Balanced anaesthesia

In the context of maintenance of general anaesthesia using volatile anaesthetic agents, reducing their concentration in the anaesthetic circuit stands as a key principle to prevent and manage cardiopulmonary depression. However, contemporary volatile anaesthetics lack analgesic properties, making it challenging to decrease their concentration in the anaesthetic circuit during general anaesthesia maintenance, especially in painful procedures. Insufficient intra-operative analgesia can lead to intra-operative nociception, poor recovery quality from general anaesthesia and post-operative pain. To address this challenge, incorporating supplementary systemic anaesthetics/analgesics can effectively reduce the dependence on volatile anaesthetics. This approach, known as balanced anaesthesia, not only mitigates the reliance on volatile anaesthetics but may also improve cardiovascular function, quality of recovery from general anaesthesia and intra- and post-operative pain management.

As already mention, balanced anaesthesia involves administering a combination of anaesthetic and analgesic agents to achieve the desired depth of anaesthesia and appropriate analgesia while minimising adverse effects of individual drugs. This strategy aims to attain the anaesthesia triad: hypnosis, analgesia, and muscle relaxation, all while ensuring that proper cardiorespiratory function is maintained (Bettschart-Wolfensberger & Larenza, 2007).

In the domain of equine anaesthesia, balanced anaesthesia protocols commonly entail the administration of both volatile and injectable anaesthetics and/or analgesic agents during the maintenance phase. This is also referred to as partial intravenous anaesthesia (PIVA). While balanced anaesthesia facilitates a lighter plane of anaesthesia due to the analgesic and relaxation properties of injectable agents, it can potentially increase the risk of awareness, a phenomenon hard to ascertain in veterinary patients.

PIVA has gained popularity among equine anaesthetists, as evidenced by the increasing number of related publications. Preliminary findings from the latest Confidential Enquiry into Perioperative Equine Fatalities (CEPEF-4) show a trend towards using volatile anaesthetics in combination with intravenous constant rate infusions (CRI) for PIVA. Among 6,000 inhalant-based general anaesthetics, 3,718 utilised PIVA (62%), compared to 2,282 (38%) using pure inhalation anaesthesia (Gozalo-Marcilla et al., 2021). Common drugs administered during PIVA protocols in horses include lidocaine, ketamine, opioids, and α2-adrenoceptor agonists (Gozalo-Marcilla et al., 2014, 2015).

Systemic lidocaine administration offers intraoperative analgesia and dose-dependent reduction in volatile agent requirements. Noted adverse responses to monitor for include central nervous system (CNS) toxicity, which can lead to ataxia during the recovery phase. Ketamine, administered at sub-anaesthetic intravenous doses, provides analgesia and decreases anaesthetic requirements, albeit requiring caution to avoid excitement during recovery due to accumulation of norketamine. Systemic full μ-opioids are justifiable for analgesia, despite debates about their inconsistent minimum alveolar concentration reduction and potential side effects. These days, α2-adrenoceptor agonists have gained significant popularity across diverse clinical contexts and have largely supplanted lidocaine, ketamine and opioids in PIVA protocols used to anaesthetise healthy patients undergoing routine surgical procedures. This shift can be credited to their notable contribution to better recovery quality from general anaesthesia. While cardiovascular effects of these agents raise concerns, appropriate CRI dose rates of short-acting drugs like xylazine, medetomidine, and dexmedetomidine can mitigate risks.

This presentation aims to critically review existing literature, discussing the rationale and constraints of balanced anaesthesia in horses, stimulating discussion on PIVA recommendations for equine patients.