Overall updates on sedation and anaesthesia in small animals

Anaesthesia can be defined as a controlled poisoning of the CNS, and general anaesthesia or sedation must be considered more than a protocol.

According to Smith (1959), “there are no safe anaesthetic agents, there are no safe anaesthetic procedures, there are only safe anaesthetists”¹.

So then, it should be the dictum for the entire anaesthetic process in
every practice¹.

Our challenge is to identify patient comorbidities and procedure risks considering as an individual, and maintaining a continuum of care: before-during-after must have the right approach to make the anaesthesia period as safe as possible for dogs and cats.

This article summarises the best approach for each patient and introduces some recent tips and protocols to help have the best anaesthesia practice.

Patient preparation

The pre-anaesthesia phase includes the choice of pre-anaesthetic sedative and analgesic drugs, but more importantly, a complete pre-anaesthetic evaluation².

Failure to record a physical exam has been shown to increase the odds of death³.

Consequently, risks specific to the patient’s size, breed, age, actual health status, expected pain degree, and the surgical or medical procedure need to be considered when making an individualised anaesthesia plan^3-5.

‘Define and communicate the patient’s ASA status’

Considering the American Society of Anaesthesiologists (ASA) Patient Status Scale (scoring of I–V) for categorising the patient will help you to make the best decisions for the patient requirements prior to anaesthesia² (Table 1).

Table 1. American Society of Anesthesiologists (ASA) patient status scoring
ASA status	Definitions
ASA I	Normal healthy animal; no detectable underlying disease.
ASA II	Mild systemic disease, but causing no obvious clinical signs or incapacity (animal compensating well).
ASA III	Severe systemic disease, causing clinical signs (animal not compensating fully, substantial functional limitations).
ASA IV	Severe systemic disease that is a constant threat to life.
ASA V	Moribund and not expected to survive without the procedure.
E	“E” to any class to denote an emergency (where a delay in treatment significantly increases the threat to life).

An increase in ASA status from I or II to III, or from III to IV or V, increased the odds of anaesthesia-related death in dogs and cats³; therefore, identifying the patient’s risk factors, and communicating those to the whole veterinary team, ensures everyone understands the priorities for that patient. The patient should be stabilised before anaesthesia, as it can exacerbate pre-existing physiologic compromise². Stabilisation is patient-specific and includes acute (for example, moderate to severe dehydration) and chronic (for example, congestive heart failure) stabilisation².

It starts at home

Modalities must be used to minimise patient stress level prior to leaving home.Anxiolytic drugs should definitely be administered for all aggressive, fearful patients and should be strongly considered for patients that develop any level of fear, anxiety or stress during a visit to the veterinary hospital^2,6. Reduced patient stress can reduce risk of harm to staff members who are restraining/handling patients^7,8; Table 2 shows drugs that are commonly used for this purpose.

Moreover, there are non-medication strategies that may play an important role in reducing patient fear and anxiety in the preanesthetic period, such as low-stress handling, pheromones, and environmental considerations (for example, cat-only wards)².

Anaesthetic protocol

The selection of premedication/sedation drugs must contribute significantly to appropriate perioperative analgesia, intraoperative physiologic parameters stability, and good quality of recovery⁹.

Currently, no individual drug has all these characteristics, and therefore combinations of drugs with different properties are employed. Table 3 shows different protocols available according to the patient’s ASA status.

Currently, the Dechra/AVA app can easily be used to find the appropriate drug and dosage for your patient. Find out more.

Anxiolytic, pre-anaesthetic and sedative drugs

These drugs are an essential component of anaesthesia care, and the choice of drugs will have a major impact on the general anaesthesia⁹.

Benefits include decreased stress/anxiety, dose reduction of induction and maintenance drugs (with dose-dependent adverse effects such as hypotension, hypoventilation, and so forth)¹⁰ and pain/nociception alleviated.

Specific drug combinations should be chosen according to desired effects (for example, mild to profound sedation or general anaesthesia) and individual patient needs (for example, degree of pain or nociception, ASA status, temperament).

They are either administered by the intramuscular route, to achieve effects well before induction, or intravenously just prior to induction, for acute dose-sparing effects of induction drugs (Table 3).

‘Sedation is not safer than GA’

The crucial point is that sedated patients also require appropriate monitoring and supportive care, just as those under general anaesthesia.

General anaesthesia often achieves greater patient safety, even for short procedures, especially if they are painful (for example, biopsy, laceration repair)².

Analgesic drugs

Pain is a complex and multi-dimensional sensation with multiple origins/causes; therefore, building an adequate analgesic protocol is an integral component of patient health and welfare.

An adequate protocol involves pre-emptive or preventive analgesia for an adequate duration and degree of pain.

Pre-anaesthetic administration of analgesic drugs lowers intra and postoperative analgesic requirements¹¹. The provision of analgesia optimises patient outcomes and attenuates pain-related adverse effects (for example, tachycardia, hypertension, slowed gastrointestinal motility, delayed wound healing, upregulation of pain and changes in behaviour)^11,12. A variety of drugs available for pain management exist, belonging to different classes of drugs, and are listed below:

Anti-inflammatory drugs decrease nociceptor activity. Inflammation is generally a significant component of acute pain. Because inflammation is also the pathology that produces most acute pain syndromes, control of inflammation decreases further tissue damage and speeds healing. An anti-inflammatory drug, like NSAIDs, should be administered to all appropriate patients.

Local anaesthetic drugs block pain signal transmission. Local anaesthetic drugs block sodium channels and provide complete pain relief from blocked nerves. This fact led to the recommendation that “local anaesthetics should be utilised, insofar as possible, with every surgical procedure”¹³.

Lidocaine is the only local anaesthetic that can be used intravenously and shows real analgesia benefits under constant rate infusion in a multi-modal approach.Currently, loco-regional is introduced in everyday practice more and more.

See further readings for more information about those techniques. Lidocaine bolus 1mg/kg to 2mg/kg IV, CRI 30μg/kg/min to 60μg/kg/min IV (this is not for cats).

Opioid drugs decrease receptor response in the central nervous system.

Although opioids do not block pain at its source or stop the transmission of pain, they are potent and rapidly acting, making them excellent for acute pain relief².

Opioid drugs decrease receptor response in the central nervous system and affect the modulation of the signal.

Opioids can be administered systemically (as a bolus or a CRI, see Table 3), or in neuraxial. Fentanyl bolus 2μg/kg to 5μg/kg IV, CRI 5μg/kg/h to 15μg/kg/h IV.

Alpha-2 agonist drugs decrease receptor response in the central nervous system.

Alpha-2 agonists are commonly used alongside other agents (such as opioids and ketamine) to produce sedation/anaesthesia, myorelaxation and analgesia⁶.

Like opioids, alpha-2 agonists can be administered either intravenously either as a bolus or a CRI, or intramuscularly.

Some anaesthetists advocate their use in nerve blocks alongside local anaesthetics. Dexmedetomidine bolus 0.5μg/kg IV, CRI 0.5μg/kg/h to 1μg/kg/h IV.

Ketamine: Although ketamine is used as an anaesthetic induction agent, its administration at a subanaesthetic dose (bolus and CRI) can decrease pain and prevent the development of central sensitisation, where this condition significantly amplifies pain intensity¹⁴. Ketamine bolus: 0.1mg/kg to 0.5mg/kg IV/IM, CRI 10μg/kg/min to 20μg/kg/min IV.

‘Is my pain plan working?’

Even though pain seems alleviated under general sedation or profound sedation, the patient may still experience nociception, characterised by the physiologic part of the pain, and still present and seen during the procedure.

Physiologic signs of nociception are defined as an increase of at least 20% of the basal physiologic parameters, seen before a noxious stimulus, and include blood pressure, heart rate, respiratory rate, or all parameters¹⁵.

Using a multiple analgesic drugs “strategy”, each acting at different pain pathway sites, alleviates or eliminates nociception from multiple sources¹⁰; therefore, analgesic drugs are applicable to each of the four phases of anaesthesia (pre-anaesthesia, induction, maintenance and recovery).

Postoperative pain management must still be considered as a continuum of our strategy. Veterinary literature abounds of reliable tools to evaluate, score the pain level and then adequately alleviate it. The “Glasgow Feline Composite Measure Pain Scale” (CMPS-Feline) and “Short Form of the Glasgow Composite Measure Pain Scale” (CMPS-SF) are two multi-dimensional pain scales validated for acute pain management and are easy and reliable to apply⁶.

Induction and maintenance

An appropriate premedication will result in lower doses of induction drugs. The patient’s level of sedation will influence the induction drug dosage and will be administered “to effect”².

Induction is most effectively and efficiently achieved by IV rather than IM administration and allows rapid airway control. Fast-acting drugs are propofol and alfaxalone:

• Propofol: 2mg/kg to 6mg/kg IV
• Alfaxalone: 1mg/kg to 3mg/kg for dogs; 2mg/kg to 5mg/kg for cats IV/IM

Tracheal intubation is an essential part of maintaining an open and protected airway; therefore, the patient airway should be secured using the placement of an endotracheal tube or supraglottic device as soon after induction as possible². This is even more important for critical patients, as they encounter airway dysfunction such as brachycephalic obstructive airway syndrome, pneumonia and upper airway collapse.

Inhalant anaesthetics delivered in O2 are typically used to maintain anaesthesia. This is dosed “to effect” ^16,17 and can be carried out with continuous infusions, intermittent doses of injectable agents or a combination of injectable and inhalant drugs¹⁸.

However, inhaled agents have dose-dependent, cardiorespiratory adverse effects like vasodilation/hypotension, autonomic system depression, bradycardia, hypoventilation and more.

To counteract or attenuate those adverse events, the use of systemic drugs (on bolus or CRI) at each step will be the best approach, as they have a sparing effect on them. Consider:

• Alfaxolone: CRI 0.05mg/kg to 0.2mg/kg/min IV.
• Propofol: CRI: 0.1mg/kg to 0.4mg/kg/min IV.

Preoxygenation

Preoxygenation must be considered a part of the preanaesthetic-induction step.

According to McNally et al, delivering 100% oxygen for 3 minutes provides almost 6 minutes of adequate saturation of haemoglobin with oxygen¹⁷.

However, in one concern of sustainability and environmental footprint reduction, preoxygenation must be used sparingly in healthy patients and reserved more for critical patients with airway disease (for example, pneumonia and asthma) and breathing difficulty (for example, upper airway dysfunction and limited thoracic movement), and in patients with expected difficult intubation (for example, upper airway collapse or foreign airway bodies).

Monitoring

Careful monitoring and interpretation of physiologic changes are critical. Monitoring decreases the odds of anaesthetic death¹⁹, whereas lack of monitoring increases the odds of anaesthetic death by a factor of 5 to 35⁴.

Both patient clinical examination and monitor appliances must be used. Monitoring respiratory function includes respiratory rate, oxygenation (percentage of haemoglobin saturated with oxygen; SpO₂), and ventilation. Blood pressure (BP), heart rate (HR) and rhythm (ECG), capillary refill time, mucous membrane colour and pulse oximetry (SpO₂) provide the best indices of cardiovascular function.

Finally, anaesthetic depth is also monitored and characterised by an absence of palpebral reflex, mild jaw tone (muscle relaxation) and lack of purposeful movement.

Recovery period

According to Brodbelt et al, 47% and 60% of all anaesthetic-related dog and cat deaths occur respectively during the postoperative period of anaesthesia^3-5; therefore, we should continue to monitor specific physiologic parameters such as HR/RR (respiratory rate), SpO₂, BP and body temperature, and closely observe until they are alert, normothermic and ambulatory.

The pain must still be controlled, and some sedatives may be required to have the smoothest recovery.

Conclusion

To conclude, anaesthesia can be considered more as the management of an individual rather than a protocol.

We can choose a protocol adapted to each patient and individualise it according to the history, signalment, breed, comorbidities, and type of procedure. ASA status will help us take this individual approach.

Tables included present examples of protocols and tips to help get the best approach, but for a more exhaustive list, see references.

• Some drugs referenced in this article are used under the cascade.