QUANTIFICATION OF CALF OXYGENATION IN PARAPLEGIC PATIENTS DURING PASSIVE LEG MOVEMENT

Abstract

Dear Editor-in-Chief: A recent article (4) demonstrated that passive leg movement can induce both EMG activity and alteration of calf oxygenation in a paralyzed lower limb of spinal cord-injured subjects. Although these results are interesting, the presentation of the muscle oxygenation data is inadequate. For measuring calf oxygenation, Kawashima et al. (4) used a multidistance spatially resolved tissue oximeter (NIRO-300, Hamamatsu Photonics, Japan) without exploiting the offered advantage to quantify muscle oxygenation directly as tissue oxy-hemoglobin (Hb) saturation (%). Tissue oxy-Hb saturation reflects the dynamic balance between O2 supply and O2 consumption and is independent on the path length of the near-infrared photons in the muscle tissue (3). Kawashima et al. (4) report the results only as changes in oxy- and deoxy-Hb values calculated relative to the resting level, and represented in micrometers. These changes should have been eventually presented at least as Δ μM*cm, because no path-length data are available for the calf of spinal cord injury patients. On the other hand, taking into account that these patients present consistent musculoskeletal degeneration (5), it would not be accurate the use of calf path-length data from healthy subjects (2). Subcutaneous adipose tissue thickness has a substantial confounding influence on in vivo muscle near-infrared spectroscopy measurements, therefore it would be essential to incorporate this factor into muscle oximetry studies in order to justify comparisons between different muscle groups (7). Kawashima et al. (4) did not report the thickness of the subcutaneous adipose tissue at the measurement position in each spinal cord-injured subject. Consistent differences in the thickness of subcutaneous adipose tissue are expected among patients, and between patients and controls. The lack of this information could compromise the results of the comparison between spinal cord injury and control subjects. Although Kawashima et al. (4) did not quantify properly muscle oxygenation, the data shown in their Figure 4 clearly suggest that the well-known postexercise hyperemic phase (evidenced as total Hb increase with respect to the pretest level) was evidenced only in the spinal cord injury patients, and it was maintained over 5 min after the end of the protocol. In addition, as previously found in walking mitochondrial disorders and muscular dystrophy patients (1,6), muscle oxygenation is not a limiting factor during passive alternate leg movement in spinal cord-injured subjects. Valentina Quaresima Marco Ferrari Department of Sciences and Biomedical Technologies University of L'Aquila L'Aquila, Italy