 To test the hypothesis that localized functional reentry maintains Afib in humans, ibutilide will be administered intravenously in patients undergoing an Afib ablation. The hypothesis of this study is that ibutilide will decrease the high frequency signals observed in Afib suggesting the presence of micro reentrant circuits as the basic mechanism of Afib, especially for the paroxysmal Afib group. The potential difference in response to the ibutilide in patients with paroxysmal versus persistent Afib may show the difference in the underlying mechanism of Afib between these two groups. 