Basic & Clinical Pharmacology & Toxicology, 2012, 110, 378–383
Doi: 10.1111/j.1742-7843.2011.00826.x
No Antidotal Effect of Intravenous Lipid Emulsion in Experimental Amitriptyline Intoxication Despite Significant Entrapment of Amitriptyline Erik Litonius1, Tomohisa Niiya1,*, Pertti J. Neuvonen2 and Per H. Rosenberg1 1
Department of Anaesthesiology and Intensive Care Medicine, Helsinki University Central Hospital and University of Helsinki, Helsinki, Finland, and 2Department of Clinical Pharmacology, University of Helsinki and HUSLAB, Helsinki University Central Hospital, Helsinki, Finland (Received 6 July 2011; Accepted 17 October 2011)
Abstract: Intravenous lipid emulsion has been used in the resuscitative treatment of intoxications caused by local anaesthetics and tricyclic antidepressants with seemingly beneficial results. We studied the effect of intravenous lipid emulsion on the plasma concentration of amitriptyline and haemodynamic recovery in a pig model of amitriptyline intoxication. Twenty pigs were anaesthetized (1% isoflurane in 21% O2) and given amitriptyline 15 mg ⁄ kg intravenously for 15 min. In random fashion immediately thereafter, either 20% lipid emulsion (ClinOleic, Lipid group) or Ringer’s acetate (Control group) was infused for 30 min.; first 1.5 ml ⁄ kg for 1 min., followed by 0.25 ml ⁄ kg ⁄ min. for 29 min. The amitriptyline concentration in total and lipid-poor plasma and haemodynamic parameters were measured until 30 min. after the infusions. Lipid infusion prevented the decrease in plasma total amitriptyline concentration, resulting in a 90% higher (p < 0.001) total concentration and significantly (p = 0.014) lower free fraction of plasma amitriptyline in the Lipid group (1.1%) compared with the Control group (3.0%) at 30 min. Haemodynamic recovery from the intoxication as measured by heart rate, arterial pressure or cardiac output was similar in both groups. However, five pigs in the Lipid group and two pigs in the Control group died. In conclusion, a marked entrapment of amitriptyline by intravenous lipid emulsion was observed but this did not improve the pigs’ haemodynamic recovery from severe amitriptyline intoxication. Care should be exercised in the antidotal use of lipid emulsion until controlled human studies indicate its efficacy and safety.
Intravenous lipid emulsion has been recommended for the treatment of severe local anaesthetic intoxication [1] and mentioned as a treatment option for amitriptyline intoxication, although with limited evidence base [2,3]. Lipid emulsion has been used in many cases of severe intoxication, and in a recently published report of severe bupropion intoxication, the patient’s haemodynamic recovery occurred within 30 min. after concurrent infusion of norepinephrine and lipid emulsion [4]. However, a recent comprehensive review describes 42 human reports of intoxications treated with intravenous lipid emulsion and underlines the possibility of positive reporting bias. The authors state that controlled studies are required to define the clinical role of lipid emulsion infusion [5]. According to the previously proposed lipid sink theory, intravenous lipid emulsion could be a useful antidote by sequestering toxins from their site of action in intoxications caused by various lipophilic drugs. In our recent study using a pig model, lipid emulsion prevented the hypotensive action Author for correspondence: Erik Litonius, Department of Anaesthesiology and Intensive Care Medicine, Helsinki University Central Hospital, PO Box 340, FI-00029 HUS, Helsinki, Finland (fax +358 9 47174017, e-mail
[email protected]). *Present address: Department of Anaesthesiology, Sapporo Medical University School of Medicine, Sapporo, Japan.
of a simultaneously infused toxic dose of amiodarone [6]. Lipid emulsion did not, however, accelerate the recovery of pigs from the cardiovascular adverse effects after infusion of toxic doses of bupivacaine or mepivacaine [7]. Amitriptyline, a highly lipophilic drug (octanol ⁄ water partition coefficient logP 4.9), is often encountered as a component in suicides and suicide attempts [8]. It easily penetrates biological membranes, is extensively (95%) bound to plasma proteins and has a large volume of distribution (15 l ⁄ kg), i.e. at steady-state, only about 0.3% of total body amitriptyline is in plasma. Amitriptyline is usually taken orally but there are formulations for intramuscular and intravenous administration. The peak plasma concentration of amitriptyline occurs within 2–8 hr after oral intake and more rapidly after parenteral administration. Amitriptyline is rapidly distributed from the blood and sequestered primarily into parenchymatous tissues [9]. Case reports of lipid rescue from amitriptyline poisoning have been published [10,11], and anionic nanosized phospholipid vesicles have been shown to have a beneficial effect on the recovery of isolated rat hearts poisoned with amitriptyline [12]. In animal studies with another equally lipophilic tricyclic antidepressant, clomipramine (logP 5.0 http:// www.vcclab.org/lab/alogps/. Accessed 2011.), treatment with intravenous lipid emulsion was effective [13,14].
2011 The Authors Basic & Clinical Pharmacology & Toxicology 2011 Nordic Pharmacological Society
LIPID EMULSION NO ANTIDOTE FOR AMITRIPTYLINE INTOXICATION
In this study, we studied the effect of intravenous lipid emulsion on plasma amitriptyline concentration and the haemodynamic recovery after amitriptyline intoxication in anaesthetized pigs. Because amitriptyline is highly lipophilic, we suggested that it would be entrapped and that the entrapment could have an effect on haemodynamic recovery from amitriptyline intoxication.
Materials and Methods The experiments were performed in the animal research laboratory at Helsinki University Central Hospital. The Finnish National Animal Experiment Board approved the experimental protocol (ESLH2008-04793 ⁄ Ym-23). The experiments were conducted in accordance with the European Community guidelines for the care and use of experimental animals. Experimental animals. Twenty landrace pigs of either sex (mean weight 29 kg [28–31 kg]) were used. They were fasted for at least 12 hr before the experiments with free access to water. No pre-medication was administered. Interventions. The pigs were anaesthetized with intravenous ketamine, first a 400 mg bolus followed by additional 50–100 mg boluses as needed. Subsequently, the trachea was intubated and volumecontrolled ventilation initiated (ServoVentilator 900C; SiemensElema, Solna, Sweden). During cannulations, 2% isoflurane in 21% oxygen was used to maintain anaesthesia. The respiratory rate was set at 20 breaths per minute, and minute ventilation was adjusted to maintain end-tidal PCO2 between 5.0% and 5.5%. During the approximately 30-min. stabilization period, Ringer’s acetate (Ringer-Acetat Baxter Viaflo; Baxter Medical, Kista, Sweden) was infused to maintain central venous pressure (CVP) at 2– 8 mmHg. The end-tidal isoflurane concentration was reduced to 1% (€ 0.1%) before the end of the stabilization period. After the stabilization period, each pig received a 15-min. infusion of amitriptyline HCl (15 mg ⁄ kg; Sigma, Darmstadt, Germany) (fig. 1). Immediately after the amitriptyline infusion, the pigs received, in random fashion (sealed-envelope simple randomization [15]), either 20% lipid emulsion (ClinOleic; Baxter S.A., Lessines, Belgium; Lipid group) or Ringer’s acetate (Control group), first 1.5 ml ⁄ kg for 1 min., then 0.25 ml ⁄ kg ⁄ min. for the following 29 min. (fig. 1), following the current recommendation for the management of clinical intoxications [1]. After the end of the treatment infusion, the pigs were monitored for 30 min. under continued anaesthesia. In case of cardiac arrest owing to ventricular fibrillation, pulseless electrical activity or asystole, external chest compressions (100 ⁄ min.) were immediately initiated, isoflurane inhalation was discontinued
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and inspiratory oxygen concentration increased to 100%. In addition, 0.5 mg boluses of adrenaline were administered as needed and external monophasic defibrillation at 150 J was performed in case of ventricular fibrillation. The resuscitative efforts were continued until the next pre-determined blood sampling time-point. At the end of the experiment, a rapid intravenous bolus of potassium chloride was used to kill the pigs. Haemodynamic measurements. When analysing the data recorded, the values at baseline (the end of the stabilization period), at the end of the amitriptyline infusion and 5, 10, 20, 30, 45 and 60 min. later were included. Blood oxygenation was monitored using a normal pulse oximeter probe attached to the tail. A 5F pulmonary artery catheter (Edwards Lifesciences LLC, Irvine, CA, USA) was inserted via jugular vein using an introducer (Cordis Corporation, Miami, FL, USA) that allowed us to continuously measure CVP and administer central venous infusions. To monitor arterial blood pressure and obtain blood samples, a femoral artery was cannulated (Arterial Cannula with FloSwitch 20G; Becton-Dickinson, Singapore, Singapore). Cardiac output was measured via the pulmonary artery catheter using the thermodilution method [16], recording the average of three measurements at each time-point. The temperature probe of the pulmonary artery catheter was also used to measure the core temperature of the animal, which was kept at 38–39C for the duration of the experiment using a movable ceiling warmer (OPN Ceiling Control Unit Type VII; Aragana, Stockholm, Sweden) and mattress warmers (Microlife AG, Widnau, Switzerland). Continuous five-lead electrocardiography (ECG) data for offline analysis were recorded using surface electrodes. The experiment was monitored using a multi-modular monitor (Datex-Ohmeda Division; Instrumentarium Corp, Helsinki, Finland). Data collection software (iCentral and S ⁄ 5 Collect; GE Health Care, Helsinki, Finland) on a laptop running Windows XP (Microsoft, Redmond, WA, USA), connected to the monitor, was used to continuously record haemodynamic values and ECG data. Blood samples and amitriptyline quantification. Arterial blood samples (15 ml each) were drawn at baseline, the end of the amitriptyline infusion and 5, 10, 20, 30, 45 and 60 min. later into heparinized tubes. The plasma was separated from the blood samples by centrifugation for 10 min. at 2500 · g and stored at )22C. Aliquots of the plasma samples from the Lipid group were further centrifuged at 20,800 · g) for 10 min. to separate the un-entrapped amitriptyline fraction, i.e. the lipid-poor aqueous fraction. The free fraction of amitriptyline was determined after separation by ultrafiltration (1200 · g for 30 min. at 25C using Centrifree Ultrafiltration Devices [Millipore Ireland Ltd, Tullagreen, Carrigtwohill, Co. Cork, Ireland]) of aliquots of the plasma samples drawn 0, 5 and 30 min. after the start of the lipid and Ringer infusions. The concentration of amitriptyline was determined using a previously described HPLC method using imipramine as an internal standard [17]. The recovery
Fig. 1. Experimental protocol. Twenty pigs were anaesthetized and instrumented. After the stabilization period, all pigs were infused 15 mg ⁄ kg amitriptyline for 15 min. in order to cause a severe intoxication. Then, in random fashion, the pigs were treated intravenously with either 20% lipid emulsion (Lipid group) or Ringer’s acetate solution (Control group). Post-treatment follow-up continued for 30 min. Haemodynamic measurements and ECG analysis were performed, and blood samples were obtained at the end of the stabilization period, at the end of the amitriptyline infusion and 5, 10, 20, 30, 45 and 60 min. after the end of the amitriptyline infusion. 2011 The Authors Basic & Clinical Pharmacology & Toxicology 2011 Nordic Pharmacological Society
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of amitriptyline was 82%, and the coefficient of variation at relevant concentrations was
