Proteinuria after burn injury

Innate Immune Activity in Glomerular Podocytes

Glomerular podocytes are specialized in structure and play an essential role in glomerular filtration. In addition, podocyte stress can initiate glomerular damage by inducing the injury of other glomerular cell types. Studies have shown that podocytes possess the property of immune cells and may be involved in adaptive immunity. Emerging studies have also shown that podocytes possess signaling pathways of innate immune responses and that innate immune responses often result in podocyte injury. More recently, mitochondrial-derived damage-associated molecular patterns (mtDAMPs) have been shown to play a critical role in a variety of pathological processes in cells. In the present mini-review, we summarize the recent advances in the studies of innate immunity and its pathogenic role in podocytes, particularly, from the perspective of mtDAMPs.

Circulating Mitochondrial DAMPs Are Not Effective Inducers of Proteinuria and Kidney Injury in Rodents

Mitochondria in eukaryotic cells are derived from bacteria in evolution. Like bacteria, mitochondria contain DNA with unmethylated CpG motifs and formyl peptides, both of which have recently been shown to be damage associated molecular patterns (DAMPs) and induce immune response and cell injury. Based on the facts that circulating mitochondrial DAMPs (mtDAMPs) are increased in the patients of trauma or burn injury who also have proteinuria, that mtDAMPs can activate immune cells which in turn secrete glomerular permeability factors, that renal intrinsic cells express a variety of DAMP receptors, and that mtDAMPs can directly increase endothelial cell permeability in vitro, we hypothesized that mtDAMPs may be novel circulating factors inducing proteinuria and kidney injury. We tested this hypothesis by directly injecting mtDAMPs into rodents and examining urinary protein and kidney histology. We prepared mtDAMP samples, including mitochondrial DNA (mtDNA) and mitochondrial debris (MTD), from rodent liver. In mice, injection of mtDNA for 20 μg/ml initial concentration in circulation (much higher than the clinical range), did not cause any renal manifestations. However, an increased dose leading to 45 μg/ml initial concentration in circulation resulted in a transient, slight increase in urinary albumin. In rats, MTD injection resulting in 450 μg/ml initial concentration of MTD protein in circulation, which was much higher than the clinical range, caused mild, transient proteinuria and lung lesions. Multiple injections of such large amount of either mtDNA or MTD into rodents on 3 consecutive days also failed in inducing proteinuria and kidney injury. In summary, clinical levels of circulating mtDAMPs do not induce proteinuria and clinically irrelevant high levels of mtDAMPs cause only a transient and slight increase in urinary protein in rodents, suggesting that circulating mtDAMPs may not be responsible for the proteinuria and kidney injury in patients with trauma, burn injury, and other diseases.

The utility of microalbuminuria measurements in pediatric burn injuries in critical care

PURPOSE

Microalbuminuria, as measured by urinary albumin-creatinine ratios (ACRs), has been shown to be a marker of systemic inflammation and an indicator of the potential severity of trauma and critical illness. Severe pediatric burns represent the best model in which to investigate the clinical utility of microalbuminuria. This study aims to ascertain whether ACR measurements have any role in predicting the severity or the intensive care requirements in the critically unwell pediatric burn population.

MATERIALS AND METHODS

A retrospective observational study was undertaken within a regional burn center with a dedicated 8-bed burn intensive care unit (ICU). This looked at 8 years of consecutive pediatric burns requiring intensive care support-a total of 63 patients after exclusions. Daily urinary ACR measurements were acquired from all patients.

RESULTS

All patients had greater than or equal to 1 ACR measurement out with the reference range, and only 8% (5/63) presented to the ICU with a normal ACR. The median day for the peak ACR measurement was day 4. The relative lack of mortalities (3/63) precluded adequate correlations between ACR and outcomes. Peak and mean ACR values correlate well with length of ICU stay, and the peak ACR also correlates with total length of hospital stay and severity of burn injury as measured by total body surface area burnt and number of organ systems requiring support. No significant differences were found when the patients were stratified by age. The peak ACR measurement was found to be independently predictive of the length of the ICU stay. As such, we have created a predictive model to prove that an ACR that remains less than 12 mg/mmol is predicative of an ICU stay of less than or equal to 7 days.

CONCLUSIONS

The clinical utilities of ACR measurements are demonstrated by their correlation with the severity of injury, length of ICU stay, and requirements for multiple organ support. Albumin-creatinine ratios raised over certain thresholds highlight to the clinician the need for closer observation and the potential deterioration of patients.

Microalbuminuria in acute burn injury

Microalbuminuria is a known finding in inflammatory states. We hypothesized that urinary albumin/creatinine ratio (ACR) would correlate with injury severity and resuscitation demands after acute burns. This pilot study evaluated 30 adults admitted within 12 hours of injury with burns > or =10% total body surface area burn injury (TBSA). The urinary ACR was calculated for each patient at 7 to 12 hours, 19 to 24 hours, and 43 to 48 hours following burn injury. Microalbuminuria was defined as a urinary ACR > or =20 mg/g. Study patients (23 males, 7 females) had a mean age of 42.9 + 14.0 years and a median TBSA burn injury of 18.8%. Inhalation injury was present in 10 of the study patients, and all patients with inhalation injury had microalbuminuria at the time of admission. One study patient died. Median time from burn injury to resuscitation was 30 hours, and the median fluid requirement was 4.2 ml/kg/%TBSA. Microalbuminuria was not uniformly present in burn-injured patients during the first 48 hours after injury. ACR values early in the hospital course correlated with higher lactate concentrations early after burn injury. However, ACR correlated with neither injury severity nor resuscitation demands after burn injury during any studied time range. Microalbuminuria does not have apparent clinical utility in burn-injured patients, and other markers of injury severity and resuscitation demands should be sought.

Microalbuminuria: A marker of endothelial dysfunction in thermal injury

INTRODUCTION

Systemic endothelial dysfunction (SED) and capillary leak occur following severe burn. SED can be assessed as low-level albuminuria (microalbuminuria) detectable only by sensitive immunoassay. This study compared the magnitude and duration of microalbuminuria with burn surface area and associated aggravating factors.

METHODS

Serial urine specimens were collected from 2 to 36 h after injury from 43 adult burn patients with a mean total body surface area (TBSA) of 32% (range 15-68%) and during 44 episodes of wound manipulation within the same period. Urinary albumin was expressed as the albumin/creatinine ratio (ACR, normal <2.3 mg/mmol).

RESULTS

Median ACR was highest 2h after injury (12.3 range 1.8-118 mg/mmol) returning to normal within 6 h. Full thickness burns (mean 17%) showed a significant association with ACR between 3 and 7h after burn. ACR was higher for up to 8 h in the presence of inhalation injury, alcohol intoxication or accelerant (p<0.05). ACR rose within 30 min of escharotomy or wound scrubbing (p<0.01).

CONCLUSION

Severe burn produces variable SED which recurs with wound manipulation. Inhalation injury, alcohol intoxication and accelerant all showed a stronger association with SED than TBSA. Microalbuminuria provides a means of monitoring microvascular integrity during the early after injury period.

Serum albumin: touchstone or totem?

A decrease in serum albumin concentrations is an almost inevitable finding in disease states, and is primarily mediated in the acute phase by alterations in vascular permeability and redistribution. This change is not disease specific but marked changes that persist are generally associated with a poorer prognosis. Critical appraisal of long-standing practices and the availability of alternative colloid solutions have led to a reduction in albumin replacement therapy, and a widespread tolerance of lower albumin concentrations in patients. The factors determining serum albumin concentrations, their measurement and the implications of hypoalbuminaemia are reviewed. The clinical value of serum albumin measurement is discussed.

A study of renal damage in seriously burned patients

The incidence of acute renal failure (ARF) in severely burned patients ranges from 1.3 per cent to 38 per cent and this complication has always been associated with a high mortality rate, of between 73 and 100 per cent. At present the exact mechanisms responsible for the onset of this complication are not well known. In order to elucidate some of these mechanisms, 20 patients with severe burns were studied for 1 year in an attempt to assess the prevailing glomerular or tubular localization of renal damage; the organic or functional pattern of renal damage and the reliability and possible prognostic significance of some renal function indices. These included the fractional excretion of sodium (FeNa), the alpha-glucosidases, the leucine aminopeptidases (LAP) and the serum and urine beta 2-microglobulin. The incidence of ARF in the patients studied was 26 per cent and in all cases it was of polyuric type. We believe that renal damage very often remains undetected when the traditional testing methods are used and that only in some patients does it become severe enough to result in ARF. In contrast, some of the tests considered in our study are extremely useful and reveal an impairment of renal function long before it becomes clinically apparent.

Burn and trauma associated proteinuria: the role of lipid peroxidation, renin and myoglobin

Ten trauma patients and 13 burns patients were studied intensively for the first 36 h and subsequent 6 days post injury in order to investigate the mechanism of trauma and burn associated proteinuria. Burns patient's initial maximum proteinuria occurred between 4 and 8 h post injury, whilst trauma patients showed greatest proteinuria within 4 h. In both groups coexisting myoglobinuria or marked elevation of serum renin activity was not found during the first 36 h. Following admission serum lipid peroxides rose in burns patients reaching a maximum between 2 and 8 h following injury, after which levels fell rapidly; in contrast trauma patient's values were initially within normal limits, but rose sharply after 12 h with peaks occurring between 16 h and 7 days post injury. Some patients within both groups showed a recurrent protein leak 2 to 5 days post injury. The data suggest that myoglobinuria or raised renin levels do not play an important role in trauma associated proteinuria.

Proteinuria following trauma

Thirteen trauma patients admitted to a major injuries unit were classified according to their injury severity. Urinary excretion of total protein, albumin and gamma glutamyl transpeptidase (GGT) activity were assessed over the following 6 days. All patients showed an initial glomerular and tubular proteinuria during the first 24 h which subsided by the second post-trauma day. The excretion of total protein and albumin was positively correlated with injury severity. Those patients with the severest injuries showed a marked recurrent total proteinuria around days 3 to 4 post-trauma which exhibited features of a tubular lesion. The recurrent proteinuria peak coincided with peak levels of serum c-reactive protein (CRP).

Proximal renal tubular dysfunction in severe burns

Proximal renal tubular function was studied in 11 patients with severe burn injury. Creatinine clearance was normal or increased in ten patients. Fractional excretion of sodium was less than 1% in ten. Fractional excretion of uric acid and amylase were increased in all but four and two cases, respectively, while absolute clearances of lysozyme and beta 2-microglobulin were increased in all but one patient. Renal threshold phosphate concentration was reduced in four patients. Twenty-four-hour urine glucose excretion exceeded 1 g in five patients, aminoaciduria was noted in eight, and proteinuria, predominantly globulinuria, was present consistently. Metabolic acidosis was seen in one patient, and transient hypokalemia occurred in two. Abnormalities of proximal tubular function were more marked in the five patients with the greatest extent of third-degree burns who died. The cause of proximal tubular dysfunction is not clear and may be related to an adaptive response to severe injury.

Urinary protein profiles after burn injury

A study of the change of urinary protein excretion has been made in 26 patients followed from the time of admission to time of discharge from the Yorkshire Regional Burn Centre (7-200 days). Total protein, IgG and albumin (as indicators of glomerular function) and alpha 1-microglobulin, beta 2-microglobulin, retinol binding protein and N-acetyl-beta-D-glucosaminidase (as indicators of proximal tubular function) as well as the patterns on SDS polyacrylamide gel electrophoresis were examined. Patients were divided into three groups according to their urinary protein profiles. The first group was 3 patients who showed little or no evidence of proteinuria; the second group consisted of 7 patients who showed a mild and transient tubular proteinuria, all of whom recovered normal function within one week. The third group, 16 patients, developed moderate to severe proteinuria usually started as a mixed pattern of glomerular and tubular proteinuria, characterized by the excretion of a high level of total protein. After a few days, this pattern changed to a typical tubular proteinuria and the latter could last for as long as 200 days; in some of these patients a secondary increase of low molecular weight protein excretion occurred either due to a latent renal complication or induced by the administration of aminoglycosides. There is a close correlation between the intensity and patterns of the proteinuria and the severity and clinical progress of the burn.