A myriad of formulas exist for calculating fluid needs during burn resuscitation, suggesting that no one formula benefits all patients. The most commonly used formula, the Parkland or Baxter formula, consists of 3 to 4 mL/kg/% burn of lactated Ringer’s, of which half is given during the first 8 hours after burn and the remaining half is given over the subsequent 16 hours. The concept behind continuous fluid requirements is simple. The burn (and/or inhalation injury) drives an inflammatory response that leads to capillary leak; as plasma leaks into the extravascular space, crystalloid administration maintains the intravascular volume. Therefore, if a patient receives a large fluid bolus in a prehospital setting or emergency department, that fluid has likely leaked into the interstitium and the patient still requires ongoing burn resuscitation according to the estimates. Continuation of fluid volumes should depend on the time since injury, urine output, and mean arterial pressure (MAP). As the leak closes, the patient will require less volume to maintain these two resuscitation endpoints. Children under 20 kg have the additional requirement that they do not have sufficient glycogen stores to maintain an adequate glucose level in response to the inflammatory response. Specific pediatric formulas have been described, but the simplest approach is to deliver a weight-based maintenance IV fluid with glucose supplementation in addition to the calculated resuscitation fluid with lactated Ringer’s.
It is important to remember that any formula for burn resuscitation is merely a guideline, and fluid must be titrated based on appropriate measures of adequate resuscitation. A number of parameters are widely used to gauge burn resuscitation, but the most common remain the simple outcomes of blood pressure and urine output. As in any critically ill patient, a target MAP of 60 mmHg ensures optimal end-organ perfusion. Goals for urine output should be 30 mL/h in adults and 1 to 1.5 mL/kg/h in pediatric patients. Because blood pressure and urine output may not correlate perfectly with true tissue perfusion, the search continues for other adjunctive parameters that may more accurately reflect adequate resuscitation. Some centers have found serum lactate to be a better predictor of mortality in severe burns,34,35 and others have found that base deficit predicts eventual organ dysfunction and mortality.36,37 Because burned patients with normal blood pressure and serum lactate levels may have compromised gastric mucosal perfusion, continuous measurement of mucosal pH with its logistical difficulties has garnered limited popularity.38,39 Invasive monitoring with pulmonary artery catheters typically results in significant excessive fluid administration without improved cardiac output or preload measurements; use of invasive monitoring seems to have variable effects on long-term outcomes.40
Actual administrated fluid volumes typically exceed volumes predicted by standard formulas.41 One survey of burn centers showed that 58% of patients end up getting more fluids than would be predicted by Baxter’s formula.42 Comparison of modern-day patients with historical controls shows that over-resuscitation may be a relatively recent trend.43 One theory is that increased opioid analgesic use results in peripheral vasodilation and hypotension and the need for greater volumes of bloused resuscitative fluids.44 A classic study by Navar et al showed that burned patients with inhalation injury required an average of 5.76 mL/kg/% burn, vs. 3.98 mL/kg/% burn for patients without inhalation injury, and this has been corroborated by subsequent studies.45,46 Prolonged mechanical ventilation may also play a role in increased fluid needs.47 A recent multicenter study found that age, weight, %TBSA, and intubation on admission were significant predictors of more fluid delivery during the resuscitation period. Those patients receiving higher fluid volumes were at increased risk of complications and death.48 Common complications include abdominal compartment syndrome, extremity compartment syndrome, intraocular compartment syndrome, and pleural effusions. Monitoring bladder pressures can provide valuable information about development of intra-abdominal hypertension.
The use of colloid as part of the burn resuscitation has generated much interest over the years. In late resuscitation when the capillary leak has closed, colloid administration may decrease overall fluid volumes and potentially may decrease associated complications such as intra-abdominal hypertension.49 However, albumin use has never been shown to improve outcomes in burn patients and has controversial effects on mortality in critically ill patients.50,51 Attempts to minimize fluid volumes in burn resuscitation have included study of hypertonic solutions, which appear to transiently decrease initial resuscitation volumes, with the downside of causing hyperchloremic acidosis.52
Other adjuncts are being increasingly used during initial burn resuscitation. High-dose ascorbic acid (vitamin C) may decrease fluid volume requirements and ameliorate respiratory embarrassment during resuscitation.53 Plasmapheresis may also decrease fluid requirements in patients who require higher volumes than predicted to maintain adequate urine output and MAP. It is postulated that plasmapheresis may filter out inflammatory mediators, thus decreasing ongoing vasodilation and capillary leak.54
One recent adjunct that has found increasing utility in other surgical ICUs has been the application of bedside thoracic ultrasound.55 Ultrasound offers the potential to make rapid, noninvasive assessments during acute changes in clinical condition. For burn patients, bedside ultrasonography may be indicated for evaluation of volume status, gross assessment of cardiac function, and diagnosis of pneumothorax. Determining patient cardiac function and volume status may guide fluid resuscitation. Cardiac function can be evaluated with three common heart views: the parasternal long axis, parasternal short axis, and apical four-chamber views.56 Volume status can be estimated by examination of cardiac function, evaluation of the inferior vena cava (IVC) diameter, and changes with respiration. Ultrasound also allows timely diagnosis of pneumothorax.57 A high-frequency probe with an adequate window between ribs permits identification of lung parenchyma against the chest well. A pneumothorax appears as a transition on ultrasound between lung parenchyma, which has a heterogeneous appearance, and air, which has a hypoechoic appearance. Further studies are warranted to identify indications for the use of ultrasound in burned patients.