Amino Acid Nutrition in the Critically-ill

Primed continuous infusions of stable isotope tracers will be applied to assess dynamic changes in whole body and hepatic protein metabolism (i.e., protein breakdown, amino acid oxidation, protein synthesis, total protein, albumin and fibrinogen synthesis) before 48 hours after beginning the intervention. During the period of isotope infusion, nutrition will be held constant. A positive protein balance (difference between protein synthesis and protein breakdown) will be used as an indicator of whole body anabolism. All isotopes will be purchased from CDN Laboratories (Montreal, Canada). Sterile solutions will be tested to be free of pyrogens. Before beginning each experiment blood and expired air samples will be collected to determine baseline enrichments of [1-13C]-ketoisocaproate ([1-13C]-KIC), [6,6-2H2]glucose, L-[2H5]phenylalanine and expired 13CO2. Retention of H13CO3- in the bicarbonate pool will be measured in each patient using the approach of Kien.

This will be measured from the rates of incorporation of L-[2H5] phenylalanine into the proteins using plasma very low density lipoprotein apolipoprotein-B100 (VLDL-apoB-100) isotopic enrichment at plateau to represent the isotopic enrichment of the phenylalanine precursor pool from which the liver synthesizes the other plasma protein. A priming dose of L-[2H5]phenylalanine (4 μmol/kg, iv) will be given followed by a six-hour infusion at 0.10 μmol/kg/min. At baseline, 3 hours, 4 hours, 5 hours, and 6 hours thereafter blood will be drawn, immediately transferred into pre-chilled tubes containing Na2EDTA and a protease inhibitor cocktail of sodium azide, merthiolate and soybean trypsin inhibitor, then centrifuged and stored at -70ºC for later analysis. 10 ml blood will be required for secretory protein synthesis studies.

Blood (one 4-ml tube per sample) will be collected at baseline, then every 12 hours in the first 48 hours, and 72 hours post initiation of amino acid administration. Measures of amino acid-sensitive inflammation include the following: serum C-reactive protein and interleukin-6.

Blood (one 2.5-ml tube per sample) will be collected at baseline, then every 12 hours in the first 48 hours, and 72 hours post initiation of amino acid administration. Using Pax-gene tubes, peripheral blood mononuclear cell (PBMC) mRNA will be isolated for detection of interleukin-6 and c-reactive protein gene expression.

Blood (one 4 ml cell separator tube per sample) will be collected at baseline, then every 12 hours in the first 48 hours, and 72 hours post initiation of amino acid administration. Measures of amino acid-sensitive cell signaling in peripheral blood mononuclear cells include: phospho- p70 S6 kinase, phospho-S6, or phospho-eiF2α by Western blot.

Plasma amino acids and markers of oxidative stress (glutathione, cysteine, related sulfhydryl) by liquid chromatography tandem mass spectroscopy. Blood will be collected at baseline, then at 24, 48, and 72 hours initiation of amino acid administration.

Investigators will measure resting energy expenditure by indirect calorimetry at the 5 hour time point of each tracer protocol (Baseline and 48 h after initiation of amino acid administration. This will permit comparison of actual energy expenditure with that estimated by weight-based nomogram used for nutritional dosing.

Duration of ICU admission in days.

Duration of ICU admission in days.

Duration of stay in ICU off mechanical ventilation.

Number of hospital-acquired infections during the hospital admission.

Death in hospital (binary value for dead or alive).