- Email: [email protected]
Analysis of altered capillary pressure and permeability after thermal injury
JOURNAL
OF SURGICAL
RESEARCH
42,693-702
(1987)
Analysis of Altered Capillary Pressure and Permeability
after Thermal
R. M. PITT, M.D., J. C. PARKER, PH.D., G. J. JURKOVICH, A.E.TAYLoR,PH.D.,ANDP.W.CURRERI,M.D. Departments
of Physiology
and Surgery,
College
of Medicine,
University
of South Alabama,
Injury
M.D., Mobile,
Alabama
Presented at the Annual Meeting of the Association for Academic Surgery, Washington, DC, November 5-8, 1986 In order to investigate the effectsof thermal injury on microvascular hemodynamics and penneability, hindpaw arterial (PA), venous (Pv), and capillary (Pc) pressures, blood (Qa) and lymph (QL) flows, and lymph (C,) and plasma (Cr.) total protein concentrations were measured before and for 3 hr after a IO-set 1OO’C scald bum in 11 dogs. Prior to injury in eight experiments (Group I-permeability analysis) venous pressure was elevated by outflow restriction until the minimal C&r was obtained. In three experiments (Group II-hemodynamic analysis) outflow was not restricted. Lymph and plasma protein fractions ranging in size from 37 to 120 8, were measured using gradient gel electrophoresis and capillary equivalent pore sizes were calculated. In the early postbum period, Pc increased from 24 -C2 (mean +_SE) to 47 -+ 5 mm Hg (P < 0.05) and precapillary resistance (R,+,)decreased from 6.6 * 0.2 to 2.5 f 0.2 mm Hg/ml/min/lOO g (P < 0.05) while postcapillary resistance (R,) remained unchanged. Pre- to postcapillary resistance (R,/RV) fell by 74%. The reflection coefficient for total proteins (calculated as c = 1 - C&Z,) decreased from 0.87 + 0.0 1 to 0.45 ? 0.02 (P < 0.0 1). Permeability ofthe postbum capillary endothelium was described by using two populations of equivalent pores. Prebum pore radii were 50 and 300 A with 13% of the capillary filtrate passing through the large pores. Pore radii increased after injury to 70 and 400 A with 49% ofthe filtrate passing through the large pores. The postbum total tissue filtration coefficient (Kr) increased to 2.4 times the control. Over the first 3 hr postbum, 53% of the increase in capillary filtration was attributable to increased capillary pressure and 47% to increased permeability. We conclude that the early rapid edema formation following thermal injury is the result of marked increases in both capillary filtration pressure and filtration through large nonsieving pores. 0 1987 Academic Press, Inc.
of early rapid edema formation. Among these are increased interstitial osmotic pressure in burned tissue and increased intravascular hydrostatic pressure caused by arteriolar dilatation or venoconstriction [5]. While previous studies have reported an almost complete loss of the sieving capacity of the capillaries for large proteins and dextrans [3], the permeability lesion has not been characterized in terms of sieving pores or of the protein osmotic reflection coefficient. The reflection coefficient determines the fraction of theoretical protein osmotic pressure that is actually manifested across the capillary membrane, either to prevent fluid movement into the tissue or to reabsorb excess fluid from the tissue. The purpose of this study was to quantify the sieving properties of the peripheral capillaries before and
INTRODUCTION
Although edema formation after thermal injury has been extensively studied, the process of early rapid edema formation remains poorly understood. In experimental scald burns maximum edema is present at 18 hr with 80% of the total edema present at 4 hr [ 11. A major factor responsible for the edema formation after thermal injury is the marked increase in microvascular permeability which occurs almost immediately after injury. However, increased permeability persists for at least 90 hr after injury [2] and the rate of early edema formation appears to exceed that expected from increased permeability alone [3, 41. Therefore, other factors in addition to increased permeability have been proposed to explain the phenomenon 693
0022-4804187 $1.50 Copyxigbt Q 1987 by Academic Fnx, Inc. All rights of reproduction in any form reserved.
694
JOURNAL
OF SURGICAL
RESEARCH:
after scald injury in terms of reflection coefficients and transcapillary equivalent pore sizes and also to obtain specific measurements of capillary filtration pressure and the longitudinal distribution of vascular resistance along the capillary. METHODS
Surgical Preparation Mongrel dogs were anesthetized with sodium pentobarbital (30 mg/kg, iv), intubated, and mechanically ventilated with room air. A cannula was placed in the jugular vein and directed centrally to withdraw blood and infuse additional pentobarbital as needed. The femoral artery and vein were exposed in the thigh and cannulated through small side branches. The left paw was shorn and the dog was placed on its right side. The lateral saphenous vein was exposed just above the ankle and a prenodal lymphatic adjacent to the saphenous vein was cannulated with PE-50 tubing and ligated distally. Cannulas in the femoral artery and vein were connected to calibrated pressure transducers (Statham Model P23) and pressures were recorded (Grass Model 7D polygraph). The ankle was secured to a stand 10 cm above the operating table and pressure transducers were zeroed at this height. The paw was mechanically flexed 60 times a minute to maintain constant lymph flow, Lymph flow (Qr) was measured by timing the meniscus movement in calibrated micropipets. Lymph and central venous blood were collected in plastic tubes with dipotassium EDTA as an anticoagulant. Samples were centrifuged and lymph (Cr) and plasma (C,) total protein concentrations were measured using a calibrated protein refractometer (American Optical). Group I-Permeability
Analysis
In eight dogs, lymph was collected until Qr and C, reached steady-state levels, usually 60-90 min. Venous pressure was then ele-
VOL.
42, NO.
6, JUNE
1987
vated in steps by tightening a tourniquet placed around the thigh underneath the femoral artery, vein, and nerve. Qr and C&Z, were measured every 30 min until C,/Cr reached a minimum value that remained unchanged in two successive determinations. It was found that the minimal Cr/C, was obtained at venous pressures near 35 mm Hg and in subsequent experiments venous pressure was elevated from baseline to near 35 mm Hg in one step. After the minimal C&r was obtained for the control period (usually after 2-3 hr), the paw was immersed to the ankle in 100°C water for 10 sec. Qr and Cr/Cr were measured 10 and 30 min postburn and then every 30 min until 3 hr postbum. Plasma and lymph samples were saved for gradient gel electrophoresis using polyacrylamide gels (PAA 4/30, Pharmacia) and scanned for UV absorbance (Helena Kwick Scan) as previously described [6]. Six protein fractions were identified and sized according to their relative migration distances. The six protein fractions were: I (37 A), II (40 A), III (44 A), IV (53 A), V (100 A), and VI (120 A). Group II-Hemodynamic
Analysis
Three animals were similarly prepared except that venous outflow was not restricted. The saphenous vein was cannulated through a small side branch, heparin ( 10,000 units, iv) was administered, and the saphenous vein was ligated just above the ankle. Saphenous vein blood was shunted through a blood flow transducer (Carolina Medical Electronics, Model EP-300, $ in. i.d.) and blood flow was measured with a calibrated square wave electromagnetic flowmeter (Carolina Medical Electronics, Model PM-50 1). Capillary pressure (PC) was measured by the venous occlusion technique. This technique has been previously demonstrated to show good correlation with capillary filtration pressure in hindlimb preparations [7]. The measurements were obtained by rapid occlusion of saphenous vein outflow which causes a rapid
PITT ET AL.: CAPILLARY
rise in venous pressure to equal capillary pressure after which pressure rises more slowly due to filling of the more compliant capillaries. The inflection point of this pressure curve is equal to capillary pressure. Total vascular resistance (&) was calculated from hindpaw blood flow (QJ, femoral artery pressure (PA), and saphenous vein pressure (Pv) as PA - pv RT= ,
QB
precapillary
resistance was calculated as PA - PC RA = ____ QB
and postcapillary
’
resistance was calculated as
Rv =
PC - pv
QB
After the experiment, the contralateral paw was disarticulated at the ankle and weighed. It has previously been determined [8] that 55% of the dog paw consists of soft tissue (skin and muscle) and resistance, blood flow, and lymph flow were normalized to 100 g of nonburned soft tissue. The experimental protocol was approved by the Institutional Animal Care and Use Committee of the University of South Alabama College of Medicine. Permeability
695
PRESSURE AND PERMEABILITY
Analysis
The reflection coefficient (a) was used to assess microvascular permeability. It is a measure of a membrane’s ability to restrict the passage of protein molecules relative to water and is a surface-area-independent coefficient describing the selectivity of the microvascular membrane. d equals zero for a membrane which is freely permeable to a protein molecule and one if the protein is totally reflected. Granger and Taylor have shown that at high filtration rates, CL/C, approaches a minimal value equal to 1 - u [9]. Therefore, this minimal CL/C, obtained by elevating venous pressure can be used to estimate d by the formula
u = 1 - C&p. Total Tissue Filtration
(1)
Coejicient
Microvascular filtration is determined by the balance of Starling forces that are related to the microvascular filtration rate (Jv) by the following equation, JV
= Kf[(pC
-
Pi)
-
dTc
-
Ti)l,
t2)
where PC and Pi are the capillary and interstitial hydrostatic pressures, 7rcand ?Tiare the capillary and interstitial oncotic pressures, u is the osmotic reflection coefficient, and Kf is the capillary filtration coefficient. If lymph flow is assumed to equal the microvascular filtration rate, lymph oncotic pressure (I=) is assumed to equal interstitial oncotic pressure, and the influence of tissue pressure is ignored. Kf can be calculated by the following equation:
QL
Kf= PC
-
Or(ac
-
TLTL) .
(3)
For the purpose of this calculation, c was calculated from the minimal CL/CP ratios obtained in the Group I experiments using Eq. (l), QL and PC were obtained from the Group II experiments, and ‘1~~and rL were calculated from the total protein concentrations in lymph and plasma obtained in Group II experiments by using the Navar equation [ lo]: a(mm Hg) = 1.4C + 0.22C2 + 0.005C3.
(4)
It should be noted that the Krcalculated from Eq. (3) is the total tissue filtration coefficient since it includes the tissue conductance as well as the conductance of the capillary membrane. K;s calculated from lymph flow data are usually lo-20 times less than those directly calculated from weight gain transients [ 111. Eflect of Increased Capillary on Capillary Filtration
Pressure
The contribution of increased microvascular pressure to microvascular fluid flux can
696
JOURNAL
OF SURGICAL
RESEARCH:
be calculated by subtracting filtration secondary to increased permeability (Jv t permeability) from total filtration (Jv total) using Eq. (2):
JV t pressure = JV total - JV t permeability. (5)
JV 7 permeability
can be calculated by substituting the preburn control capillary pressure (Pc control) for the measured capillary pressure (Pc measured) for each set of postburn data. Thus,
JV total - JV t permeability = Kr[(Pc measured - Pi) - a(~~ - ri)] - &[(Pc Rearranging
control - Pi) - c(?T, - xi)].
(6)
yields
JV total - JV t permeability
VOL.
42, NO.
6, JUNE
1987
Davis equation [ 131. The large pore curve was subtracted from the 1 - u values for the smaller molecules to correct for filtration of the smaller molecules through the large pores and the resulting 1 - u values were then fit with a theoretical small pore curve. The intercept for each curve is the relative contribution of each set of pores to the total membrane conductance. The relative hydraulic conductance through the small (FS) and large (FL) pores was used to calculate the ratio of large to small pore areas (A,/&) and numbers (NJ&) by assuming Poiseuille flow through the pores and using the following equations: AL/&
=
(rilr%f'dFd
NLIK
=
kWk)@flri).
Statistical Analysis
= Kr(Pc measured - PCcontrol),
(7)
JV t pressure = Kf(PC measured - PC control).
(8)
and
Thus the relative contribution of increased capillary pressure to microvascular filtration depends only upon the filtration coefficient and the change in capillary pressure.
Data in the text, tables, and figures are presented as means + SE. Comparisons between pre- and postburn data were performed by one-way analysis of variance (ANOVA) and the Newman-Keuls test. P < 0.01 was accepted as significant for the permeability analysis group (n = 8) and P < 0.05 was accepted as significant for the hemodynamic analysis group (n = 3). RESULTS
Estimation of Equivalent Pore Dimensions and Filtration Fractions Samples of lymph and plasma obtained immediately before and 2 hr postburn were analyzed using gradient gel electrophoresis, and the concentrations of six endogenous protein fractions with radii of 37,40,44, 53, 100, and 120 A were measured. The osmotic reflection coefficient (a) for each protein fraction was calculated. The graphical analysis of Renkin et al. [ 121 was used to analyze the data. For all six protein fractions, 1 - d was plotted as a function of molecular size. The larger molecules (fractions IV, V, and VI) were fit by a theoretical large pore curve which was generated using the Drake and
Figures la and 1b show the effect of the burn on lymph flow (QL) and the minimal lymph to plasma total protein concentration ratio (CL/C,) in the elevated venous pressure experiments (Group I) used for the permeability analysis. CL/C, was significantly increased at 10 min postburn and continued to increase until 2 hr postburn. Lymph flow increased in a biphasic pattern with a transient peak at 10 min followed by a slower rate of increase during the remainder of the experimental period. That the total protein WC, increased from 0.13 f 0.0 1 (means f SE) to 0.55 & 0.03 despite a fivefold increase in lymph flow indicates a marked increase in microvascular permeability.
PITT ET AL.: CAPILLARY
600
0 1
2
-
b
04
I P ; Burn 1:
697
PRESSURE AND PERMEABILITY
3
I al k Burn s ”
Time (hours)
1
2
3
Time(hours)
FIG. 1. Effect of thermal injury on lymph flow (Qi,) and the lymph to plasma total protein concentration ratio (C&r) in the elevated venous pressure experiments. Values are means -CSE. (*) P < 0.01.
Since venous pressure was elevated to obtain the filtration-rate-independent minimal CL/C, prior to injury and was kept constant throughout the experiments, the osmotic reflection coefficient (a) could be calculated for total protein and six endogenous protein fractions. Table 1 indicates the minimal CJCr’s and the calculated reflection coefficients for total protein and the six protein fractions prior to injury and at 2 hr postbum. There was a marked decrease in u for all protein fractions identified. In particular, the decrease in (T for the largest fractions (V and VI) with molecular radii of 100 and 120 A
TABLE
indicates the development of very large pores after thermal injury. Figures 2a and 2b are logarithmic plots of 1 - d as a function of molecular radius for the six protein fractions. The solid lines indicate the theoretical small and large equivalent pore curves that best fit the data for prebum (Fig. 2a) and two hr postburn (Fig. 2b) lymph. The postburn microvascular endothelium is described by two populations of equivalent pores with radii of 70 and 400 ii compared to 50 and 300 ii for the prebum controls. Table 2 summarizes the calculated small (rs) and large (I~) pore radii and the
1
B
G/G Fraction Total protein I II III IV V VI
Preburn 0.13 0.17 0.15 0.15 0.12 0.08 0.07
f * + + f f f
0.01 0.02 0.02 0.02 0.01 0.01 0.01
Note. Values are means + SE. *P < 0.01
2 hr Postbum 0.55 0.64 0.60 0.53 0.45 0.37 0.34
f f + + f f +
0.03* 0.03* 0.05* 0.04* 0.03* 0.06* 0.07*
Prebum
2 hr Postbum
Molecular radius (A)
0.87 0.83 0.85 0.85 0.88 0.92 0.93
0.45 0.36 0.40 0.48 0.55 0.59 0.64
37 40 44 53 100 120
698
JOURNAL
OF SURGICAL
RESEARCH: VOL. 42, NO. 6, JUNE 1987
1.00 I--
0.80 0.00
b
l.
.
-\ 0.20 O’(O: ‘:
4001
FIG. 2. Plot of 1 - (r (closed circles) versus molecular radius for the six protein fractions identified. Data are for prebum (a) and 2 hr postbum (b) lymph. The analysis predicts prebum pore radii of 50 and 300 A and postbum pore radii of 70 and 400 A. The large pores are responsible for 49% of the postbum capillary filtration compared to 13% prior to injury.
small (Fs) and large (FL) pore filtration fractions as well as the ratios of small to large pore areas (As/&) and numbers (NJ&). Large and small pore radii increased after injury and there was a 3.8-fold increase in the filtration fraction (F’) passing through the large pores. This was accompanied by an 86% decrease in the ratios for both pore area (As/&) and number (Ns/N,). Because of the increase in large pore numbers and the filtration rate, total filtration through the large nonsieving pores (calculated as QL X FL) increased from 9.8 to 177.8 &min, an 18.1-
fold increase. At the same time filtration through the small pore population (calculated as QL X Fs) increased from 65.3 to 185.1 &min, or only 2.8-fold. Figure 3 shows the effect of injury on arterial (PA), venous (Pv), and capillary (Pc) pressures in the Group II experiments. Capillary pressure increased from 24.0 -t 2.0 mm Hg prior to injury to a maximum of 47.7 f 5.6 mm Hg at 30 min and remained significantly elevated at two hr postburn. Arterial pressure was unchanged while venous pressure was significantly elevated only at 10 min postbum.
TABLE 2 SUMMARYOFPOREANALYSISRESULTS
Prebum 2 hr postbum
50 A 70A
3OOA 400A
0.86
0.13
0.51
0.49
240 33
8300 1200
Note. rs and rL are the small and large pore radii. Fs and FL are the small and large pore filtration fractions. As /AL and N&/N,. are the ratios of small to large pore areas and numbers.
PITT
01
ET
AL.:
CAPILLARY
PRESSURE
4
aL E Burn s
1
3
2 Time (hours)
FIG. 3. Effect ofthermal
injury
on arterial
(P"), and capillary
(PC) pressures in Group means + SE. (*) P < 0.05.
(PA), venous II. Values are
Figure 4 illustrates the effect of injury on blood flow (&), total vascular resistance (Rr), pre- (RA) and postcapillary (R,) resis-
699
PERMEABILITY
tance, and the ratio of pre- to postcapillary resistance (R,J&). Blood flow increased to a maximum of 2.2 times the control at 30 min and then gradually declined to a value of 1.5 times the control at 3 hr. The increase in blood flow was accompanied by a decrease in the total vascular resistance to 46% of the control. The fall in RT was entirely secondary to a decrease in precapillary resistance since RA fell to 38% of the control while Rv was unchanged. This resulted in a 74% decline in the ratio of pre- to postcapillary resistance. Figure 5 illustrates that, as expected, lymph flow and the lymph to plasma total protein concentration ratio increased after injury in the Group II experiments. CL/C, increased from 0.32 f 0.05 to a maximum of 0.75 f 0.13 at 3 hr postburn while lymph flow increased from 37.7 f 8.2 to 338.0 f 22.1 pl/min/ 100 g. Because venous pres-
0
:: Time (hours)
8
11 ‘R”
0
AND
E tBurn 0s
t ’
1
2
Burn
5 E s
Burn
1
2
3
Time (hours)
0
3
Time (hours)
FIG. 4. Effect of thermal postcapillary (Rv) resistance, + SE.(*) P < 0.05.
i 5 $ s
injury on blood flow (Qr,), total and the ratio of pre- to postcapillary
I 1
2
3
Time (hours)
vascular resistance (RT), pre- (&), and resistance in Group II. Values are means
700
JOURNAL
OF SURGICAL
RESEARCH:
VOL. 400-
42, NO.
6, JUNE
b
Time (hod
lime
FIG. 5. Effect of thermal injury on lymph flow (QL) and the lymph ratio (C,/Cr) in Group II. Values are means + SE. (*) P < 0.05.
sure was not elevated by outflow restriction in this group, the control and postburn CJCr’s are higher while the control and postburn lymph flows are lower than in Group I. Figure 6 shows the time course of the change in the total tissue filtration coefficient as calculated from Eq. (3). & increased from a preburn value of 0.0049 to 0.012 ml/min/ mm Hg/lOO g at 3 hr, a 2.4-fold increase. Figure 7 is a plot of the effect of thermal injury on the capillary filtration rate and illustrates the contribution of increased capillary pressure (Jv t pressure) to total filtration (Jv total) as calculated from Eq. (8). At 30 min postburn, increased capillary pressure was responsible for 69% of the increase in
1987
to plasma
total protein
(hours)
concentration
capillary filtration. This declined to 28% at 3 hr. By integrating the curve in Fig. 7, it was determined that increased capillary pressure was responsible for 53% of the increase in capillary filtration in the first 3 hr postburn. DISCUSSION
Postbum paw lymph is characterized by a loss of sieving of the large molecular weight protein fractions in plasma so that lymph concentrations approach those of plasma. The appearance of large gaps in postcapillary venules has been described in studies of leak sites with fluorescent dextrans within 10 to 350 -
Jv Total
300. ,015
250. I
JvtPemmaMny
g $
200. 150.
r 100. 50.
9Llrnt cmlrd Burna 1’ ’ 2 ’ 3’ s TIME (hours)
FIG. 6. Effect of thermal injury on the total tissue filtration coefficient (Kr) as calculated from Eq. (3).
FIG. 7. Relative contribution of increased capihary pressure and increased permeability to total capillary filtration in the first 3 hr postbum as calculated from Es. (8).
PITT ET AL.: CAPILLARY
PRESSURE AND PERMEABILITY
15 min after bum injury [3]. A later phase of injury peaks 2 to 3 hr after burn and is characterized by large gaps between the endothelial cells of capillaries as well as venules [ 141. In the present study, this loss of sieving capacity was specifically described in terms of two equivalent pore populations. These pores increased in size from 50 and 300 A to 70 and 400 A but there was also a significant shift of microvascular filtration to the larger pores. Fully 49% of the microvascular filtrate passed through the large 400 A pores after injury compared to 13% prior to injury. The postbum increase in permeability was also described by an increase in the total tissue filtration coefficient to 2.4 times the control. The prebum total tissue filtration coefficient of 0.0049 ml/min/mm Hg/lOO g as calculated from Eq. (3) based on lymph flow data is close to the value of 0.0043 ml/min/ mm Hg/ 100 g which can be calculated from the data in Chen et al. [8]. This compares to values for the capillary filtration coefficient of 0.035 ml/min/mm Hg/lOO g in the cat hindpaw [5] and 0.028 ml/min/mm Hg/lOO g in the dog hindpaw [8]. We observed a sustained increase in the total tissue capillary filtration coefficient over the first 3 hr postbum. Arturson and Mellander [5] observed only a transient increase in the capillary filtration coefficient with a return to near control values at 1 hr postbum. This discrepancy may be due to differences in the severity of the burn injury, 75°C for 20 set as compared to 100°C for 10 set in the present study. The increase in blood flow to the burn wound is consistent with previous studies using labeled microspheres [ 15, 161. This increase in flow was the result of a decrease in precapillary resistance to 38% of the control since arterial pressure and postcapillary resistance remained unchanged. The decrease in precapillary resistance also contributed to the increase in postbum capillary pressure. Arturson and Mellander [5] also observed a decrease in the postburn total vascular resistance but since capillary pressures were not measured, the impact of the marked de-
701
crease in precapillary resistance was not appreciated. In the present study, the increase in capillary pressure was responsible for 53% of the increased capillary filtrations over the first 3 hr postburn. Although the rate of edema formation was not directly measured, the observation that capillary pressures were beginning to return to prebum levels at 3 hr (a period that corresponds to the phase of rapid edema formation) supports the hypothesis that a transient increase in capillary pressure in the face of a sustained increase in permeability is responsible for the early rapid edema formation after experimental scald bums. Considering the high microvascular pressure and low protein reflection coefficients observed here, a postulated increase in interstitial osmotic pressure secondary to the release of cell components is not required to explain the observed filtration rates and could not exert a significant osmotic pull across the damaged capillary membrane. Increased permeability alter bum injury is thought to be caused by the effects of direct heat damage or by mediators released from injured tissue. Pharmacological interventions designed to block the effects of these potential mediators have usually been met with only modest success [ 171. To our knowledge, no previous studies have attempted to determine the impact of changes in capillary pressure on postbum edema formation. The data from the present study indicate that this impact is substantial and this may explain why interventions designed to attenuate increases in permeability have had such limited effects on early postbum edema formation. REFERENCES Demling, R. H., Mazess, R. B., Witt, R. M., and Wolberg, w. H. The study of bum wound edema using dichromatic absorptiometry. J. Trauma 18: 124, 1978. Ryan, P., Katz, A., Lalonde, C., and Demling, R. H. Effect of microvascular hydrostatic pressure and local prostanoid production on early and late postbum edema formation. JBCR 7: 15, 1986. Arturson, G. Microvascular permeability to macro-
702
4. 5. 6.
7. 8. 9.
10.
JOURNAL
OF SURGICAL
RESEARCH: VOL. 42, NO. 6, JUNE 1987
molecules in thermal injury. Actu Physiol. &and. 463: 111, 1979. Leape, L. L. Initial changes in bums: Tissue changes in burned and unburned skin of rhesus monkeys. .Z Trauma 15: 969, 1970. Arturson, G., and Mellander, S. Acute changes in capillary filtration and diffusion in experimental bum injury. Acta Physiol. &and. 62: 457, 1964. Perry, M. A., Navia, C. A., Granger, D. N., Parker, J. C., and Taylor, A. E. Calculation of equivalent pore radii in dog hindpaw capillaries using endogenous lymph and plasma proteins. Microvasc. Res. 26: 250, 1983. Korthuis, R. J., Granger, D. N., and Taylor, A. E. A new method for estimating capillary pressure. Amer. J Physiol. 246: H880, 1985. Chen, H. I., Granger, H. J., and Taylor, A. E. Interaction of capillary, interstitial, and lymphatic forces in the canine hindpaw. Circ. Rex 39: 245, 1976. Granger, D. N., and Taylor, A. E. Exchange of macromolecules across the microcirculation. In E. M. Renkin and C. C. Michel (Eds.), Handbook ofphysiology, Sect. 2: The Cardiovascular System. Bethesda, MD: Amer. Physiol. Sot., 1984. Vol. 4, Part 1, p. 467. Navar, P. D., and Navar, L. G. Relationship between colloid osmotic pressure and plasma protein
concentration in the dog. Amer. J. Physiol. 233: H295, 1977. 11. Taylor, A. E., Rippe, B., Townsley, M. I., Korthuis, R., and Parker, J. C. Interstitial lung tissue fluid resistance. In D. G. Garlick and P. I. Komer (Eds.), Frontiers in Physiological Research. Canberra: Australian Academy of Science, 1984. P. 147. 12. Renkin, E. M., Watson, P. D., Sloop, C. H., Joiner, W. M., and Curry, F. E. Transport pathways of fluid and large molecules in microvascular endothelium of dog’s paw. Microvasc. Res. 15: 205, 1977. 13. Drake, R., and Davis, E. A corrected equation for the calculation of reflection coefficients. Microvasc. Res. 15:259, 1978. 14. Cotran, R. S. The delayed and prolonged vascular leakage in inflammation. II. An electron microscope study of the vascular response after thermal injury. Amer. J. Pathol. 46: 589, 1965. 15. Kramer, G. C., Wells, C. H., and Hilton, J. G. Blood flow and blood circulation spaces in the acute second degree bum wound. Fed. Proc. 37: 3 15, 1978. 16. Owen, D. A. A., and Fair&ton, H. E. Inflammation and the vascular changes due to thermal injury in rat hindpaws. Agents Actions 6: 622, 1976. 17. Demling, R. H. Bums. In N. C. Staub and A. E. Taylor (Eds.), Edema. New York: Raven Press, 1984. P. 579.
OF SURGICAL
RESEARCH
42,693-702
(1987)
Analysis of Altered Capillary Pressure and Permeability
after Thermal
R. M. PITT, M.D., J. C. PARKER, PH.D., G. J. JURKOVICH, A.E.TAYLoR,PH.D.,ANDP.W.CURRERI,M.D. Departments
of Physiology
and Surgery,
College
of Medicine,
University
of South Alabama,
Injury
M.D., Mobile,
Alabama
Presented at the Annual Meeting of the Association for Academic Surgery, Washington, DC, November 5-8, 1986 In order to investigate the effectsof thermal injury on microvascular hemodynamics and penneability, hindpaw arterial (PA), venous (Pv), and capillary (Pc) pressures, blood (Qa) and lymph (QL) flows, and lymph (C,) and plasma (Cr.) total protein concentrations were measured before and for 3 hr after a IO-set 1OO’C scald bum in 11 dogs. Prior to injury in eight experiments (Group I-permeability analysis) venous pressure was elevated by outflow restriction until the minimal C&r was obtained. In three experiments (Group II-hemodynamic analysis) outflow was not restricted. Lymph and plasma protein fractions ranging in size from 37 to 120 8, were measured using gradient gel electrophoresis and capillary equivalent pore sizes were calculated. In the early postbum period, Pc increased from 24 -C2 (mean +_SE) to 47 -+ 5 mm Hg (P < 0.05) and precapillary resistance (R,+,)decreased from 6.6 * 0.2 to 2.5 f 0.2 mm Hg/ml/min/lOO g (P < 0.05) while postcapillary resistance (R,) remained unchanged. Pre- to postcapillary resistance (R,/RV) fell by 74%. The reflection coefficient for total proteins (calculated as c = 1 - C&Z,) decreased from 0.87 + 0.0 1 to 0.45 ? 0.02 (P < 0.0 1). Permeability ofthe postbum capillary endothelium was described by using two populations of equivalent pores. Prebum pore radii were 50 and 300 A with 13% of the capillary filtrate passing through the large pores. Pore radii increased after injury to 70 and 400 A with 49% ofthe filtrate passing through the large pores. The postbum total tissue filtration coefficient (Kr) increased to 2.4 times the control. Over the first 3 hr postbum, 53% of the increase in capillary filtration was attributable to increased capillary pressure and 47% to increased permeability. We conclude that the early rapid edema formation following thermal injury is the result of marked increases in both capillary filtration pressure and filtration through large nonsieving pores. 0 1987 Academic Press, Inc.
of early rapid edema formation. Among these are increased interstitial osmotic pressure in burned tissue and increased intravascular hydrostatic pressure caused by arteriolar dilatation or venoconstriction [5]. While previous studies have reported an almost complete loss of the sieving capacity of the capillaries for large proteins and dextrans [3], the permeability lesion has not been characterized in terms of sieving pores or of the protein osmotic reflection coefficient. The reflection coefficient determines the fraction of theoretical protein osmotic pressure that is actually manifested across the capillary membrane, either to prevent fluid movement into the tissue or to reabsorb excess fluid from the tissue. The purpose of this study was to quantify the sieving properties of the peripheral capillaries before and
INTRODUCTION
Although edema formation after thermal injury has been extensively studied, the process of early rapid edema formation remains poorly understood. In experimental scald burns maximum edema is present at 18 hr with 80% of the total edema present at 4 hr [ 11. A major factor responsible for the edema formation after thermal injury is the marked increase in microvascular permeability which occurs almost immediately after injury. However, increased permeability persists for at least 90 hr after injury [2] and the rate of early edema formation appears to exceed that expected from increased permeability alone [3, 41. Therefore, other factors in addition to increased permeability have been proposed to explain the phenomenon 693
0022-4804187 $1.50 Copyxigbt Q 1987 by Academic Fnx, Inc. All rights of reproduction in any form reserved.
694
JOURNAL
OF SURGICAL
RESEARCH:
after scald injury in terms of reflection coefficients and transcapillary equivalent pore sizes and also to obtain specific measurements of capillary filtration pressure and the longitudinal distribution of vascular resistance along the capillary. METHODS
Surgical Preparation Mongrel dogs were anesthetized with sodium pentobarbital (30 mg/kg, iv), intubated, and mechanically ventilated with room air. A cannula was placed in the jugular vein and directed centrally to withdraw blood and infuse additional pentobarbital as needed. The femoral artery and vein were exposed in the thigh and cannulated through small side branches. The left paw was shorn and the dog was placed on its right side. The lateral saphenous vein was exposed just above the ankle and a prenodal lymphatic adjacent to the saphenous vein was cannulated with PE-50 tubing and ligated distally. Cannulas in the femoral artery and vein were connected to calibrated pressure transducers (Statham Model P23) and pressures were recorded (Grass Model 7D polygraph). The ankle was secured to a stand 10 cm above the operating table and pressure transducers were zeroed at this height. The paw was mechanically flexed 60 times a minute to maintain constant lymph flow, Lymph flow (Qr) was measured by timing the meniscus movement in calibrated micropipets. Lymph and central venous blood were collected in plastic tubes with dipotassium EDTA as an anticoagulant. Samples were centrifuged and lymph (Cr) and plasma (C,) total protein concentrations were measured using a calibrated protein refractometer (American Optical). Group I-Permeability
Analysis
In eight dogs, lymph was collected until Qr and C, reached steady-state levels, usually 60-90 min. Venous pressure was then ele-
VOL.
42, NO.
6, JUNE
1987
vated in steps by tightening a tourniquet placed around the thigh underneath the femoral artery, vein, and nerve. Qr and C&Z, were measured every 30 min until C,/Cr reached a minimum value that remained unchanged in two successive determinations. It was found that the minimal Cr/C, was obtained at venous pressures near 35 mm Hg and in subsequent experiments venous pressure was elevated from baseline to near 35 mm Hg in one step. After the minimal C&r was obtained for the control period (usually after 2-3 hr), the paw was immersed to the ankle in 100°C water for 10 sec. Qr and Cr/Cr were measured 10 and 30 min postburn and then every 30 min until 3 hr postbum. Plasma and lymph samples were saved for gradient gel electrophoresis using polyacrylamide gels (PAA 4/30, Pharmacia) and scanned for UV absorbance (Helena Kwick Scan) as previously described [6]. Six protein fractions were identified and sized according to their relative migration distances. The six protein fractions were: I (37 A), II (40 A), III (44 A), IV (53 A), V (100 A), and VI (120 A). Group II-Hemodynamic
Analysis
Three animals were similarly prepared except that venous outflow was not restricted. The saphenous vein was cannulated through a small side branch, heparin ( 10,000 units, iv) was administered, and the saphenous vein was ligated just above the ankle. Saphenous vein blood was shunted through a blood flow transducer (Carolina Medical Electronics, Model EP-300, $ in. i.d.) and blood flow was measured with a calibrated square wave electromagnetic flowmeter (Carolina Medical Electronics, Model PM-50 1). Capillary pressure (PC) was measured by the venous occlusion technique. This technique has been previously demonstrated to show good correlation with capillary filtration pressure in hindlimb preparations [7]. The measurements were obtained by rapid occlusion of saphenous vein outflow which causes a rapid
PITT ET AL.: CAPILLARY
rise in venous pressure to equal capillary pressure after which pressure rises more slowly due to filling of the more compliant capillaries. The inflection point of this pressure curve is equal to capillary pressure. Total vascular resistance (&) was calculated from hindpaw blood flow (QJ, femoral artery pressure (PA), and saphenous vein pressure (Pv) as PA - pv RT= ,
QB
precapillary
resistance was calculated as PA - PC RA = ____ QB
and postcapillary
’
resistance was calculated as
Rv =
PC - pv
QB
After the experiment, the contralateral paw was disarticulated at the ankle and weighed. It has previously been determined [8] that 55% of the dog paw consists of soft tissue (skin and muscle) and resistance, blood flow, and lymph flow were normalized to 100 g of nonburned soft tissue. The experimental protocol was approved by the Institutional Animal Care and Use Committee of the University of South Alabama College of Medicine. Permeability
695
PRESSURE AND PERMEABILITY
Analysis
The reflection coefficient (a) was used to assess microvascular permeability. It is a measure of a membrane’s ability to restrict the passage of protein molecules relative to water and is a surface-area-independent coefficient describing the selectivity of the microvascular membrane. d equals zero for a membrane which is freely permeable to a protein molecule and one if the protein is totally reflected. Granger and Taylor have shown that at high filtration rates, CL/C, approaches a minimal value equal to 1 - u [9]. Therefore, this minimal CL/C, obtained by elevating venous pressure can be used to estimate d by the formula
u = 1 - C&p. Total Tissue Filtration
(1)
Coejicient
Microvascular filtration is determined by the balance of Starling forces that are related to the microvascular filtration rate (Jv) by the following equation, JV
= Kf[(pC
-
Pi)
-
dTc
-
Ti)l,
t2)
where PC and Pi are the capillary and interstitial hydrostatic pressures, 7rcand ?Tiare the capillary and interstitial oncotic pressures, u is the osmotic reflection coefficient, and Kf is the capillary filtration coefficient. If lymph flow is assumed to equal the microvascular filtration rate, lymph oncotic pressure (I=) is assumed to equal interstitial oncotic pressure, and the influence of tissue pressure is ignored. Kf can be calculated by the following equation:
QL
Kf= PC
-
Or(ac
-
TLTL) .
(3)
For the purpose of this calculation, c was calculated from the minimal CL/CP ratios obtained in the Group I experiments using Eq. (l), QL and PC were obtained from the Group II experiments, and ‘1~~and rL were calculated from the total protein concentrations in lymph and plasma obtained in Group II experiments by using the Navar equation [ lo]: a(mm Hg) = 1.4C + 0.22C2 + 0.005C3.
(4)
It should be noted that the Krcalculated from Eq. (3) is the total tissue filtration coefficient since it includes the tissue conductance as well as the conductance of the capillary membrane. K;s calculated from lymph flow data are usually lo-20 times less than those directly calculated from weight gain transients [ 111. Eflect of Increased Capillary on Capillary Filtration
Pressure
The contribution of increased microvascular pressure to microvascular fluid flux can
696
JOURNAL
OF SURGICAL
RESEARCH:
be calculated by subtracting filtration secondary to increased permeability (Jv t permeability) from total filtration (Jv total) using Eq. (2):
JV t pressure = JV total - JV t permeability. (5)
JV 7 permeability
can be calculated by substituting the preburn control capillary pressure (Pc control) for the measured capillary pressure (Pc measured) for each set of postburn data. Thus,
JV total - JV t permeability = Kr[(Pc measured - Pi) - a(~~ - ri)] - &[(Pc Rearranging
control - Pi) - c(?T, - xi)].
(6)
yields
JV total - JV t permeability
VOL.
42, NO.
6, JUNE
1987
Davis equation [ 131. The large pore curve was subtracted from the 1 - u values for the smaller molecules to correct for filtration of the smaller molecules through the large pores and the resulting 1 - u values were then fit with a theoretical small pore curve. The intercept for each curve is the relative contribution of each set of pores to the total membrane conductance. The relative hydraulic conductance through the small (FS) and large (FL) pores was used to calculate the ratio of large to small pore areas (A,/&) and numbers (NJ&) by assuming Poiseuille flow through the pores and using the following equations: AL/&
=
(rilr%f'dFd
NLIK
=
kWk)@flri).
Statistical Analysis
= Kr(Pc measured - PCcontrol),
(7)
JV t pressure = Kf(PC measured - PC control).
(8)
and
Thus the relative contribution of increased capillary pressure to microvascular filtration depends only upon the filtration coefficient and the change in capillary pressure.
Data in the text, tables, and figures are presented as means + SE. Comparisons between pre- and postburn data were performed by one-way analysis of variance (ANOVA) and the Newman-Keuls test. P < 0.01 was accepted as significant for the permeability analysis group (n = 8) and P < 0.05 was accepted as significant for the hemodynamic analysis group (n = 3). RESULTS
Estimation of Equivalent Pore Dimensions and Filtration Fractions Samples of lymph and plasma obtained immediately before and 2 hr postburn were analyzed using gradient gel electrophoresis, and the concentrations of six endogenous protein fractions with radii of 37,40,44, 53, 100, and 120 A were measured. The osmotic reflection coefficient (a) for each protein fraction was calculated. The graphical analysis of Renkin et al. [ 121 was used to analyze the data. For all six protein fractions, 1 - d was plotted as a function of molecular size. The larger molecules (fractions IV, V, and VI) were fit by a theoretical large pore curve which was generated using the Drake and
Figures la and 1b show the effect of the burn on lymph flow (QL) and the minimal lymph to plasma total protein concentration ratio (CL/C,) in the elevated venous pressure experiments (Group I) used for the permeability analysis. CL/C, was significantly increased at 10 min postburn and continued to increase until 2 hr postburn. Lymph flow increased in a biphasic pattern with a transient peak at 10 min followed by a slower rate of increase during the remainder of the experimental period. That the total protein WC, increased from 0.13 f 0.0 1 (means f SE) to 0.55 & 0.03 despite a fivefold increase in lymph flow indicates a marked increase in microvascular permeability.
PITT ET AL.: CAPILLARY
600
0 1
2
-
b
04
I P ; Burn 1:
697
PRESSURE AND PERMEABILITY
3
I al k Burn s ”
Time (hours)
1
2
3
Time(hours)
FIG. 1. Effect of thermal injury on lymph flow (Qi,) and the lymph to plasma total protein concentration ratio (C&r) in the elevated venous pressure experiments. Values are means -CSE. (*) P < 0.01.
Since venous pressure was elevated to obtain the filtration-rate-independent minimal CL/C, prior to injury and was kept constant throughout the experiments, the osmotic reflection coefficient (a) could be calculated for total protein and six endogenous protein fractions. Table 1 indicates the minimal CJCr’s and the calculated reflection coefficients for total protein and the six protein fractions prior to injury and at 2 hr postbum. There was a marked decrease in u for all protein fractions identified. In particular, the decrease in (T for the largest fractions (V and VI) with molecular radii of 100 and 120 A
TABLE
indicates the development of very large pores after thermal injury. Figures 2a and 2b are logarithmic plots of 1 - d as a function of molecular radius for the six protein fractions. The solid lines indicate the theoretical small and large equivalent pore curves that best fit the data for prebum (Fig. 2a) and two hr postburn (Fig. 2b) lymph. The postburn microvascular endothelium is described by two populations of equivalent pores with radii of 70 and 400 ii compared to 50 and 300 ii for the prebum controls. Table 2 summarizes the calculated small (rs) and large (I~) pore radii and the
1
B
G/G Fraction Total protein I II III IV V VI
Preburn 0.13 0.17 0.15 0.15 0.12 0.08 0.07
f * + + f f f
0.01 0.02 0.02 0.02 0.01 0.01 0.01
Note. Values are means + SE. *P < 0.01
2 hr Postbum 0.55 0.64 0.60 0.53 0.45 0.37 0.34
f f + + f f +
0.03* 0.03* 0.05* 0.04* 0.03* 0.06* 0.07*
Prebum
2 hr Postbum
Molecular radius (A)
0.87 0.83 0.85 0.85 0.88 0.92 0.93
0.45 0.36 0.40 0.48 0.55 0.59 0.64
37 40 44 53 100 120
698
JOURNAL
OF SURGICAL
RESEARCH: VOL. 42, NO. 6, JUNE 1987
1.00 I--
0.80 0.00
b
l.
.
-\ 0.20 O’(O: ‘:
4001
FIG. 2. Plot of 1 - (r (closed circles) versus molecular radius for the six protein fractions identified. Data are for prebum (a) and 2 hr postbum (b) lymph. The analysis predicts prebum pore radii of 50 and 300 A and postbum pore radii of 70 and 400 A. The large pores are responsible for 49% of the postbum capillary filtration compared to 13% prior to injury.
small (Fs) and large (FL) pore filtration fractions as well as the ratios of small to large pore areas (As/&) and numbers (NJ&). Large and small pore radii increased after injury and there was a 3.8-fold increase in the filtration fraction (F’) passing through the large pores. This was accompanied by an 86% decrease in the ratios for both pore area (As/&) and number (Ns/N,). Because of the increase in large pore numbers and the filtration rate, total filtration through the large nonsieving pores (calculated as QL X FL) increased from 9.8 to 177.8 &min, an 18.1-
fold increase. At the same time filtration through the small pore population (calculated as QL X Fs) increased from 65.3 to 185.1 &min, or only 2.8-fold. Figure 3 shows the effect of injury on arterial (PA), venous (Pv), and capillary (Pc) pressures in the Group II experiments. Capillary pressure increased from 24.0 -t 2.0 mm Hg prior to injury to a maximum of 47.7 f 5.6 mm Hg at 30 min and remained significantly elevated at two hr postburn. Arterial pressure was unchanged while venous pressure was significantly elevated only at 10 min postbum.
TABLE 2 SUMMARYOFPOREANALYSISRESULTS
Prebum 2 hr postbum
50 A 70A
3OOA 400A
0.86
0.13
0.51
0.49
240 33
8300 1200
Note. rs and rL are the small and large pore radii. Fs and FL are the small and large pore filtration fractions. As /AL and N&/N,. are the ratios of small to large pore areas and numbers.
PITT
01
ET
AL.:
CAPILLARY
PRESSURE
4
aL E Burn s
1
3
2 Time (hours)
FIG. 3. Effect ofthermal
injury
on arterial
(P"), and capillary
(PC) pressures in Group means + SE. (*) P < 0.05.
(PA), venous II. Values are
Figure 4 illustrates the effect of injury on blood flow (&), total vascular resistance (Rr), pre- (RA) and postcapillary (R,) resis-
699
PERMEABILITY
tance, and the ratio of pre- to postcapillary resistance (R,J&). Blood flow increased to a maximum of 2.2 times the control at 30 min and then gradually declined to a value of 1.5 times the control at 3 hr. The increase in blood flow was accompanied by a decrease in the total vascular resistance to 46% of the control. The fall in RT was entirely secondary to a decrease in precapillary resistance since RA fell to 38% of the control while Rv was unchanged. This resulted in a 74% decline in the ratio of pre- to postcapillary resistance. Figure 5 illustrates that, as expected, lymph flow and the lymph to plasma total protein concentration ratio increased after injury in the Group II experiments. CL/C, increased from 0.32 f 0.05 to a maximum of 0.75 f 0.13 at 3 hr postburn while lymph flow increased from 37.7 f 8.2 to 338.0 f 22.1 pl/min/ 100 g. Because venous pres-
0
:: Time (hours)
8
11 ‘R”
0
AND
E tBurn 0s
t ’
1
2
Burn
5 E s
Burn
1
2
3
Time (hours)
0
3
Time (hours)
FIG. 4. Effect of thermal postcapillary (Rv) resistance, + SE.(*) P < 0.05.
i 5 $ s
injury on blood flow (Qr,), total and the ratio of pre- to postcapillary
I 1
2
3
Time (hours)
vascular resistance (RT), pre- (&), and resistance in Group II. Values are means
700
JOURNAL
OF SURGICAL
RESEARCH:
VOL. 400-
42, NO.
6, JUNE
b
Time (hod
lime
FIG. 5. Effect of thermal injury on lymph flow (QL) and the lymph ratio (C,/Cr) in Group II. Values are means + SE. (*) P < 0.05.
sure was not elevated by outflow restriction in this group, the control and postburn CJCr’s are higher while the control and postburn lymph flows are lower than in Group I. Figure 6 shows the time course of the change in the total tissue filtration coefficient as calculated from Eq. (3). & increased from a preburn value of 0.0049 to 0.012 ml/min/ mm Hg/lOO g at 3 hr, a 2.4-fold increase. Figure 7 is a plot of the effect of thermal injury on the capillary filtration rate and illustrates the contribution of increased capillary pressure (Jv t pressure) to total filtration (Jv total) as calculated from Eq. (8). At 30 min postburn, increased capillary pressure was responsible for 69% of the increase in
1987
to plasma
total protein
(hours)
concentration
capillary filtration. This declined to 28% at 3 hr. By integrating the curve in Fig. 7, it was determined that increased capillary pressure was responsible for 53% of the increase in capillary filtration in the first 3 hr postburn. DISCUSSION
Postbum paw lymph is characterized by a loss of sieving of the large molecular weight protein fractions in plasma so that lymph concentrations approach those of plasma. The appearance of large gaps in postcapillary venules has been described in studies of leak sites with fluorescent dextrans within 10 to 350 -
Jv Total
300. ,015
250. I
JvtPemmaMny
g $
200. 150.
r 100. 50.
9Llrnt cmlrd Burna 1’ ’ 2 ’ 3’ s TIME (hours)
FIG. 6. Effect of thermal injury on the total tissue filtration coefficient (Kr) as calculated from Eq. (3).
FIG. 7. Relative contribution of increased capihary pressure and increased permeability to total capillary filtration in the first 3 hr postbum as calculated from Es. (8).
PITT ET AL.: CAPILLARY
PRESSURE AND PERMEABILITY
15 min after bum injury [3]. A later phase of injury peaks 2 to 3 hr after burn and is characterized by large gaps between the endothelial cells of capillaries as well as venules [ 141. In the present study, this loss of sieving capacity was specifically described in terms of two equivalent pore populations. These pores increased in size from 50 and 300 A to 70 and 400 A but there was also a significant shift of microvascular filtration to the larger pores. Fully 49% of the microvascular filtrate passed through the large 400 A pores after injury compared to 13% prior to injury. The postbum increase in permeability was also described by an increase in the total tissue filtration coefficient to 2.4 times the control. The prebum total tissue filtration coefficient of 0.0049 ml/min/mm Hg/lOO g as calculated from Eq. (3) based on lymph flow data is close to the value of 0.0043 ml/min/ mm Hg/ 100 g which can be calculated from the data in Chen et al. [8]. This compares to values for the capillary filtration coefficient of 0.035 ml/min/mm Hg/lOO g in the cat hindpaw [5] and 0.028 ml/min/mm Hg/lOO g in the dog hindpaw [8]. We observed a sustained increase in the total tissue capillary filtration coefficient over the first 3 hr postbum. Arturson and Mellander [5] observed only a transient increase in the capillary filtration coefficient with a return to near control values at 1 hr postbum. This discrepancy may be due to differences in the severity of the burn injury, 75°C for 20 set as compared to 100°C for 10 set in the present study. The increase in blood flow to the burn wound is consistent with previous studies using labeled microspheres [ 15, 161. This increase in flow was the result of a decrease in precapillary resistance to 38% of the control since arterial pressure and postcapillary resistance remained unchanged. The decrease in precapillary resistance also contributed to the increase in postbum capillary pressure. Arturson and Mellander [5] also observed a decrease in the postburn total vascular resistance but since capillary pressures were not measured, the impact of the marked de-
701
crease in precapillary resistance was not appreciated. In the present study, the increase in capillary pressure was responsible for 53% of the increased capillary filtrations over the first 3 hr postburn. Although the rate of edema formation was not directly measured, the observation that capillary pressures were beginning to return to prebum levels at 3 hr (a period that corresponds to the phase of rapid edema formation) supports the hypothesis that a transient increase in capillary pressure in the face of a sustained increase in permeability is responsible for the early rapid edema formation after experimental scald bums. Considering the high microvascular pressure and low protein reflection coefficients observed here, a postulated increase in interstitial osmotic pressure secondary to the release of cell components is not required to explain the observed filtration rates and could not exert a significant osmotic pull across the damaged capillary membrane. Increased permeability alter bum injury is thought to be caused by the effects of direct heat damage or by mediators released from injured tissue. Pharmacological interventions designed to block the effects of these potential mediators have usually been met with only modest success [ 171. To our knowledge, no previous studies have attempted to determine the impact of changes in capillary pressure on postbum edema formation. The data from the present study indicate that this impact is substantial and this may explain why interventions designed to attenuate increases in permeability have had such limited effects on early postbum edema formation. REFERENCES Demling, R. H., Mazess, R. B., Witt, R. M., and Wolberg, w. H. The study of bum wound edema using dichromatic absorptiometry. J. Trauma 18: 124, 1978. Ryan, P., Katz, A., Lalonde, C., and Demling, R. H. Effect of microvascular hydrostatic pressure and local prostanoid production on early and late postbum edema formation. JBCR 7: 15, 1986. Arturson, G. Microvascular permeability to macro-
702
4. 5. 6.
7. 8. 9.
10.
JOURNAL
OF SURGICAL
RESEARCH: VOL. 42, NO. 6, JUNE 1987
molecules in thermal injury. Actu Physiol. &and. 463: 111, 1979. Leape, L. L. Initial changes in bums: Tissue changes in burned and unburned skin of rhesus monkeys. .Z Trauma 15: 969, 1970. Arturson, G., and Mellander, S. Acute changes in capillary filtration and diffusion in experimental bum injury. Acta Physiol. &and. 62: 457, 1964. Perry, M. A., Navia, C. A., Granger, D. N., Parker, J. C., and Taylor, A. E. Calculation of equivalent pore radii in dog hindpaw capillaries using endogenous lymph and plasma proteins. Microvasc. Res. 26: 250, 1983. Korthuis, R. J., Granger, D. N., and Taylor, A. E. A new method for estimating capillary pressure. Amer. J Physiol. 246: H880, 1985. Chen, H. I., Granger, H. J., and Taylor, A. E. Interaction of capillary, interstitial, and lymphatic forces in the canine hindpaw. Circ. Rex 39: 245, 1976. Granger, D. N., and Taylor, A. E. Exchange of macromolecules across the microcirculation. In E. M. Renkin and C. C. Michel (Eds.), Handbook ofphysiology, Sect. 2: The Cardiovascular System. Bethesda, MD: Amer. Physiol. Sot., 1984. Vol. 4, Part 1, p. 467. Navar, P. D., and Navar, L. G. Relationship between colloid osmotic pressure and plasma protein
concentration in the dog. Amer. J. Physiol. 233: H295, 1977. 11. Taylor, A. E., Rippe, B., Townsley, M. I., Korthuis, R., and Parker, J. C. Interstitial lung tissue fluid resistance. In D. G. Garlick and P. I. Komer (Eds.), Frontiers in Physiological Research. Canberra: Australian Academy of Science, 1984. P. 147. 12. Renkin, E. M., Watson, P. D., Sloop, C. H., Joiner, W. M., and Curry, F. E. Transport pathways of fluid and large molecules in microvascular endothelium of dog’s paw. Microvasc. Res. 15: 205, 1977. 13. Drake, R., and Davis, E. A corrected equation for the calculation of reflection coefficients. Microvasc. Res. 15:259, 1978. 14. Cotran, R. S. The delayed and prolonged vascular leakage in inflammation. II. An electron microscope study of the vascular response after thermal injury. Amer. J. Pathol. 46: 589, 1965. 15. Kramer, G. C., Wells, C. H., and Hilton, J. G. Blood flow and blood circulation spaces in the acute second degree bum wound. Fed. Proc. 37: 3 15, 1978. 16. Owen, D. A. A., and Fair&ton, H. E. Inflammation and the vascular changes due to thermal injury in rat hindpaws. Agents Actions 6: 622, 1976. 17. Demling, R. H. Bums. In N. C. Staub and A. E. Taylor (Eds.), Edema. New York: Raven Press, 1984. P. 579.