Relationship between work rate and oxygen uptake in mitochondrial myopathy during ramp-incremental exercise

We determined the response characteristics and functional correlates of the dynamic relationship between the rate (Δ) of oxygen consumption (VO₂) and the applied power output (work rate = WR) during ramp-incremental exercise in patients with mitochondrial myopathy (MM). Fourteen patients (7 males, age 35.4 ± 10.8 years) with biopsy-proven MM and 10 sedentary controls (6 males, age 29.0 ± 7.8 years) took a ramp-incremental cycle ergometer test for the determination of the VO₂ on-exercise mean response time (MRT) and the gas exchange threshold (GET). The ΔVO₂/ΔWR slope was calculated up to GET (S₁), above GET (S₂) and over the entire linear portion of the response (S_T). Knee muscle endurance was measured by isokinetic dynamometry. As expected, peak VO₂ and muscle performance were lower in patients than controls (P < 0.05). Patients had significantly lower ΔVO₂/ΔWR than controls, especially the S₂ component (6.8 ± 1.5 vs 10.3 ± 0.6 mL·min^-1·W^-1, respectively; P < 0.001). There were significant relationships between ΔVO₂/ΔWR (S_T) and muscle endurance, MRT-VO₂, GET and peak VO₂ in MM patients (P < 0.05). In fact, all patients with ΔVO₂/ΔWR below 8 mL·min^-1·W^-1 had severely reduced peak VO₂ values (<60% predicted). Moreover, patients with higher cardiopulmonary stresses during exercise (e.g., higher Δ ventilation/carbon dioxide output and Δ heart rate/ΔVO₂) had lower ΔVO₂/ΔWR (P < 0.05). In conclusion, a readily available, effort-independent index of aerobic dysfunction during dynamic exercise (ΔVO₂/ΔWR) is typically reduced in patients with MM, being related to increased functional impairment and higher cardiopulmonary stress.