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 (G≥2; 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 G≥2 (P<0.001). The amount of UFH to achieve the first ACT ≥300 seconds was significantly higher for G<2 versus G≥2 (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.