Routine Anesthesia
By Dr. Elana Rybak, CVMA, Imaging and Anesthesia Veterinarian
We talk a lot about anesthesia for ‘non-routine’ patients - brachycephalics, those with cardiac disease, geriatric patients, etc. but what about our normal, routine cases? Let’s talk about what a routine anesthesia protocol looks like for a healthy patient without significant co-morbidities (ie-ASA I).
Your patient is Camden, a 4 year old, MN beagle that presents to you for neck pain. Camden is compliant, but very nervous and tense for his exam. He appears to have moderate neck pain on your exam and is guarding his neck during evaluation for range of motion and palpation. Camden also very much dislikes when his paws are touched for conscious proprioceptive testing. Based on your evaluation of Camden, you conclude that he needs an MRI of his neck to further evaluate for causes of his neck pain.
Bloodwork is run (CBC/Chemistry/Electrolyte panel) to make sure there are no metabolic issues or surprises for this patient. This bloodwork is unremarkable.
So let’s come up with a good sedation and anesthetic protocol for Camden. We know he is painful, tense, and does not like his paws touched. This means that he will likely benefit from heavy sedation prior to attempting to place an IV catheter, and needs a good analgesic on board. The MRI will likely take about an hour, so we need medications that will help keep our patient in a good anesthetic plane for that period of time.
I like to think of how I select drugs in the following way for balanced anesthesia:
What opioid will I use?
I always use an opioid in my sedation or anesthesia protocols. Why? I am either using them to treat or prevent pain, or I am using a drug like Butorphanol for its good sedative properties and synergistic effects with other medications.
What additional sedative will I use, if any?
In a patient that does not have a contraindication for it, my go-to is often Dexmedetomidine. This drug has excellent sedative properties and allows me to greatly decrease the dosage of induction drugs and inhalant that I will need to use. Dexmedetomidine does have some degree of analgesia as well which can help with some of our more painful patients.
Other options for a complementary sedative could include Midazolam (caution with young patients - may cause paradoxical excitation), lower doses of Alfaxalone, and even adding low dose Ketamine into the mix may be helpful.
What will I use to induce my patient?
My general go-to is Midazolam and Propofol, however for painful healthy patients I often do a combination of Midazolam with Ketamine and then top off with Propofol if needed. Midazolam and Alfaxalone is also an excellent induction combination.
Additionally, if it has been some time between my initial sedation medications and the effects have somewhat worn off, I may top-off my sedation cocktail with either the same, or a reduced dose as needed.
So let’s go back to Camden! Camden is painful so my opioid choice is going to be a pure Mu agonist. I would likely select either Methadone (0.3mg/kg) or Hydromorphone (0.1mg/kg) for this case. Due to the length of anesthesia for MRI, I would preferably not start with Fentanyl since it is quite short-acting and would need repeated dosing or a CRI to really be continually effective.
Camden is healthy, but nervous (and hates his paws being touched), so he will likely need an additional sedative to be able to place an IV catheter comfortably and with minimal stress. My #1 choice here will be Dexmedetomidine. It will allow our patient to relax, become sedate, and be compliant for IV catheter placement without hurting himself or the people restraining him.
Now that Camden is sedated, has his IV catheter placed, and has received Cerenia IV (all anesthetic cases receive Cerenia in our practice), it is time for induction.
Camden is pre-oxygenated for 5 minutes via mask. He is compliant for this as he was nicely sedate from his pre-medication. In general, if a patient is fighting pre-oxygenation, I will often give a small dose of Propofol (or Alfaxalone if that is my induction drug) to get the patient calm enough to allow effective pre-oxygenation. A top-up of a premed (for example Dexmedetomidine) could also be used.
For Camden’s induction, I will use a combination of Midazolam and Lower-dose Ketamine. Generally Midazolam at 0.2mg/kg and Ketamine at 0.5-1mg/kg. If this is not quite sufficient, Propofol (or Alfaxalone) is titrated to effect to allow intubation.
In general, this combination (Opioid + Dexmedetomidine premed, Midazolam + Ketamine +/- Propofol induction) is very effective and lends to a very smooth sedation, induction, and anesthetic maintenance for at least a ~1 hour long procedure. Ultimately, using lower dosages of several drugs allows us to beneficially use a medication, while avoiding some of the more unwanted side effects that can occur with higher doses. Don’t be afraid to use multiple drugs in your anesthesia plans - these combinations can be very safe and highly effective for a balanced anesthetic event!