Persistent Mitochondrial Dysfunction Linked to Prolonged Organ Dysfunction in Pediatric Sepsis

Abstract

Objectives: Limited data exist about the timing and significance of mitochondrial alterations in children with sepsis. We therefore sought to determine if alterations in mitochondrial respiration and content within circulating peripheral blood mononuclear cells were associated with organ dysfunction in pediatric sepsis.

Design: Prospective observational study SETTING:: Single academic PICU.

Patients: One-hundred sixty-seven children with sepsis/septic shock and 19 PICU controls without sepsis, infection, or organ dysfunction.

Interventions: None.

Measurements and main results: Mitochondrial respiration and content were measured in peripheral blood mononuclear cells on days 1-2, 3-5, and 8-14 after sepsis recognition or once for controls. Severity and duration of organ dysfunction were determined using the Pediatric Logistic Organ Dysfunction score and organ failure-free days through day 28. Day 1-2 maximal uncoupled respiration (9.7 ± 7.7 vs 13.7 ± 4.1 pmol O2/s/10 cells; p = 0.02) and spare respiratory capacity (an index of bioenergetic reserve: 6.2 ± 4.3 vs 9.6 ± 3.1; p = 0.005) were lower in sepsis than controls. Mitochondrial content, measured by mitochondrial DNA/nuclear DNA, was higher in sepsis on day 1-2 than controls (p = 0.04) and increased in sepsis patients who had improving spare respiratory capacity over time (p = 0.005). Mitochondrial respiration and content were not associated with day 1-2 Pediatric Logistic Organ Dysfunction score, but low spare respiratory capacity was associated with higher Pediatric Logistic Organ Dysfunction score on day 3-5. Persistently low spare respiratory capacity was predictive of residual organ dysfunction on day 14 (area under the receiver operating characteristic, 0.72; 95% CI, 0.61-0.84) and trended toward fewer organ failure-free days although day 28 (β coefficient, -0.64; 95% CI, -1.35 to 0.06; p = 0.08).

Conclusions: Mitochondrial respiration was acutely decreased in peripheral blood mononuclear cells in pediatric sepsis despite an increase in mitochondrial content. Over time, a rise in mitochondrial DNA tracked with improved respiration. Although initial mitochondrial alterations in peripheral blood mononuclear cells were unrelated to organ dysfunction, persistently low respiration was associated with slower recovery from organ dysfunction.

Figure 1.

Mitochondrial respiration and content in control and sepsis patients by study day. A, Mitochondrial respiration measured using high-resolution respirometer. Data are presented as oxygen flux in pmol O₂/s/10⁶ cells, which is directly proportional to oxygen consumption rate. B, Citrate synthase activity as nmol/min/mg cellular protein and (C) mitochondrial DNA/nuclear DNA (mtDNA/nDNA) presented as expression fold change in sepsis relative to controls. Data are presented as mean ± sd. **p < 0.01 and *p < 0.05 compared with controls after Bonferroni correction for multiple comparisons. ATP = adenosine triphosphate, ETS_max = maximal uncoupled respiration through the electron transport system, LEAK = proton leak after inhibition of adenosine triphosphate synthase, SRC = spare respiratory capacity.

Figure 2.

Association of patterns of change in mitochondrial spare respiratory capacity (SRC) with duration of organ dysfunction. A, Organ failure-free days out of 28 for the 72 sepsis patients with respiration measured at all three timepoints stratified by pattern of change of SRC, with group 1 defined as initially normal SRC without decrease over time, group 2 as normal SRC with decrease over time, group 3 as low SRC with an increase over time, and group 4 as low SRC without an increase over time (Table 1 for group definitions). Data are presented in box plot analysis with the central line indicating the median and boxes indicating the interquartile range. Group 2 had significantly fewer organ failure-free days than group 1 (p = 0.03). B, Kaplan-Meier survival analysis comparing time to resolution of all organ dysfunction (censored for discharge or death) between sepsis patients with persistently low SRC (groups 2 and 4) versus recovery of SRC (groups 1 and 3) over time. Log-rank p = 0.15.

Figure 3.

Longitudinal changes in mitochondrial DNA/nuclear DNA (mtDNA/nDNA) stratified by trajectory of spare respiratory capacity (SRC). mtDNA/nDNA values for sepsis patients on each study day stratified by persistently low versus recovery of SRC over time. There were no differences in the longitudinal change between groups (repeated measures analysis of variance p > 0.05), but mtDNA/nDNA increased significantly from day 3–5 to 8–14 in sepsis patients who recovered SRC (*p = 0.005) but not in those with a persistently low SRC (p = 0.51).