The Link between Hypermetabolism and Hypernatremia in Severely Burned Patients

Abstract

Hypernatremia is common in critical care, especially in severely burned patients. Its occurrence has been linked to increased mortality. Causes of hypernatremia involve a net gain of sodium or a loss of free water. Renal loss of electrolyte-free water due to urea-induced osmotic diuresis has been described as causative in up to 10% of hypernatremic critical ill patients. In this context, excessive urea production due to protein catabolism acts as major contributor. In severe burn injury, muscle wasting occurs as result of hypermetabolism triggered by ongoing systemic inflammation. In this retrospective study, severely burned patients were analysed for the occurrence of hypernatremia and subsequent signs of hypermetabolism. The urea: creatinine ratio-as a surrogate for hypermetabolism-sufficiently discriminated between two groups. Four of nine hypernatremic burn patients (44%) had a highly elevated urea: creatinine ratio, which was clearly associated with an increased urea production and catabolic index. This hypermetabolism was linked to hypernatremia via an elevated urea- and reduced electrolyte-fraction in renal osmole excretion, which resulted in an increased renal loss of electrolyte-free water. In hypermetabolic severely burned patients, the electrolyte-free water clearance is a major contributor to hypernatremia. A positive correlation to serum sodium concentration was shown.

Keywords: burn; electrolyte-free water clearance; hypermetabolism; hypernatremia.

Figure 1

Day of NaS incline dependent group analysis presented as daily median and interquartile range of (a) Urea:Creatinine ratio (b) Catabolic index (c) Urea generation rate (d) Urine osmolality ratio (urea:not urea) (e) Sum of urine sodium and potassium concentrations and (f) electrolyte-free water clearance (EFWC).

Figure 2

ICU-day dependent group analysis presented as daily median and interquartile range from admission to day 25 (day of latest NaS peak) of (a) Energy expenditure (b) Calorie intake (c) Protein intake (d) Nitrogen balance (e) Fluid balance and (f) Sodium balance. Vertical lines indicate median day of NaS peak (solid) and beginning of NaS incline (dotted). * incomplete values due to partially extubated patients.

Figure 3

Correlation between EFWC and NaS during hypernatremic phase in (a) group high and (b) group low.