EFFECTS OF INTRAVENOUS ANESTHETICS ON ATRIAL WAVELENGTH AND ATRIOVENTRICULAR NODAL CONDUCTION IN GUINEA-PIG HEART - POTENTIAL ANTIDYSRHYTHMIC PROPERTIES AND CLINICAL IMPLICATIONS

Background: Supraventricular tachydysrhythmias such as atrial fibrilla tion frequently complicate the perioperative period. Two electrophysio logic factors critical to the pathogenesis of supraventricular tachydy srhythmias are: 1) atrial wavelength, the product of atrial conduction velocity (CV) and effective refractory period (ERP), and 2) atriovent ricular nodal conduction. Modulation of these factors by drugs has imp ortant clinical ramifications. The authors studied the effects of prop ofol, thiopental, and ketamine on atrial wavelength and atrioventricul ar nodal function in guinea pig isolated atrial trabeculae and hearts, respectively. Methods: Electrocardiogram recordings in superfused atr ial tissue were obtained using hanging microelectrodes. A stimulating and two recording electrodes were placed on a single atrial trabecula, and the interelectrode distance was measured. Atrial ERP determinatio ns were made using a premature stimulus protocol. The time (t) require d for a propagated impulse to traverse the interelectrode distance (d) was measured. Conduction velocity was calculated as d/t. Langendorff- perfused guinea pig hearts were instrumented for low atrial pacing (cy cle length = 300 ms) and for measurements of stimulus-to-His bundle in terval, an index of atrioventricular nodal conduction. To investigate the frequency-dependent behavior of the atrioventricular node, compute r-based measurements were made of Wenckebach cycle length (WCL) and at rioventricular nodal ERP. Results: Thiopental significantly prolonged atrial ERP in a concentration-dependent manner, whereas propofol and k etamine had no significant effect on atrial refractoriness. In contras t, ketamine caused a dose-dependent decrease in atrial CV, but propofo l and thiopental had no significant effect on CV. Therefore, thiopenta l, ketamine, and propofol caused an increase, a decrease, and no chang e, respectively, in atrial wavelength. All anesthetics caused a concen tration-dependent prolongation of the stimulus-to-His bundle interval, atrioventricular nodal ERP, and WCL. However, on an equimolar basis, significant differences in potencies were found. The concentrations of drug that caused a 20% increase in ERP (ERP(20)) and WCL (WCL(20)) fo r propofol, thiopental, and ketamine were 14 +/- 2 mu M, 26 +/- 3 mu M , and 62 +/- 11 mu M, and 17 +/- 2 mu M, 50 +/- 1 mu M, and 123 +/- 19 mu M (mean +/- SEM), respectively. Therefore, the rank order of poten cy for frequency-dependent atrioventricular nodal effects is propofol > thiopental > ketamine. Conclusion: The authors' results indicate tha t propofol would be most effective at filtering atrial impulses during supraventricular tachydysrhythmias, whereas thiopental would be most effective at preventing atrial reentrant dysrhythmias. In contrast, ke tamine may be most likely to promote atrial reentry while having minim al effect on atrioventricular nodal conduction.