1. A symptomatic diuresis, polyuria, sometimes accompanies paroxysmal tachycardias in man. A study was undertaken in patients with inducible supraventricular tachycardia, irrespective of the symptom of polyuria, to measure simultaneous changes in urine output and plasma hormone levels during the arrhythmia.

2. During 14 episodes of induced tachycardia in 10 patients, there was a significant increase in mean urine flow, which was independent of the state of patient hydration. During a mean tachycardia duration of 30 ± 3 min, the mean group urine flow increased from 3.2 ± 2.3 ml/min to 7.6 ± 3.7 ml/min (P<0.001). This was characterized by a consistent increase in free water excretion and a variable increase in sodium excretion.

3. The plasma atrial natriuretic peptide level (five patients) significantly increased from 7.6 ± 4.6 pg/ml to 34.6 ± 21.7 pg/ml (P<0.02) during the arrhythmia. There was no significant change in the plasma argininevasopressin level and a non-significant reduction in plasma renin activity.

4. This study has shown that induced supraventricular tachycardias in man are accompanied by a significant diuresis. The mechanisms responsible remain unknown, but do not appear to involve the plasma arginine-vaso-pressin level alone.

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