Abstract
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.
Highlights
The rate (Δ) of change in oxygen consumption ( O2) relative to power output during rampincremental exercise depends on the ability of the cardiocirculatory system to deliver, and the muscle capacity to extract, O2 from arterial blood [1,2]
This study has provided novel evidence that a readily available index of “aerobic efficiency” during ramp-incremental cycle ergometer exercise (Δ O2/ΔWR) is typically reduced in patients with Mitochondrial myopathy (MM), in the supra-gas exchange threshold (GET)
The Δ O2/ΔWR was associated with worsening maximal and submaximal ( O2GET) markers of aerobic impairment and reduced peripheral muscle performance
Summary
The rate (Δ) of change in oxygen consumption ( O2) relative to power output (work rate, WR) during rampincremental exercise depends on the ability of the cardiocirculatory system to deliver, and the muscle capacity to extract, O2 from arterial (capillary) blood [1,2]. For instance, have shown that a flatter-than-normal Δ O2/ΔWR is a sensitive marker of abnormal O2 delivery and utilization in a number of systemic pathological conditions ranging from congestive heart failure to systemic lupus erythematosus [6,7,8,9,10,11]. Patients with MM commonly present hyperventilatory and hypercirculatory patterns of response to dynamic exercise [16,17,18,19,20]. These abnormalities are likely to be related to the fundamental pathophysiological abnormality in MM, i.e., impaired O2 extraction.
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