The spectrum of exercise tolerance in mitochondrial myopathies: a study of 40 patients.
Level 4 - case-series / case-control
Case-control cross-sectional physiological study
PubMed 12538407 · doi:10.1093/brain/awg028
What was done
Evaluated oxidative capacity along with circulatory and ventilatory responses during maximal cycle exercise in 40 patients with biochemically or molecularly defined mitochondrial myopathy and varying exercise tolerance. Responses were compared against healthy sedentary control individuals. Measured outcomes included peak work capacity, peak oxygen uptake (VO2), peak systemic arteriovenous oxygen difference (a-vO2), cardiac output response relative to VO2 (DeltaQ/DeltaVO2), ventilatory response (peak VE/VO2), and skeletal muscle mutant mtDNA heteroplasmy levels.
What was found
Compared with healthy controls, patients with mitochondrial myopathy had significantly lower mean peak work capacity (0.88 +/- 0.6 vs. 2.2 +/- 0.7 W/kg, P < 0.01; range 0.17-3.2 W/kg) and peak VO2 (16 +/- 8 vs. 32 +/- 7 ml/kg/min, P < 0.01; range 6-47 ml/kg/min). Peak systemic a-vO2 difference was reduced in patients (mean 7.7 +/- 3.5 ml/dl, range 2.7-17.6 vs. 15.2 +/- 2.1 ml/dl in controls) and correlated linearly with peak VO2 (r2 = 0.69). Patients showed exaggerated circulatory (mean DeltaQ/DeltaVO2 = 15.0 +/- 13.6 vs. 5.1 +/- 0.7) and ventilatory responses (mean peak VE/VO2 = 65 +/- 24 vs. 41.2 +/- 7.4, P < 0.01). DeltaQ/DeltaVO2 and peak VE/VO2 were negatively and exponentially related to peak systemic a-vO2 difference (r2 = 0.92 and r2 = 0.53, respectively). In patients with heteroplasmic mtDNA mutations, muscle mutant mtDNA proportion was inversely correlated with peak oxygen extraction (r2 = 0.70).
Why it matters
This paper demonstrates that exercise limitation in mitochondrial myopathy is driven by impaired peripheral oxygen extraction rather than central delivery deficits, and that hyperdynamic circulatory and ventilatory compensations scale exponentially with the severity of oxidative impairment and mtDNA mutation burden.
Limits
The abstract does not state the sample size, age, or sex distribution of the healthy control group. The cross-sectional design cannot assess longitudinal disease progression or track physiological changes over time. Results are reported collectively without disaggregating outcomes across specific biochemical defect subgroups or individual gene mutations in the abstract.
Cited by
- supports Patients with mitochondrial myopathies exhibit an exaggerated cardiac output response to exercise, increasing cardiac output by 10 to 20 liters per liter of oxygen uptake compared to normal values of around 5 liters.