Hypertonic saline does not reverse the sodium channel blocking actions of lidocaine: evidence from electrophysiologic and defibrillation studies.

Studies have shown that increasing extracellular sodium concentration can partially reverse sodium channel blockade. However, there is conflicting in vitro evidence in this regard for lidocaine. The effects of lidocaine on cardiac electrophysiology and defibrillation were studied in a basal and hypernatremic state to determine reversibility of sodium channel blockade. Electrophysiologic studies measured right ventricular effective refractory period at 350 ms pacing cycle length and QRS interval, JT interval, and monophasic action potential duration during sinus rhythm and right ventricular pacing (350 ms cycle length) in 14 pentobarbital-anesthetized swine (25-30 kg). Defibrillation threshold (DFT) was measured by quantitating successful conversion of sustained ventricular fibrillation to normal sinus rhythm. Each pig was randomly assigned to a treatment group with three study phases; group 1 = baseline, lidocaine (20 mg/kg/h), and lidocaine plus placebo (D5W; n = 7); and group 2 = baseline, lidocaine, and lidocaine plus hypertonic saline (2-3 mM/kg/h; n = 7). In groups 1 and 2, lidocaine infused alone significantly (p < 0.01) increased DFT values from baseline (9.8 +/- 3.9 to 15.7 +/- 5.8 J and 8.9 +/- 2.9 to 14.7 +/- 5.4 J, respectively) and increased QRS duration from baseline during right ventricular pacing (89 +/- 6 to 109 +/- 10 ms; p < 0.01; and 87 +/- 6 to 103 +/- 12 ms; p < 0.01). Lidocaine alone reduced right ventricular action potential duration (APD) in groups 1 and 2 (214 +/- 18 to 206 +/- 20 ms; p < 0.10; and 228 +/- 8 to 212 +/- 8 ms; p < 0.05), respectively, and it reduced paced JT interval in both groups (194 +/- 20 to 184 +/- 18 ms; p < 0.10; and 200 +/- 12 to 183 +/- 16 ms; p < 0.05), respectively. When hypertonic saline was added to lidocaine, DFT and QRS duration values were unaffected (14.7 +/- 5.4 to 16.1 +/- 3.7 J and 103 +/- 12 to 100 +/- 11 ms, respectively). However, APD and JT intervals returned to basal values when hypertonic saline was added to lidocaine (212 +/- 8 to 225 +/- 13; p < 0.05; and 183 +/- 16 to 192 +/- 18; p < 0.05, respectively). When D5W was added in the control group, no changes occurred in DFT or electrophysiologic values. Lidocaine slowed ventricular conduction velocity and reduced APD. The administration of hypertonic saline to increase extracellular sodium concentrations failed to reverse the effect of lidocaine on conduction-velocity slowing or elevated DFT values. Hypertonic saline did reverse the effects of lidocaine on repolarization parameters. These data suggest that shortening of repolarization is not a mechanism by which lidocaine makes it more difficult to defibrillate the heart.