Impact of International Normalized Ratio and Activated Clotting Time on Unfractionated Heparin Dosing During Ablation of Atrial Fibrillation

Background For ablation of atrial fibrillation, it is unclear how baseline international normalized ratio (INR) affects the dosing of unfractionated heparin (UFH). Methods and Results A retrospective review of 170 consecutive patients undergoing atrial fibrillation ablation with baseline activated clotting time (ACT) and INR values was performed. Patients were grouped according to INR <2.0 (G<2; n=129) and INR 2.0 (G2; n=41). Clinical variables, UFH doses, and ACT values were recorded. An equation was derived to calculate the first bolus of UFH required to achieve an ACT 300 seconds, and this was subsequently assessed in 168 patients. For the initial 170 patients, the baseline INR (2.47 +/- 0.31 versus 1.53 +/- 0.31) and ACT (185 +/- 26 versus 153 +/- 30 seconds) were significantly greater in G2 (P<0.001). The amount of UFH to achieve the first ACT 300 seconds was significantly higher for G<2 versus G2 (9701 +/- 2390 versus 8268 +/- 2366 U; P=0.0001). Baseline INR, ACT, and weight were predictors of the UFH dosage to achieve an ACT 300 seconds. An equation derived to achieve an ACT 300 seconds after a single bolus of UFH met this end point in 160 of 168 patients (95%). Conclusions Baseline INR and ACT, in addition to weight, are the only predictors of UFH dosage needed to achieve an ACT 300 seconds. A derived equation predicted the UFH dosage to achieve an ACT 300 seconds.