Electrocardiographic changes in thyrotoxic periodic paralysis

The electrocardiograms (ECGs) of 30 patients with hypokalaemic thyrotoxic periodic paralysis during and after paralysis were studied. During paralysis, typical features of hypokalaemia were seen in all patients with serum potassium levels of 2.8 mmol/l or less; above this level, the ECGs varied from non-diagnostic to those showing typical features of hypokalaemia. It was not possible to accurately predict the serum potassium level from the ECG except when either sinus arrest or heart block was present. Although extrasystoles have been reported to be common in hypokalaemia, none of the patients in this study had extrasystoles. Sinus arrest occurred in two patients and second degree atrio-ventricular block occurred in three patients, a finding which has not been reported in hypokalaemia.