Effects of the volatile anesthetic agents on sinus node function and atrioventricular conduction in dogs: A comparison with chloralose anesthesia

PM101: a cyclodextrin-based intravenous formulation of amiodarone devoid of adverse hemodynamic effects

Intravenous amiodarone (Amiodarone i.v.) is widely used to treat cardiac arrhythmias. The most frequent clinical adverse event associated with Amiodarone i.v. administration is systemic hypotension which has been attributed to the cosolvents used in the formulation, polysorbate 80 and benzyl alcohol. To minimize hypotension Amiodarone i.v. is diluted in 5% dextrose in water prior to administration and slowly infused. PM101 is a novel intravenous formulation that uses sulfobutylether-7-beta-cyclodextrin to solubilize amiodarone, and thus should be devoid of the untoward hemodynamic effects associated with polysorbate 80 and benzyl alcohol. Beagle dogs (n=7/group) were anesthetized with morphine and alpha-chloralose and instrumented to assess aortic blood pressure, cardiac output, cardiac contractility, and heart rate. Animals were treated with the U.S. approved human-equivalent loading dose (2.14 mg/kg) of Amiodarone i.v., PM101, and their respective vehicle controls. Administration of Amiodarone i.v. rapidly and significantly decreased mean aortic pressure, cardiac output, and cardiac contractility. A significant increase in heart rate was also observed as was a transient, but not significant, decrease in systemic vascular resistance. A similar pattern of rapid and significant hemodynamic changes was produced by the Amiodarone i.v. Vehicle (polysorbate 80/benzyl alcohol) alone. In marked contrast, PM101 and its vehicle produced no significant hemodynamic effects. This study provides a useful model for the continued search for a safe and effective intravenous amiodarone formulation devoid of the hypotensive risk associated with the current commercial formulation.

The effects of intrinsic nitric oxide on cardiac neural regulation in cats

In this study, we aimed to elucidate the effects of intrinsic nitric oxide (NO) on cardiac neural regulation. Twenty-two cats were anesthetized with 1.5% isoflurane and allocated to Group I (intact; n = 7), Group D (denervated baroreceptors and vagi; n = 8), or Group B (autonomic blockade with i.v. hexamethonium, propranolol, and atropine; n = 7). Cardiac sympathetic nerve activity (CSNA), mean arterial pressure (MAP), sinus heart rate (HR), and A-H and H-V intervals during pacing (150 bpm) were measured before and after i.v. administration of a NO synthase inhibitor, NG-nitro-L-arginine (L-NNA, 30 mg/kg) and after reversal with an excessive dose of L-arginine (300 mg/kg), before and during intermittent electrical stimulation of the posterior hypothalamus. L-NNA significantly increased MAP in Groups I and B, but not in Group D. L-NNA significantly decreased HR and lengthened A-H in Group I, but not in other groups. L-arginine further decreased HR and lengthened A-H unexpectedly. The reasons for these findings could not be determined in this study. L-NNA did not change CSNA. Hypothalamic stimulation did not potentiate L-NNA-induced changes in CSNA, hemodynamic variables, and atrioventricular conduction. In conclusion, intrinsic NO may modulate atrioventricular conduction and sinus rate through a vagal cholinergic, rather than a nonautonomic mechanism.

IMPLICATIONS

Elucidating the roles of intrinsic nitric oxide (NO) on cardiac neural regulation is important. In intact, vagotomized, and baroreceptor-denervated or pharmacologically autonomic blockaded cats, an NO synthesis inhibitor was administered, and atrioventricular conduction and cardiac sympathetic neural discharge were measured. The results suggest a vagal cholinergic mechanism of intrinsic NO.

The effects of dobutamine and phenylephrine on atrioventricular conduction during combined use of halothane and thoracic epidural lidocaine

The purpose of this study was to measure cardiac sympathetic nerve activity (CSNA) and atrioventricular (AV) conduction and to test the effects of dobutamine (DOB) and phenylephrine (PHE) on AV conduction during combined use of halothane and thoracic epidural lidocaine. Cats were anesthetized with 1 % halothane and an epidural catheter was inserted through T-9 laminectomy. His bundle and atrial electrocardiograms were obtained and atrial electric stimulation was performed using quadripolar catheter electrodes. Cats underwent left thoracotomy, and CSNA was recorded directly from the left ventrolateral or ventromedial nerve. In addition to sinus cycle length (SCL) measurement during spontaneous beating, the functional refractory period (FRP) of the atrioventricular node (AV node), effective refractory period (ERP) of the atrium, atrium-His (A-H) intervals were determined just before and 10, 20, and 30 min after epidural administration of 1% lidocaine (0.2 mL/kg) in Group C. DOB 5 micrograms/kg/min (Group DOB) and PHE 0.5-1.0 micrograms/kg/min (Group PHE) were intravenously administered from 12 to 22 min after epidural lidocaine. CSNA and mean arterial pressure (MAP) were markedly decreased and SCL, FRP of AV node, ERP of atrium and A-H interval were significantly prolonged after epidural lidocaine. MAP increased to baseline level during DOB or PHE infusion. Worsening of cardiac electrophysiological variables was improved with DOB infusion, but did not change with PHE infusion. We conclude that thoracic epidural lidocaine during halothane anesthesia almost eliminates CSNA, and thereby attenuates sinus node automaticity and AV node function. DOB restored normal cardiac electrophysiological variables, and therefore is preferable to phenylephrine as a pressor drug.

Additive cardiac depression by volatile anesthetics in isolated hearts after chronic amiodarone treatment

Some patients undergoing general anesthesia may be chronically receiving the antidysrhythmic drug amiodarone. The half-life of this drug is very long and it may not be advisable or possible to discontinue its administration prior to anesthesia. We examined depressant effects of three volatile anesthetics in hearts isolated from guinea pigs chronically treated with amiodarone. Hearts were isolated and perfused retrogradely through the aorta with oxygenated Krebs-Ringer solution at 37 degrees C at constant pressure. Variables measured in 26 hearts were heart rate (HR), atrioventricular, intraatrial, and intraventricular conduction times (AVCT, IACT, IVCT) during pacing at 240 bpm, coronary flow, and left ventricular pressure (LVP). Amiodarone (20 mg intraperitoneally) or placebo (Group 1) was given once daily for 1 (Group 2) or 4 (Group 3) wk. Cardiac tissue concentrations of amiodarone were similar (12.1 micrograms/g wet weight) in hearts in Groups 2 and 3 but serum levels were twice as high in hearts in Group 3 as in Group 2 (0.33 vs 0.17 microgram/mL). Before anesthetic exposure, all variables for hearts in Group 2 were not significantly different from those in Group 1. Significantly, for hearts in Group 3, compared to those in Group 1, HR was slower (-14%), conduction times were longer (IACT + 5 ms, IVCT + 4 ms, AVCT + 9 ms), coronary flow was higher (+23%), and LVP was lower (-12%). After control measurements, hearts were exposed to 0.5 and 1 minimum alveolar anesthetic concentration (MAC) halothane, enflurane, and isoflurane in random order.(ABSTRACT TRUNCATED AT 250 WORDS)

Propofol-fentanyl versus isoflurane-fentanyl anesthesia for coronary artery bypass grafting: effect on myocardial contractility and peripheral hemodynamics

To avoid intraoperative awareness and postoperative respiratory depression from high-dose opioid anesthesia, propofol (P), or isoflurane (I) has been combined with moderate-dose opioid with varying results. However, the effects of both P and I on myocardial contractility and left ventricular afterload have not been completely quantified. The end-systolic pressure-diameter relationship (ESPDR) of the left ventricle (LV) is a reliable method to quantitatively assess LV contractility because it is relatively independent of changes in preload and incorporates afterload changes. The purpose of this study was to quantify the cardiodynamic effects of propofol-fentanyl (PF) anesthesia in comparison with isoflurane-fentanyl (IF) anesthesia in patients undergoing coronary artery bypass grafting (CABG). Thirty patients with normal or moderately impaired LV function (ejection fraction > or = 40% with LV end-diastolic pressure < or = 18 mmHg, no preoperative akinesia or dyskinesia) undergoing elective CABG were studied. After premedication with flunitrazepam, 2 mg orally, all patients were induced with thiopental, 1 mg/kg, fentanyl, 20 micrograms/kg, and vecuronium, 0.1 mg/kg, and were ventilated with oxygen/air (F(1)O2 0.6). Anesthesia was maintained throughout the procedure with a zero-order intravenous (IV) continuous infusion of P, 3 mg/kg/h (PF group), or with isoflurane inhalation of 0.6% (IF group), supplemented by intermittent boluses (5 micrograms/kg) of fentanyl (up to a total maintenance dose of 30 micrograms/kg). After intubation, a cross-section of the LV was visualized by two-dimensional transesophageal echocardiography and an m-mode echocardiogram was obtained at the maximum anterior-posterior diameter. The radial artery pressure tracing and the ECG were simultaneously recorded with the M mode.(ABSTRACT TRUNCATED AT 250 WORDS)