Abstract Background Exercise intolerance in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Long COVID (LC) is associated with impaired systemic oxygen extraction (SOE) by invasive cardiopulmonary exercise testing (iCPET), suggesting either peripheral left-to-right shunting, intrinsic skeletal muscle mitochondrial dysfunction, or both. Using iCPET and frozen skeletal muscle biopsies, we aimed to determine the contributions of each. Methods Frozen vastus lateralis needle muscle biopsies from thirty-five ME/CFS and ten LC patients demonstrating poor SOE on iCPET were analyzed for mitochondrial biomass and electron transport chain (ETC) function (Baylor Genetics Laboratory, Houston, TX). Samples were assayed for citrate synthase (CS) activity, mtDNA content, and ETC complexes I-IV activities, each expressed as percent activity relative to healthy control reference values. Mitochondrial variables were non-normally distributed and are presented as medians (IQR). Between-group comparisons used a Mann-Whitney-type approach with Cliff’s δ as an effect size, and Spearman correlations evaluated relationships between SOE and mitochondrial parameters. CS activity data from patients with genetically confirmed oxidative phosphorylation (OXPHOS) disease (n = 232) provided a comparative reference. Results For ME/CFS + LC, at peak exercise VO2 = 72.1 ± 10.7 % predicted, Qc = 95.2 ± 13.3 % predicted, pRAP = 1.5 ± 3.0 mmHg, and pSOE = 0.72 ± 0.07. ME/CFS and LC median CS activity = 66 % (47-74) and mtDNA copies= 55.5 % (43-70). Lower Complex I (ρ = -0.28, p = 0.060) and combined Complex II+III (ρ = -0.33, p = 0.028) activities correlated with worse SOE, with the latter reaching statistical significance, while CS activity and mtDNA did not (ρ = -0.10 and 0.35; both p 0.1). Compared with OXPHOS disease, ME/CFS + LC exhibited significantly lower CS activity (62 47-74 % vs 162 128-191 %; Cliff’s δ ≈ 0.95, p 0.001). Conclusions Impaired systemic oxygen extraction in ME/CFS and Long COVID is associated with diminished skeletal muscle ETC complex activity, but not with mitochondrial biomass. Although overall CS activity was lower than in genetic mitochondrial myopathies, neither CS nor mtDNA correlated with SOE, indicating that functional rather than quantitative mitochondrial defects likely underlie the impaired oxygen extraction phenotype. This abstract is funded by: Open Medicine Foundation
Squires et al. (Fri,) studied this question.