Transthoracic impedance (TTI) is a major determinant of current flow in defibrillation, and it is therefore important to understand the factors that determine TTI. Our purpose was to evaluate the effect of atrial and ventricular arrhythmias on TTI. In anesthetized, closed-chest dogs we measured TTI by means of a technique previously validated by us, which did not require administration of actual shocks. Measurements were made at baseline (sinus rhythm) and during rapid atrial pacing (atrial fibrillation), rapid ventricular pacing, and electrically induced ventricular fibrillation (VF) with respiration discontinued. TTI was unchanged by rapid atrial or ventricular pacing. When VF was induced and respiration was discontinued, TTI fell immediately from 51.6 +/- 4.3 ohms to 45.6 +/- 4.7 ohms (p < 0.01) and did not change thereafter. The drop in TTI was probably due to respiratory arrest and decreased chest size with full exhalation; when VF was induced but respiration was continued TTI did not change, whereas discontinuing respiration caused TTI to fall even if VF was not induced. We conclude that TTI is not altered by arrhythmias.