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: ObjectivesWe have examined the acute effects (<45 min) of aldosterone antagonism on heart rate variability and baroreflex sensitivity, markers of cardiac vagal control, in 13 healthy subjects.: BackgroundEvidence for the beneficial effects of aldosterone antagonists comes from studies showing increased survival rates following their addition to standard heart failure therapy. Many mechanisms have been suggested for this action, including effects upon the autonomic nervous system.: MethodsHeart rate variability and baroreflex sensitivity were examined 30 min following the administration of potassium canrenoate (intravenous) (aldosterone antagonist) or saline (control).: ResultsActive treatment reduced resting heart rate (−6 ± 1 beats/min [mean ± standard error mean]) compared to control (0 ± 1 beat/min) (p < 0.001) and increased measures of high frequency (HF) heart rate variability. Root mean square of successive RR interval differences increased by 21 ± 5 ms versus −6 ± 5 ms control (p < 0.001); HF power increased by 1,369 ± 674 ms2with aldosterone antagonism compared to −255 ± 431 ms2 following saline infusion (p < 0.01). Baroreflex sensitivity (alpha-HF) was increased after active treatment (+4 ± 2 ms/mm Hg vs. 0 ± 1 ms/mm Hg control [p < 0.05]). No changes in plasma potassium levels were observed.: ConclusionsThese results provide evidence that aldosterone antagonists acutely improve cardiac vagal control, irrespective of any diuretic effects, and may in part explain their beneficial effects in treatment of heart failure. [Copyright &y& Elsevier]