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Study objectives: This study assessed whether the antiplatelet agent cilostazol, which has potent cyclic nucleotide phosphodiesterase type-3 inhibitory activity, affects the ventricular escape rate and neurohumoral factors in patients with third-degree atrioventricular block. Design: Prospective, but nonrandomized, study. Setting: Cardiology division of an acute care hospital. Patients: We studied 12 patients with third-degree intra-His or infra-His atrioventricular block who were in functional class II or Ill of the New York Heart Association classification. None of the patients had experienced Adams-Stokes attacks. Interventions: These patients were given cilostazol orally at a dose of 200 mg daily for at least 1 week. Measurements and results: Before and after treatment with cilostazol, continuous 24-h ECG monitoring and measurement of plasma natriuretic peptide concentrations were performed. Cilostazol significantly increased the mean (± SEM) total 24-h QRS count from 57,300 ± 2,800 to 74,400 ± 3,200 beats (p = 0.001) and significantly decreased the maximum geometric mean R-R interval over a 24-h period from 1,900 ms (95% confidence interval [CI], 1,700 to 2,100 ms) to 1,600 ms (95% CI, 1,400 to 1,900 ms; p = 0.02), although none of the patients showed the abolishment of the atrioventricular conduction abnormalities. The total 24-h count of premature ventricular beats was not different before treatment (15 beats; 95% CI, 5 to 44 beats) and after treatment (12 beats; 95% CI, 5 to 30 beats; p = 0.57). Treatment with cilostazol significantly decreased the concentration of plasma atrial natriuretic peptide from 88 pg/mL (95% CI, 49 to 160 pg/mL) to 51 pg/mL (95% CI, 32 to 80 pg/mL; p = 0.007) and of brain natriuretic peptide from 166 pg/mL (95% CI, 71 to 389 pg/mL) to 77 pg/mL (95% CI, 30 to 178 pg/mL; p = 0.02). Conclusions: Cilostazol significantly increased the ventricular escape rate and significantly decreased the level of circulating... [ABSTRACT FROM AUTHOR]