Purpose: The aim was to identify additional noninvasive tools allowing to detect and to quantify the metabolic impairment in patients with mitochondrial myopathies (MM) or McArdle's disease (McA).
Methods: Kinetics of adjustment of pulmonary oxygen uptake (V˙O2 kinetics) during transitions to constant-load moderate-intensity cycle ergometer exercise were determined on 15 MM, 8 McA, 21 patients with signs and/or symptoms of metabolic myopathy but a negative biopsy ("patient controls"; P-CTRL), and 22 healthy untrained controls (CTRL).
Results: V˙O2 kinetics were slower in MM and in McA versus P-CTRL and CTRL, slower in McA versus MM, and not significantly different between P-CTRL and CTRL. The time constants (τ) of the monoexponential function describing the V˙O2 kinetics were (X¯ ± SE) 59.2 ± 8.5 s in MM, 87.6 ± 16.4 s in McA, 36.9 ± 3.1 s in P-CTRL, and 35.4 ± 1.9 s in CTRL. In a subgroup of the patients (eight MM and seven McA), τ of V˙O2 kinetics were negatively correlated with two variables determined in a previous study (Grassi B, Marzorati M, Lanfranconi F, et al. Impaired oxygen extraction in metabolic myopathies: detection and quantification by near-infrared spectroscopy. Muscle Nerve. 2007;35:510-20): a) a muscle oxygenation index, obtained by near-infrared spectroscopy, estimating the peak capacity of skeletal muscle fractional O2 extraction; and b) V˙O2 peak.
Conclusions: In MM and McA patients, analysis of pulmonary V˙O2 kinetics during moderate-intensity exercise allows to identify and to quantify, noninvasively, the impairment of skeletal muscle oxidative metabolism. In these patients, the slower V˙O2 kinetics can be considered a marker of the impaired exercise tolerance. The present data could be useful for clinicians who need an objective, quantitative, and longitudinal evaluation of the impairment to be used in the follow-up of these patients as well as in the assessment of therapeutic interventions.
1Department of Science and Biomedical Technologies, University of Udine, Udine, ITALY; 2Department of Science and Biomedical Technologies, University of Milano, Segrate (MI), ITALY; 3Institute of Bioimaging and Molecular Physiology, CNR, Segrate (MI), ITALY; and 4Department of Neuromuscular Diseases, Istituto Neurologico "Carlo Besta" IRCCS, Milano, ITALY
Address for correspondence: Bruno Grassi, M.D., Ph.D., Dipartimento di Scienze e Tecnologie Biomediche, Università degli Studi di Udine, Piazzale M. Kolbe 4, I-33100 Udine, Italy; E-mail: firstname.lastname@example.org.
Submitted for publication December 2008.
Accepted for publication April 2009.