Research paper
Total intravenous anesthesia with alfaxalone, dexmedetomidine and remifentanil in healthy foals undergoing abdominal surgery

https://doi.org/10.1016/j.vaa.2019.01.003Get rights and content

Abstract

Objective

To evaluate effects of anesthesia induced with alfaxalone and maintained with alfaxalone, dexmedetomidine and remifentanil infusions in foals.

Study design

Prospective, experimental study.

Animals

A group of six healthy foals [median (range) 11 (8–33) days] undergoing abdominal surgery.

Methods

Intravenous (IV) dexmedetomidine (3–7 μg kg−1) provided sedation for insertion of a pulmonary artery catheter. IV anesthesia was induced with alfaxalone (2 mg kg−1) and maintained with alfaxalone (6 mg kg−1 hour−1), dexmedetomidine (1 μg kg−1 hour−1) and remifentanil (3 μg kg−1 hour−1). Foals were endotracheally intubated and lungs were mechanically ventilated with oxygen. Cardiac output (thermodilution), heart rate and systemic arterial pressure were measured. Arterial and mixed venous blood was analyzed for PO2 and PCO2, and glucose, lactate and electrolyte concentrations. Anesthetic depth was subjectively assessed. Systemic vascular resistance (SVR), oxygen utilization and intrapulmonary shunt were calculated. Preinduction (PB) or 10 minutes postinduction (+10B) data were used as baselines with one-way analysis of variance for repeated measures. Data are mean ± standard deviation; significance was p ≤ 0.05.

Results

Duration of anesthesia was 129 ± 22 minutes. One foal was administered additional alfaxalone (0.5 mg kg−1) following induction. Cardiac index decreased to 107 ± 31 and 87 ± 21 mL kg−1 minute−1 at 60 and 80 minutes, respectively, compared with PB (157 ± 33 mL kg−1 minute−1). SVR increased to 1223 ± 166 dynes second−1 cm−5 at 80 minutes compared with +10B (704 ± 247 dynes second−1 cm−5). Mean arterial pressures were 63–128 mmHg. Time from stopping infusions to standing was 46–106 minutes. All foals were hypothermic (<36 °C) and three foals were administered atipamezole (0.05 mg kg−1) intramuscularly during recovery.

Conclusion

and clinical relevance Combined alfaxalone–dexmedetomidine–remifentanil provided suitable anesthesia to permit laparotomy in foals. At the doses evaluated, prolonged recovery may occur.

Introduction

Compared with adult horses, foals <4 weeks of age have an immature sympathetic nervous system and lower myocardial compliance. Therefore, cardiac output (Q˙t) becomes more dependent on heart rate (HR) to compensate for reduced myocardial contractility. As a result, anesthetic agents or pathological conditions may exert a greater impact on cardiovascular function in young foals (Robertson, 2005, Fischer and Clark-Price, 2015). In addition, young foals have immature visceral organ systems and increased body water content that may alter drug kinetics (Baggot & Short 1984).

Alfaxalone is an anesthetic neurosteroid molecule that interacts with the γ-amino butyric receptor in the central nervous system. Alfaxalone produces smooth induction and a reliable plane of anesthesia in adult horses and butorphanol-premedicated foals (Goodwin et al., 2011, Goodwin et al., 2012). In adult horses, alfaxalone is included in total intravenous anesthesia (TIVA) for castration (Goodwin et al., 2013, Aoki et al., 2017, Deutsch et al., 2017) and electrical noxious stimulation tests (Ohmura et al. 2016). The authors have reported that alfaxalone, when infused with other agents such as medetomidine, detomidine and butorphanol, produces an acceptable quality of anesthesia for surgery (Goodwin et al., 2013, Aoki et al., 2017, Deutsch et al., 2017). No reports have been identified that describe a TIVA protocol incorporating alfaxalone for anesthesia of young foals undergoing surgery, although it has been reported in adult horses (Goodwin et al. 2019).

The α2-adrenoceptor agonist, dexmedetomidine, may be administered to adult horses during anesthesia to improve analgesia, muscle relaxation, inhalation anesthetic-sparing, and recovery quality (Gozalo-Marcilla et al. 2013). The effects of dexmedetomidine have not been described in young foals. Remifentanil is an ultrashort-acting μ-opioid receptor agonist that is rapidly metabolized by blood and tissue esterases with little or no accumulation (Lehner et al. 2000). No adverse cardiovascular effects are observed during remifentanil and dexmedetomidine infusions in isoflurane-anesthetized horses, and the recovery from anesthesia is considered good (Benmansour et al. 2014). Although used for clinical cases at this institution, there have been no published reports of remifentanil use in young foals.

We hypothesized that a combination of alfaxalone, dexmedetomidine and remifentanil infusions would provide good quality anesthesia and analgesia for abdominal surgery in healthy foals.

Section snippets

Animals

Approval for animal use was obtained from the University of Saskatchewan, Saskatoon, Canada (no. 20160008). A total of six Quarter horse foals (three male and three female) aged 11 (8–33) days and weighing 65 (48–158) kg [median (range)], were enrolled in the study and were scheduled to undergo general anesthesia in a concurrent surgical research project involving laparotomy and initiation of intra-abdominal adhesions. Foals were considered healthy prior to the study based on physical

Results

Induction of anesthesia was smooth and tracheal intubation was successful in all foals. An additional bolus of alfaxalone (0.5 mg kg−1) IV was required in one foal 10 minutes after tracheal intubation to abolish limb movement. During surgery, no additional alfaxalone was administered, and no spontaneous blinking or nystagmus occurred in any foal. Palpebral reflexes appeared brisker in all foals after the start of surgery, when compared with palpebral reflexes during the first hour of anesthesia

Discussion

The combination of IV alfaxalone, dexmedetomidine and remifentanil delivered at the infusion rates used in this study allowed successful completion of a laparotomy in these foals. Induction of anesthesia with alfaxalone was smooth and endotracheal intubation was easily performed. One foal required extra alfaxalone and the waning level of sedation from lengthy PA catheter placement probably was a factor.

The alfaxalone infusion rate was based on kinetic data obtained from a previous study of

Conclusion

This TIVA combination using alfaxalone (6 mg kg−1 hour−1), dexmedetomidine (1 μg kg−1 hour−1) and remifentanil (3 μg kg−1 hour−1) enabled abdominal surgery in foals. Foals were at a subjectively lighter plane of anesthesia after the start of surgery, therefore, diligent monitoring of anesthetic depth is recommended. Hypothermia or accumulation of anesthetic drugs may have contributed to the prolonged recovery.

Acknowledgements

This research was funded by L David Dube and Heather Ryan Veterinary Health and Research Fund (417288). The authors thank Andrew H Duke and Jocelyn McKenzie for their assistance and animal care.

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