Glycemic control, insulin therapy, and insulin resistance in burn patients

Overview¶

Stress hyperglycemia is one of the most predictable metabolic consequences of a major burn. Hyperglycemia and insulin resistance have long been recognized in severely burned patients, and the response appears even in those who were never diabetic. ^[1] It is driven by the counter-regulatory hormonal environment of the hypermetabolic response rather than by any failure of the pancreas, and for that reason it behaves as a distinct entity rather than a simple marker of illness. ^[36]

The practical importance is that dysglycemia is not benign. Hyperglycemia is a critical determinant of worse prognoses after burn injury, associated with more infection, more sepsis, more graft loss, and higher mortality. ^[16][4][46] Insulin remains the gold-standard treatment, but the same drug that lowers glucose also produces hypoglycemia, which itself worsens outcomes. ^[20][11] Managing glucose in the burn ICU is therefore a problem of finding a target range and a monitoring strategy that capture insulin's benefit without paying its hypoglycemia cost, and of weighing newer adjunct agents that may shift that balance.

Epidemiology¶

Glucose intolerance is common rather than exceptional after a significant burn. In a prospective cohort, two-thirds of burn patients met criteria for impaired fasting glucose, impaired glucose tolerance, or diabetes, demonstrating that some degree of glucose intolerance is the rule in this population. ^[6] During acute resuscitation the burden is striking: in one series, blood glucose exceeded 140 mg/dL in 64% of measurements, and peak values surpassed 140 mg/dL in nearly every patient. ^[5]

Pre-existing diabetes is also over-represented and consequential. Patients with diabetes make up a meaningful fraction of burn admissions, and a population-linked cohort found burn survivors carried roughly twice the rate of diabetes-related hospital admissions compared with an uninjured cohort. ^[7] That elevated risk was concentrated in the first five years after discharge, which the investigators identified as a critical window for incident diabetes admissions. ^[7] Severity and host factors shape who becomes hyperglycemic: burn size, age, and body fat percentage have been associated with the area under the glucose curve, while time post-burn and lean mass were inversely associated. ^[45]

Pathophysiology¶

The hypermetabolic response to a large burn is characterized by marked and sustained increases in catecholamines, glucocorticoids, and glucagon. ^[37] These counter-regulatory hormones augment hepatic glucose production and catecholamine-mediated glycogenolysis, so the liver keeps pouring out glucose while peripheral tissues fail to take it up. ^[3] The result is a glucose-rich state that severely burned patients actively maintain because glucose is the fuel "ready to use" for wound healing and the inflammatory response. ^[44]

Insulin resistance in burns is characterized as a post-receptor signaling defect, not simply a relative insulin deficiency. In a rodent (rat) thermal-injury model, insulin-stimulated glucose transport into skeletal muscle was impaired, confirmed by reduced uptake of labeled deoxyglucose into soleus muscle strips. ^[2] In that model the lesion sat downstream of the insulin receptor: insulin-stimulated PI3-kinase activity, pivotal for GLUT4 translocation, was decreased, and tyrosine phosphorylation of the insulin receptor and IRS-1 was attenuated. ^[2] These alterations in post-receptor insulin signaling are thought to be responsible for the insulin resistance after thermal injury. ^[2]

A leading mechanistic account links this resistance to ectopic fat. The post-injury hyperglycemic response is accompanied by significant alterations in fat metabolism, and the data suggest insulin resistance relates to triglyceride storage in ectopic sites such as liver and muscle. ^[1] Deposition of triglyceride in those sites follows from increased free-fatty-acid delivery driven by catecholamine-induced lipolysis, and the resulting intracellular lipid products may in turn impair insulin signaling. ^[1] Notably, the catabolic drive is not transient: muscle catabolism is sustained long after the wound has healed, which points to mechanisms beyond wound repair. ^[39]

Assessment¶

Glucose in the burn patient is monitored to titrate insulin, but the measurement itself is a source of error. Point-of-care glucometers can read falsely high in burn patients, and treating that fictitious hyperglycemia with insulin can precipitate iatrogenic hypoglycemia and even seizures. ^[29] High-dose ascorbic acid, used by some centers in resuscitation, introduces a measurable performance bias on point-of-care glucose testing, which matters when those values drive insulin dosing. ^[38]

Beyond the single glucose value, the shape of the glucose record carries prognostic information. Glycemic variability, rather than the mean glucose level alone, is an important factor associated with sepsis and hospital mortality in critically ill patients. ^[14] In burn cohorts specifically, greater glycemic variability has been associated with increased rates of mortality and sepsis, so monitoring strategies that quantify variability add information beyond spot checks. ^[15] The feasibility ceiling is real: intensive regimens are ultimately limited by how frequently serum glucose can be measured, which is part of why continuous monitoring is an area of interest. ^[41]

Management¶

Insulin is the standard of care for the hyperglycemia associated with the hypermetabolic response. ^[20] Its rationale extends beyond glucose lowering, because in burns insulin also promotes muscle anabolism and modulates the systemic inflammatory response. ^[4] Mechanistic work confirms a direct anabolic action: extremity hyperinsulinemia significantly increased leg blood flow and the rate of muscle protein synthesis, demonstrating that insulin directly stimulates muscle protein synthesis in severely injured patients. ^[32] In a controlled study, seven days of insulin plus glucose reduced donor-site healing time from 6.5 to 4.7 days and increased collagen type IV staining, indicating these high doses could be administered safely to improve wound matrix formation. ^[33]

The central question has been how tightly to control. Early enthusiasm came from critical-care trials in which intensive insulin therapy reduced morbidity and mortality, and tight glucose control changed the way many burn centers practiced ICU care. ^[12] In burns, several studies supported a benefit from controlling glucose, though the strongest signal comes from pediatric cohorts: in severely burned children, patients held near 130 mg/dL showed attenuated hypermetabolic and inflammatory responses and significantly lower rates of infection, sepsis, and mortality than those with poor control, and intensive treatment decreased infections and sepsis. ^[8][23] On that basis, some authors suggested targeting a blood glucose near 130 mg/dL in severely burned patients, and a review concluded there is a signal that targeting roughly 130 to 150 mg/dL is beneficial for morbidity and mortality without the hypoglycemia risk of stricter targets. ^[8][12]

Because tight targets buy hypoglycemia, adjunct and alternative agents have been studied. Metformin attenuates hyperglycemia and reduces net muscle protein catabolism after burn injury, and in a randomized comparison it produced far less hypoglycemia than insulin while still increasing muscle protein synthesis. ^[19][11] A large analysis reported that treatment with metformin after burn was associated with reduced morbidity and mortality compared with insulin. ^[20] GLP-1-based therapy is the other active frontier: exenatide given to maintain glucose between 80 and 140 mg/dL achieved comparable control while cutting administered insulin from 76 to 22 units per patient per day, with no excess hypoglycemia. ^[18] Part of the appeal is a built-in safety margin: in an animal (rat) burn model, GLP-1 infusion did not drive glucose below roughly 70 mg/dL, so the hypoglycemia risk that accompanies insulin is reduced. ^[47] The current literature suggests GLP-1 agonists yield less insulin dependence with similar glucose control and hypoglycemic event rates compared with a basal-bolus regimen. ^[21] These agents sit alongside the broader pharmacologic toolkit for the hypermetabolic response, which includes propranolol, oxandrolone, and recombinant growth hormone, though those target catabolism rather than glycemia directly. ^[48]

Complications¶

The dominant iatrogenic hazard of glucose management is hypoglycemia. Insulin treatment predisposes burn patients to hypoglycemia, which increases morbidity and mortality, and in a large analysis insulin administration improved infection and sepsis outcomes but caused a four- to five-fold increase in hypoglycemia that was itself associated with a nine-fold increase in mortality. ^[3][11] Hypoglycemic episodes are not rare and correlate with injury severity and inhalation injury; patients with one or more episodes had longer hospitalization and more frequent infection, sepsis, multiple organ failure, and death. ^[10] Traditional tight-control protocols have reported severe hypoglycemia rates as high as 19%, whereas a standardized continuous-insulin algorithm significantly lowered hypoglycemia incidence. ^[30][42]

Hyperglycemia carries its own complication burden, principally infection. Burn-associated hyperglycemia leads to increased infection, with pneumonia among the most prominent; systemic glucose above 150 mg/dL was associated with higher rates of pneumonia, infection, sepsis, and longer ventilation. ^[16] Hyperglycemia was the only independent predictor of bacteremia in one cohort and also predicted pneumonia and urinary tract infection. ^[17] Daily mean glucose and measures of variability have been independently associated with proven infection, with a daily mean above 150 mg/dL showing the strongest association. ^[15] A practical corollary is diagnostic: a greater-than-25% rise in daily insulin dosing can precede other clinical signs of sepsis by up to 48 hours and may serve as an early marker. ^[31]

Special Considerations¶

Children are a distinct population in whom controlled trials exist. In a randomized trial of 239 severely burned pediatric patients, intensive insulin treatment decreased the incidence of infections and sepsis compared with controls. ^[23] Controlling glucose at or below 120 mg/dL improved insulin sensitivity and mitochondrial oxidative capacity while decreasing resting energy expenditure in severely burned children. ^[24] The metabolic derangement is also durable in children: insulin resistance secondary to the hypermetabolic stress response persists when burn wounds are at least 95% healed. ^[25]

Patients with pre-existing diabetes face worse trajectories. Diabetic burn patients had a significantly higher incidence of wound infection and severe renal impairment, longer length of stay, and more operations, and across cohorts diabetes was associated with increased total morbidity regardless of how well it had been controlled before injury. ^[27][26] Diabetes also raised the rate of amputation compared with no-diabetes and prediabetes groups in one series. ^[28] One analysis suggested the protective "obesity paradox" of higher BMI in burns may be compromised by insulin resistance. ^[43]

Outcomes and Prognosis¶

Across burn cohorts, the direction of the association is consistent: worse glycemia tracks with worse outcomes. Burn survivors experiencing complex glycemic derangements in the acute period are at significantly increased risk of worse outcomes. ^[13] Tight glycemic control in the burn ICU was associated with a more than ten-fold lower mortality than poor control in one study, while better control reduced sepsis incidence. ^[9] Phase-specific analysis adds nuance: during the flow phase, both hyperglycemic and hypoglycemic rates independently predicted mortality in severe burn patients, suggesting that reducing the dysglycemic rate in that phase could improve outcomes. ^[34]

The newest signal concerns GLP-1 receptor agonists used before injury. In recent cohort studies, prior GLP-1 receptor agonist use was associated with significantly lower mortality, fewer infectious complications, and reduced critical-care utilization, independent of BMI, with one analysis reporting 54% reduced odds of mortality and meaningful reductions in sepsis, pneumonia, and MRSA infection. ^[22][40] GLP-1 agonist exposure was associated with lower rates of readmission and mortality at both 90 days and one year post-burn, and with lower soft-tissue infection at 90 days, which remained numerically but not significantly lower at one year. ^[22] These are observational findings and have not yet been tested prospectively in burns.

Controversies and Evidence Gaps¶

The core unresolved question is how tight glucose control should be. Tight glycemic control with insulin has not consistently shown benefit in critically ill patients, and in burns specifically a tight-control protocol has yielded inconsistent mortality and morbidity outcomes. ^[35][13] Although most critical-care investigations favor intensive control, conflicting recommendations exist, and a systematic review found two studies showed a mortality benefit while two showed none, with most studies reporting higher hypoglycemia under tight control. ^[13] The same review concluded that an individualized, patient-centered approach factoring comorbidities and injury characteristics is the defensible position rather than a single universal target. ^[13]

Several deeper gaps follow. The literature on managing hyperglycemia in severely burned patients is sparse and dominated by small studies, and early intensive-control protocols in mixed ICU populations encountered increased hypoglycemic complications that later studies could not reconcile with the initial benefit. ^[9][36] A subphenotype signal has emerged: in pediatric data, the hyperinflammatory subgroup appeared to derive the mortality benefit of tight control, raising the possibility that benefit is not uniform across patients. ^[35] Whether acute glucose dysregulation matters more than a diabetes label for predicting infection is itself unsettled, with at least one cohort suggesting the acute disturbance is more important. ^[17] Finally, the adjunct agents that look most promising for sidestepping hypoglycemia, metformin and GLP-1 agonists, rest largely on observational and small-trial evidence and await prospective confirmation in burns. ^[20]

References¶

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