24-Hour protein, arginine and citrulline metabolism in fed critically ill children - A stable isotope tracer study

Abstract

Background & aims: The reference method to study protein and arginine metabolism in critically ill children is measuring plasma amino acid appearances with stable isotopes during a short (4-8 h) time period and extrapolate results to 24-h. However, 24-h measurements may be variable due to critical illness related factors and a circadian rhythm could be present. Since only short duration stable isotope studies in critically ill children have been conducted before, the aim of this study was to investigate 24-h appearance of specific amino acids representing protein and arginine metabolism, with stable isotope techniques in continuously fed critically ill children.

Methods: In eight critically ill children, admitted to the pediatric (n = 4) or cardiovascular (n = 4) intensive care unit, aged 0-10 years, receiving continuous (par)enteral nutrition with protein intake 1.0-3.7 g/kg/day, a 24-h stable isotope tracer protocol was carried out. L-[ring-²H₅]-phenylalanine, L-[3,3-²H₂]-tyrosine, L-[5,5,5-²H₃]-leucine, L-[guanido-¹⁵N₂]-arginine and L-[5-¹³C-3,3,4,4-²H₄]-citrulline were infused intravenously and L-[¹⁵N]-phenylalanine and L-[1-¹³C]leucine enterally. Arterial blood was sampled every hour.

Results: Coefficients of variation, representing intra-individual variability, of the amino acid appearances of phenylalanine, tyrosine, leucine, arginine and citrulline were high, on average 14-19% for intravenous tracers and 23-26% for enteral tracers. No evident circadian rhythm was present. The pattern and overall 24-h level of whole body protein balance differed per individual.

Conclusions: In continuously fed stable critically ill children, the amino acid appearances of phenylalanine, tyrosine, leucine, arginine and citrulline show high variability. This should be kept in mind when performing stable isotope studies in this population. There was no apparent circadian rhythm.

Clinical trial register: NCT01511354 on clinicaltrials.gov.

Keywords: 24-Hour pattern; Amino acid metabolism; Circadian rhythm; Critical illness; Pediatric; Protein metabolism.

Figure 1. Study design

Study design of 24-hour stable isotope tracer protocol, as used in 8 critically ill children. Black triangles indicate blood samples, the white arrow indicates administration of tracer prime.

Figure 2. Individual tracer enrichments and plasma amino acid concentrations over 24-hour

Measurements over 24-hour are shown for 8 critically ill children individually per panel. Time is shown in clock hours (e.g. 8 hours = 8 am), values above 24 represent the next day. Within every panel, the upper graph shows plasma amino acid concentrations, the lower graph shows stable isotope tracer enrichments with IV tracers on the left Y-axis and EN tracers on the right Y-axis. The mode of nutrition delivery is shown in the right upper corner of the panel. AA, amino acids; [Arg], plasma arginine concentration; Arg2, L-[¹⁵N₂]-arginine; [Cit], plasma citrulline concentration; Cit5, L-[¹³C-²H₄]]-citrulline (Cit5); EN, enteral; [Leu], plasma leucine concentration; Leu1, L-[¹³C]-leucine; Leu3, L-[²H₃]-leucine; [Phe], plasma phenylalanine concentration; Phe1, L-[¹⁵N]-phenylalanine; Phe5, L-[ring-²H₅]-phenylalanine; [Tyr], plasma tyrosine concentration; Tyr2, L-[²H₂]-tyrosine.

Figure 3

Figure 4. Individual phenylalanine kinetics over 24-hour

Measurements over 24-hour are shown for 8 critically ill children individually per panel. Time is shown in clock hours (e.g. 8 hours = 8 am), values above 24 represent the next day. AA kinetics are shown on the left Y-axis, AA concentration on the right Y-axis. AA, amino acid; [Arg], plasma arginine concentration; [Cit], plasma citrulline concentration; de novo Arg, de novo arginine synthesis; NO synth, NO synthesis; RaArg, rate of appearance of arginine; RaCit, rate of appearance of citrulline.

Figure 5. 24-hour whole body protein balance in 8 critically ill children

Measurements over 24-hour are shown for 8 critically ill children, every line represents one patient. Time is shown in clock hours (e.g. 8 hours = 8 am), values above 24 represent the next day.