Interaction of vitamin K antagonists with heparin affect monitoring by activated clotting times.
BACKGROUND
Unfractionated heparin is recommended during atrial fibrillation (AF) ablation to achieve activated clotting time (ACT) above 250-300 s to prevent clot. Many patients on therapeutic international normalised ratio (INR) undergo AF ablation procedures; however, it is unknown whether they require less heparin to achieve similar ACT levels.


METHODS
During AF ablation, the ACT was measured before and 10 min after administration of i.v. unfractionated heparin in patients with and without anticoagulation. The association of INR, heparin, pre-procedure ACT and body weight with ACT after heparin administration was tested using multivariable linear regression models.


RESULTS
The subjects of this study were 149 patients undergoing AF ablation, among them 40 (27%) with subtherapeutic INR < 2, 79 (53%) with an INR between 2 and 3, and 30 (20%) patients with INR > 3. Baseline ACT was associated with INR (r = 0.33, p < 0.001). After a mean of 8,685 +/- 2,015 U (range, 5,000-15,000 IU) unfractionated heparin, univariate predictors of ACT were baseline INR (p < 0.001), heparin dose (p = 0.012) and baseline ACT (p = 0.027). In the multivariable model, baseline INR (part r = 0.64, p < 0.001) and heparin dose (part r = 0.33, p < 0.001) strongly predicted post-heparin ACT. Estimated from the regression model, the heparin dose reductions by approximately one third in those with an INR of 2-3 and by at least two thirds in those with an INR above 3 may be favourable. Over the following 3 months, no thromboembolism and acute bleeding were observed.


CONCLUSIONS
The INR was the strongest predictor of post-heparin ACT, even more important than the heparin dose itself. The reduction of heparin dose by one third if INR is between 2-3 and by two thirds if INR is above 3 may be favourable.
Muntwyler
Jorg
J
Cardiovascular Center, 8027 Zurich, Switzerland.
Attenhofer Jost
Christine H
CH
Diefenbacher
Werner
W
Beer
Jürg H
JH
Nikolic
Rada
R
Amanpour
Feri
F
Faeh-Gunz
Anja
A
Naegeli
Barbara
B
Straumann
Edwin H
EH
Maurer
Dominik
D
Candinas
Reto
R
Dang
Lam
L
Scharf
Christoph
C
D000925:Anticoagulants; D016477:Artifacts; D004347:Drug Interactions; D005260:Female; D006493:Heparin / Q000008:administration & dosage*; D006801:Humans; D019934:International Normalized Ratio / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014812:Vitamin K / Q000037:antagonists & inhibitors*; D014914:Whole Blood Coagulation Time / Q000379:methods*
D000925:Anticoagulants; D014812:Vitamin K; D006493:Heparin