Hyperthyroid Hypercalcæmia on Lithium Therapy
Report:
Sinus tachycardia 163/min
Borderline right atrial abnormality
Borderline short QT interval 0.22â
Nonspecific T wave changes
Comment:
Marked tachycardia with short QT interval gives an illusion of atrial flutter. Indeed, the patient was treated with digoxin and amiodarone for flutter, without success! She was also given lithium carbonate, as the serum level was subtherapeutic (0.2); the level rose to mildly toxic range (1.4). Serum calcium was 3.8 mmol/L! She became obtunded and hypercarbic and was transferred to ICU, where therapy was repeatedly delayed by (of all people) her Endocrinologist197!
Thyroid hormones were grossly elevated: T4 76 [8-22], T3 4.3 [1-2.4], with suppressed TSH 0.04 [0.17-2.9]. Thus she had two reasons for hypercalcæmia: hyperthyroidism and lithium therapy198. She had three reasons for obtundation: hypercalcæmia, thyroid storm itself, and the lithium toxicity.
The QT interval is often difficult to measure. The QTc obtained for the rate 163/min and QT 0.22â was 0.40â (normal) by one method and 0.367â (minimally shortened) by another (Bazettâs).
QTc = QT + 1.75(rate-60) = 400.25 msec
QTc = QT/Square root RR = 0.367 sec
ECG is not a very useful tool in this setting, although it was virtually diagnostic of thyrotoxicosis in the setting of unexplained, striking sinus tachycardia. The QRS complex is probably mildly prolonged, increasing the measured QT interval; the PR interval is similarly prolonged, making the T-P junction the end of T wave in most leads; in addition, large variety of factors may be actually prolonging the QT interval in a sick elderly patient. One such factor is thyrotoxicosis itself199.
The RAA may be spurious, since hyperthyroidism increases all ECG voltages (opposite of better known reduction of ECG amplitude in myxÅdema). At least in part, increased P wave amplitude is due to tachycardia itself.
266. 29 year old woman with hypokalæmia, hypercalcæmia and hypermagnesæmia due to near-drowning. Explain the phasic reversal of her T wave polarity
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