Transcapillary colloid osmotic pressures in injured and non-injured skin of seriously burned patients

Plasma volume expansion and capillary leakage of 20% albumin in burned patients and volunteers

Background

Burn injury is associated with a long-standing inflammatory reaction. The use of albumin solutions for plasma volume support is controversial because of concerns of increased capillary leakage, which could aggravate the commonly seen interstitial oedema.

Methods

In the present open controlled clinical trial, an intravenous infusion of 20% albumin at 3 mL/kg was given over 30 min to 15 burn patients and 15 healthy volunteers. Blood samples and urine were collected for 5 h. Plasma dilution, plasma albumin and colloid osmotic pressure were compared. Mass balance calculations were used to estimate plasma volume expansion and capillary leakage of fluid and albumin.

Results

The patients were studied between 4 and 14 (median, 7) days after the burn injury, which spread over 7–48% (median, 15%) of the total body surface area. The albumin solution expanded the plasma volume by almost 15%, equivalent to twice the infused volume, in both groups. The urinary excretion exceeded the infused volume by a factor of 2.5. Capillary leakage of albumin occurred at a rate of 3.4 ± 1.5 g/h in burn patients and 3.7 ± 1.6 g/h in the volunteers (P = 0.61), which corresponded to 2.4 ± 1.0% and 2.5 ± 1.2% per hour of the intravascular pool (P = 0.85). The median half-life of the plasma volume expansion was 5.9 (25th–75th percentiles 2.7–11.7) h in the burn patients and 6.9 (3.4–8.5) h in the volunteers (P = 0.56).

Conclusions

Albumin 20% was an effective volume expander in patients at 1 week post-burn. No relevant differences were found between burn patients and healthy volunteers.

Trial registration

EudraCT 2016-000996-26 on May 31, 2016.

A review of the use of human albumin in burn patients

This review article examines the use of human albumin (HA) in burn treatment. Generally, there are two scenarios where HA may be administered: acutely as a volume expander during burn shock resuscitation and chronically following resuscitation to correct hypoalbuminemia. Although colloids were the cornerstone of the earliest burn resuscitation formulas, HA was in fact rarely used. More recently however, with the recognition of fluid creep, HA usage during resuscitation has increased. Animal studies demonstrate that during acute fluid resuscitation, administration of colloids, including albumin (ALB), have no ability to arrest the formation of burn wound edema, but they do reduce edema formation in the nonburn soft tissues and help preserve intravascular volume and reduce resuscitation fluid requirements with no apparent increase in extravascular water accumulation in the lung. Human studies suggest that immediate use of ALB during acute resuscitation achieves adequate resuscitation using a lower total overall volume requirement, transiently provides better maintenance of intravascular volume and cardiac output, produces less overall edema gain than crystalloid resuscitation alone but may be associated with increased extravascular lung water accumulation during the first postburn week. However, many questions remain unanswered, and modern, large-scale prospective studies are desperately needed. Maintenance of normal serum ALB levels through continuous supplementation of HA following burn resuscitation is even less well understood. Although this approach makes physiologic sense, the limited amount of available data from human burn studies reveal that chronic ALB supplementation is expensive and may not result in any major clinical benefits. Again, modernized prospective studies are greatly needed in this area.

[Treatment of serious burns]

BACKGROUND

Treatment of patients with large burns is challenging.

MATERIAL AND METHOD

The article is based on clinical experience, and a non-systematic review in PubMed.

RESULTS

In patients with burns covering more than 10 - 15 % of the total body surface area, fluid resuscitation should be initiated early. Fluid induces edema, and facial burns may necessitate early orotracheal intubation to secure the airways. Reduced ventilation and-/or peripheral circulation due to deep burns should be managed by early escharotomy (and, more seldom, fasciotomy) at the primary hospital. Respiratory distress is most often due to vigorous fluid resuscitation, secretions, pneumonia and-/or sepsis. Fiber bronchoscopy may reveal inhalation injury and enables removal of secreted material from the airways. In the acute initial phase, hypotension is usually caused by hypovolemia. Subsequently a massive inflammatory response (SIRS) causes vasodilatation, hypotension and increased cardiac output. Wound and airway infections are common. SIRS may cause CRP levels above 100 and a body temperature of 38 - 39 degrees C, which makes it difficult to find the right time to start antibiotic treatment. Nevertheless, prophylactic use of antibiotics is not encouraged. Definitive surgery, excision and transplantation, should be performed early, preferably within the first week.

INTERPRETATION

Patients with large burns should be treated according to general principles for intensive medical care, preferably in units with special experience in treatment of burns.

Resuscitation of severe thermal injury with hypertonic saline dextran: effects on peripheral and visceral edema in sheep

BACKGROUND

Edema of tissue not directly injured by heat is a common complication after resuscitation of burn shock. Hypertonic 7.5% NaCl 6% dextran (HSD) infusion reduces early fluid requirements in burn shock, but the effects of HSD on peripheral and visceral tissue edema are not well-defined.

METHODS

We measured the microcirculatory absorptive pressures of burned and nonburned skin and tissue water content of skin and other tissues in anesthetized sheep after 70% to 85% total body surface area scald and resuscitation. Fluid infusion was initiated 30 minutes after injury using 10 mL/kg HSD (n = 11) or lactated Ringer's (LR) (n = 12), with infusion rates titrated to restore and maintain preburn oxygen delivery (DO2). Thereafter, both groups received LR infusions as needed to maintain DO2 until the study's end at 8 hours. Colloid osmotic pressure was measured in plasma, and combined interstitial colloid osmotic and hydrostatic pressures were measured in skin.

RESULTS

Both treatments successfully restored DO2, but fluid requirements were less with the HSD group than with the LR group (43+/-19 mL/kg vs. 194+/-38 mL/kg, respectively, p < 0.05). The peripheral and visceral tissue water contents at 8 hours postinjury until the end of the study in both burn groups were significantly higher than in nonburn controls. However, HSD-treated sheep had significantly less water content in the colon (less 28%), liver (less 9%), pancreas (less 55%), skeletal muscle (less 21%), and nonburned skin (less 12%) compared with LR-treated sheep (p < 0.05 for each). HSD-treated sheep maintained significantly higher (3 to 5 mm Hg) plasma colloid osmotic pressure than LR-treated sheep.

CONCLUSION

There were no observed differences in edema in burn skin between the two treatment groups. The early volume-sparing effect of HSD and reduction in tissue edema are likely attributed to an increased extracellular osmolarity and a better maintenance of the plasma oncotic pressure.

Fluid resuscitation following a burn injury: implications of a mathematical model of microvascular exchange

A validated mathematical model of microvascular exchange in thermally injured humans has been used to predict the consequences of different forms of resuscitation and potential modes of action of pharmaceuticals on the distribution and transport of fluid and macromolecules in the body. Specially, for 10 and/or 50 per cent burn surface area injuries, predictions are presented for no resuscitation, resuscitation with the Parkland formula (a high fluid and low protein formulation) and resuscitation with the Evans formula (a low fluid and high protein formulation). As expected, Parkland formula resuscitation leads to interstitial accumulation of excess fluid, while use of the Evans formula leads to interstitial accumulation of excessive amounts of proteins. The hypothetical effects of pharmaceuticals on the transport barrier properties of the microvascular barrier and on the highly negative tissue pressure generated postburn in the injured tissue were also investigated. Simulations predict a relatively greater amelioration of the acute postburn edema through modulation of the postburn tissue pressure effects.

[Is albumin administration useful in critical care for burnt patients?]

The most typical reaction of the organism after a major burn consists in transcapillary shift from plasma into interstitial space. Capillary hyperpermeability, but also changes in colloid osmotic gradient and decrease in interstitial hydrostatic pressure, explain the fluid shift to burned and, at minor importance, non burned areas during the first post-burn day. The extent of capillary hyperpermeability results in inefficiency of colloid infusions in reducing fluid shift to burned areas. Some groups advocate colloid supply from the eighth post-burn hour, on when hyperpermeability decreases, in non burned sites. However, for most groups, restoration of a functional interstitial space has priority during the 24 first post-burn hours, justifying crystalloid supply without colloids. Furthermore, colloid infusion could be responsible for delayed pulmonary oedema, in the first days following initial fluid replacement. After 24 post-burn hours, in patients experiencing severe albumin depletion, infusion of human albumin is justified, in order to favour oedema resorption.

A model of fluid resuscitation following burn injury: formulation and parameter estimation

A dynamic compartmental model is developed to describe the redistribution of fluid and albumin between the circulation and the intact and injured interstitia following burn injury in humans. Transcapillary fluid and albumin exchange is described by a coupled Starling mechanism, while the effect of the burn is represented by time-dependent perturbations to all three compartments. The unknown model parameters are determined for two groups of patients, having less than and greater than 25% total body surface area burns, by statistical fitting of model predictions to patient data from two sources. The parameters include the perturbations to the fluid filtration coefficients in uninjured and injured tissue, GkF,Tl and GkF,BT, respectively, the relaxation coefficient, r, which describes the exponential decay of the perturbations, and the exudation factor, EXFAC, which relates the protein concentration in the exudate to that in the injured tissue. Perturbations to other parameters, including the membrane permeability-surface area product and the albumin reflection coefficient in the injured and uninjured tissues, are determined based on interrelationships with GkF,Tl and GkF,BT. The values of GkF,BT, when corrected for tissue destruction and decreased post-injury perfusion, are in reasonable agreement with the limited experimental data available from the literature. The model and its parameters are further validated by comparing the simulated patient responses to the clinical data used in the parameter estimation as well as to data available from two additional sources.

Pathogenesis of edema formation in burn injuries

One of the obvious acute features of cutaneous thermal injury is the swelling of the involved tissue. This swelling is caused by a fluid shift from circulating plasma. Along with the evolution of intravenous fluid therapy in trauma and surgery, the implementation of such therapy to burn victims has improved survival. Edema generation aggravated by fluid therapy may, however, represent a source of increased morbidity. This paper presents a review of the literature on postburn edema generation, focusing mainly on fluid physiology. It is well documented that fluid is lost from the circulation into burned tissue because of a moderate increase in capillary permeability to fluid and macromolecules and a modest increase in hydrostatic pressure inside the perfusing microvessels. Recently it was discovered that a very negative interstitial pressure develops in thermally injured skin. This pressure constitutes a strong "suction" adding markedly to the edema generating effect of increased capillary permeability and pressure.

Thermal skin injury: effect of fluid therapy on the transcapillary colloid osmotic gradient

The effects of fluid therapy on interstitial colloid osmotic and hydrostatic pressures in thermally injured skin were investigated in anesthetized rats subjected to full-thickness scald burns to 40% of the body surface area and resuscitation for 3 hr by either lactated Ringer's or plasma. Interstitial fluid hydrostatic pressure (Pif) was reduced from -2 mm Hg to -20 to -40 mm Hg after injury, which will profoundly increase transcapillary filtration. Following the onset of fluid therapy, Pif increased to slightly positive values. In control, colloid osmotic pressure in plasma (COPp) was 20.6 +/- 0.4 mm Hg and in interstitial fluid (COPif) 13.7 +/- 0.3 mm Hg (means +/- SEM). The transcapillary oncotic pressure gradient (COPgrad = COPp-COPif) was 6.9 +/- 0.4 mm Hg. Following nonresuscitated thermal injury, COPp declined to 18-19 mm Hg (P less than 0.05) and COPif was reduced to 10.4 +/- 0.5 mm Hg (P less than 0.05). Fluid therapy by lactated Ringer's markedly reduced COPp (12.3 +/- 0.3 mm Hg; P less than 0.05), and COPgrad was almost abolished (2.6 +/- 0.7 mm Hg; P less than 0.05). In contrast, plasma infusion maintained COPp, whereas COPgrad increased significantly (11.1 +/- 1.2 mm Hg; P less than 0.05). Noncolloid saline solutions have been preferred for the initial fluid therapy for burns. The present study provides evidence that this will reduce both COPp and COPgrad, a situation in which edema formation will be favored.

Time-related changes in the Starling forces following extracorporeal circulation

The intra- and postoperative variations of the transcapillary forces [colloid osmotic pressure of plasma (COPpl), colloid osmotic pressure of interstitial fluid (COPif), average hydrostatic pressure in the interstitium (Pif)] were studied in the subcutaneous tissue as a function of time in 13 patients operated on for coronary artery disease using extra-corporeal circulation (ECC). The measurements were performed before operation, during ECC, and during the first 24 hours postoperatively. COPif was measured subcutaneously on the chest both by the wick method and by a noninvasive blister suction method. The latter technique allowed several consecutive measurements in the same individual during the postoperative period. Pif was measured by "wick-in-needle" technique in the same area as the COPif measurements. COPpl was measured in a blood sample collected from a cubital vein. COPpl was reduced about 50% during ECC returned to pre-ECC level within the first 6 hours postoperatively. During ECC COPif was higher than COPpl, reaching its minimum level 4 to 5 hours postoperatively. Measurements performed following ECC showed return of the transcapillary COP-gradient to the normal direction (COPpl greater than COPif). Pre-ECC level of COPif was not entirely obtained during the first postoperative day. Pif increased gradually during ECC and continued to increase the first 2 to 3 hours following ECC. Pre-ECC level was reached within 24 hours postoperatively. The present investigation has demonstrated major dynamic variations in the transcapillary forces in patients undergoing open heart surgery with ECC. There was an increased net capillary filtration (F) intraoperatively predisposing to interstitial edema formation in subcutaneous tissue until several hours following the termination of ECC.