Body composition studies in critical illness

Abstract

The systemic inflammatory response which is common to critically ill patients with major traumatic injury or severe sepsis precipitates a metabolic response which leads to rapid and marked derangements in body composition. Our laboratory, designed to measure total body protein (TBP), total body fat (TBF), total body and extracellular water (TBW and ECW), and total body potassium (TBK) in these patients while under the care of the intensive care unit (ICU), has characterized these derangements over a 3-week period from soon after admission to the ICU. Of the 33 patients studied, 21 were admitted to the ICU with blunt injury and 12 with peritonitis secondary to perforation of an abdominal viscus. Body composition assessments were carried out using in vivo neutron activation analysis, dual-energy X-ray absorptiometry, tritium and bromide dilution, and whole-body counting as soon as patients were haemodynamically stable. These measurements were repeated at regular intervals for 21 days. Massive TBP loss occurred which was derived principally from proteolysis of skeletal muscle in the early catabolic phase with later contribution from visceral organs. An aggressive approach to nutritional support of these patients ensured that TBF was unchanged over the course of the study. Trauma patients had retained >6 L and sepsis patients >12 L of resuscitative fluids, mainly in the extracellular compartment, by the time they were haemodynamically stable. After this time diuresis occurred, but in elderly patients (>60 years) the period of ECW expansion was markedly prolonged compared to younger patients (<40 years). The time course of changes in TBK, a measure of the cellular mass of the body, followed a very similar profile to that of TBP. The catabolic features of trauma and sepsis were very similar reflecting the impact of a systemic inflammatory response which is initiated by quite different insults.