Updated: Oct 14, 2020
Optimising intra-operative analgesia
Veterinary practitioners are currently facing a shortage of isoflurane, leading many to wonder how to manage without this drug that we have come to rely on closely.
There are many ways to reduce or spare our volatile agent use – and analgesia is the main factor to address. By reducing volatile agent usage we will see other positives - less cardiovascular depression – mainly noted by an improvement in blood pressure and an improvement in patient comfort levels related to improved analgesia.
One aim of premedication is to smooth the maintenance phase of anaesthesia. With a perfect premed you can markedly reduce reliance on isoflurane. Firstly it is important to identify monitoring techniques used to verify depth of anaesthesia is appropriate.
Questions to ask ourselves are;
Is there a blink reflex? Is there jaw tone? Is the pulse rate steady or does it vary with surgical stimulus? Same applies to respiratory rate. Does blood pressure monitoring support an appropriate anaesthetic depth? Check the ear-flick (good for cats).
This will allow us to judge our ability to turn the vaporiser down. As a rule, if your iso vaporiser is calibrated and serviced annually, if you need it set above 2%, your patient requires further analgesia and your premed needs attention!
Consider premedication as the foundation for the subsequent anaesthetic. We are aiming for analgesia and ‘background’ depth advantage – if your sedation is poor prior to induction then you are unlikely to achieve this depth advantage whereby the effect of the premedicants reduces our volatile reliance.
Premedicants that are well-suited to reducing volatile agent reliance are the alpha-2-agonists and if you are not already using these, now is probably the perfect time to make that switch. Medetomidine and dexmedetomidine provide excellent, dose-dependent sedation and also offer analgesia which is synergistic with opioids. Doses of either drug range from 1mcg/kg (0.001mg/kg) to 20mcg/kg (0.02mg/kg) in both dogs and cats. Lower doses produce mild sedation whereas higher doses will give a profound effect. Understanding the physiology of these drugs is important. Alpha-2-agonists produce vasoconstriction and bradycardia – blood pressure as a result of the vasoconstriction is good. Provided volatile agent levels are kept low, it is unlikely for hypotension to result – but may occur in anaesthetics lasting over 2 hours where the effect of the alpha-2-agonist will decline over time. The usual cautions to using alpha-2-agonists with certain cardiac pathologies apply, such as mitral valve disease and dilated cardiomyopathy.
Practical tip– select your dose of alpha-2-agonist. If the desired effect is not achieved, repeat. Give induction agents slowly (over at least 60 seconds). Reduce your usual vaporiser setting. Monitor depth closely and titrate volatile agent accordingly.
Acepromazine is not as useful as a volatile sparing agent as the alpha-2-agonists. Increasing the dose of acepromazine rarely improves sedation level – simply prolongs duration of sedation. There is no analgesia offered. If you are comfortable using acepromazine, you could consider adding small doses of alpha-2-agonists to your acepromazine-opioid premed combination to improve level of sedation – try 2-5mcg/kg IM or IV.
Benzodiazepines – remember that these are not analgesic and often do not have a significant volatile sparing effect.
The addition of ketamine to either an acepromazine-opioid or an alpha-2-agonist premed will enhance the degree of sedation and smooth the anaesthetic. Doses such as 1mg/kg will give a sedative effect plus a short-duration analgesic effect. At these doses there should be no effect on quality of recovery from anaesthesia. Prolonging this effect with a continuous rate infusion is discussed below.
Opioids are the mainstay of premedication and in combination with alpha-2-agonists (and acepromazine) produce good to excellent sedation. Analgesic effect is dose dependent so one method of improving analgesia and prolonging duration of effect is to use a higher dose. This is more applicable for methadone that buprenorphine. The clinically useful dose range for methadone in dogs and cats is 0.1-0.5mg/kg IM or IV. If you are using a lower end of this dose range it would be worth considering an increase in dose.
Practical tip– use your usual premed dose of methadone then monitor depth closely. At signs of the depth lightening, rather than increase the isoflurane, give a top up of methadone 0.1mg/kg IM or IV.
Adverse effects associated with methadone in dogs include panting, whining and nausea and in cats, salivation. If you see such effects with higher doses, reduce the dose next time methadone is due. You could also consider switching opioids.
Total blockade of the nociceptive (painful) input to the central nervous system is the gold standard for analgesia provision and the only class of drugs that achieve this fully are the local anaesthetics. With an effective local technique your patient cannot sense anything from the surgical site and your aim is simply to anaesthetise the ‘head end’ with your volatile agent. Read this introduction to using local anaesthesia here or view the Zero Pain Philosophy lecture series.
Simple blocks to start right away
Incisional (line) block – this video shows how easy this block is to perform. Infiltrate lidocaine subcutaneously (between skin and linea alba) after your first prep of the patient. With 4mg/kg lidocaine you will have plenty of volume available – most celiotomy incisions in dogs require 2-4ml. Reaction to skin incision will be reduced and the patient will be more comfortable in recovery. Evidence shows that local infiltration does not increase wound infection nor wound breakdown.
Castration – combine an incisional block with intra-testicular lidocaine along with some perineural infiltration (like you would do for a calf castrate).
The concept of balanced anaesthesia means using a variety of agents and techniques to achieve an overall balance – thereby reducing reliance of any one agent, which comes with the risk of adverse effects. Running analgesic infusions alongside your volatile agent is a simple method of providing balanced anaesthesia.
A simple and highly effective option is a ketamine continuous rate infusion. We all stock ketamine and it is a cost-effective option for excellent analgesia. There are very few contra-indications to using ketamine at these analgesic doses and I would add this as an adjunct to a whole variety of anaesthetic protocols. Read this article for information on starting out with ketamine infusions.
The alpha-2-agonists medetomidine and dexmedetomidine provide sedation and analgesia. Studies into dexmedetomidine in dogs suggest the dose rate for provision of analgesia is higher than that for sedation – this being 3mcg/kg/hr (this same rate will be effective for medetomidine too). At this rate you will see sedation to provide a background for your anaesthetic. Overall the GA will be smoother and recovery calm. It is not necessary to give atipamezole unless sedation in recovery is profound. An understanding of the physiological effects of these drugs is important – bradycardia and vasoconstriction are to be expected – although effects may be reduced at these low doses.
Fentanyl is a lesser-used opioid in UK general practice but widely used in specialist anaesthesia. Fentanyl is a short acting full mu opioid agonist which is used as a continuous rate infusion. Running fentanyl alongside our volatile agent will allow a volatile-sparing effect which is dose dependent. Recommended doses range from 5-20 mcg/kg/hr. At the higher end of this range you may see hypoventilation requiring controlled mandatory ventilation (IPPV). If you have capnography you can use this to document hypoventilation – defined as an end tidal carbon dioxide of > 45mmHg.
Nitrous oxide has long been a useful component of balanced anaesthesia. Nitrous oxide is an analgesic with opioid actions and interactions with descending inhibitory pathways, as well as NMDA activity. When used as a component of the carrier gas to vaporise isoflurane, reductions in isoflurane requirements are seen.
Other key factors in sparing volatile agent
Reduce oxygen flow
Oxygen is required to remove carbon dioxide from the breathing system. Regardless of flow rate (1L/min versus 4L/min) we are delivering 100% oxygen. Therefore turning up your oxygen in response to a dip in SpO2 is ineffective.
The higher the oxygen flow the greater the isoflurane output from the vaporiser. Turning the oxygen flow down is a perfect way to save on isoflurane, as discussed here. Capnography can help you. Register for this webinar to update your knowledge on capnography.
Service your vaporiser
We all work better when we are looked after. Do not overfill your vaporiser (never go above the line) as this affects output.
Optimising your analgesia is a priority for patient comfort and is the basis of the Zero Pain approach. The other advantages with reduced volatile reliance are clear.
Increase the analgesia, turn down the vaporiser!