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Methoxyflurane Anesthesia Augments the Chronotropic and Dromotropic Effects of Verapamil

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Methoxyflurane Anesthesia Augments the Chronotropic and Dromotropic Effects of Verapamil J Pharm Pharmaceut Sci (www.ualberta.ca/~csps) 2 (1):30-35, 1999 Methoxyflurane Anesthesia Augments The Chronotropic And Dromotropic Effects Of Verapamil. Manuscript received March 15, 1999; Revised April 30, 1999, Accepted April 30, 1999. Patrick R. Mayo, Fakhreddin Jamali Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta, Canada ABSTRACT. Introduction: Inhalation anesthetics increase to 17.6%. The time for the ECG to have been shown to have electrical suppressant normalize after exposure to VER, MF and VER + effects on excitable membranes such as the cardiac MF was 37.5 ± 15.1 min, 69.8 ± 5.3 min, and 148.5 ± conduction system. Therefore, the anesthetized 6.6 min respectively Conclusion: General anesthesia patient or laboratory animal may respond differently with MF enhances the dromotropic and chronotropic to cardiac drugs when compared with their conscious effect of VER. This should be considered when MF- counterparts. The purpose of this study was to assess anesthesia is used in experimental procedure. the effects of anesthesia with methoxyflurane (MF) on the dromotropic and chronotropic effects of INTRODUCTION verapamil (VER) in the rat. Methods: A lead I ECG was measured using subcutaneous electrodes placed Inhalation anesthetics have been shown to have anti- both axilli and over the xyphoid process in male arrhythmic or arrhythmogenic potential in both Sprague-Dawley rats. Dromotropic effect was animals and man (1, 2). For example, halothane and measured using the PR-interval which indicated the enflurane prolong refractory periods that may electrical spread across the atria to the AV-node and suppress re-entrant ventricular arrhythmia in the post- chronotropic effects were determined using RR- myocardial infarction (MI) patient (3), whereas interval. A total of six animals were randomized to methoxyflurane (MF) has been shown to increase the receive 10 mg/kg s.c. of verapamil in the presence or automaticity of the atrial conducting system (4). It absence of general anesthesia containing has also been noted that a majority of patients methoxyflurane. In addition, PR-interval and RR- experience some form of arrhythmia during surgical intervals were determined in the presence of only anesthesia due to multiple causes ranging from methoxyflurane and at rest without any drug hypoventilation to the type of anesthesia (5). In exposure. The time for the ECG to normalize after addition, the anesthetized patient is significantly exposure to methoxyflurane and/or verapamil was different from the conscious patient. For example, the also determined. Results: Exposure to verapamil addition of antiarrhythmic drugs for ventricular alone resulted in a 5% prolongation in PR-interval ectopy may actually increase the incidence of fatal and 6% prolongation in RR-interval. Methoxyflurane ventricular arrhythmia (6). This results in two major alone had a larger effect than verapamil problems. First, the unconscious patient or the demonstrating a 14.5% prolongation in PR-interval unconscious laboratory animal differs physiologically and a 12.3% in RR-interval which was statistically from their conscious counterparts. Most notably, significant. The combination of MF + VER resulted autonomic tone is altered by general anesthesia (6). in a synergistic prolongation in PR-interval to 28.7% Thus, the reaction to drugs may differ from the while the effect on RR-interval was additive with an conscious to the unconscious state. For example, it has been shown with verapamil (VER) that the minimum effective antiarrhythmic dose is 0.5 mg/kg iv in anesthetized animals and 2 mg/kg iv in 1 Corresponding author: F. Jamali, Faculty of Pharmacy conscious animals (7). The second problem involves & Pharmaceutical Sciences, University of Alberta, the possibility of a drug interaction between the Edmonton, Alberta, Canada. T6G 2N8 anesthetic agent and the test drug. A good example is email:[email protected] 30 J Pharm Pharmaceut Sci (www.ualberta.ca/~csps) 2 (1):30-35, 1999 the synergistic interaction between class I The study followed the University of Alberta antiarrhythmics such as lidocaine and the inhalation guidelines established for ethical handling of live anesthetic halothane (8). An unknown drug animals. Adult, male, Sprague-Dawley rats (Charles interaction could confound the results for the basic River Colony, n = 6, 311 ± 23 g) were used in the researcher and have fatal results for the patient. study. The animals were acclimated to a 12-hour day- Therefore it is important to conduct research in both night cycle, housed in rodent cages, and fed standard anesthetized and unanesthetized animals to determine rodent chow. if differences and/or drug interactions exist. On the morning of the experiment ECG electrodes Verapamil (VER) is a phenlyalklyamine L-type Ca2+ were placed under light anesthesia with MF. The channel blocker. It decreases electrical conduction placement of the ECG electrodes required no more across the atria and slows conduction through the than 10 minutes in all animals. The animals were AV-node. Therefore, it is widely used for placed in a Plexiglas restraining cage and allowed to supraventricular tachyarrthymias, and has a well- recover for a two-hour period. All animals had their described concentration effect relationship (9, 10). ECG's recorded every 10 mins during this 2-h Methoxyflurane (MF) is a volatile inhalation recovery period to ensure that the animal was free of anesthetic that is no longer used in humans due to any residual effects of the anesthesia. The ECG was renal toxicity, but is commonly used for surgical then recorded for an additional 2-h period to obtain anesthesia in animals. Several structurally similar baseline values. The animals were then randomized inhalation anesthetics such as halothane, isoflurane to each of three treatments: Methoxyflurane (MF) and enflurane have been shown to inhibit the animals had surgical anesthesia induced with 0.15% Na+/Ca2+ exchange and Ca2+ channels in the heart MF and maintained for 2 hours while recording their (11). It is therefore possible that MF could interact ECG. The MF was then discontinued and the animal with VER increasing the possibility of AV-block or allowed to regain consciousness while their ECG was supraventricular arrhythmia. In addition, monitored until baseline values were re-established. pharmacokinetic-pharmacodynamic animal studies Surgical anesthesia was defined as an absence of foot require the surgical placement of cannulae for serial withdrawal to painful stimuli; absence of ear blood sampling. Therefore, it is important to movement to ear prick and lack of eyelid response to understand the combined effects anesthesia and test brushing accompanied with deep rhythmic breathing. drug on Pharmacodynamics. The objective of this VER group received VER 10 mg/kg s.c. and had their study was to determine the effect of MF anesthesia ECG's recorded over a two hour period. The VER + alone or in combination with VER on the ECG of the MF group was placed under surgical anesthesia with rat. In addition, the time required to return to baseline 0.15% MF then VER 10 mg/kg s.c was administered. values after treatment with VER, MF and MF + VER The anesthesia was maintained for a 2-h period then was determined. discontinued. ECG’s were recorded until the baseline values were restored. Animals were then crossed over METHODS AND MATERIALS to the remaining treatments such that each animal served as its own control and at the end of a four-day Verapamil hydrochloride was a gift from G.D. Searle period an animal had received all four treatments. (Skokie, Ill), manufactured by Knoll Pharmaceuticals Each treatment effect was determined by comparing (Stuttgart, Germany). Methoxyflurane (Metofane) the ECG measurements recorded over a two hour was purchased from Janssen pharmaceuticals, period and calculating the percent change from veterinary division (North York, Canada). The baseline. The study is summarized in Figure 1. braided stainless steel, Teflon coated electrodes were purchased from Cooner Wire Co (Chatsworth, California). 31 J Pharm Pharmaceut Sci (www.ualberta.ca/~csps) 2 (1):30-35, 1999 Place Recovery Measure Treatment Monitor Statistical Analysis Electrodes Baseline Return to Baseline Data are expressed as the mean ± SEM. The ANOVA 10 min 2 h 2 h 2 h 2 h with Fishers protected least significant difference if post-hoc test was used to test for differences amongst necessary the treatment groups. Statistical significance was set at α = 0.05. Figure 1: Study Protocol. The ECG was recorded using a Honywell ECG RESULTS amplifier (Honywell Electronics for Medicine, Edmonton, Canada) and the data recorded on a chart The MF and VER both demonstrated significant recorder. Cardiac intervals were measured with dromotropic effects as measured by PR-interval (Figure 4). VER alone produced a significant 5% calipers (NewCon, Edmonton, Canada). The mean of seven cycles was taken for the measurement of PR, prolongation in PR-interval. QRS and RR intervals. Figure 3 demonstrates a a 30 typical rat electrocardiogram. PR 25 20 a 15 10 a 5 0 VER MF VER+ MF 20 a 18 RR 16 a 14 12 10 a 8 6 4 2 % Change from Baseline from Change % 0 VER MF VER+ MF 10 9 QRS a 8 7 b Figure 2: Electrode placement for the rat ECG. b 6 a, augmented; V, voltage; R, right; L, left; F, foot 5 4 3 2 R 1 0 VER MF VER+ MF T Figure 4: Effect of verapamil (VER), methoxyflurane P anesthesia (MF) and verapamil plus methoxyflurane S anesthesia (VER+MF) on cardiac indices. Error bars represent SEM; Significantly different (p<0.05) afrom Q baseline and other treatments, bfrom baseline and VER+MF. The MF group demonstrated a 14.5 % prolongation Figure 3: Typical rat ECG illustrating P-Wave, QRS in PR-interval. However, VER + MF clearly complex and T-Wave. demonstrated additive effects with a 28.7% prolongation in PR-interval.
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