Muscle-derived Amino Acids in Sepsis

Rationale: Skeletal muscle wasting is a common and major problem in the intensive care unit. The resulting ICU-acquired weakness is associated with increased mortality/morbidity, prolonged weaning from ventilator support and persistent functional impairment lasting far beyond the time of ICU discharge. It is conceived by many that muscle protein breakdown during critical illness is a phylogenetic phenomenon resulting in enhanced release of amino acids into the circulation to serve as substrate for central protein synthesis (acute phase response) or rapidly dividing cells including immune cells. However, the actual metabolic fate of amino acids released during muscle breakdown has been poorly characterized in critically ill patients. By tracing amino acids derived from protein breakdown, we aim to better characterize changes in AA kinetics in ICU patients and gain insight in the (patho)physiology and drivers of protein catabolism in sepsis.

Objective: To study the metabolic fate of amino acids derived from muscle protein breakdown in septic patients.
Study design: non-therapeutic interventional cohort study.

Study population: 21 patients with sepsis admitted to the ICU, 18 - 75 years old. Patients are eligible if they have an arterial catheter and a central venous catheter in the femoral vein.
Intervention: Patients participating in this study will receive a primed continuous intravenous infusion with non-radioactive stable isotope tracers after a 6 hour fast. 2H5 phenylalanine, 2H2 tyrosine and 13C glutamine will be infused for 6.5 hours. Both arterial and femoral venous blood samples will be collected through indwelling catheters and analyzed for tracer enrichments. Repeated bedside ultrasound of the leg will be performed to quantify muscle loss over time on the day 1, 5, 7 and 12.

Main study parameters/endpoints: Main study parameters will be protein breakdown and glutamine release from the leg (assessed by a two-pool model using AV leg gradients), albumin synthesis assessed by 2H5 phenylalanine incorporation. In addition we will study the incorporation of 13C in urea, glucose and citrate cycle intermediates such as citrate, fumarate, malate in peripheral leukocytes as a marker of glutamine utilization by the immune system. Moreover we will measure 13CO2 enrichment in exhaled air as a measure of total glutamine oxidation. Finally we will quantify muscle loss due to sepsis using repeated ultrasound measurements of the leg muscles.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Stable isotopes are non-radioactive molecules, which have been extensively used in both healthy and critically ill patients. Phenylalanine, tyrosine and glutamine are amino acids that are part of normal nutritional formulas used in the ICU and will not pose any risk to the participants. In addition, the intravenous infusion will be performed using a tracer dosage, ensuring visualization but no metabolic alteration of the substrate metabolites. Both infusion and sampling will take place through indwelling catheters, meaning no additional punctures or line placements are required. The total amount of blood sampled will be approximately 72 ml (~1.0% of total circulating volume). This amount of blood sampling has been proven safe in previous studies in ICU patients and is unlikely to pose any risk or burden to the participant. Ultrasound measurements of the leg pose no harm or risk to the participant.

We hypothesize that muscle protein breakdown (i.e. amino acid release) is related to increased glutamine utilization by immune cells, gluconeogenesis and synthesis of acute phase proteins.

Plasma will be sampled before tracer infusion, when a steady state is reached (after 1,5 hours) every hour until 5 hours after steady state conditions.
Muscle ultrasound will be performed on day 1, 5, 7 and 12 of participation

Patients participating in this study will receive a primed continuous intravenous infusion with non-radioactive stable isotope tracers after a 6 hour fast. 2H5 phenylalanine, 2H2 tyrosine and 13C glutamine will be infused for 6.5 hours. Both arterial and femoral venous blood samples will be collected through indwelling catheters and analyzed for tracer enrichments. Repeated bedside ultrasound of the leg will be performed to quantify muscle loss over time on the day 1, 5, 7 and 12.