Usefulness of isoproterenol during atrial fibrillation in evaluation of asymptomatic Wolff-Parkinson-White pattern

The asymptomatic individual with a Wolff-Parkinson-White (WPW) pattern is considered at risk for ventricular fibrillation if a rapid ventricular response (shortest RR interval ≤ 250 ms) is observed during induced atrial fibrillation (AF) in the laboratory. It has been suggested that isoproterenol administration during AF may more accurately define the patient at risk. Consequently, the effect of isoproterenol on ventricular response during AF was studied in 21 asymptomatic individuals with WPW pattern to assess the potential of isoproterenol to identify patients at risk for sudden death. An electrophysiologic study that included elective induction of AF was performed. The shortest and mean RR intervals between 2 consecutive preexcited and normal QRS complexes, the average RR interval and the proportion of preexcited QRS complexes were measured in the control state and after bolus injections of isoproterenol (0.5, 1.0, 2.0 and 4.0 μg) during AF. Both atrioventricular nodal and accessory pathway conductions were enhanced proportional to isoproterenol dose. Isoproterenol had a greater effect on the atrioventricular node, as reflected by significantly greater changes in the shortest RR between normal complexes (339 ± 70 vs 255 ±21 ms, mean ± standard deviation, p < 0.001) than the shortest RR between preexcited complexes (264 ± 39 vs 219 ± 34 ms, p < 0.001) and a decrease in percentage of preexcited complexes (65 ± 37 vs 50 ± 33%, p < 0.01). After isoproterenol, 67% of patients had a shortest RR between preexcited complexes ≤ 250 ms, the traditional marker of risk for ventricular fibrillation, versus 33% in the control state. These results suggest that isoproterenol predictably accelerates the ventricular response in patients with asymptomatic WPW syndrome primarily by enhancement of atrioventricular nodal conduction. Shortening of the shortest RR interval between preexcited complexes to 250 ms occurred in 67% of patients and would be expected to have poor specificity as a potential prognosticator of ventricular fibrillation.