To investigate the effect of theophylline on cardiac electrophysiology, electrophysiologic testing was performed before and during aminophylline (theophylline ethylenediamine) infusions in 10 male patients with stable COPD. This produced a mean plasma theophylline concentration of 15.6 +/- 0.9 micrograms/ml (mean +/- SEM) and increases in plasma catecholamine concentrations consistent with activation of the sympathetic nervous system. Plasma epinephrine increased from 40 +/- 6 to 57 +/- 9 pg/ml (p less than 0.05), and plasma norepinephrine increased from 406 +/- 63 to 522 +/- 90 pg/ml (p less than 0.01) during the aminophylline infusion. Significant reductions were seen in the atrioventricular (81.4 +/- 6.8 to 73.0 +/- 5.6 ms, p less than 0.05) and His-Purkinje (40.7 +/- 4.3 to 36.1 +/- 3.8 ms, p less than 0.02) conduction intervals, sinoatrial conduction time (81.3 +/- 10.3 to 65.0 +/- 7.6 ms, p less than 0.05), corrected sinus node recovery time (199.0 +/- 13.4 to 148.2 +/- 22.9 ms, p less than 0.05), shortest atrial pacing interval producing 1:1 atrioventricular conduction (391.0 +/- 25.5 to 325.0 +/- 15.5 ms, p less than 0.001), and atrial effective refractory period (ERP) (224.0 +/- 7.0 to 201.0 +/- 7.2 ms, p less than 0.01), but there was no change in the intraatrial (PA) conduction interval or ventricular ERP. Although no arrhythmias were induced, 5 patients had symptoms (3 presyncope and 2 chest pain) with rapid atrial pacing during but not prior to the aminophylline infusion. These changes could be caused by direct action of theophylline on cardiac conduction tissue or they may be caused at least in part by the associated increase in sympathetic tone.(ABSTRACT TRUNCATED AT 250 WORDS)