was measured a few hours after sampling. Determination of Interstitial Colloid Osmotic Pressure (ni) (a) In tissue fluid collected by implanted wicks. The material was a threestranded nylon thread consisting of 315 filaments each about 25 pm in diameter (Enkalon 3 x 3, Enka bv., Arnheim, Holland). This thread was laid double and sewn into the tissue by a 75mm straight needle without cutting edges (Acufirm, W. Germany). A knot was tied at the end of the wick to prevent it from slipping under the skin (Fig. la). After insertion the area was covered with sterile plastic film (Wound Cleansing Pack, Molnlycke, Sweden) to prevent evaporation from the wick ends. After a 60-min equilibration period the wicks were pulled out and the middle parts were quickly dropped into a tube containing paraffin (Fig. la). Wick fluid was isolated from the wick by centrifugation of the paraffin tube for about 20 min at 1500 rpm. The wick fluid droplet was then collected by a hematocrit tube. Only clear and pink wicks were accepted, and wicks colored red by blood were discarded (Noddeland, 1982a). (b) In tissue juid collected by empty wick catheters. The wick catheter was I

a

C

b

,Dhcron

wick

z-

-

Dacron

wck

_~

FIG. I. Schematic representation of methods for collection of interstitial fluid (a and b) and for direct measurement of interstitial colloid osmotic (c) and hydrostatic (d and e) pressures in human subcutaneous tissue. (a) Implanted nylon wick; (b) empty wick catheter; (c) implantable colloid osmometer; (d) wick-in-needle; (e) wick catheter. PT represents pressure transducer. Suction in empty wick catheters varied between zero and -20 cm H20.

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made of a 30-cm-long polyethylene tube (PE-50, Clay Adams, Parsipanny, N.J.). In one end of the catheter a lo-mm double 4-O-Dacron suture (Mersilene, Ethicon Inc., Somerville, N.J.) was pulled in and anchored by a 6-O nylon monofilament tether. The 3 mm of Dacron suture protruding from the catheter was frayed (Fig. lb). Before insertion the wick was wetted. Sterile isotonic saline was allowed to enter the first lo-20 mm of the wick catheter. Then the wick was held against a sterile cloth to suck out excess saline, until the meniscus was exactly at the end of the wick in the catheter. The amount of saline contained in the wick catheter was determined in six wick catheters. The fibers protruding from the tip were cut away, and the wick catheters were weighed before and after the wetting procedure. The wick catheter was inserted through a lbgauge intravenous catheter placement unit (Cathlon IV Jelco Labs., Raritan, N.J.) (Hargens et al., 1981). The skin was lifted up and penetrated with the catheter placement unit. Then the metal stylet was withdrawn a few millimeters to the opening of the surrounding plastic sheath which was advanced bluntly 4-5 cm under the skin (Fig. lb). The metal stylet was replaced by the wick catheter, and finally the plastic sheath was removed. The wick catheter was taped to the skin, and the free end connected to a suction device (see Protocols I and II). After a 60min sampling period the wick catheter was clamped and pulled back. The sample was analyzed similar to samples from implanted wicks. Red samples were discarded (Hargens et al., 1981). (c) By an implantuble colloid osmometer. The colloid osmometer consisted of a hollow-fiber dialysis tube (Model HI DP-10, Amicon Corp.) threaded over a steel cannula provided with a side hole (Fig. lc). The colloid osmometer was sealed with nail-polish and sterilized in a steam autoclave at 120°C for 15 min, and connected to a pressure transducer (Hewlett-Packard 128OC)and recorder (Hewlett-Packard 7754A) via a 30-cm-long polyethylene catheter (PE-20) (Reed, 1979). Colloid osmometers were inserted via a 14-guage intravenous placement unit by a procedure similar to the wick catheter insertion. Implantation time varied. In general the colloid osmometer was pulled out when recorded pressure apparently had leveled off. After each implantation, the colloid osmometer was tested in a 40 g/liter albumin solution. Only measurements from colloid osmometers giving reproducible readings in this solution were accepted. The colloid osmometer records the sum of interstitial hydrostatic and colloid osmotic pressure. To obtain rr, the average Pi measured with wick catheter and wick-in-needle was subtracted from the pressure read by the osmometer. Measurement of Interstitial Fluid Hydrostatic Pressure (P,) (a) By wick catheter technique. Wick catheters and implantation procedure were identical to those used for tissue fluid sampling. Wick catheters were connected to a pressure transducer (Hewlett-Packard 128OC) and recorder (Hewlett-Packard 7754A), and the system was filled with sterile isotonic saline (Fig. Id). Communication between wick catheter and the interstitial fluid was ascertained by compression and decompression of the catheter with a screw clamp. The amount of fluid expelled from the catheter by compression was about 1 ~1. The measurement was accepted when the pressure difference with and without clamp was less than 1 mm Hg.

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(b) By wick-in-needle technique. The wick-in-needle consisted of a steel cannula filled with a loose bundle of nylon filaments (Enkalon). The cannula had an outer diameter of 0.6 mm and was provided with a 3-mm-long side hole about 5 mm from the tip (Fig. le). The wick-in-needle was connected to a polyethylene catheter (PE-20) and filled with sterile isotonic saline. The pressure transducer, recorder, and test for communication were the same as described for the wick catheter. Wick-in-needles were inserted through the skin without anesthesia (Fadnes et al., 1977). Protocol I: Comparison of Methods Four implanted wicks and two empty wick catheters were implanted on the right side of the thorax. Empty wick catheters were connected to a constant suction pressure of - 10 cm Hz0 immediately after insertion. On the left side of the thorax one wick catheter, one wick-in-needle, and one or two colloid osmometers were inserted after a zero-pressure recording at the same height level. A blood sample was collected, and both implanted wicks, and empty wick catheters were removed after 60 min in tissue. All P, recordings were repeated to obtain duplicate values. Protocol II: Effect of Suction Pressure on Samples of Interstitial Fluid Obtained by Empty Wick Catheters Altogether two implanted wicks and eight (nine) empty wick catheters were inserted on both sides of thorax and left in tissue for 60 min. Pairs of empty wick catheters were connected to different suction pressures: no suction, -3, - 10, and - 20 cm HzO. In addition, blood was collected for rrp measurement, and Pi was determined twice by one wick catheter. Statistics Paired data were analyzed by the Wilcoxon-Mann-Whitney test. Groups I and II were compared by the Wilcoxon two-sample test. All calculated P values were two tailed. Standard deviations of methods were calculated from the average difference between duplicate measurements (AD) by the formula SD = t T”~ . AD. RESULTS In Group I the Pi averaged -0.2 mm Hg both when measured with the wick catheter technique and with the wick-in-needle (Table 1). Mean 7~~was 27.6 mm Hg. Interstitial colloid osmotic pressure (ri) in fluid from implanted nylon wicks was 15.6 mm Hg. The two other methods gave statistically significant lower mi values; 10.5 mm Hg with empty wick catheters (P < 0.01) and 9.8 mm Hg with implantable colloid osmometer (P < 0.05) (Table 1). However, implantation time of the colloid osmometer varied from 10 to 54 (mean 30) min, and a significant correlation (P < 0.01) was found between ni and implantation time (Fig. 2). In Group II Pi was measured by the wick catheter technique and averaged -0.6 (SD 1.1) mm Hg, and mean 7~~was 26.9 (SD 2.8) mm Hg, and ri measured by implanted nylon wicks averaged 15.9 (SD 2.2) mm Hg. None of these values was statistically different from the corresponding values in Group I, but in empty

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TABLE COLLOID

HUMAN

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SUBCUTIS

I

OSMOTIC PRESSURE IN PLASMA (P,) AND INTERSTITIAL FLUID (P,), AND INTERSTITIAL HYDROSTATIC PRESSURE (P,) IN GROUP I”

FLUID

p,

Subject 1

2 3 4 5 6 7 8 Mean SD “ b ’ *

Empty wick catheter”

Implanted wick

‘TTP

15.5 14.8 13.0 17.3 12.5 19.8 16.3 15.8 15.6 2.3

25.7 24.9 28.0 25.5 26.5 29.3 29.5 31.2 27.6 2.3

Implantable colloid osmometer recording f p,

Wick catheter

Wick-inneedle

-0.5 0 - I.0 -0.5 I.0 0.3 0.3 -0.8 -0.2 0.7

- 1.0 0.8 -0.5 0 ~ I.0 1.5 -0.5 - 1.0 -0.2 0.9

-

10.5 10.0 11.8 14.0 8.5 11.5 10.3 7.5 10.5* 2.0

12.4 9.8 10.5 10.4 9.2 6.4 9.8* 2.0

All pressures in mm Hg. Suction pressure, - 10 cm HzO. Reflection coefficient of 0.75. P < 0.02 vs implanted wick.

wick catheters with - 10 cm Hz0 suction ri was significantly higher in Group II (mean 12.9 (SD 1.4) mm Hg) than in Group I (P < 0.02). Colloid osmotic pressure in fluid sampled with empty wick catheters was inversely related to the suction pressure: -20 cm HZ0 gave a mi of 11.6 (SD 1.5) mm Hg, - 10 cm Hz0 gave 12.9 (SD 1.4) mm Hg, and - 3 cm Hz0 gave 14.9 (SD 1.1) mm Hg. However, 20 y=O25x+l67

F

r:O93

mm Hg

/’

n=6

15

ptOOl

,’

/

i Q

10

rn3

/ ‘ 5

t

0 ’ 0

IO 20 IMPLANTATION

30 TIME

LO

50

60

minutes

FIG. 2. Interstitial colloid osmotic pressure (P,) in human subcutaneous tissue as a function of implantation time in Group I. Squares represent measurements obtained in one subject by implantable colloid osmometer. The numbers (3 and 4) show that the values are based on three and four implantations in each subject. Mean rr, ? 1 SD obtained by implanted nylon wicks in the same subjects in Group I (closed circle) and in previous experiments (Noddeland, 1982a) (open circles) are shown for comparison. Continuous line is calculated with least-squares method, and the broken line indicates extrapolation to 60-min implantation time.

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20 l.5 mm Hg

01,

3 0 -3

-10

SUCTION PRESSURE

-20 cm Ii20

FIG. 3. influence of suction pressure on colloid osmotic pressure (n,) in tissue fluid sampled by empty wick catheters (triangles) in Group II. lnterstitital colloid osmotic pressure measured by implanted nylon wicks is shown for comparison (circle). Values represents mean 2 1 SD. Asterisks indicate P < 0.02 vs implanted nylon wicks. Possible dilution error (see Results) is indicated by d.

paradoxically empty whick catheters without suction gave a 7Tiof 12.3 (SD 2.0) mm Hg (Fig. 3). A linear regression analysis based on all empty wick catheters in Group I gave the following equation: y = 0.091~ + 13.6, r = 0.43, n = 25, P < 0.05. The same calculation disregarding zero-suction samples showed: y = 0.18x + 15.2, r = 0.70, n = 19, P < 0.001. Thus, the intercept of 13.6 or 15.2 mm Hg indicated the ni value without influence of suction. In spite of a wide variation in sample size the general impression was that greater suction produced larger samples. Some of the samples were smaller than 5 ~1. By weighing of empty wick catheters where the protruding fibers had been cut away, it was found that the wetting procedure added approximately 0.9 (SD 0.2) ~1 saline. This indicates that about one-fifth of a 4- to 5-11.1 sample is saline. The capillarity of an empty wick catheter was determined by placing the tip vertically in saline. A I2-mm fluid column was measured when a stable level was reached. The fraction of clear and pink samples was 68% in empty wick catheters and 93% in implanted nylon wicks (Table 2). Red cells in implanted wick samples were typically hemolyzed whereas those in empty wick catheters were not hemTABLE 2 NUMBER

OF DISCARDED AND ACCEPTED INSERTIONS”

Empty wick catheters Discarded No sample Red sample Accepted Pink sample Clear sample Sum

Implanted nylon wicks

(8%)

6 18

(24%)

4

(7%)

13 37 74

(18%) (50%) (100%)

12 37 53

(23%) (70%) (100%)

a All implanted wicks and empty wick catheters in Groups I and II are included.

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olyzed. Standard deviations of the methods were 2.6 mm Hg for empty wick catheters and 1.9 mm Hg for implanted wicks. DISCUSSION Interstitial Colloid Osmotic,Pressure (ri) Implanted nylon wicks gave the same mean values in Group I and II, and these values are not different from those previously obtained in other healthy volunteers (Noddeland, 1982b). One 5-cm-long wick usually yields a sample sufficient for ri measurement (about 5 pl), and by inserting four wicks it is always possible to obtain parallel samples from different wicks even if two wicks are discarded due to bleeding. The colloid osmotic pressure difference between duplicate wicks is small compared to the intersubject variations. Empty wick catheters and implantable colloid osmometers in Group I gave significantly lower ri values than implanted nylon wicks. In some wick catheters suction was less than - 10 cm H,O due to air leakage, and our impression was that lower suction produced smaller and more concentrated samples. This observation raised the suspicion that suction pressure might influence colloid osmotic pressure of the sample and this explains the different 7Tiobtained by wicks and empty wick catheters. Protocol II was designed to investigate this possibility. The results from Group II confirm that suction lowers colloid osmotic pressure of the sample. When this effect is accounted for by linear extrapolation back to zero suction, empty wick catheters give values in the same range as implanted wicks even if the samples with zero suction and possible dilution error are included in the calculation (Fig. 3). The capillarity of empty wick catheters explains why tissue fluid enters wick catheters without suction. However, there is no obvious reason why empty wick catheters give significantly higher values (P < 0.02) in Group II than in Group I. The average 7~~value measured by implantable colloid osmometers is significantly lower (P < 0.05) than by implanted nylon wicks (Table 1). The reason for this is evident from Fig. 2: a significant positive correlation between implantation time and the ri value obtained from the implanted colloid osmometer. This finding was unexpected at the time of the experiments since previous experience with the implanted colloid osmometer in rats (Reed, 1979) gave stable readings after 1.5-20 min which lasted for more than I hr. These recordings agreed with samples collected by implanted wicks in rats (Fadnes and Aukland, 1977). However, in man, implanted nylon wicks have later shown increasing 7Ti from 30 to 120 min after insertion (Noddeland, 1982a). The wick data obtained at 30- and 60-min implantation time are plotted in Fig. 2. They agree well with the pressure readings from the implantable colloid osmometer (mean implantation time, 30 min) and explain the apparent difference between nylon wicks and implanted colloid osmometers. In light of these results it may be more pertinent to interpret this time relationship as changes in interstitial colloid osmotic pressure following the mechanical trauma of wick or colloid osmometer insertion. Inflammation elicited by the mechanical trauma may change both capillary hydrostatic pressure and the properties of the capillary wall. However, the impact on vi is not obvious: If transcapillary water transport increases more than protein transport, the total effect is a lowering of ri. On the other hand, a rise in

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transcapillary protein transport exceeding the rise in water transport will raise ri above the “true” level. We believe that the three methods give the same ri in the same area of human subcutaneous tissue provided equal implantation time and little or no suction in empty wick catheters. Still, the true ri level of undisturbed subcutaneous tissue remains uncertain. It should be emphasized that in man there is not one true ri level valid for subcutaneous tissue all over the body. In the feet pi is consistently 5 mm Hg lower than in the thorax, except when the person stays in bed for several days (Noddeland, 1982b). As implanted nylon wicks, empty wick catheters, and implantable colloid osmometers give similar results, the choice of method will depend on the purpose. For measurements in human subcutaneous tissue the implanted nylon wicks are preferable since no suction is needed and since the wicks produce relatively large samples. However, empty wick catheters may be better for sampling of interstitial fluid in skeletal muscle. The implantable colloid osmometer in its present form is not well suited for measurements in man, mainly because respiratory movements and changes in position during the measurement influence the recorded pressure. These are problems not experienced in the anesthetized animal, and we think further methodological studies are necessary for common use in human tissues. Interstitial

Hydrostatic

Pressure (PJ

The same Pi value was obtained with wick-in-needle and wick catheter technique, and the variation between the two methods was similar to the variation between duplicate measurements with one method. Our experience is that the wick catheter technique is the better for clinical purposes. A stable pressure reading is obtained more readily with a wick catheter than with a wick-in-needle. Also, it is easier to provide surgical sterility with wick catheters. ACKNOWLEDGMENTS This research was supported by the Veterans Administration, USPHYNIH Grants AM-25501 and AM-26344, and by a Research Career Development Award (AM-00602) to A.R.H. The project also received financial support from the Norwegian Research Council for Science and the Humanities.

REFERENCES K. AND JOHNSEN, H. M. (1974). A colloid osmometer for small fluid samples. Acta Physiol. 90, 485-490. AUKLAND, K., AND NICOLAYSEN, G. (1981). Interstitial fluid volume: Local regulatory mechanisms. Physiol. Rev. 61, 556-643. BRAMKAMP, R. G. (1935). The protein content of subcutaneous edema fluid in heart disease. J. Clin. Invest. 14, 34-36. CROCKETT, D. J. (1956). The protein levels of oedema fluids. Lancer, 1179-I 182. FADNES, H. O., AND AUKLAND, K. (1977). Protein concentration and colloid osmotic pressure of interstitial fluid collected by the wick technique: Analysis and evaluation of the method. Microvasc. Res. 14, 11-25. FADNES, H. O., REED, R. K., AND AUKLAND, K. (1977). Interstitial fluid pressure in rats measured with a modified wick technique. Micvovasc. Res. 14, 27-36. GUYTON, A. C., GRANC~ER, H. C., AND TAYLOR, A. E. (1971). Interstitial fluid pressure. Physio[. Rev. 51, 527-563. AUKLAND,

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HARGENS, A. R., MUBARAK, S. J., OWEN, C. A., GARETTO, L. P., AND AKESON, W. H. (1977). Interstitial fluid pressure in muscle and compartment syndromes in man. Microvasc. Res. 14, l-10. HARGENS, A. R., COLOGNE, J. B., MENNINGER, F. J., HOGAN. J. S.. TUCKER,B. J., AND PETERS,

R. M. (1981). Normal transcapillary pressures in human skeletal muscle and subcutaneous tissues. Microvasc. Res. 22, 177-189. NODDELAND, H. (1982a). Colloid osmotic pressure of human subcutaneous interstitial fluid sampled by nylon wicks: Evaluation of the method. Stand. J. C/in. Lab. Invest., in press. NODDELAND, H. (1982b). Influence of body posture on transcapillary pressures in human subcutaneous tissue. Stand. .I. C/in. Lab. Invest., in press.

REED, R. K. (1979). An implantable osmometer: Measurements in subcutis and skeletal muscle of rats. Microvasc. Res. 18, 83-94. SCHOLANDER, P. F., HARGENS,A. R., AND MILLER, S. L. (1968). Negative pressure in the interstitital fluid of animals. Science 161, 321-328. STARLING E. H. (1896). On the absorption of fluids from the connective tissue spaces. J. Physiol. London 19, 3 12-326. WELLS, H. S., YOUMANS, J. B., AND MILLER, D. G.. JR. (1938). Tissue pressure (intracutaneous,

subcutaneous, and intramuscular) as related to venous pressure, capillary filtration, and other factors. J. CIin. Invesf. 17, 489-499.