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Assessment of muscle membrane properties using muscle velocity recovery cycles in patients with critical illness polyneuromyopathy

Introduction

Muscle weakness and atrophy due to critical illness polyneuromyopathy (CIPM) is common in long-stay intensive care patients. Recent nerve excitability studies suggest that the recovery cycle after a single supramaximal stimulus provides useful information about axonal membrane potential and ion channel function in neuropathies. We previously found that critical illness polyneuropathy is associated with nerve depolarization, and that this depolarization is strongly correlated with serum potassium in patients with renal failure [1]. We have adapted this method to human muscle fibres, by measuring the changes in conduction velocity of muscle fibres [2]. The muscle relative refractory period (RRP) increases and supernormality (SN) decreases in ischaemia, suggesting that these measures may be indicators of membrane potential also in the muscle [2]. The aim of this study was to evaluate muscle RRP and SN in patients with CIPM.

Methods

Nine patients (age 44 to 73 years) with electrophysiologically proven CIPM were studied on two occasions within 1 week. Multifibre responses to direct muscle stimulation through needle electrodes were recorded from the brachioradialis, and the latency changes measured as conditioning stimuli were applied at interstimulus intervals of 2 to 1,000 ms. RRP and SN were compared with an age-matched control group.

Results

In patients with CIPM, muscle RRP was abnormally prolonged (6.25 ± 2.74 ms (mean ± SD) vs. 3.27 ± 0.45 ms in healthy subjects; P = 0.0015) and supernormality was reduced (3.6 ± 3.1% vs. 9.3 ± 3.4%; P = 0.013). Moreover, during renal failure (8/18 measurements), muscle supernormality correlated strongly with serum potassium (R = 0.95, P = 0.0004).

Conclusion

Muscle fibres are depolarized in CIPM. If, as we have previously suggested, nerve membrane depolarization is an important cause of neuropathy in critical illness, it seems likely that muscle membrane depolarization may be an important cause of myopathy. Serum potassium is an important factor for muscle membrane depolarization in patients with renal failure.

References

  1. 1.

    Z'Graggen WJ, et al.: Nerve excitability changes in critical illness polyneuropathy. Brain 2006, 129: 2461-2470. 10.1093/brain/awl191

  2. 2.

    Z'Graggen WJ, et al.: Velocity recovery cycles of human muscle action potentials and their sensitivity to ischemia. Muscle Nerve 2009, in press.

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Z'Graggen, W., Brander, L., Tuchscherer, D. et al. Assessment of muscle membrane properties using muscle velocity recovery cycles in patients with critical illness polyneuromyopathy. Crit Care 13, P111 (2009). https://doi.org/10.1186/cc7275

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Keywords

  • Conditioning Stimulus
  • Serum Potassium
  • Critical Illness Polyneuropathy
  • Recovery Cycle
  • Human Muscle Fibre