Most radiofrequency energy delivery systems provide a baseline (pre-ablation) impedance measurement; however, the application of this value, particularly in avoiding catheter overheating and coagulum formation, has not been described. We evaluated the ability of the product of the power output P and the baseline impedance Z (P × Z) to predict the likelihood of an impedance rise and coagulum formation during radiofrequency energy delivery in 62 consecutive patients undergoing successful catheter ablation of the slow atrioventricular (AV) nodal pathway or an accessory pathway. The mean P × Z during the 114 pulses resulting in an impedance rise was 3,770 ± 846 W Ω; only 42 impedance rises in 14 patients occurred at a P × Z < 3,500. For comparison, the P × Z during the single radiofrequency pulse that resulted in loss of pre-excitation or the slow AV nodal pathway in the entire patient group was 3,118 ± 590 (p = 0.001) and in only 9 patients was the P × Z > 3,500. This data suggest that adjusting the power during each radiofrequency pulse to maintain the P × Z < 3,500 should enable the operator to avoid most impedance rises.