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 (VO(2)) 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 VO(2) on-exercise mean response time (MRT) and the gas exchange threshold (GET). The ΔVO(2)/ΔWR slope was calculated up to GET (S(1)), above GET (S(2)) and over the entire linear portion of the response (S(T)). Knee muscle endurance was measured by isokinetic dynamometry. As expected, peak VO(2) and muscle performance were lower in patients than controls (P < 0.05). Patients had significantly lower ΔVO(2)/ΔWR than controls, especially the S(2) component (6.8 ± 1.5 vs 10.3 ± 0.6 mL·min(-1)·W(-1), respectively; P < 0.001). There were significant relationships between ΔVO(2)/ΔWR (S(T)) and muscle endurance, MRT-VO(2), GET and peak VO(2) in MM patients (P < 0.05). In fact, all patients with ΔVO(2)/ΔWR below 8 mL·min(-1)·W(-1) had severely reduced peak VO(2) values (<60% predicted). Moreover, patients with higher cardiopulmonary stresses during exercise (e.g., higher Δ ventilation/carbon dioxide output and Δ heart rate/ΔVO(2)) had lower ΔVO(2)/ΔWR (P < 0.05). In conclusion, a readily available, effort-independent index of aerobic dysfunction during dynamic exercise (ΔVO(2)/ΔWR) is typically reduced in patients with MM, being related to increased functional impairment and higher cardiopulmonary stress.

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 percent 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.