Comparison between the three models in their ability to account for the effects of changes in serum potassium levels on multiple measures of nerve excitability.

<p>Data from Medium K data was fitted to nerve model, and then adjusted for different potassium levels according to Models 1, 2 and 3. Discrepancies score difference between model and recorded data and discrepancy reductions score improvement over no allowance for potassium. For each data set Model 3 provides lowest discrepancy (figures in bold).</p

Multiple excitability measurements recorded from normal subjects: motor axons in the median nerve were tested at the wrist and compound muscle action potentials recorded from the abductor pollicis brevis muscle.

<p>A–C: Mean +/− SD for all 38 subjects. D–F: Comparisons between means of Lower K (grey) and Higher K (black) groups. A, D: Threshold electrotonus, <i>i.e.</i>, threshold changes during and after polarizing currents set to +40 (top), +20, −20 and −40% (bottom) of threshold. B, E: Recovery cycle showing successive phases of refractoriness, superexcitability, and late subexcitability. C,F: Current-threshold (I/V) relationship.</p

Threshold electrotonus (top row) and recovery cycle (bottom row) waveforms generated by Models 1–3 for values of extracellular potassium corresponding to the Lower K (grey) and Higher K (black) groups.

<p>Threshold electrotonus (top row) and recovery cycle (bottom row) waveforms generated by Models 1–3 for values of extracellular potassium corresponding to the Lower K (grey) and Higher K (black) groups.</p

Examples of nerve excitability measures showing significant relationship to serum potassium levels.

<p><b>A</b>: Superexcitability, <b>B</b>: TEd20(90–100 ms) threshold decrease at end of 20% depolarizing current, <b>C,D</b>: TEh20(90–100 ms) and TEh40(90–100 ms) threshold decrease at end of 20% and 40% hyperpolarizing current (NB Negative threshold decrease indicates threshold were increased by hyperpolarization).</p

Potassium dependence of 2 nerve excitability measurements predicted by Models 1 (red line), 2 (green line) and 3 (blue line) compared with mean measurements for Higher K (▴), Medium K (•) and Lower K (▪) groups, and ellipse representing 1 SD limits for 9 patients with chronic renal failure (reproduced from Kiernan et al.).⁹

<p>Only Model 3 predicts an appropriate drop in superexcitability with increasing potassium level.</p

Mean values of excitability parameters derived from the multiple measures of nerve excitability performed on the median nerve in 38 normal subjects.

<p>First column shows mean ± standard deviation (SD). Second column shows Pearson product moment correlation coefficient between excitability measure and serum potassium. Third column shows p values (* = P<0.05, ** = <i>P</i><0.01, *** = <i>P</i><0.001). SDTC: strength-duration time constant. TEd20 and TEh20: threshold electrotonus changes due to depolarizing and hyperpolarizing currents respectively, set to 20% of control threshold; TEd40, TEh40 same, but for 40% polarizing currents; expressions in square brackets indicate times after start of 100 ms current, early [10–20 ms], late [90–100 ms] or around peak threshold change [ peak]. I/V: current-threshold. RRP: relative refractory period.</p