522 Scientific Reports: Original Study JAVMA, Vol 229, No. 4, August 15, 2006 SMALL ANIMALS M aintenance of anesthesia by use of an inhalant agent has been routinely used in clinical practice. Inhalant anesthesia has become popular in veterinary practice because anesthetic depth is easily and rapidly adjusted by changing vaporizer settings and fresh gas flow rates. Additionally, inhalant anesthetics have a favorable pharmacokinetic profile, allowing relatively rapid induction and recovery from anesthesia because anesthetic gas uptake and elimination occurs mainly via the lungs. However, one of the main concerns is the progressive cardiorespiratory depression observed with high doses of inhalant agents such as isoflurane. 1 In most instances, the cardiovascular depression caused by inhalant agents at doses adjusted to maintain a moder- ate level of anesthesia is well tolerated in healthy ani- mals undergoing elective procedures. However, high- risk patients or animals with severe systemic disease may have excessively depressed cardiovascular function if anesthesia is maintained with an inhalant alone. In this situation, balanced anesthesia techniques achieved by combining inhalant agents with drugs such as opi- oids or local anesthetics administered systemically may provide better cardiovascular stability by reducing inhalant agent requirements during anesthesia. 2-8 Fentanyl is a short-acting synthetic opioid agonist at μ receptors that has high lipid solubility and is approximately 100 times as potent as morphine. 9 Because fentanyl has a rapid onset and a short duration of action, it is suitable for continuous infusion regi- mens. Although vagally mediated bradycardia often occurs, cardiovascular stability is present even when the drug is administrated in high dosages. 4 Several studies 3,4,8,10-13 reveal that fentanyl significantly reduces inhalant requirements in a variety of species, and there is evidence that greater hemodynamic stability is achieved when fentanyl is combined with inhalant agents in a balanced anesthesia technique. Lidocaine is a local anesthetic that reversibly inhibits nerve conduction by blocking Na channels. It has been widely used in regional anesthesia tech- niques, such as nerve blocks and epidural anesthesia. It is also commonly used in the treatment of ventricu- lar arrhythmias. 14 There has been a renewed interest in the use of lidocaine infusions during anesthesia in dogs because its use reduces the MAC of inhalant anesthetics in several species. 2,5-7,15 Even though stud- ies 2-7,10-13,15 reveal that lidocaine and fentanyl infusions reduce the amount of volatile agent required to main- tain anesthesia in the laboratory setting as measured by use of classical MAC determinations, to our knowl- edge, there are no reports evaluating the inhalant- Evaluation of the isoflurane-sparing effects of lidocaine and fentanyl during surgery in dogs Paulo V. M. Steagall, MV; Francisco J. Teixeira Neto, MV, PhD, Bruno W. Minto, MV; Daniela Campagnol, MV; Mariana A. Corrêa, MV, MSc Objective—To evaluate the isoflurane-sparing effects of lidocaine and fentanyl administered by constant rate infusion (CRI) during surgery in dogs. Design—Randomized prospective study. Animals—24 female dogs undergoing unilateral mas- tectomy because of mammary neoplasia. Procedures—After premedication with acepromazine and morphine and anesthetic induction with ketamine and diazepam, anesthesia in dogs (n = 8/group) was maintained with isoflurane combined with either saline (0.9% NaCl) solution (control), lidocaine (1.5 mg/kg [0.68 mg/lb], IV bolus, followed by 250 μg/kg/min [113 μg/lb/min], CRI), or fentanyl (5 μg/kg [2.27 μg/lb], IV bolus, followed by 0.5 μg/kg/min [0.23 μg/lb/min], CRI). Positive-pressure ventilation was used to maintain eucapnia. An anesthetist unaware of treatment, end- tidal isoflurane (ETiso) concentration, and vaporizer concentrations adjusted a nonprecision vaporizer to maintain surgical depth of anesthesia. Cardiopulmonary variables and ETiso values were monitored before and after beginning surgery. Results—Heart rate was lower in the fentanyl group. Mean arterial pressure did not differ among groups after surgery commenced. In the control group, mean ± SD ETiso values ranged from 1.16 ± 0.35% to 1.94 ± 0.96%. Fentanyl significantly reduced isoflurane requirements during surgical stimulation by 54% to 66%, whereas the reduction in ETiso concentration (34% to 44%) observed in the lidocaine group was not significant. Conclusions and Clinical Relevance—Administration of fentanyl resulted in greater isoflurane sparing effect than did lidocaine. However, it appeared that the low heart rate induced by fentanyl may partially offset the improvement in mean arterial pressure that would be expected with reduced isoflurane requirements. (J Am Vet Med Assoc 2006;229:522–527) ABBREVIATIONS MAC Minimum alveolar concentration CRI Constant rate infusion SAP Systolic arterial pressure DAP Diastolic arterial pressure MAP Mean arterial pressure ETCO 2 End-tidal carbon dioxide ETiso End-tidal isoflurane HR Heart rate From the Department of Veterinary Surgery and Anesthesiology, School of Veterinary Medicine and Animal Science, São Paulo State University, Botucatu, SP 18618-000, Brazil. Presented in part at the Association of Veterinary Anaesthetists Autumn Meeting, Newmarket, United Kingdom, September 2005. Supported in part by Fapesp (Fundação de Amparo à Pesquisa de São Paulo). Address correspondence to Dr. Teixeira Neto.