ANESTHETICS AND AUTOMATICITY IN LATENT PACEMAKER FIBERS .2. EFFECTS OF HALOTHANE AND EPINEPHRINE OR NOREPINEPHRINE ON AUTOMATICITY OF DOMINANT AND SUBSIDIARY ATRIAL PACEMAKERS IN THE CANINE HEART

Knowledge of anesthetic effects on the automaticity of dominant and subsidiary cardiac pacemakers is fundamental to an understanding of mechanisms of arrhythmia during anesthesia, as well as to the management of patients with sinus node dysfunction or atrioventricular (AV) conduction block. Among potential pacemakers of the heart are subsidiary atrial pacemakers (SAP), which are located outside the classic sinoatrial (SA) node region but still within the right atrium. SAP have a higher inherent rate of automaticity than AV junctional pacemakers, may contribute to a multicentric atrial pacemaker complex, and can control the rhythm of the heart when the SA node is absent or inhibited. How halothane, epinephrine (E), or norepinephrine (NE), alone or in combination, would affect the relation between the automaticity of the SA node and SAP was tested using an isolated, perfused canine right atrial preparation (n = 78). This preparation was perfused via the SA node artery with Krebs' solution (36.0 +/- 0.5-degrees-C) equilibrated with 97% oxygen-3% carbon dioxide. Delivered concentrations of halothane of 1 or 2% corresponded to measured perfusate concentrations of 0.50 +/- 0.02 or 0.80 +/- 0.04 mm in experiments with E (n = 24) and 0.45 +/- 0.02 or 0.75 +/- 0.04 mm in experiments with NE (n = 54). E or NE perfusate concentrations were 1, 2, and 5-mu-g/l or 2, 5, and 10-mu-g/l, respectively. To determine the site of earliest activation (SEA), extracellular recordings were made from the SA node region and distal sites (approximately 1, 2, and 3 cm) along the sulcus terminalis, the previously reported locations of SAP. For control (absence of drugs), SEA was always the SA node. Alone, 1 or 2% halothane did not produce a significant number of pacemaker shifts to SAP sites. Without halothane, increasing concentrations of E or NE did produce shifts in SEA to SAP sites (P < 0.05). The magnitude of shifts to increasingly distal sites (1, 2, or 3) was normalized per number of experiments to produce a normalized magnitude score. The effect of increasing E or NE to increase normalized magnitude scores was not affected by exposure to 1 or 2% halothane. It is concluded that E or NE augment the automaticity of SAP more than that of the SA node, with or without halothane. Further, ectopic atrial rhythms with halothane require E or augmented adrenergic tone (NE).