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

• Main Authors: A.C. Gimenes, J.A. Neder, S. Dal Corso, C.R. Nogueira, L. Nápolis, M.T. Mello, A.S. Bulle, L.E. Nery
• Format: Article
• Language: English
• Published: Associação Brasileira de Divulgação Científica 2011-04-01
• Series: Brazilian Journal of Medical and Biological Research
• Subjects: Muscular diseases; Oxygen consumption; Cardiopulmonary exercise testing; Skeletal muscle; Pathophysiology; Mitochondrial myopathy
• Online Access: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-879X2011000400013
• ISSN: 0100-879X; 1414-431X

We determined the response characteristics and functional correlates of the dynamic relationship between the rate (&#916;) of oxygen consumption ( <img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1188">O2) 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 <img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1189">O2 on-exercise mean response time (MRT) and the gas exchange threshold (GET). The &#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1190">O2/&#916;WR 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 <img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1191">O2 and muscle performance were lower in patients than controls (P < 0.05). Patients had significantly lower &#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1192">O2/&#916;WR 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 &#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1193">O2/&#916;WR (S T) and muscle endurance, MRT-<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1194">O2, GET and peak <img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1195">O2 in MM patients (P < 0.05). In fact, all patients with &#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1196">O2/&#916;WR below 8 mL·min-1·W-1 had severely reduced peak <img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1197">O2 values (<60% predicted). Moreover, patients with higher cardiopulmonary stresses during exercise (e.g., higher &#916; ventilation/carbon dioxide output and &#916; heart rate/&#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1198">O2) had lower &#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1199">O2/&#916;WR (P < 0.05). In conclusion, a readily available, effort-independent index of aerobic dysfunction during dynamic exercise (&#916;<img border=0 width=11 height=13 src="548_files/image001.gif" v:shapes="_x0000_i1200">O2/&#916;WR) is typically reduced in patients with MM, being related to increased functional impairment and higher cardiopulmonary stress.